Firstly, is there a specific forum for LOHAN ideas?
Secondly, instead of releasing LOHAN at a pre-deteremined altitude, why not rig up a device to launch LOHAN as soon as the baloon bursts? That way you would get maximum altitude...
How about after burst? Can the parachute be made big enough that the loss in altitude will be less than having to chop the lifting stage short? If it drops 500 feet before the rocket fires, you're still in the money compared with just saying "okay, 60,000 feet" or whatever. Plus you still get to launch if there's a balloon failure before any kind of preset altitude.
Some possible ideas re LOHAN project
First off, I apologize (sorry, apologise—I’m American) if this forum is the wrong place to post these ideas—I’ve tried emailing Mr. Haines directly a couple of times but with no response.
1.) Aircraft configuration:
I realize that by now the Southampton people have probably finalised aircraft design, but I might call your attention to the family of low aspect ratio lifting bodies commonly called Facetmobiles. (As it happens, I was the test pilot for the first human-carrying version, which—among other things—was flown from southern California to the famous Oshkosh, Wisconsin, airshow and back under its own power). See www.facetmobile.com. Note that small-scale unpowered Facetmobiles are regularly dropped from LOHANesque altitudes with scientific instrument payloads and return autonomously to their launch sites; those dropped above 100,000 feet probably exceed M1 during the initial descent.
Facetmobiles offer glide ratios on the order of 6:1, very low structural weight, simple construction (flat surfaces only), and relatively immense internal volume for payload.
Due to its design, a Facetmobile will automatically assume a controllable flight attitude (technical term: carefree re-entry) even if the initial encounter with sensible atmosphere is in an unfavorable attitude (technical term: arsy-versy or, in the USA, ack-basswards).
2.) Launch stabilization:
How about a small but relatively massive (say 50 gm) gyro wheel and brushless electric motor (micro model plane motor) rigidly mounted inside the spacecraft (axis of rotation parallel to spacecraft longitudinal axis). This could be spun up immediately prior to launch (from an inclined rail) by a battery that remains on the launch truss; it would continue spinning long enough to stabilise during powered flight. This would provide rigidity in pitch and yaw until adequate q is attained. As for roll, so long as the thrust vector is properly oriented through the C/G of the vehicle, who cares? (see “carefree re-entry” above).
I call your attention to the Ruby autopilot (www.uthere.com). Weighs about 60 gm, includes its own GPS, plugs into regular RC model servos, costs $345. I’m sure there are many others—I suggest checking websites for the burgeoning FPV (first person video) segment of RC modeling. Note that for the present there are stupid security restrictions that prevent the Ruby from working outside the USA (which means putative terrorists could still use one to control some engine of destruction aimed at domestic targets but not foreign ones…go figure).
Given the planned release altitude, I would hope LOHAN could return to her launch site, metaphorically wagging her tail behind her, even if the balloon system drifts quite a long way downwind during ascent. That said, and considering the export restrictions above, I’d like to close by suggesting…
4.) Where to fly:
The central plains of the USA are not only vast and flat, but crisscrossed by roads at regular one-mile intervals. Not the most dramatic scenery, but ideal for tracking and recovering Unofficial Flying Objects.
Better yet, I’d suggest the annual Burning Man Festival in the Nevada desert (www.burningman.com). Not only is it an (at least) once in a lifetime event to visit, but it includes an ephemeral airstrip on which visitors arrive in everything from Ultralights to turboprops—and there are almost sure to be a couple of helicopters whose owners would very likely be willing to aid in LOHAN recovery.
Best of luck to you all!
You know how distended the balloon can get before it is at risk of bursting, so that's what you aim for, not for a specific altitude. A couple of overlapping conductors, such as aluminium foil, can be attached so that at a certain point of distension they are no longer in contact. Cover them with an elastic membrane to keep them flat and in contact in the wind and you're done: before the balloon bursts, a circuit breaks and you're done: simple.
In the spirit of KISS - and I know it's nowhere near as fun as GPS or pressure related ideas, why don't you just set a timer to trigger the launch after the time you estimate it'll take the the balloon to reach roughly the desired height? It won't be an exact science but it should be close enough for jazz. If you have any data from PARIS on how long it took the payload to reach it's max height, that should help you make an educated guess as to when to set the timer to go bang.
It sounds too easy, am I being thick?
I'm still concerned about the single launch shaft.
During ascent if the wind buffeting is heavy enough it could cause the wings to smash into the teflon-coated girders and cause some significant damage.
Any thought of extending the teflon strips down until they are just standing off the wing surface? That should minimize the chance of wing damage due to buffeting. You wouldn't want them constantly touching the wing due to the chance of freezing, but a small ( 0.5") gap should give the chance for the wings to move still, freeing them up from any potential stickiness, but reducing the overall roll movement so as to prevent damage.
Good point. If it flaps about it might bend the titanium rod as well. Is there anything to stabilise the plane along the roll-axis while on the launcher?
Perhaps you could have the trailing edge of the wings rest inside a couple of forward-facing (teflon-coated?) U-shaped brackets while in the launch position. No need for them to touch, but it might help to stabilise the plane in case of turbulence.
Not as elegant as Tempest8008's idea, only U-brackets would distribute the restraining force over both wings instead of just one.
So what are your plans regarding the on-board GNC? Have you thought about the controller hardware/software you are going to put into Vulture 2? How about sticking a Raspberry Pi in there (assuming you can get your hands on one – I’m sure you can pull some strings). Would be a good way to engage with a wider community and possibly get some nice coverage for all of you. Would also be nice to open source some of the control software and let the community chip in with ideas/suggestions. I seem to recall reading about some guys using ‘FlightGear’ (as an aero model) to train their auto-pilot software? How about something like that?
Doesn't look too stable to me. Where I live every house has an aluminium channel embedded in the concrete ceiling by the windows, in which nylon glides slide to hold the curtains.
If said channel could be supported the length of the truss, and the glides secured to the LOHAN, it might be a readily available, stable launch runway.
...or rod, or whatever you're sliding along, just to avoid arguments.
I've just suggested this in the last SPB article, but now there's a proper forum for dissecting ideas:
What about using chemical heat pack powder inside a hollow launch guide? The powder will get "up to" 45C according to most heat pack instructions, though the instructions also tend to state "do not disassemble". The ingredients are rust, activated charcoal and water anyway though, so hardly a major risk.
How much thermal expansion the rig will go through can be tested on the ground with a frozen sleeve and heated launch guide. The heat packs are good for anything between 8 and 24 hours when used as sold, depending on what pack you buy. I've noticed the 24 hour ones will take maybe an hour to get to full temperature though.
I did also suggest using resistive wire to make heat, but the various modes of failure plus a honking great battery to melt ice over a 5 or 10 minute period would probably make the chemical powder idea better.
Launch should occur on a clear day therefore the dew point will be below ambient at all elevations. Although the rod will cool rapidly, it should be slightly warmer than the ambient temp so there will be no condensation on the rod or tube. No freezing will occur. The balloon must not pass through any clouds. Stay away from grease of any kind. A light sprinkling of powdered graphite wouldn't hurt. Stay with the single rod. I know of no amateur rocket people that use a tandem rail. Anyone who has ridden in a balloon knows there is no buffeting. Phone up Sir. Richard B., I'm sure he'll concur.
Manual telescope tracking is unlikely to work , but automatic telescope tracking could be used if you are using the right tracking scope, usually the camera is set to track a pinpoint light, but in this case I'm sure a white sphere could be tracked against the blue sky if you were to play with the camera software a bit...
I'm kind of with others in wondering why the big solid backplate too. The rocket is probably going to provide a hell of a thump to that plate when it goes off, and I can't see how it'll do anything but push the whole platform back, especially if there's a halfway-reasonable rocket engine providing the punch.
Now you could possibly re-use exhaust gas pressure, but that would involve swing wing designs or some way of slotting the whole thing inside a launch tube. Engine goes off, provides pressure in the tube which should fire the rocketplane out of the end like a bullet from a gun. Wings can flip out and sabots can fall off as the aircraft leaves the barrel. Methinks the SPB team are going for something with less moving parts though, as lovely as the idea sounds, and as epic as the launch footage would be.
but wouldn't that require the 'fixed' portion of the gun to have sufficient mass to balance the reaction effect. If not it is as likely to propel the tube, girder and balloon assembly backwards as it is to push the plane forwards. And as previously stated that could swing the girder and alter the angle of the launch.
Better not to have the back plate and rely on the reaction motor entirely...
.. or go back to the vertical launch paradigm. (three balloons, vertical launch tube through the middle, swing wing glider at the bottom of the tube, discarding sabot to reuse the gas pressure in the launch tube (nice)).
The design can be widely adjusted for reaction effects and responsiveness. If there is no blast plate at all, there is a small drag force (from friction) in the direction the plane launches. Installing a blast plate across the exhaust would capture some of the thrust energy and redirect it perpendicularly, canceling it out.
Another option is to enclose the exhaust gases in a piston-launcher, which turns almost 100% of its initial thrust to pushing on both the launcher and plane; this makes a much stronger reaction force. That kicks against the launcher and violently jolts it — which gives a launch pad that retains stability in free-fall, and also protects nearby delicate bits extremely well.
I have photos that demonstrate the thrust containment; I have built a black-powder piston launcher made entirely of combustible materials. It's untouched (save the blast containment tube, which is a bit cooked) after three launches. It launches 1:48 scale F-104 plastic models with Estes engines.
A couple of issues occur to me regarding the use of a launch rod and backplate, at least as shown in the Reg diagrams.
Re the launch rod: First, whilst using a Teflon insert that completely encircles the launch rod to reduce friction seems like a good idea there are a couple of potential problems with this. The major problem is that if anything sticks to the launch rail, from ice build-up due to frozen condensation to tiny bits of grit, then by completely wrapping the launch rail with the insert you've maximised the surface area where anything stuck to the launch rail can jam against it.
A better idea would be to use just two long and thin strips (thin to minimise the swept area and long to bring the contact area back up enough to lower the contact pressure), angled about 45 deg either side of vertical, at the top of the hanger loops (and if you were to go this way then you should also cut/trim them so that they come to a point at the front, giving them a chance to divert anything stuck to the rod around them or, alternatively divert them around anything that won't shift)
An even better idea though, would be to ditch the cantilevered launch rail entirely and just use some curtain rail, which could be attached along its entire length, with a couple of roller glides which would roll over anything but a catastrophic build up of crud. Make sure you completely degrease the rail and roller glides though; any lubricant is likely to freeze and jam (this was a trick learned by photographers in the arctic/antarctic, where the lube in their cameras was prone to freeze)
Re the backplate: This is a _really_bad idea. Y'know rockets work by equal and opposite reaction? Well the rocket will go forwards because of all the stuff it's chucking out the back but all this stuff coming out the back will hit the backplate, forcing it and the entire truss backwards. However, because the truss is suspended from above what will happen is that the truss will pivot backwards, swinging the launch rail downwards just as the plane is moving along it.
Do away with the backplate entirely and just use a 'stop' on the launch rail to hold it in place and prevent it dropping off the end.
Oh yeah - re the plane swinging about uncontrollably in the wind: won't this only be an issue at low altitudes? I thought that high altitude winds were relatively 'smooth' and not very turbulent, so just launch when the low-alt weather is fairly calm; once it gets up high it'll be moving _with_ the relatively un-turbulent air.
Analysis of the previous launch shows that the balloon horizontal speed increased up to a specific altitude after which the rise was almost vertical. Indicating that the wind speed at launch should be negligible.
As far as the backplate is concerned I still maintain that the vertical launch tube is the bets option. The veritcal arragment prevents issues of launch swing and the tube can be used to reduce freezing problems. It would be possible to seal the tube completely by blocking the 'muzzle' end with wax paper, cling film or a light cap so preventing ingress of moisture. This blockage (or could we say hymen) would be broken on launch either by the launch vehicle or it's exhaust gasses. The tube could be loaded in a dry as atmosphere as possible and something like silica gel could be included to further prevent moisture build up. The tube can be insulated by expanded polystyrene to reduce the freezing effects further. maybe chemical hand warmers could be included too.
A vertical launch tube is a nice idea except that you'd have to rig it between at least three balloons for any degree of stability and it would need to be (at a rough guess) at least twenty metres long, to reach from the bottom of the tethers up past the mid-point of the balloons, to ensure that the end of the tube clears the hugely expanded balloons. This tethering rig would have keep the tube centered throughout the considerable expansion range of the balloons. It could be done but it would add a _lot_ of weight, not to mention a folding wing system for the aircraft (which would mean both extra weight and complexity).
There's a simple problem with putting any sort of air-tight cap on the launch tube too. Any cap that's flimsy enough for the aircraft to simply fly through would have burst at a relatively low altitude i.e. < 30,000 ft due to the outside air-pressure drop.
A couple of other people have worried about achieving lift from the wings during the rocket burn phase. This is actually a bit of a reverse issue because if the wings generate any real lift during rocket burn then there's a risk of the plane looping. This is because sub-sonic lift is proportional to airspeed whilst the attitude of the plane, whilst under thrust, is pretty much irrelevant; the lift will always act nearly perpendicularly to the wings so, for example, if the plane were to reach the vertical then any wing-generated lift would tend to pull it back past the vertical to inverted - not good. The wings really don't want to generate too much lift and are really only there for getting back down in a controlled manner after the rocket has burned out. For similar reasons, the launch rail need only be long enough to stabilise the plane whilst the rocket thrust stabilises. Once the rocket is burning steadily that's really the only force that's significant - all other forces will be tiny in comparison.
I agree about the length of the launch tube. probably a show stopper but given the difficulties of the PARIS separation (attributed to freezing) wouldn't the complexity be worth while? As for the pressure differential, we are only concerned with ingress of air to launch tube and as the pressure would generally be dropping throughout the accent a one way valve would fix the problem.
What about a mixed solution with a launch tube suspended at an acute angle a long way below a single balloon? The angle should be high enough to ensure that the rock expends most of it's thrust gaining altitude and the distance below the balloon would be enough to ensure that the upward trajectory did not intersect with the balloon.
"As for the pressure differential, we are only concerned with ingress of air to launch tube..." umm... I think you may have got that the wrong way around. It'll be the _outside_ air pressure that's dropping, meaning that the pressure inside the tube will be increasing relative to the outside; the issue won't be _ingress_ of air in to the tube but air trying/wanting to leave the tube, to equalise with the outside pressure.
This presents another possible problem: when you get a a sudden pressure drop in a system containing any water vapour you're likely to get the formation of clouds i.e. water droplets, which could then freeze.
Another potential issue with using a long tube is that the plane will need to displace the air in the tube ahead of it as it traverses the tube, effectively acting like a piston, which will cost it launch energy. In an open air launch though, i.e. not in a tube, the air can also be displaced around the plane/rocket. To be sure, the expanding gas from the rocket will ameliorate against this factor and aid in pushing the air out of the tube but someone (a rocket scientist?) would have to work the numbers to find the trade-off balance.
Issues with the back plate being asymmetrically attached to the launch rail...the effect of the thrust, (I agree with many of the posters here), will be to apply a rotational moment on the rail. The solution may be to have the back plate not being flat. A V shaped back plate, perhaps with a small central perforation will collect the exhaust from the rocket and be more effectively propelled directly backwards, acting more like the recoil of a gun.
In the electronics enclosure on the truss
could you (for sheer interest sake) put a thermometer, and a pressure gauge and transmit/store the data so it can be recorded for the purpose of a beautiful looking graph (or 2) post mission?
happy to help with this one, would probably be easier to store the data rather than transmit it and the whack it into excel/other spreadsheet post mission to build the graphs
Is the launch rail long enough to build sufficient airspeed for the wings to generate lift in the thin atmospher?? or just relaying on rocket power to keep the nose pointing upwards?
if the aircraft stalls as it leaves the end of the rod and pitches downwards, will the flight systems apply elevator to correct the attitude, or will the arcraft rocket propell its self nose down??
With negligible air for the wings to work with, could swinging them back out of the way be beneficial? If so, here's an idea.
How about pivoting the wings so that just momentum swings them back when the rocket is burning. A small amount of spring loadedness could bring them back to position for the glide phase.
Another possibility would be to mount the rocket in such a way that it's forward thrust against a mechanism (rack and pinion kind of thing) swings the wings back.
Problems: complicated and heavy for little benefit. If the plane was being launched on a big rocket and was going supersonic during upward cruise, then folding the wings away would be useful. But since the aerodynamic effects prior to engine ignition are negligible, the weight and complexity are not worthwhile.
On a model scale I'm not sure if a swing wing would really be that complex. You need a thick enough peg-like spring to bear the wing (or one length of metal coiled in the right places to mount both wings to), and some fishing line attached to a servo somewhere via a couple of pullys off a model yacht or whatever. Shape the body with the first couple of inches of wing built-in so that the sprung parts are supported and won't wobble about on the single spring holding them on. Obviously the mounting points for the springs will need reinforcement, but the wing struts should be pretty reinforced for a rocket plane anyway shouldn't they? This also means that you can tuck the wings right back against the body for the launch, and then use airspeed and altitude to decide when to start slowly loosening the wire.
Now if only I had money, a laser sintering thingummybobsit and some time I'd test it myself!
The moment resistance of a fixed wing can come from the mechanical properties of the skin, which will be far lighter than a hinge (because the opposite edges will be further apart and therefore subjected to much smaller tensile/compressive forces). Additionally this would obviate the requirement for energy storage for unfolding the wing — and using the engine thrust to open the wing is the worst possible approach, since that would mean unfolding the wing near the maximum airspeed. If you were to unfold the wing you would want it to happen near apogee, where the airspeed stresses are minimal.
The two advantages of swing-wings would be a more compact body at launch (which, given the essentially unlimited space around the craft, is an insignificant benefit) and the nullification of lift forces during the high-speed portion of the flight. But those could be zero anyway, if an airfoil section which is nearly symmetrical is chosen and flaperons are positioned at a slightly negative angle for the launch. Come to think of it, they needn’t even be flaperons; they could be standard ailerons with a lightweight, simple device to give slight negative displacement for the launch.
I'm strongly inclined to agree.
The benefits of folding wings are relatively small when set against the increased complexity and the resulting increased potential for things to go wrong.
Not only is there the wing swing/folding mechanism to consider but also the aileron control linkages running inside the wings (so far, the images seem to suggest that LOHAN will have ailerons).
RL swing/wing aircraft use hydraulics to actuate the wing-mounted flight control surfaces but this isn't an option on LOHAN - it'll have to use electro-mechanical servos located in the fuselage with push/pull linkages and cams. Note to the SPB team: you'll probably want to degrease any servos and cams to prevent freezing - after all, you're not going for longevity.
Well as I was suggesting, there isn't all that much extra complexity on a model scale. The transmission can basically be two fishing lines attached to a relatively strong servo toward the tail that's powerful enough to counteract a spring that'll keep the wings straight at 50, 60mph or whatever speed you're going to go to "fully extended". Having wings of a reasonable size would mean a much better glide ratio, even if you don't think drag during the rocket burn on smaller fixed wings will be a problem. I'm not sure how much wind resistance there is at 80,000 feet, but once the rocket has been burning for a couple of seconds I'm pretty sure LOHAN's velocity will be enough for even that rarified atmosphere to start tugging on any sticky-out bits with quite a force.
As for the aileron linkages, any decent model has a seperate servo for each control surface, usually with one for each aileron mounted inside the wing, forward of the aileron. These can be mixed either with a physical onboard mixer or in the transmitter (and presumably in the open source autopilot the SPB team are apparently using). It also allows ailerons to be flaperons (and elevators to be elevons) with a bit of clever mixing. At 9g or less for a decent micro-servo it's not going to be a bother on a craft of LOHAN's, erm, proportions.
Having ailerons would also mean you don't need a V tail, plus I've seen models land safely after losing one of their elevators completely. Little harder, especially for an autopilot, to do that after getting a whole wing torn off.
Also, try and make the autopilot aim straight up half a second second after leaving the platform. Use some kind of umbilical jack lead, or maybe a powerful magnet stuck to two contacts on the aircraft as an easy way to detect a launch. You also get to keep the aircraft's lightweight batteries topped up with something more heavy duty in the launch system that way. Yes, it probably won't give us much additional altitude and yes, there isn't much air up there but it's going to have some effect and it'd still look cool on a camera. Plus it might limit the damage of an odd launch angle.
Last thing, uhm, have you considered apogee detection? I'm sure you'd like to go from burn mode to glide mode in the most efficient way you can.
As to “how much atmosphere”: we can expect only a few percent of sea-level air pressure. 80,000 feet is about 24km, which is the lower stratosphere. The pressure at 20km is about 5500 Pa or about 5.5% of sea level pressure.
Now, using the formula $F_D = 1/2 /rho v^2 C_d A$  for incompressible flow aerodynamics (LOHAN is not supersonic), we can see the drag is proportional linearly to the air density $/rho$ and quadratically proportional to the airspeed $v$. It’s also linearly proportional to the frontal cross-section area (which would be improved by folding wings) and the drag coefficient (which would probably be impaired by folding wings causing a lumpier shape).
A servo motor would not be particularly heavy, but if it was located in the tail as you suggest it would wreak havoc on the balance even for a light motor — but there is really no need for that motor to be anywhere but near the hinge point. However it would take a long time, probably 30 seconds or so, to extend the wings. By that point the most optimistic benefits of folding wings are certainly gone.
You do have a good idea, though, having a relatively heavy battery in the launcher; it could be used as a heater for the touchy bits of the plane. It would also add mass to the launcher, which would add to dynamical stability on launch.
I agree that having folding/swing wings would allow longer wings and a better glide ratio but I still don't think that the added complexity and risk would be worth it because it's not going to be doing much gliding anyway. At the proposed altitude you'd not only need unfeasibly long, high-aspect ratio wings, but a pretty high airspeed as well, to gain sufficient lift for a meaningful glide.
I'm afraid I don't understand a couple of your other comments: "Having ailerons would also mean you don't need a V tail...". A V-tail doesn't replace ailerons but just reduces the number of flight/control surfaces in the tail empennage by combining the horizontal stabilisors with the fin i.e. two surfaces instead of three. Yes, you then need to mix the pitch control with the rudder control, but that's not rocket science.
Then: "plus I've seen models land safely after losing one of their elevators completely. Little harder, especially for an autopilot, to do that after getting a whole wing torn off". Do you mean the entire hstab on one side was lost, or just the elevator on one side? In either case, as long as the remaining hstab/elevator retains enough control authority then it'll still be controllable, albeit not as controllable as as you'd like (the A-10 was designed to be flyable after losing one entire hstab & rudder). But why would one of LOHAN's wings get torn off? And if one wing is torn off then you're going to be stuffed anyway (although I've heard one story of a Japanese F-15 that managed to land after getting _most_ of one wing torn off in a collision).
WRT v-tail, I was on about losing the entire stabiliser. Given independant servos, you've a chance of bringing an aircraft in with half the stabiliser gone if it's a nice airframe. Lose half of a v-tail and you're pretty buggered.
Anyway, the glide ratio doesn't need to be massively brilliant at 100,000 feet. With the wings swept back you could go for a high-speed descent, tearing toward the landing site like some NASA black ops test vehicle until the atmosphere becomes thick enough to support a more gentle glide that won't tear the bottom of the aircraft off when it hits the floor.
Plus, you know, added awesome, and all.
I agree that’s a possibility — so I refer people once more to my sketch here:
What’s not really visible in plan view is the tail structure, which is in fact TWO opposed T-tails: one pointing up, the other pointing down. This will (a)provide the largest possible aerostabilization surface, (b) leave the sides of the fuselage clear for boosters/gyros, (c)provide redundancy if one of the tails snaps off, and (d) provides a nice automatic-landing feature: when it reaches the ground, even if it’s flying nearly horizontally, the tail will strike first and drop the nose to the ground, inducing a large negative AOA and firmly landing the craft with no guidance needed.
When will you guys be ordering your radiation hardened electronics for the mission? (http://en.wikipedia.org/wiki/RAD750). I suppose you could save a few bucks by buying the stuff from Radio Shack but you'll cry when you reach the Van Halen belt. The rays you'll encounter are unlikely to be the Ray of Hope. Better to shell out some $$$'s and get the proper stuff.
When the rocket motor is activated it will reduce the weight of the balloon payload, causing the balloon to rise. As rockets tend to accelerate very slowly to start with, will the Vulture 2 craft acelerate quickly enough to escape the launch rail at all?
I suggest that instead of a launch rail, Vulture 2 is attached to the payload spar with slow-burning fuse, ignited by the rocket motor (with appropriately fire-proof connections to the craft). This will achieve separation of the components irrespective of relative acceleration.
Otherwise my guess is that the balloon will accelerate as quickly upwards ar Vulture 2 does, so you might as well attach the rocket motor to the payload spar and release Vulture 2 when it has ceased firing. Vulture 2 could then be a glider with better aerodynamics and less weight.
Just a thought but what about if the glider/rocket/launch mechanism was mounted parlty inside the balloon?
The motor ignition could be could held back by the pressure inside the balloon. When the balloon bursts the pressure in lost and the rocket fires, right at the point of maximum altitude. Yes, it would be tricky to arrange but the advantages might be worth while.
The balloon would protect the rocket and launch system from the damp and the wind. The heavy and tricky air pressure launch trigger would no longer be required. Launch could be vertical (or very nearly) and rotation and swing effects would be minimised. Also it could be guaranteed that the balloon would not interfere with the launch and maximum launch altitude would be attained.
I'm thinking of a sealed box or canister containing the rocket/glider. the lid of the canister is hinged upwards and is held open with a spring. The lid is also rigged as the trigger for the rocket firing mechanism. The lid is held closed while air is pumped out of the canister. The lid is then prevented from opening by the outside air pressure. The canister is then fitted into the neck of the balloon so that the top and lid of the canister are inside the balloon (and subject to the balloon pressure) and the bottom is outside the balloon with the instrument package tethered to the base. The canister is then held on a nominally vertical position between the lift of the balloon and the weight of the instrument package. A timer may be required to prevent the accidental launch in the early stages of the accent. but at some point the lid of the canister prevented from opening only by the difference in the pressure between the balloon and the canister. At some point the balloon bursts (explosively) and the pressure difference is lost, the lid 'pops' and the rocket is ignited.
Yes it would weigh a lot and that would reduce the maximum altitude that the balloon could take it to. On the plus side though there would be no need for the air pressure trigger so a bit or weight is saved and there would be no need to make provision to launch the rocket before the balloon bursts. If the balloon bursts before that rocket is fired the whole mission will fail so a significant margin for error must be allowed. It could be that the current safe launch altitude is less that the maximum attainable by the same balloon with heavier payload. Also the vertical launch has to be the best way to get maximum vertical distance form the rocket engine doesn't it?
Maybe just using a pipe from the neck of the balloon connected to a pressure switch of some kind as the rocket motor trigger is a sensible compromise?
A pressure switch a really silly idea. Why? Balloons hold essentially zero gage pressure, just enough to be balanced by the (very small) inward component of the tension of the thin layer of latex. It would be really hard to detect that.
What would be far easier to detect? The change from ~1g gravitational force to ~0g as the balloon bursts and the rig starts to fall. That would also do with a sensor right next to the microcontroller (maybe even built into it) rather than requiring a long (and thus heavy and interference-prone) lead to a delicate and iffy sensor.
How is the rocket assisted, high altitude, low air pressure part of the flight controlled? The control surfaces won't work at that altitude will they? In my head I see the rocket soaring vertically upwards past the balloon onto the last reaches of the Earth's surly bonds but what is to stop it soaring vertically downwards or in ever decreasing circles or back into the balloon canopy? Would the addition of a long stick, like a firework rocket work, help.
Yet another suggestion from Stupid Suggestions Inc..
The rocket/glider is suspended vertically by a 100 meters of fishing line. This line passes down through the nose of the rocket/glider right through the body and down to the rocket motor firing mechanism below. Initially the line is coiled onto a reel so the rocket/glider is suspended just below the rest of the payload. At the rocket/glider launch altitude the fishing line is released and the glider begins to drop. Some kind of damper mechanism on the reel slows the decent slightly so keeping the nose suspended and vehicle vertical. at the end of the 100 metre drop the line stops unreeling and snags. This has two effects, It fires the rocket motor and drops the fishing line tether. The glider/rocket is then in free flight 100 meters below the balloon and accelerating vertically upwards. I would have thought that at that distance it would be unlikely to hit the balloon on its accent and the problems of freezing onto the launch rail are removed.
On the down side the rocket is accelerating downwards at launch and the reel mechanism my freeze but I would have thought the reel mechanism would be easier to protect or artificially heat than the rail.
Assuming that the rocket doesn't end up with any appreciable amount of swing or spin, once the tether has been reeled, out it'll still be travelling at the same lateral speed as the balloon. Result: if it ascends vertically it'll be highly likely to hit the balloon (which will be a pretty big target once it's expanded).
Afterthought: even if its swinging it'll still be pointing towards the balloon when the engine ignites.
Keep 'em coming though, but simpler ideas are better.
Dangling the aircraft from a string gives no dynamical stability whatsoever on engine ignition. That means it will immediately try to turn around backwards — but with a rocket engine firing, it will simply tumble at random and probably break the airframe. Launch stability is an absolute requirement, and for that we MUST use the mass of the payload package and the effects of a stabilizer rail.
"no dynamical stability whatsoever".
I see what you mean; for the period between the tether being dropped and the rocket gaining forward motion the craft is in uncontrolled free fall. However that leads on to my other post. I was under the impression that the aerofoil surfaces will have a negligible effect at the launch altitude due to the thin atmosphere. What is going to stop the rocket spinning after launch?
Conventional rockets use gyro-controlled gimbal mounted engines don't they? Fireworks use the moment of inertia control of a long stick. Maybe instead of a launch rail the glider should be equipped with a long stick from the tail stuck down a long tube on the launch platform.
Given the tenuousness of the atmosphere, I figure some sort of vectored thrust is mandatory. Of course, a swiveling nozzle is unfeasible, but the V-2 (and later, the Redstone, which was a sort of super-V-2) had directable vanes that projected into the exhaust and provided a modicum of thrust vectoring.
Note the “jet vane mounting plates” here: http://www.myarmyredstonedays.com/Photos/page8/shell_04.html
And the vanes themselves: http://www.myarmyredstonedays.com/Photos/page8/shell_10.html
“During powered-flight phase, the combined effects of the jet vanes in the exhaust stream of the rocket motor and air rudders on the thrust unit produced the necessary control torque to reposition the missile.” — http://www.myarmyredstonedays.com/page12.html
But I still think the only way to get an initial kickoff with any alacrity at all, is to use a piston-launcher and use inertial reaction to get that plane moving forward.
... How much height is the rocket expected to add to the final altitude?
Is it is not much more than the balloon alone would reach then it is not worth too much worry about launch and control. Just getting the thing to altitude and firing is probably enough. A straight, upward thrust phase would be nice but up, down or in circles is not truly significant to the whole operation.
On the other hand if it is then then adding a proper vectored thrust rocket control is critical. It needs to be properly controlled in both thrust and glide phases and this requires two separate control systems. This obviously adds weight and and a lot of complexity. It needs to be thought of a rocket with wings more than a glider with an engine. This does mean however that if the rocket is properly able to control it's flight direction then the launch isn't so much of a dilemma. If the launch becomes the most critical part of the mission then the launch method that is least likely to fail is the most suitable. Any shortcoming will at least be partly corrected by the smarter vehicle.
Take a look at the setup in profile -- the image on the right side of the picture (vulture_2_launch.png). If that system were stiff, when the rocket fires the plane could slide up right off the rod. But because it's hanging from a tether, it's not stiff. When the rocket fires, the force of the thrust will create a torque that makes the entire structure want to rotate about its center of gravity, which will probably end up someplace inside the truss. The truss is stiff, but that torque will cause a rotation about the one point whose rotation about the axis pointing straight out of the picture is not constrained... which is the where the three tether lines intersect. The entire structure (truss, rod, plane) will probably rotate counterclockwise... in which case the structure may start to rotate before the plane slides off the rod, resulting in the plane coming off at a significantly different angle than intended... or not coming off at all and just spinning around.
If the rod fitted in a hole 'drilled all the way through' the body of the plane you wouldn't have to worry about icing etc. as it would be protected.
Why bother with the rod at all? It doesn't do anything for directional stability in the fraction of a second it takes for the plane to be popped off by the rocket.
I'd take a coat hanger, cut the horizontal bit and form two elbow hooks to go under/behind the wings. Tilt the hooks so the plane points up, dangle from a string and off you go. No beam even simply tandem balloons.
For heavy rockets, those in the biz use a square Aluminium C rail. That rail could replace the apex of your beam. So, the flat side of the beam faces up, and the apex points down. Then a pair of Delrin rail buttons* are screwed to the top of the plane, and they slot into the open end of the rail. The bottom end of the rail just needs a thru-bolt to stop the buttons from sliding all the way through. That also eliminates the need for a plate and bumper at the back of the plane.
Hang a drogue (sea anchor style or just a paper streamer) off of the bottom end of the beam so that it always points into the wind, and then you shouldn't have trouble with the plane twisting and binding in the channel. Any pilot knows you take off into the wind.
*For example, see: http://stores.whatsuphobby.com/-strse-Components-cln-Railbuttons/Categories.bok
The flight profile of the PARIS balloon show it ascending vertically in the last phase of it's travel. This would indicate virtually no wind. Take this together with the very low air pressure at that altitude probably makes any aerodynamic components unworkable, they would need to be so large that they would either be too heavy to lift or too fragile to survive the early accent.
Ah but rockets like to take off down wind. I tried launching a rocket into the wind once, and it became a very low-flying cruise missile which struck the house at the end of the field. Slightly downwind is the best option.
However as the balloon will be moving at the same speed as the wind, I don't see how a drogue will help in any way.
The effect you describe is called weathercocking (which is comedic gold, but I digress). It’s described in this handy NASA bulletin: http://www.grc.nasa.gov/WWW/k-12/rocket/rktcock.html
This will not be an issue for LOHAN. As you mention, balloon airspeed will be negligible. Also, weathercocking depends on extremely large tail fins and does not happen appreciably with airplanes whose major airfoils are the wings mounted amidships.
I didn't read this comment earlier, but I just want to make a note:
Even normal airplanes DO suffer from weathercocking. It's barely noticable at moderate to normal speeds, but at slow speeds the effect can be very strong. This is because at low speeds, the vertical stabilizer and rudder act like a vane. Once speed builds up they keep the pointy end pointed in the direction of travel, but at low speeds, it wants to point the pointy end into the wind. Gliders on their start-roll have been blown completely off course by a stray gust of wind. This even leading to injuries and probably even a few fatalities.
Re making the Teflon insert slightly larger than the diameter of the launch rod: this might actually make things worse for two reasons.
The first reason, and one that applies more at lower altitudes is that by leaving a slight gap between the insert and the rod you're leaving space for the ingress of wind-blown dust between the insert and the rod and once the dust is in there it won't get blown out again.
"Wind-blown dust?", I hear you say. Yes, the stuff that water droplets, which we see as clouds, nucleate around, which brings us to the more serious problem with leaving a gap between the insert and the rod and which applies more at higher altitudes.
I assume that you're going to launch on a cloudless day, so you can at least watch LOHAN for a good part of its journey, and lack of clouds will imply low dust levels: all well and good. However, this doesn't necessarily mean that there won't be much moisture in the upper atmosphere; it just might mean that there's nothing for the moisture to nucleate around/condense upon. The trouble is that when LOHAN drifts up through this air then the water will be able to condense upon it and in this case, contrary to the first mental image that may be conjured up, a moist LOHAN is not going to be a good thing.
With a relatively long launch rod, and assuming that the moisture isn't already super-cooled, any moisture that condenses upon the rod will run down it and into the slight gap between the insert and the rod and once it's there capillary action will keep it it there, almost guaranteeing freezing between the rod and the insert as LOHAN rises even further (if the moisture is already super-cooled then it'll freeze as soon as it condenses). Either of these conditions could be tricky, if they actually occur and the longer the launch rod, the worse the problems would be.
One possible solution is to (once again) do away with the single launch rod and instead use two 'L' shaped launch rods hanging down from the truss. You would then use a much shorter 'running length' than currently planned, let's say about 4-6 inches in old money, but then you'd be able to completely enclose them at the front, preventing any water ingress. Obviously, the vertical length of the two 'L' shaped hanger would have to be unequal, with the rear hanger being longer than the front so that the rear attachment point on the aircraft clears the front hanger.
Re the backplate: Whilst making it thinner/narrower is a good idea, which will reduce the back-force acting upon it, if it's going to support a long cantilevered launch rod it'll still have to be quite substantial. Going for the two 'L' shaped launch hangers would remove the need for it entirely though, as you could then put the 'stop' on one of the hanger runners, within the covering shroud, or even extend one of the covering shrouds backwards to act as a stop upon one of the hanger uprights.
This sounds like an increasingly sane proposal.
THEREFORE, I am going to take a number of previously-mentioned points and amalgamate them into one coherent counterproposal to El Reg’s approach.
Centrally, the core flaw in the Register’s Official Design (the ROD) is the rod itself. It’s too prone to jamming due to icing or to the tube freezing in place; it’s going to be too flexible to provide significant guidance since (a) the launcher will be dangling in mid-air, not anchored, and (b) it will only be attached at a single point at the tail end; it’s going to be rather heavy no matter what, if it’s long enough to hold the plane in a straight line as the craft launches; finally, it will impart a wild and unpredictable rotational moment on the launcher platform due to inevitable friction when the plane launches. The last point will render the direction of launch entirely unpredictable and will make the launcher useless for capturing takeoff video.
Therefore I suggest a totally different approach to the launcher, a π-shaped configuration with a transversely-mounted equipment bay; four short longitudinal trusses bracketing the wings; a piston-launcher which both contains initial exhaust to protect the instruments while providing dynamical stability that utilizes the launcher mass as a launch anchor; and gyro-stabilization for both the launcher and the plane, powered by batteries in the launcher.
As the plane approaches launch altitude, the gyros will spin up. These will lock in the direction of the launch, and the ones in the launcher could in principle be used to provide launch directional control (if they are spun up first). The gyro on the plane will be powered by a tether line; on launch this will separate and the drop in input voltage can trigger a timer that will, after a few seconds, engage a braking circuit in the fashion of an H-bridge which will slowly stop the gyro and allow the autopilot to perform aerodynamic control as the craft gains speed.
Once the gyros are are spun up, their momentum should be maintainable without too much power input, so this can confer directional stability even if the balloon unexpectedly blows. An accelerometer should easily detect this event, and can trigger immediate launch if the gyros are at speed and the device suddenly finds itself in low gravity.
Initial launch energy will be substantially amplified using a piston-launcher, which will (a) contain launch exhaust to protect the launcher payload, (b) magnify the initial thrust by at least an order of magnitude, and (c) lock the position of the tail w.r.t the launcher at the time of ignition, providing a positive lock on launch direction.
The legs of the π consist of a pair of short trusses around the wing bases with PTFE-coated rails that enforce the initial direction of separation, two-axis constraint can be achieved with rails along the fuselage as well. While these will be much, much bulkier than a launch rod, they will not be heavier because they can be built in a stiff space-frame style rather than relying on the rigidity of a slender rod or tube: they can be MUCH stiffer for the same weight.
It’s also worth considering that a compact payload will be easier to transport to the launch site than a long slender one; it also provides camera mounting points on either side of the aircraft, permitting better launch video and still images. Finally, by having a launcher that symmetrically encloses the tail of the aircraft rather than dangling the plane below it, the problem of torque moments causing wild gyrations is greatly reduced.
If any interest is shown in this proposal I would be happy to draw up schematic proposal plans.
Your diagram shows the truss having two short tethers that join a little way above it [the truss] and then a long single tether up to the balloon. I'd suggest using two long tethers direct to the balloon instead.
The reasons for this are that a long single tether will have very little resistance to twisting and will be prone to winding itself up and, in addition, will also be more prone to penduluming/swinging.
The result of wind-up will be spinning of the truss (in the horizontal plane) and swinging is also undesirable as it will be changing the angle at which LOHAN is actually launched. Twin tethers won't stop this entirely, of course, but it should reduce it somewhat.
Dunno what you're planning to use for the tethers but something non-stretchy would be a good idea: consider some heavy-test Dacron fishing line i.e. the stuff they use for marlin and sailfish sport fishing.
My minimal experience with ground-level rocketry and extensive experience repairing fine mechanisms would suggest that if you lube your rod very lightly, (more to repel moisture than reduce friction) frozen droplets will be easily dislodged. Judge the size of the tube just right, too tight will build up lube and bind, too loose will just bind from offset thrust. (Talk to Goldilocks ...)
Should get LOHAN off like shit off a shiny shovel.
On the diagram, you show a rubber pad on the backplate -- presumably there to keep the nozzle /tail from getting damaged by bumping into the aluminum plate. (If this assumption is incorrect, you can probably ignore the rest of this post.)
I might suggest -- either along with or in place of the plate-bumper -- a stop on the launch rod designed to hold the plane away from the plate. Two possibilities that occur are:
1 -- a ring with a set-screw (so that you can adjust its position on the rod, then lock it into place) with a cushioning pad between the ring and the launch-glides, or;
2 -- a segment of the same tube used to line the launch-glides, sufficiently long to rest on the plate and to hold the tail of the rocket away from it, glued onto the rod.
Using a segment of the tube glued securely onto the rod -- but ONLY at the base near the backplate -- could have a secondary advantage: Since it appears to be reinforced with spirals of stiff filament or wire, then there should be some mechanical friction if the ends of two sections of tube should butt against each other, somewhat in the manner of lock-washers. This rotational friction between the ends of the buffer tube and the launch-guide tube, and the resistance of the reinforcing spiral fibers to uncoil may (I think!) serve to damp some of the swing of the plane on the launch-rod that seems to have so many commentators concerned. Attaching the buffer tube at the base, while leaving the end free to rotate slightly, then to "recoil" from the increased tension in the spiral filaments, should tend to resist the airplane's rotation and to push it back to a "neutral" position on the rod.
The launcher/rocket will be swing underneath the balloon like a pendulum. Launch I guess should take into consideration attitude, altitude and the phase of the pendular swing. Launch triggered as the apparatus is moving backwards would not be as efficient as if it occurred in the forward swing. This probably most effectively be detected by another pendulum on the launcher, or much more simply by incorporating strain gauges in the the suspending cables.
If you're worried about pendular swing, the easiest solution would be to damp it by hanging a weight from the bottom point of the whole megillah (from the bottom of the backplate?), using a long cord of a different length from the main balloon tether. A long, thin fiberglas rod with a weight on the end, hanging from a swivel shackle off of the backplate might be even better -- you just want something with a different period from the main tether and any oscillations should damp each other. You could even use two rods, of different lengths shackled together with the weight hanging from the bottom of the lower rod to really break the rhythms up, but that may be overkill. Of course, this DOES add weight to the ascender, but has the advantage of NOT adding significant complexity.
First off, from the last article I see you are planning to mount the teflon "wing guides" on rubber backing to "give some bounce" to LOHANS mighty lift-generators flapping around. I am curious how you plan to do this mounting. The advantage of Teflon is that nothing really sticks to it. The disadvantage is that this includes any type of glue you can think off. (And I mean ANY glue. I've tried about 20 different manufacturer recommended products in a previous job. All of them failed with little force. NONE would be good to -60 degrees C. Mounting the strips with bolts kinda defeats the purpose of the rubber.
Secondly, in all these plans I see LOHAN's launch rod only supported at one end. (For good reason). But why not make the rod-runner separate from LOHAN and support the other end of the rod too? This would give a lot more support. The runner moves with the plane and has some sort of simple friction or force based release mechanism (my recommendation is a very small ty-rap which breaks easily enough when shock-loaded) Once the runner reaches the end of the launch rod the runner suddenly stops. LOHAN keeps going, breaking the connection to the runner and is then free to keep going.
I've had a bit of a think about the launch rod v. Launch rail conundrum but I've got to ammend my statement about support for a rail. It makes things much more complex, provides more contacts area (and thus more friction and more chances for jamming).
Thus my suggestion becomes making the "runner" not a full tube but consist of a 2 or 3 sections which are clamped by some strong springs. This, if needed the bushing can expand to accommodate rubble but will normally provide a nice tight fit on the rod.
Next, and this is just the engineer in me going, but WHY??: Why did you choose this tubing over fabricating some bushings out of PTFE bar stock on a lathe? This would allow you to choose the fit on the rod exactly as you want it. Setting the play on these tubes is going to be a lot harder. (But can be solved by my previously mentioned multisegment clamp design. Or if the tube if flexible enough, a single slit and some circular springs) How do you plan on affixing these bushings on LOHAN (or the rod-runner, see my first bit). I advise against using the ubiquitous Omega pipe clamps (random google pic example: http://store.eberliron.com/images/products/omega.jpg) They are prone to being overtightened which would seize up the bushings and a slight shift due to buffeting could likewise cause problems. Personally I'd have the LOHAN design boffins design some mountings into her and screw some self designed solid PTFE bushings into/onto those.
(Final note: I pretend to be a mechanical engineer during the day. The above contains some problems I've encountered on the job. Also, as my username implies, I might be wrong ofcourse ;)
Final Final note: when are we going to get some info on LOHANS internals (electrics) so we can shoot some holes into that design too? ;)
The Launch isn't so much of a problem. Tubes, rods, rails, whatever can all be made to work I'm sure.
The big problem is aerodynamic surfaces at low pressures.
1. How to control the flight when the existing control surfaces have no effect? I think it has been suggested that this involves gyro's, vanes and complexity.
2. What happens if at high velocity they do start to have an effect? If the rocket achieves a great enough velocity that even the thin atmosphere gives some lift to the wings then rocket control may be compromised or the crafts glide controls may be damaged.
Probably safest to keep all control surfaces neutral during rocket burn. Biggest risk is that the rocket over speeds the airframe causing damage.
I once bungee launched an RC glider - the bungee was too strong, and as soon as i applied up elevator, the nose pitched up, and the wings snapped off! the fuselage went off into the distance like a missile! very funny..
I don't see why everyone concludes so easily there won't be enough atmosphere for aerodynamic control. The Kaman line lies at 100 km (official edge of space, the altitude at which the speed of an aerodynamic lifting body needs to be equal to escape velocity to produce sufficient lift to support it's weight, thus being in orbit, not in aerodynamic flight) Even if we very generously say LOHAN will reach 40 km altitude, this is not even halfway to that point. There is PLENTY of atmosphere to reach controllable aerodynamic flight (and in fact, if LOHAN is going to enter a proper glide it WILL reach those speeds due to simple dynamics)
Using full control inputs during rocket burn is not a good idea. Just like it isn't a good idea in any aircraft to use full control inputs at speed. However, a simple output scalar that limits the maximum control surface deflection as a function of indicated airspeed will be sufficient to solve that problem. The problem here is simply one of accelerations (g-forces) and the LOHAN design boffins should be able to calculate those long before the model is even constructed.
So it would be possible to get the rocket to control itself into a vertical flight during it's burn phase and then revert to glide control afterwards?
In that case rails, rods and bumpers become an unnecessary risk factor to a successful release. Simply drop the glider and ignite the rocket.
Possible yes, without risk, most certainly not. During gliding (if the design boys/girls do their homework) LOHAN will be aerodynamically stable at a certain airspeed, with all control surfaces neutral. Then to get up to a vertical climb once the rocket starts, the control surfaces need to actively steer her into that direction. That means that in case of dead control electronics LOHAN remains in a horizontal flightpath and is propelled to very high speeds, potentially until she overspeeds and breaks up.
When launching vertically from a guide rod, LOHAN should again be stable and maintain attitude on her own during rocket burn. (This can be achieved with some smart balancing to account for lost propellant weight, the center of aerodynamic pressure and center of gravity. Then after the burn return to stable glide once speed bleeds off.
The rudder, elevator and ailerons can be actively used to keep the rocket stable and pointed in the right direction during rocket burn, but they should not be necessary. Using them to transition from horizontal to vertical is a risky proposition as you risk losing the aicraft in case of a controls failure. (Control surfaces suddenly moving to full deflection during launch risks overstressing the aircraft too. Hence limiting the deflections to a percentage of maximum in relation to either Indicated airspeed, accelerometer data or both if the control surfaces are used to provide added stability)
And finally a few more questions for the Special Project Team
- How are you planning on tracking LOHAN during the flight? Is "telescope guy" (can't be bothered to search for his name) going to give it another try?
- Will there be a test launch of LOHAN at ground level to check all mechanics and dynamic behaviour of the rig? (I really hope so, not a good idea to go into this blind and hope for the best
I think we are going in circles here.
I was under the impression that a rocket would not achieve a stable trajectory without help. Because of the distance between the centre of pressure and the centre of gravity any off centre forces would be bound to produce a turning moment. Some rockets use fins to produce aerodynamic stability by creating a restoring force at a similar distance from the centre of gravity. That would require a symmetrical tail fin arrangement (which could be arranged) and a suitable airflow over the fins.
If the air pressure is enough to produce aerodynamic forces then the wings will tend to destabilise the rocket. If not then it won't be stable at all.
Despite her backronym (which has been recognized as inaccurate from the time it was suggested) nobody says that LOHAN will be anywhere near the Kaman line or anything like in orbit. However, at the altitudes we expect her to launch the atmosphere is exceedingly tenuous — 3–5% of sea level pressure, tops. That’s thicker than the Martian atmosphere at ground level, but not by a whole lot.
In order to achieve aerostabilization in this tenuous air, the craft will require airspeed and lots of it. Since the craft will necessarily be unstable until this high speed is reached, unless it is otherwise stabilized it will tumble and wil simply never get to a speed where it IS stable.
So how do we stabilize the craft initially? I can see four possible approaches:
• Launcher: stabilize the craft mechanically while it accelerates, like ground level rocket launchers.
• Gyroscopes: an internal gyro maintains the craft’s heading as it accelerates; shortly after aerostabilization occurs the gyro is halted so as not to impede control.
• Thrust vectoring, which in my view is only attainable with vanes impinging on the rocket exhaust.
• Reaction control thrusters — this is almost certainly impractical.
Disadvantages of these approaches:
• Mechanical stability during launch must be done without an anchored launcher; this means it must use inertia, which means it must be heavy, which limits the balloon’s maximum altitude. It also implies a large launcher, since it will take some distance for the aircraft to reach speed. This can be mitigated ONLY by using a piston launcher. (I feel like I am talking to a wall here; is anyone reading these? I am the only person to utter the words “piston launcher” or to suggest capturing and utilizing otherwise-lost initial thrust energy.)
• Gyroscopes are necessarily massive and/or bulky (in order to get a large moment of inertia) and they require a strong, rigid mounting to the airframe; this could be tricky. On the other hand, when the aircraft reaches aerostable speed, it’s simple to stop a gyroscope using an H-bridge — just short the terminals of the motor. (It will coast freely while an open circuit is maintained.)
• Thrust vectoring will require making some pretty serious components; I imagine a ring with vanes being steered by servos. Problem is, it will place a heavy component at the extreme aft of the craft, which is the worst possible location for a massive component as far as aerodynamic stability goes.
And because of that tenuous atmosphere she reaches the required speed much faster, due to lower drag. I'm still not convinced aerodynamic control is impossible, but I'll let the boffins give the final word on that matter. I'm a mechanical engineer, not an aerodynamicist.
I don't see the advantage of a piston launcher. Jup, you capture a bit of lost energy. Big woop, it comes at the cost of a heavier system, which WILL be more prone to failure than a simple launch rail. And then there's the problem that this piston launcher needs to push against something. Which it can't, because the girder doesn't provide enough inertia to stay even relatively still. (I'm guessing the girder might at most be about 2x the weight of LOHAN. Any heavier and they need to rethink matters as the girder is NOT the main payload and any weight there "is wasted") Thus the kick of a piston launcher would kick the girder as much as LOHAN, possibly causing trouble with the launch or overstressing LOHAN structurally.
Another option for stability control you forgot is active reactive gyro stabilization. Spin up a gyro to roll one way, brake the gyro to move the other way. (Although you would still need a pretty heavy set of gyros to get enough reactive force and the system requires lots of power)
Thrust vectoring is not going to be as difficult as you describe. 3 vanes in a triangle around the exhaust centre axis would be enough. Then the servos to actuate those can be moved higher up in the rocket and Bowden cables or carbon fibre push-pull rods used to control them. No reason to think this would require the heavy bits to be at the bottom. With some clever design you could potentially even control these with the control surface servos, cutting the weight some
Beer, because its friday, and I feel we are kinda discussing pub physics
I think we have a kind of consensus here.
Simple aerodynamic control may be possible, with careful design, but only at speed. Thrust control is possible but complex and heavy.
Aerodynamic control has the advantage of simplicity. Minimal changes to the design to make a symmetrical set of tail tins but it cannot be tested at low altitudes and the launch is liable to be effected by the cold.
Thrust control is complex and heavy but at least it can be tested at ground level.
I'm not convinced about a piston launcher as it still relies on a reaction force from the free swinging girder.
Would it help if the glider and rocket engine were separate sections so the rocket could drop away after use?
The advantage of the piston launcher is that it will provide a consistent launch effect whether it’s suspended by the balloon or in free-fall (after the balloon pops).
The reaction mass for the piston launcher is not so much the girder (which will be even less significant, as it can be a lot shorter) but the equipment (firmly) attached to it: batteries, sensors, cameras and so on. If these are mounted near on the structure to the piston tube, the structural stresses will be manageable, and it will be possible to make the launch forces pass near the centre of mass of the launcher/payload — this will eliminate torsional moments almost entirely.
Yes, it will give a hard kick to the cameras — but it will give an equal kick to the plane, which really, really needs it!
“…because of that tenuous atmosphere she reaches the required speed much faster, due to lower drag. I'm still not convinced aerodynamic control is impossible, but I'll let the boffins give the final word on that matter. I'm a mechanical engineer, not an aerodynamicist.”
As someone with (a little) experience launching rocket planes, I will tell you right now that even at or below sea level, aerodynamic drag would never be the limiting factor — simple dynamic equilibrium, Newton’s Third Law, is the problem.
Look at the chart at http://0.u.is/_7p42 — the curve of interest is the one labeled G12-RCT. (They have said they are using a more powerful motor, but let’s use this for now). The initial thrust, right after ignition, ramps up to max power in a couple of milliseconds (see how the curve appears to intersect the vertical axis on the graph) so we can consider the thrust to be 8 lbs, roughly 40 newtons.
Given an aircraft weight of only 500 grams, which will be MUCH less than it really is, how fast could it get over three metres’ acceleration if we ignore all drag sources? Well, 40N/0.5kg = 80 m·s^-2 or about 8g; this sounds impressive (and it is), but let’s see how fast it’s going after three metres’ acceleration at 80 m·s^2:
x = v0*t + .5*a*t^2 but v0 = 0, therefore t = sqrt(2*x/a) = sqrt(2*3/80) = 0.274 seconds
Its speed will be 80*.274 = 21.9 m/s = 78 km/h or about 50 mph.
Can we agree that this is basically the minimum possible speed we can expect aerostabilization — that it would be silly to expect a rocket-plane in 5% of an atmosphere to be stable at under 80 km/h? Then it means we have to keep the mass of the plane below (500g/80N)*[initial thrust of final chosen engine] for it to work. And I don’t think that’s feasible.
Now what happens if we put in a piston launcher? The initial thrust gets immensely magnified until it’s essentially an impulse of huge magnitude, 10 times greater at least. With my small model (and a ground-based launcher) I got the plane up to ~60 km/h in about 30cm. The benefit of an air-launched one would diminish since the pad would launch itself backward — it would depend on the mass ratios of the two parts.
Do you really think they can make LOHAN's mighty lifing apendages strong enough to endure 80g launch accelerations? I would hope not. (Probably 8g is already pushing it in terms of structural loads.) Yes, a piston launcher gives more force (like I admitted already) but A: it could be too much force, and B: you'll be putting a lot of that energy into kicking the girder+payload backwards, wasting a lot of it. (Even at 5 kilograms girder+payload weight, thats only a factor of 10 LOHAN to launcher ratio, assuming 500 grams weight for LOHAN, which might be a bit low)
A piston launcher also provides much more sliding surfaces to seize up, goes against the KISS principle and would be heavier than a launch rail/rod. (Every bit of mas you don't have to lug up to altitude means better climb speed and thus less drift, or more payload weight available for other useful stuff)
"Can we agree that this is basically the minimum possible speed we can expect aerostabilization — that it would be silly to expect a rocket-plane in 5% of an atmosphere to be stable at under 80 km/h?"
Nope, I'm afraid I think it wouldn't be silly to expect that. I've seen model planes weighing close to a kilogram being stably controlled at less than walking speed (maybe 2 or 3 km an hour) at our normal sea level atmosphere. With some proper design and some sufficiently large control surfaces 80 km/h COULD be sufficient for aerostabilization at 5% atmosphere.
Like I said, it might be best if we could get an answer on the matter from the LOHAN design boffins themselves. Afterall, they are receiving/have received the proper education and tools to simulate, predict and calculate all these factors.
The 80g will not be borne by the aircraft; it will be divided up between the launcher and the airplane according to their respective masses. If the aircraft weights twice as much as the launcher, it will undergo 1/3 of the acceleration while the rest will be taken by the launcher. However, it is never a zero-sum game: any capture of initial exhaust gases to improve the initial flight speed will be beneficial. Finally, it’s easy to reduce the kick of the system, simply by adding small vents to the pistons. Any craft that can sustain the stresses of high-speed flight will be able to handle a 10g longitudinal impulse without a problem.
"Can we agree that this is basically the minimum possible speed we can expect aerostabilization — that it would be silly to expect a rocket-plane in 5% of an atmosphere to be stable at under 80 km/h?"
In your response to this, you say it would be silly and then state — just as I suggested — that we could expect minimum aerostabilized speeds of 80 km/h. That means if we don’t block ALL instabilities BELOW that speed, we will never REACH that speed because we’ll tumble and waste all the thrust from the main engine. And a piston launcher is a device that will help there.
Another is gyros, but they have undesirable side-effects at high speed. Therefore I propose a two-stage design: there will be the long-burning, low-thrust main engine and four, fast-burning JATO boosters strapped to the rear fuselage. Now, stability is still an issue — but not if we have gyros IN THE BOOSTER PACKS which are ejected when the boosters burn out. The gyros will be spun up by batteries in the launcher, and the booster packs will drop to ground with a parachute or drag streamer.
This way we have positive stabilization and hard off-the-line acceleration (two mass-intensive parts) that are only present while needed; and when the plane hits 80–100 km/h (say, 25 m/s or higher) and can manage its own thrust, the booster-stabilizers go their own way. This will happen automatically if they are held in place by their own thrust — drag will drop them off passively, or we can use ejection charges.
Finally, we can use the hard-point at which the boosters join the fuselage to (slightly) steer the booster roskets to provide a modicum of thrust vectoring in the critical early stages.
I didn't say we should expect MINIMUM 80 km/h. I'm saying that with good design 80 km/h could be sufficient. But so could 60, or 50.
And if you say that under that speed aerodynamic stability can't be reached then wouldn't there simply be insufficient external forces to make LOHAN start tumbling before she reaches that speed in the first place? With the rocket aligned to thrust through the CG there would simply be no forces big enough acting on her to cause a tumble before she reaches sufficient speed for positive aerodynamic control to kick into effect.
Even at 1/3 the acceleration, thats still close to 27g. An airframe designed for high speed flight does not necessarily be able handle 10g longtitudinal. Infact, most airplane designs are remarkable fragile in that direction. Because they never have to experience an acceleration that big in that direction and the entire mass-moment inertia of the wing has to be taken up over the small distance between the mounting points (Which would be even smaller than the wings root chord and not that much compared to the wing length. Aerodynamic drag in the longitudinal direction is by far not as big as the forces created by acceleration forces in that direction. I'm still very sceptical about trying to raise the initial launch acceleration.
Once again, the design boffins will have to give the final word, they have the exact design (and probably some shiny Finite Element Analysis software) and can calculate the maximum stresses and related accelerations.
Your launch rail has a circular cross-section, allowing the plane to rotate along its bank axis during launch and potentially bringing the wings into contact with the truss. Why not go with an oval or rectangular cross section which would prevent the plane from banking during launch? Alternatively the rail could be shaped like an inverted "T".
The rail would have to be exceedingly wide in order to impart any stabilizing moment — and if it was, then any roll motion it’s intended to correct would either damage the rail or jam the launch.
Far better to use separate, widely spaced rails (such as lightweight trusses guiding the wings directly).
We need to get LOHAN moving really fast off the line. Unfortunately, since she’s a boost-glider, that is antithetical to the design of her engine which is aimed at a long, smooth burn and not an almighty kick in the pants.
I have suggested a piston launcher as a possible remedy. But I have another idea now.
What if the aircraft is suspended @ CG (its wing roots) on a rocket-trolley? The plane and cart engine ignitions are simultaneous in a sort of cluster launch (this will require a lot of kick into the igniters, probably a capacitive-discharge ignition). But the launch-cart would have the fastest-burning engines available, and as many of them as is feasible. When they burn out, the JATO cart will simply fall off the airframe and parachute down (deployment triggered by aircraft separation).
This will accomplish a few goals: (1) bring up initial thrust to maximize off-the-line speed. (2) Keep aircraft weight and bulk down by jettisoning the boosters, while allowing a long burn for the sustainer engine as is desirable for a boost-glider. (3) Eliminate the reaction kick of the piston launcher.
See my reply slightly further up at "launch-smaunch" faster acceleration is going to get you in trouble due to massive structural loads. The 8g or maybe 10g push of LOHANS mighty thruster itself is going to push the structure to the limits I suspect. MAYBE you could boost it to 20, but that's probably going to be the max.
As with any launch method discussed here there is essentially no fixed point. Any forces on the launch mechanism can only be counteracted by the inertia from the mass of the rest of the payload (which is deliberately kept to a minimum) and the balloon itself (little mass, negative weight and little air resistance at altitude). Even it doesn't freeze it will have to be remarkably friction free to prevent the rocket simply pulling the whole girder/balloon combo along with it as there is so little 'pulling the other way'. Without sufficient mass there is nothing to keep a rail pointing in the same direction all the time either.
Without a fixed point to launch from the craft must simply be released and be able to set its own course.
Not quite, Magnus — you are right, the launcher should be considered free-floating, and the balloon will have negligible effect on the launch dynamics. But there are still plenty of ways in which the effects of the plane and launcher can be adjusted to allow a more positive launch than release at zero airspeed (which has ~0 chance of success).
There doesn't have to be, a rocket provides it's own reaction force and doesn't need anything to push against. Only if you start adding other contraptions to the design do you "need" the inertia of the lifting rig.
I'm very much against the idea about strapping more boosters to either LOHAN or the launch rig (Or complicating the launch rod any further than a simple rod and runner). Every bit of complexity you add means another possible point of failure. And every possible point of failure means a bigger chance of something going wrong. Better adhere to KISS. Possibly get a bigger rocket with a special design combustion chamber (yes a solid fuel rocket has one too) to provide high initial thrust and then drop to sustained boost. Adding drop away boosters just gives Murphy's Law too much chance to come into effect.
My point is that there is nothing holding the rod still or at a fixed direction for the plane to run off it.
If the mass of the girder+payload is zero, the air resistance is zero and the friction of plane against the rod is any value greater than zero then the rod will remain with the plane. In most designs the power is out of line with rod so a turning moment is created. . In real life the values are all small and so small differences are significant. If the friction is higher than the mass can balance then the plane may drag the girder. If the mass is too low the girder may rotate about its centre of gravity. It will fail to constrain or even impede the launch.
The rod (or any other device attached to the girder) is unlikely to add stability to the launch but is likely to cause problems.
The inertia of the launch rig is enough to overcome that friction I'd wager.
And no, the right won't be fully stable, but there's no way to improve that. Launching without initial guidance is not really an option. Even if you don't know exactly which direction the rig will be pointing at launch, the launchrod will still provide a constraint in terms of elevation and roll angles. Just free-launching LOHAN means you lose orientation contraints in just about all rotational angles.
"If the mass of the girder+payload is zero, the air resistance is zero..."
This is not true. Air resistance has no relationship with mass and is only dependent upon cross-sectional area, form (streamlining) and total surface area. Cross-sectional area is by far the greatest factor, at all speeds, and form becomes a significant factor as the speed increases. Total surface area isn't much of an issue and in any case, form tends to increase it anyway (longer = better form but also = greater total surface area).
Magnus seems to think the only thing potentially stabilizing the launcher would be a physical anchoring; this makes me think he or she has not studied dynamics.
The positional stabilization will not be absolute in the sense that a ground-mounted launcher would be, where reaction forces and moments are absorbed by the earth. Rather, the reaction forces will be neutralized at a designed balance point; the mass ratio of the plane and launcher will determine that. As for the moments/torques, those can be controlled by altering the moment of inertia of the launcher — if there is a long boom, then mounting a camera or two on its tip will greatly increase the moment of inertia and thereby reduce gyrations (while giving really neat camera angles).
As far as the boosters go, I seriously doubt a boost-glider engine will suffice. They are designed to supply low thrust and long burn durations, which is the opposite of LOHAN’s initial requirements because she MUST GET MOVING FAST, at least off-the-line, or she will never get momentum at all because she’ll be tumbling; in that case we should skip the rockets entirely and we have a glorified PARIS. Therefore short-duration boosters with beefy gyros which are dropped off on burnout seem like an ideal design.
Here is a plan view of my booster configuration: http://autographic.ca/TheRegister/LOHAN_plan.png [Warning: big fat PNG file]
Poor Coco: Thanks for answering my query. I still contend that the masses involved are purposefully low and this makes it difficult to create balancing forces. I suppose all that can be tested at ground level so not so much of a gamble.
I suggest that in order to maximise inertia in three dimensions a good design might be to arrange the payload into four equal masses mounted on the end of relatively long rods of equal length and arranged to form a tetrahedron centred on the base of the launch rod. This might also lend itself to some good camera angles.
"If the mass of the girder+payload is zero, the air resistance is zero and the friction of plane against the rod is any value greater than zero"
My point was not that air resistance is related to mass - but that both are very low so that the total of the forces available to act against friction are also very low. At the point of firing the rocket, before the plane has any speed relative to the rod, it cannot overcome even a small force due to friction so any icing or damage to the mechanisms will result in the plane dragging the launch platform. Also there is very little mass to build inertial forces to prevent the launch platform turning so any out of balance forces will render the rod critically unstable.
I'm not getting much support for my theories and there is a lot of real knowledge on this forum so I am probably wrong.
"At the point of firing the rocket, before the plane has any speed relative to the rod, it cannot overcome even a small force due to friction"
I think this is where you're going wrong. If this were true then _nothing_ would be able to accelerate from zero and move away from its initial position. Think about it; anything accelerating from zero has to start by moving very slowly, so for example, a parked (stationary) automobile has to overcome not only air resistance but the rolling resistance of its tyres* to start moving. The fact that automobiles accelerate much less quickly than our a rocket doesn't seem to stop them though (you need the M25 for that).
* As a pneumatic tyre rolls it is deformed and this uses energy.
Well, no, that’s not exactly correct. I think the point is that static friction is always at least as great (at its maximum) than sliding friction — which is true, pure and simple. I guess the objection was that a launcher floating in space would not present a great load to a plane launching off of it, in order to overcome static friction (including potentially frozen-together components).
However, while I agree that a slender guidance rod is completely wrong here, it’s not due to static friction considerations (which could be overcome by the launcher’s inertia or, worst case, by a heavily detuned piston launcher) but because a guidance rod would be barkin’ useless for something as heavy as LOHAN, especially one cantilevered into an aluminum plate. In addition to having great propensity for freezing to the launch lug, it would be floppy as hell and not direct the plane effectively. Indeed, launch rods are ONLY used in ground-based launchers with rockets weighing a couple of hundred grams tops — anything bigger than that uses a rail or tube launcher.
Well, it's correct insofar as Magnus referred to insufficient speed:
" before the plane has any speed relative to the rod..."
but I agree that static friction will always be greater than kinetic friction i.e. it'll take more force to overcome static friction and get it moving than it'll take to keep it moving once it's started.
Whilst I still also agree that a long cantilevered rod isn't a good idea, I don't think that icing will be as great a problem as I used to believe it might be: a thin layer of low-temp grease along the rod will act as a release barrier between any ice and the rod, and unless there's a really heavy build-up of ice along the rod then the force of the rocket should easily dislodge it.
Seems as though the SPB are fixated on a rod though, so I just hope they'll come around to the idea of shortening it quite a bit; it just doesn't need to be very long, given the very rapid build-up of thrust from the motor, and unless they somehow mount it way off or misaligned with the axis, or it burns very unevenly, neither of which are likely, then it'll fly pretty damn straight once the motor is going (contrary to common belief, the only reason it should divert from straight flight, once the rocket motor is burning, is if another additional force acts upon it).
“(contrary to common belief, the only reason it should divert from straight flight, once the rocket motor is burning, is if another additional force acts upon it).”
Well, the real problem is that the plane is dynamically *unstable* until stabilization moments are created by fast airflow; prior to that point any disruption that causes a minor change in the direction of the nose will lead to continually increasing deviations leading to tumbling and a total waste of the engine impulse. So, yes, it will require a destabilizing force — but those forces are EVERYWHERE, from asymmetry between the line of thrust and the CG of the plane; from turbulence in the exhaust gases; from launch rod friction; the list goes on and on. Unless non-aerodynamic stabilizers are used — and a launch rod WILL NOT WORK I will repeat that a million times — there is no realistic chance of an interesting powered flight.
"...the real problem is that the plane is dynamically *unstable*..."
This is a flawed assumption. In general, aircraft are designed to be dynamically stable, the exception being some of the relatively recent fighter aircraft, which have been designed to be unstable in an effort to improve manoeuvrability. In this case though, the students working on the aerodynamic design aren't going to purposely design an inherently unstable aircraft.
Of all the the various forces that will be acting upon the aircraft at launch, by far the greatest will be the kick up its rear from the rocket motor. However, not only is there no reason to suppose that the motor will be mounted so badly off-axis that it'll impart an unintended turning moment but the list of additional external forces does not "go on and on". In fact, the only other significant force you mention is the launch rod friction, which is already considered in the design, and which will only act along the axis of the rod i.e. it might slow the aircraft down but it can't result in an off-axis deviation. Neither will turbulence in the exhaust gases be an issue. If the there was any significant turbulence in the exhaust gases as they leave the rocket nozzle, such that it produced off-axis thrust, then no rockets would work. Sure, the gases may become turbulent _after_ they've left the nozzle but that's irrelevant because they're heading in the opposite direction, obviously, and will be behind the aircraft where they can't affect it.
Unless the launch is going to occur in intrinsically turbulent conditions, then that's about it for external forces.
Remember too that this aircraft is going to be made to very tight tolerances; it's not going to be hand-made but 'printed' so there's just no reason to suppose that there's going to be any large deviation between it's predicted (modelled) and actual flight characteristics.
It really is worth taking a look at some of the unguided amateur rocket launches on youtube to see just how stable their flight is: during the burn phase LOHAN is going to be pretty much the same.
How the fuck am I supposed to say “yes, it matches” or “no, it doesn’t” IF YOU DO NOT SUPPLY A URL, TROLL? I am not the one stalling.
Of course the physics are the same, but if you are too cowardly to post an example of what you refer to, then please go away and allow a useful discussion to occur.
Guess he doesn’t actually have any examples of rockets taking off without a launcher guiding them — I sure haven’t seen any on YouTube — and I DO have an example of a rocket taking off without initial guidance.
It was a (dis)proof of concept, with three engines at the nose of a finless rocket, intended to verify the Pendulum Fallacy (http://en.wikipedia.org/wiki/Pendulum_rocket_fallacy) after that was pointed out to me and others at the start of the LOHAN design. If the fallacy was indeed fallacious, the rocket — launched utterly without guidance other than tractor-configuration mini-rockets — would not fly straight.
Guess what happened? It went upward maybe a maximum of two inches, and flew essentially sideways in a totally uncontrolled tumble.
Conclusion: active stabilization IS MANDATORY.
As much as I hate having to resort to one of Rupert the unbearable's publications:
Troll calling the kettle black - heh heh heh...
Now down to specifics: The vast majority of results from a youtube search for 'amateur rockets' show successful launches; sure, you can search for unsuccessful launches (additional keyword "fail"), of which there are, and must be, some (mostly accounted for by the aforementioned home-made nozzles and motors), but the vast majority of amateur unguided rocket launches do not display the kind of instabilities that you profess to be inevitable, despite your professed experience of 'having built over one hundred, and designed over thirty' rocket powered vehicles.
Are you now going to try to divert attention away from the issue and on to incorrect punctuation?
Oh yeah - NO NEED TO SHOUT!!!
Right, I thought so. You simply don’t have a clue what you’re on about, or you’re trolling.
You see, there are NO videos on YouTube of rocket-planes being launched from balloons, as we’re trying to design here — because as far as we know THIS HAS NEVER BEEN DONE BEFORE. So every launch video you can watch on YouTube is either (a) from a ground-based launcher and therefore irrelevant to LOHAN or (b) of a lightweight rocket, and therefore irrelevant to LOHAN.
Try paying a tiny bit of attention to what we are actually trying to do here, Lee.
I have to say that while I can see that there is lots of evidence of ground based rockets launching successfully (and more than a few unsuccessful ones). The additional problems of an air launch and a rocket-that-must-also-function-as-a-plane make these all but irrelevant to the LOHAN project.
It's my belief that a test launch at ground level will make the differences abundantly clear. Especially if that launch is from a suspended platform.
A ground-level launch will *only* be relevant if the launcher is dangling on a thread — the enormous difference that you don’t seem to see is the ability of the ground to absorb reaction forces from the launch. Though you seem to think it’s minor, that is a HUGE difference, if you have a heavy craft (which our plane, unlike a rocket, is) which has to get moving fast — your claims these factors are irrelevant are meaningless handwaving.
How do you plan to compensate for the 20-times-thicker atmosphere in testing? What I’m saying is this: while a totally unstable test at lower altitude means we’re definitely in trouble, a stable test tells us nothing about a launch in the stratosphere.
"the enormous difference that you don’t seem to see"
That's what this bit was about:
"Especially if that launch is from a suspended platform."
Perhaps it would have been clearer to say 'free moving suspended platform'.
My point was that the problems of stabilising the plane/rocket combo (or plocket (tm)) and the non-fixed launched platform WILL show that we are in trouble BEFORE we go adding in low air density and low temperatures to the list of problems.
Thought I'd be quiet for a while, to see additional guff you'd come up with - wasn't disappointed either.
So let's go through this argument, shall we?
It really started in your 5th May post when you said that the aircraft is dynamically unstable, asserting that without fast airflow to stabilise it the aircraft will immediately diverge into uncontrolled flight because of a list of external forces acting upon it that "go on and on". Oh yes, you also said, quite unequivocally, that a launch rod would not work, and offered to repeat yourself one million times.
In response, I pointed out that there was no reason for the aircraft to be dynamically unstable and that the external forces acting upon it at launch were both few and small. I addressed the specific issues that you did mention i.e. "asymmetry between the line of thrust and the CG of the plane; from turbulence in the exhaust gases; from launch rod friction". Trying to be helpful, I suggested that you take a look at the numerous successful launches of unguided amateur rockets on you-tube that use nothing more than a simple and slender launch rod (of the type that you claim will never work) for guidance during the initial phase of thrust build-up.
However, instead of discussing the issues of external forces or giving any other reason to justify your assertion that the aircraft will immediately diverge into uncontrolled flight due to off-axis thrust or other external forces, you just took umbrage at my suggestion to view the successful launch vids.
When I then asked if your rockets don't fly as well as those shown in the vids you complained that I hadn't posted urls - something I hadn't thought necessary as it only took a single search for "unguided amateur rocket launches" on youtube to find rather a lot of them.
By this point you'd stopped talking about LOHAN, you'd failed to support your assertions with reasons or explanations and completely failed to address any of the points I had made. Instead you just resorted to insults.
Your next post, after I didn't respond, you then went on to say that you had "an example of a rocket taking off without initial guidance" - what, just the one? However, this turned out to be related to the pendulum fallacy, which is totally irrelevant as there has never been a suggestion by the SPB team that they were going to try this scheme. In a great leap of illogicality, you conclude that active stabilisation is an absolute prerequisite for any rocket launch (better tell that to all the people who have used unguided rockets, from the Chinese in the 13th centuary, through WW2, to the unguided Zuni rockets used by the US military in to the 1970's).
After starting your next relevant post with more insults, you then say that there are "NO videos on YouTube of rocket-planes being launched from balloons". This is quite right but I couldn't figure out the relevance of you so emphatically making the point; I certainly hadn't claimed that there were any such videos, so who were you arguing against there? You still had a tendency to shout though ...sigh. Finally, you assert that that the launches on youtube (aha - so you did manage to find them after all) are irrelevant because they are either "(a) from a ground-based launcher and therefore irrelevant to LOHAN or (b) of a lightweight rocket, and therefore irrelevant to LOHAN"
Ok - let's deal with these two last points: why is a ground based launch irrelevant to LOHAN? What differences are there? Sure, LOHAN is going to be moving, but then the Earth is moving too. Remember that because LOHAN is going to be hanging from a balloon it's not going to be travelling _through_ the air but moving _with_ it; it's [LOHAN's] relative airspeed [to the surrounding air] is going to be insignificant. There will a considerable relative inertia difference between the Earth and LOHAN's launch platform but unless there's significant binding between the two then it doesn't make the slightest bit of difference; the force supplied by the rocket will not be acting upon the launch platform but upon the aircraft, which is free to move. Umm... as to your point (b) err.. LOHAN _is_ a lightweight rocket.
Two irrelevancies: (1) a ground-based launcher will transmit moments and forces to the ground in a way that a balloon-suspended launcher would be incapable of doing; (2) a launch at altitude will require a huge margin of safety w.r.t a low-altitude launch because of the 5% air pressure at altitude.
I like the way that NOW — after trolling for days — you act all innocent and explain what you have been keeping to yourself. Classy move.
There are two reasons why a launch rod, which works fine for rockets, will not for LOHAN. (1) A rocket has all of its mass concentrated in a linear structure close to the rod; LOHAN will have wings that will greatly increase the stabilization forces from the wire. Thus, a slender wire will be inadequate even for a lightweight plane. (2) Even with slender rockets, above a certain mass and power threshold launch rods are useless; that is why high-power launchers use RAILS. Try watching some YouTube videos of launches with real engines (as in J power or up). (3) The proposed launch rod mounting was a cantilever onto an aluminum plate; this provides roughly as much directional control as a used Kleenex.
In reply to your points:
1&3: Thats why I suggested a carrier/runner on the rod, detachable from lohan, so that the end of the rod can be supported too. Then you suddenly have 4 times the stiffness.
2: Most large size rockets I see launched barely have a launch rail to begin with. The top mount is out of the guide at maybe half a rocket-length. Not all that much guidance to begin with. It's there more to aid in erecting the rocket on the launch pad than provide launch guidance.
An extra note from me: Because Lohan has wings and is heavier than the normal Estes A motor hobby rockets, she'll automatically be more stable. More mass further from the CG means more inertia (both in mass and moment). Thus more resistance to tumbling. The reason those tiny rockets need such a long rod relative to their length is BECAUSE of their lack of weight. Even the tiniest force is enough to send them seriously out of balance. Larger rockets need less guidance at launch because A: Their larger weight resists small outside influences better and B: Their engines are better designed, burn more symmetrically and provide much more even thrust.
Like I said earlier, the only way to really put these concerns to rest is with a low altitude launch. Perhaps hang the girder from a long bungy cord to simulate the balloon's reaction forces. (Although in the split second it takes to launch, it's not really going to make much of a difference)
To Summarise then:
1. A rod/rail launch may or may not produce a straight line launch from a free swinging platform
2. A rod/rail launch may or may not actually slide due to static friction vs small inertial forces
3. A rocket-plane may or may not require giro-stabalisation due to out of line forces
4. No one really reads the detail and meaning of the other posts.
I back iamanidiot: The arguing phase is over and proof is required. Either hard maths or testing.
I think a suspended test would be more useful.
If you can't scrounge a crane to suspend the launch platform from its tether(s) then use a tethered balloon to suspend it i.e. three tethers to hold the balloon in place, which then suspends the launch platform at a convenient height above the ground. You'll have to wait for a windless day to try it because in the actual launch the balloon will be moving with the surrounding air and not stationary wrt the ground.
Hmm... you could also use this rig to try various balloon-platform tethers too. IIRC, the Vulture 1 vid seemed to show that it was spinning quite a lot and I mentioned in an earlier post that a single long tether from the balloon to two short attachment tethers joined just above the launch platform would be subject to wind-up i.e. if the launch platform acquired any spin during its ascent then a single long tether would offer little resistance to that spin but would store that spin energy like a spring; when the balloon reached smoother air this 'spring' would start to unwind, transferring its energy into the launch platform in the form of rotational momentum, which then causes the tether to wind up again (sure, much of the energy will go into spinning the balloon too, but reducing the total amount of energy stored in the tether as much as possible is a Good Thing). Most people will have already seen this phenomenon for themselves, one way or another, but it can be demonstrated by dangling a weight on a bit of string, manually winding it up and then releasing the weight; the string won't just unwind and then stop but, after transferring its stored energy to the weight, will wind itself up again (in the opposite direction, of course).
Dunno how many attachment points you have to the balloon but I doubt it's just, in effect, a bit of string tied around the neck (is there not a clamp?). Consider attaching a (relatively) short (say ~30 cm) spreader bar just beneath the balloon on two short tethers, such that the spreader bar forms the hypotenuse of a right-angled triangle [with the two short tethers], and then use two long tethers, attached at each end of the spreader bar to the ends of the launch platform. Should reduce the horizontal spin of the launch platform quite a lot.
(1) the thrust from the rocket will be acting upon the vehicle, not the launch platform: this applies whether it's launched on the ground or in the air. The only issue here is the risk of binding between the aircraft and the launch rod such that energy from the rocket _is_ transferred to the launch platform. The SPB has given a lot of thought to this specific problem though, and remember that the pics from Vulture 1showed little sign of heavy ice build-up.
There _was_ an issue, when the SPB were still planning to use a fairly substantial back 'plate' to hold the cantilevered launch rod which, because the back plate would have an appreciable area, would mean that the thrust from the rocket would act against it. They now recognise this problem though, so now we just have to wait and see how they're going to get around it (personally, I don't think that simply reducing the size of the backplate, to make it much narrower, will be adequate because in the very early stages of rocket burn, when the nozzle is very close to the support, the exhaust jet will be equally narrow and concentrated.
Anyway, it can be argued that a ground launch is more problematic in terms of the rocket exhaust upsetting things because, unless you mount the rocket above a pit to divert the gases away from the immediate launch area, as they do with the big rockets, the gases have nowhere else to go except back up towards the rocket. In a free-air launch though i.e. without a backplate then there's nothing obstructing the exhaust gases from continuing back and away from the rocket.
(2) I'm not sure what aspect of safety you're referring to wrt to the ambient air-pressure. The rocket motor has its own oxidiser, so as long as it can be ignited it's going to burn. Dunno if the low pressure or temps could result in degradation or breakdown of the fuel/oxidiser mix, or ignition issues, but that's what REHAB is for.
I don't know where you got the idea that the launch rod was going to be a "slender wire" - it looked like it was ~8mm diameter to me, and even though Titanium is a little more flexible than steel, at that diameter it's just not going to flex at all.
The thrust from the rocket motor MAY act solely on the plane or it MAY act partially on the launcher — do the words “piston launcher” ring a bell at all? The design of the enclosure for the motor(s) on the launcher, in addition to protecting the launcher structure from exhaust gas exposure, can be used to provide a DESIGNED amount of positive-separation force. It’s not an all-or-nothing choice.
Your arguments about ground launchers are totally specious and entirely dependent on launcher design, anchoring, launch angle, magnitude of the exhaust gas plume, bla bla bla bla bla. But the common factor of ALL ground launchers which you are dancing around is — ready for this? — THEY ARE ON THE GROUND.
The safety factor I was referring to is the MUCH larger aerostabilizing that is required at 80,000 feet than is required at the ground. With 20 times denser atmosphere, a successful ground launch using aerostabilizing is pretty meaningless. However, an UNSTABLE launch with 1 atmosphere ambient pressure tells us we haven’t a hope in hell at altitude. This question can only be settled with wind-tunnel tesing using a Reynolds number appropriate to the tenuous atmosphere.
And my use of the term “slender” is the engineering use — the rod will be really long compared to its diameter, which means the moments are intensely focused at the mount point. You point out correctly that suspending the rod at both ends quadruples the stiffness — while making a sticky problem of letting go and getting the support out of the way just in time. A launch RAIL, being continuously mounted to the launch structure, will be (a) many times stiffer; (b) may be made of thinner material so it’s lighter; (c) presents no tricky problems with clearance at launch.
As I've been reading up on UAV rules, regulations and electronics out if personal interest I've got another few questions for the LOHAN Development of Control Unit Programming and Systems (LOHAN D-CUPS ?) team.
Have you checked out the rules and regulations on flying a UAV vehicle (Which LOHAN will be considered to be if it features any kind of return to target or stabilization electronics)? In the Netherlands for instance it's basically forbidden for any civilian, unless you work through a metric ton of paperwork. The plane needs to be controllable by a human pilot at all times, constant monitoring to prevent collisions with other air traffic, stay within a designated zone, etc.
What kind of controllerboard will LOHAN use? Ardu-Pilot, UAV-dev board? (For the UAV-Dev board, I'll point to this site which is pretty good info and source material: http://code.google.com/p/gentlenav/)
Hmmmm, seems like I'm coming up with a lot of questions lately, maybe I should just get out some time. I'll get my coat...
I'm curious to find out what sort of control system they're going to use too.
I've done an awful lot of work on PID controller based autopilots and FCS's (in a sim only, I hasten to add) and from that experience I'm assuming that the aero students designing the airframe have got a good FDM with which to do some [a lot of] testing. Tuning a PID controller cascade can be fiendishly hard work.
> There's no final decision yet on the electronics payload.
On the assumption that this won't be El Reg's last high altitude venture, and following the maxim that there are no experimental failures, just more data, I suggest (if there's space) including an accelerometer and solid state gyroscope to monitor just how much buffeting this system gets. It could save a lot of arguments on the next project, or indeed this one if you go ahead with a launch trial run.
I figure accelerometers and gyros are pretty much a given for any autopilot. There will certainly be space: a solid state 6-axis accelerometer/gyro combination is a single integrated circuit. It seems to me that the best option, though, would be to put 3-axis accelerometers in the wingtips and perhaps one in the tail, and integrate their inputs to determine overall vehicle motion.
Agreed: the autopilot is going to need inputs to work with. In addition to pitch, roll & yaw, it'll also really need GPS (for lat, lon & altitude) and airspeed.
Pitch, roll, yaw & GPS could be handled by a smart-phone, which could also run the autopilot/FCS s/w, but airspeed could be a bit trickier; dunno if there are already suitable airspeed sensors available from the RC world and, if so, how you would feed the input into the s/w.
Actually, if the airframe FDM is really accurately modelled I think you might be able to derive the airspeed by comparing the actual pitch, roll & yaw rates against the corresponding pitch, roll & yaw control outputs i.e control surface deflections, set in the context of air density, which is established via the GPS altitude (basically, comparing what you're doing with what's actually happening as a result of what you're doing, and comparing that with what _should_ be happening as a result of what you're doing - at some point along the curve there will be a correlation, and that's where you actually are). A real airspeed sensor would still be a better idea though, as it gives the autopilot/FCS real data to work with.
You're most certainly going to need an IAS (Indicated Airspeed) sensor. Real airspeed is not actually measurable directly. Speed over ground, as indicated by a GPS is completely useless unless you know the exact windspeed of the air around the plane, which you don't. (In strong winds, speed over ground can be 0 or even negative! Especially for a light craft such as LOHAN.)
Yes, airspeed sensors for RC applications are already available for large scale RC gliders. They even have Total Energy compensated variometers nowadays.
For altitude, a barometric pressure sensor is going to be much more accurate at giving you an altitude than GPS. General rule of thumb, GPS is about 5 times less accurate in altitude as it is in horizontal position. This error increases further at greater altitude and most commercial GPS chips have a hard time even getting a good fix above 15 km or are programmed not to give data above this altitude (the US considers any GPS chips capable of getting a good fix above 18km to be weapons components, specifically guided rocket components, and bans trade of components and knowledge thereof under ITAR regulations. the LOHAN D-CUPS team might find this link interesting: http://bear.sbszoo.com/construction/gps/LP/LP_GPS.htm).
Surprisingly enough, barometric pressure sensors are pretty damn accurate. More so than GPS at lower altitudes (So for landing control and such, you'd want a pressure sensor).
Using a smartphone instead of a dedicated electronics platform (the UAV-dev and Ardupilot boards being prime candidates) is not really going to provide much advantage. Coding is often more complicated, not to mention actually linking the thing to I/O's . The accelerometers in smartphones are often not that precise and suffer from quite a bit of jitter. Not really noticeable when controlling a game on a smartphone, but enough to cause problems in a controls-algorithm. Most UAV boards have high-precision IMU's (Also handy for dead-reckoning in case of loss of GPS signal. More accurate IMU means better navigation accuracy in that case)
Beer: because I'm slightly inebriated while typing this.
I've no experience of the accuracy of the accelerometers built into smartphones so I'll take your word for it. I bet that the varios for RC gliders are rather expensive.
It's correct that you can't measure absolute airspeed directly, which is why the instruments include the 'indicated' proviso. The problem with using IAS though, in the context of autopilots/FCS, is that for a constant airspeed (speed through the surrounding air) the IAS will drop quite dramatically as altitude increases and at the very high altitude that LOHAN is expected to reach the IAS will be lucky to hit double digits even whilst under power; I'm pretty certain that once it's gliding its IAS at high altitude will be in single digits, which is not good news for the autopilot/FCS (it could well have to cope with an IAS range that is in the region of, or even exceeds 1:100).
I'm not sure that a barometer will give a more accurate altitude indication than GPS because you can't be sure that, when you calibrate the barometer at ground level on the day of the flight, which will be for the local weather system, those low level weather system conditions will apply at the much higher altitudes that LOHAN will reach; doing so is fine for light aircraft/GA but that's because they don't go very high and stay in the same weather system. One the other hand, whilst a GPS will only be accurate to two or three hundred metres at high altitude I think this could be better than what you'd get from a barometer calibrated on the ground in a relatively low altitude weather system. However, a barometer will be great for air density, which will be useful to the autopilot/FCS.
That’s a good point about IAS, Lee. Fortunately I think it would be possible to apply a transfer function in the microcontroller to estimate the actual airspeed in a straightforward way, by encoding a hopefully accurate model atmosphere.
So really, with a Pitot tube, we can gain three measurements: static pressure, IAS and corrected actual airspeed. Combined with accelerometer and GPS data, one could build a nice system that uses primarily inertial guidance up high, GPS at moderate altitudes supplemented with barometric altimetry when the air thickens (as indicated by static pressure). The saved data could make for some interesting analysis after the fact as well.
I have been thinking about Lester’s comments about the engine; I think I may know what their engine idea is — if I am right, it will be brilliant.
Having thought about it a bit more, I think that the soton students might just go for an entirely dynamics based solution, based upon accelerometer feedback and orientation. For example, when all the accelerometer rates are low and you're upright and moderately level you can assume that you're in controlled flight without needing to know airspeeds, pressures and descent rates etc. For navigation you just add monitoring of where you are with where you want to be, make a control input and monitor the results from the accelerometers to keep inside the controlled flight envelope. Such a system wouldn't be good for following a set flight-path or arriving at a certain point at a certain time, which would need airspeed etc. but that's not really needed for LOHAN anyway; it just needs to get back on the ground in roughly the right place.
The more I think about it, the better it sounds, and it does away with some instruments that have quite a lot of potential to not only go wrong but also add weight. For example a frozen pitot tube is a real risk (I'm sure there won't pitot heating on Vulture 2) and the barometer, which will be affected by local weather systems could be dumped too. Sure, they could still be carried, but would only provide data of academic interest.
We'll see though...
I'm not sure where you got the "local weather system" idea from, its not really relevant.
(And I'm saying that as a pilot. Granted, I only have a glider pilot license, but that seems all the more appropriate here)
Normal aviation knows 3 main altimeter settings. QFE where the altimeter is set to 0 (Field elevation) at take-off (As in, the local airpressure is input into the pressure compensation). This is only used for local flights in flat terrain and when air pressure can be assumed no to fluctuate more than a few mbar over the duration of the flight.
Then there's QNH in which the altimeter is set so it would indicate 0 at sea-level under current local weather conditions/air pressure. (This is the altimeter setting used by all commercial heavy metal below "transition level" which is normally 3000 feet)
Then lastly there is the Flight Level setting. Above Transition Level every plane HAS to set their altimeter to the standardized atmospheric pressure of 1013 mbar. This means all aircraft use the same altimeter setting and that 10,000 feet for one aircraft is also 10,000 feet for another aircraft.
While for high altitude a barometric pressure sensor might also have a hard time giving accurate data, like I said in my previous post, most GPS units won't give ANY data above 18 km. My concern, and the reason I suggest packing a pressure sensor, is for lower altitude navigation. Once LOHAN approaches the ground, she can go from free-flight (higher speed, to provide better safety margins and transition any turbulence faster) to "landing mode" (lower the speed, more careful turning, etc) The error margin for a GPS unit is not really good enough to do this. A good pressure sensor could just about tell you when to flare out for landing if you know the ground-elevation of your landing site. The LOHAN flight would happen on a clear day, with relatively stable weather (And thus relatively stable air pressure). By setting the pressure altitude to 0 at ground level (QFE), LOHAN will know how high above the start site she is (And I'm assuming she'd be "returning home" to land there too) Because the pressure at the landing site would not have changed over the 1 or 2 hours the flight might take, 0 will still be 0.
My flightlogs from some glider flights I've made myself tell me that a SiRFstar3 GPS chip can vary well over 10 meters in altitude when standing still on the ground. And that was on a good day.
Then there's the airspeed matter. Indicated Airspeed is all you need. The funny thing is, for all intents and purposes, if LOHAN needs to cruise at 40 km/h indicated at sea-level, she'll still need to fly 40 km/h indicated at 30,000 meters in the air. Because the air is thinner, she needs to fly faster relative to the air. (Higher True Airspeed) And because the air is thinner, this higher True Airspeed is still the same Indicated Air Speed. This holds true almost all the way to space. (There's some complications with Aeroelastic flutter at top speed and stall speed does rise a bit, but I'm not going to get that complicated here)
Pitot icing could be a problem. I won't deny that. But getting stable controlled flight without having an indication of IAS is going to be pretty tough. As I said in my last post, GPS speed is worse than useless. Substituting a Pitot pressure sensor with a stall-horn and some sort of overspeed detector (Spring loaded flap or something) might be a fair compromise though. Then you would know the 2 truly relevant flight conditions to be avoided. Low speed stall and overspeeding. Any speed in between those 2 could be considered good enough.
The problem remains, airspeed is one of those really relevant variables when flying an airplane.
Thanks for the information from an actual pilot’s mouth! You’re right, a glider pilot is exactly who we need in this conversation. One thing I would like to bring up: I believe the GPS cutoff is for a *combination* of great altitude and high speed; slow-moving objects over 18,000' can still use the signal, I believe. Is this accurate?
As far as the autopilot goes, I suppose we need system specs before we can decide if a pitot is optional. Keep in mind, we can build the static and ram-air ports into the actual fuselage design, so they could have quite different port locations: ram-air on the nose and static on the belly, for example, or multiple static ports. It may not add much weight at all — even if the information is “purely academic” and not used for flight control. A log in the black box could be quite interesting to analyze along with accelerometer and GPS data.
Finally, I can’t help but see how pitot-tube data can’t help the autopilot. Suppose the airplane flies through a front; as the wind direction switches, the inertial sensors would detect a sudden plunge in altitude but with the same horizontal motion (which would be hard to diagnose), but a pitot tube would measure the drastic airflow changes (which would make decisions easier).
I only have good friend who is a glider pilot, but anyway, the issue of local weather systems is relevant in the context of calibrating a barometer that is expected to be used in LOHAN because these weather systems do not extend up to the altitude that's expected to be reached. The local weather system prevailing at the time of launch will dictate the ground level air pressure and because the playmonaut won't be able to recalibrate it during the flight it can only be set once, at ground level.
I'm happy to accept that a GPS won't be usable above 18km (a little over 59k ft) but I suspect that a barometer that was calibrated at ground level won't be very useful either, as LOHAN approaches the altitude that PARIS achieved, which was nearly 90k ft.
If airspeed is going to be an element of the autopilot scheme then the issue of pitot freezing cannot be ignored and must be addressed because the loss of such a critical input to the autopilot would be show-stopper. However, pitot heating in Vulture 2 is not really going to be a viable option due to weight, complexity and energy requirements.
Whilst you will certainly need to do a good landing, on an airstrip, in your glider, because you'll want to use it again without having to carry out major repairs, this isn't so much of an issue with Vulture 2. Being much smaller and lighter, it'll be landed like a free-flight model and, being made out of nylon, will probably suffer less damage.
"A good pressure sensor could just about tell you when to flare out for landing if you know the ground-elevation of your landing site" Sure, if by 'good' you mean one that's accurate to +/- 1ft at field level. In practice you'd be lucky to achieve +/- 20ft with a 'good' one - just don't try flaring at +20ft agl (or -20ft agl either, for that matter).
Because of the potential problems with ensuring that instruments like a pitot and barometer provide reliable data to the autopilot/FCS over the full range of the environmental envelope I strongly suspect that they'll not be used at all and a purely dynamic FCS, more akin to a lightweight missile guidence system, such as you'd find in air-to-air or ground-to-air missiles, rather than an aircraft type autopilot, will be used instead.
Lee, one point I wish to stress is that barometric measurement, GPS and accelerometers are NOT mutually exclusive; having a GPS on board does not preclude taking inertial measurement too, and allow that to be periodically refreshed by GPS readings; and, when the ground is near, taking aerodynamic measurements.
One way to avoid or mitigate tube icing problems is to have multiple ram and static ports; say a ram air port on the nose and somewhere on the wing leading edge of each wing. If the ports are built into the plane they would be really quite light.
Just had a quick think about the icing matter, one solution could be to go from a positive Energy pressure (Pitot tube with port facing forward) to a negative Energy Pressure. Just let the port point backwards. Freezing is going to be much less likely in that situation.
Hmm, I have my doubts that a back-facing port would work; the opening would not detect laminar flow but turbulence from the tip itself, would it not?
What about multiple ram-air ports; if one or more (but not all of them) freeze up they will simply be sealed off and the remainder will continue to measure ram air pressure. If we had a row of ports along the wing outboard leading edge, with a common chamber connecting them, this would work, would it not? (It would respond more sluggishly than a small ram-air chamber, I admit.)
Certainly, barometric, GPS and accelerometer instruments are not mutually exclusive but that isn't really the issue here. Neither GPS nor accelerometers on their own would provide a working solution because accelerometers can't give you location and GPS can't give you attitude rates; they comprise two halves of a single system that need to integrated to produce a working solution.
Sure, you could also carry a barometer but integrating that into the autopilot/FCS not only increases weight but also complexity (in addition to the complexity overhead of integrating the baro data you'll also have to arbitrate between that data and the GPS data when they inevitably disagree).
I'm not sure what you mean by "inertial measurement" here; the accelerometers will be providing inertial data but Vulture 2 isn't going to be carrying a relatively massive inertial reference platform, which you'd need for inertial navigation. In any case, there's the GPS for that (yes, it may not work above 59k ft but that still leaves you those 59k ft to glide back to the landing area once Vulture 2 has descended to that altitude, which should be enough)
I don't think that ram ports built into the airframe will work. I believe that the reason they're on tubes is because they need to take readings from outside the boundary layer whereas sticking them in the nose or the leading edge of the wing, which is where you get the greatest accelerations of air around the airframe, would produce some wildly skewed readings. Having multiple sensors is no guarantee against icing failure either because the conditions that cause one sensor to ice up will apply to all the others too.
I can't see why you would want to switch between flight control and navigation systems in such a relatively simple vehicle just because you're nearer to the ground; it would be as effective as forming a coalition government from two fundamentally different ideologies.
A conventional pitot tube works by ram air pressure, which is combined with static pressure to derive IAS and establishing static pressure in itself is not simply a question of making a hole in the side of the airframe and sticking a sensor in there; the boundary layer airflow across the opening of the hole results in a venturi type effect so the static pressure needs to be calibrated along with the ram pressure. Would not a reversed "negative energy" pitot tube just give you a variation on static pressure? Also, would it guarantee that you get no icing? I've not heard of such a device and unless they already exist in a suitably small, light weight, low power and inexpensive package then I suspect that devising one, and then calibrating it across the environmental envelope of Vulture 2, is going to be a bit beyond the scope of the project.
For the word “inertial” I mean any guidance system based on input from accelerometers and/or gyros (optical or mechanical). It would not be able to fly the plane to a specific destination very well due to rounding error, but it will be useful first-level feedback for control inputs. It will be easy to distinguish various roll angles, for example (comparing g-sensors at the wingtips, differentially, is a great method) and so this is the system that would be used to fly the plane from moment to moment.
It would be a minor conceptual change to take a system that aims for a certain behaviour (say, uniform gliding in a spiral or straight line) and an autopilot capable of seeking some specified goal. “By the time the plane reaches 50,000 feet, it should be somewhere near location X.” Then, again supposing the plane can pick up a GPS reading at that time or earlier, the goal-specifying navigation software can update the current goals for the autopilot to aim for.
This is how I see the resolution of conflicts between systems; inertial measurements will attempt to keep track of the aircraft state, while periodic GPS data can be inserted to correct for rounding error or drift. If we have useable barometric indicators, the correction factor for altitude may be the best — especially if the plane can radio for ground conditions at location X: weather, elevation, stay-the-hell-out-of-here-if-possible warnings, etc.
I think we're on the same page here. To summarise: a GPS to tell it in which direction to fly (navigation) and a MEMS gyro & accelerometers to detect and control aircraft attitude, keeping it within the controlled flight envelope, whilst it heads there.
You wouldn't need accelerometers in the wingtips though, and just looking at differential readings from one in each tip would only tell you how much the wing was flexing. However, whilst this wouldn't be important for the actual flight it might be of interest to the aero and material science students working on the airframe design as it would provide useful comparison data between the predicted and actual structural behaviours.
I think an active uplink to the aircraft would be fun but would add weight, complexity and power draw, and would be another potential point of failure, not only by simply not working but by latching on to the 'wrong' signal, should one just happen to be present (you'd have to ensure a secure and authenticated connection).
Actually, the height difference between the wingtips would be an excellent approximation of the actual bank of the craft. There would be a small displacement due to the flexure of the wings, but (a) this disappears in differential measurement and (b) by symmetry the angle of bank at the fuselage is parallel to the angle between the wingtip sensors — certainly within the tolerance of the sensors. Furthermore, by separating the accelerometers by a metre or whatever, they should be able to achieve greater precision than a solid-state gyro in a compact enclosure.
As far as variations in GPS altitude, we should be able to refine the reading by averaging out all the values over the past few seconds. We will know (from the IMU) when we are in a steady gradual drop vs. a sudden gut-wrenching air pocket. Therefore we can use the data stream, integrating it with weighting more recent values more strongly, to get a pretty decent absolute altitude measurement. That can be used to update the IMU periodically.
I REALLY don't see any advantages in placing accelerometers or gyros in the wingtips over simply placing them in the fuselage. The bank angles remain exactly the same. Yes, accelerations get somewhat increased, but mostly about the wrong axis (roll becomes a vertical acceleration) thus needing a lot of extra filtering and introducing much more overhead. Simply placing the gyros as close to the CG as possible is going to make things much easier. Also, don't forget we are talking about a low power system (probably 5 volt). Have you got any idea what a 5v signal does when going through a 1 meter cable (even a shielded one?) the signal quality will most likely be horrible. And even a shielded cable at high altitude will be very susceptible to working as an antenna, scrambling your signal even further. My vote is to keep wires short, and keep the electronics in the central cargo package.
Good point about antenna pick-up effects of long wires. I agree that sending analog acceleration signals would be lousy at best. But I don’t see why digital signals couldn’t make it; if RF interference is an issue then you can make the ground line for the outboard electronics a brass tube, and run the signal and power wires inside. That would be one serious Faraday cage for almost the entire signal length.
It is probably a good idea to have shielded servo signal lines, while we’re at it. That would really only mean adding one wire to the above shielding tube (the PWM position signal). Install a wired female plug (GND/PWR/PWM) in the servo bay to make servicing or parts replacement easy.
@Poor Coco, even digital signals over long wires suffer from signal degradation. This is mostly due to voltage drop over the wire due to resistance but also the wire capacitance. This capacitance is going to be made even worse if you run the wire inside a tube. As a lot of digital equipment relies on reliable high and low signal strength (or even sometimes sharp rise and fall signal flanks) a long wire for any low voltage system is always to be avoided. Always keep signal wires as short as possible. When you really need long wires, proper termination is critical.
How much signal degradation do you expect from a wire that is at absolute most 1 metre long? With a digital system the parasitic capacitance will only reduce your maximum bit rate, which will NOT be an issue here, we’re just getting accelerometer data — not downloading Torrents.
If the interference is so enormous that a continuously shielded cable 18" long is unable to transmit a digital signal, then NOTHING will work. Funny thing, I see balloon flights with bog-standard digital cameras on them working just fine thank you. How’s THAT possible?
I'm not saying its impossible, just hard. A bog standard digital camera is not at the end of an 18" cable and usually has a much stronger signal. Solid State IMU's create a rather weak signal at best and are very susceptible to noise on the feedlines. I'm not saying this just out of theory. I have a friend who's into both glider aerobatics and the aerodynamics involved. He's made an IMU unit where he ran into this exact problem. Due to space constraints he had placed his IMU package at the end of a roughly 1 meter cable (he wanted the IMU at the CG, but couldn't fit the entire electronics package above the main wingspar) The data reception was unreliable while working perfectly with a shorter cable.
Yeah, no, thats not how mosfets work. If you'd said Schmitt-triggers you'd be closer to a correct answer. A mostfet doesn't "straighten out" the signal and just amplifies the noise basically. A Schmitt-trigger would give you back something much closer to your original signal IF the noise is low enough compared to the high and low voltages of your chip.
Don't need to watch them, already did.
What I'm saying is that in a long line, your logic high signal as sent by the chip is not going to be a clean logic high any more, and a logic low might not even be a logic low immediatly. Thus your signal falling outside the noise margin. Amplifying the signal is not going to do anything! (Also, amplify it to what? Your uC still needs 5v so amplifying is not going to do anything. You need a filter to remove noise. A mosfet doesn't do that! A schmitt trigger (when correctly configured) does.
Read http://www.vagrearg.org/?p=transline for an explanation of what I'm on about. And this is with a good signal generation in a low noise environment, not a low power MEMS chip signal 30.000 meters up in the air
Seriously, if you are that convinced I'm wrong, give me an argument that shows it, instead of swearing and telling me I should go watch a course on basic electronics. I might be a mechanical engineer but I'm not an electrics idiot!
And I remain at my standpoint that even with a reliable signal, placing accelerometers at the wingtips is not going to tell you anything extra over a IMU placed at or near the CG. It's just going to give you a lot more calculating headaches to calculate roll rates from accelerations. Something a gyro at the centerline could tell you directly. It doesn't add anything!
@Poor Coco, even digital signals over long wires suffer from signal degradation. This is mostly due to voltage drop over the wire due to resistance but also the wire capacitance.
Voltage drop along the length of the wire comes from V=I*R, and while the resistance may be several ohms (in case of lousy wire and contacts), current (into the controller input) doesn't have to exceed 0.1mA, so your voltage loss from that bit is negligible.
Signal shape is affected by the wire's capacitance, which will be in the order of 100pF for a meter of shielded signal wire like RG316. Current digital logic is well able to deal with that kind of load, and there's always the option to go for an interface driver that uses I2C (availabe on most microcontrollers), which has been developed for use in automotive environments.
Hmmm, spent quite some time typing a reply this morning, but this seems to have not been posted and/or disappeared.
My point is that with a baro sensor you don't NEED the GPS altitude indication and can just ignore it within the control algorithms. The sensor itself is not really that big, in fact, there are SMT components that can do the trick. (For example: http://www.sparkfun.com/products/9603, although this one only goes down to 300 hPa, I've seen models that go to lower pressures/higher altitudes) I'm pretty sure there's UAV controller boards already out there that simply integrate this thing on the board.
The biggest advantage from a pressure sensor it seems is not even in accuracy, it's in stability of readings. A GPS can jump up and down 10 or 20 meters in altitude in a few dozen seconds or even less. Some uav controllers simply don't like that sort of behaviour. It certainly makes reading the data more difficult.
@LeeE, a set of 3 gyros and 3 accelerometers, each aligned with the major axes of rotation/translation can more simply referred to as an Inertial Measurement Unit. Your smartphone is already an IMU. An IMU does not have to be "relatively massive". Even a smartphone can be used for inertial navigation, there's quite a few bot-building enthusiasts who have proven that. (And most smartphone IMU's are not that great/accurate) Problem with Inertial Navigation is cumulative error. The larger your moment-by-moment error, the larger your navigational error is going to be over time as errors stack up. Thus needing the GPS to correct the positional data. Inertial is a good backup for GPS black-out emergencies though.
LeeE is correct that ram-air-pressure ports on the airframe itself don't really work. Not because they wouldn't read a ram air pressure but because this pressure is dependant on the airflow around the model/wing. A variation in angle of attack alters the flow around the model and alters the pressure point, meaning a measurement taken in the boundary layer will not be accurately describing the "free-air" conditions of the air. (just get one of those airfoil simulator "games" and look at what the airflow around the leading edge does as the AoA changes)
This neatly segues into the question about static ports. Static pressure usually IS measured directly at the skin of the aircraft. And there's a very nice trick used when doing it. Most craft have several static ports along the fuselage (often symmetrically placed on either side of the fuselage) On gliders the usual pattern is 2 on the nose, 2 on the tailboom, sometimes another set further back.) The placement of these ports is done so that as the airflows around the aircraft vary with speed, the resultant pressure in the static pressure piping stays as close to the true static pressure as possible. All this can nowadays be determined with some fancy FEM flow-modelling. In the crotchety grandpa "in my days" times it was often a combination of designer experience, trial and error and dumb luck. Some older gliders use a so-called venturi tube, which is a necked tube pointed into the airstream, with the static port at an appropriate location along the venture to obtain the correct pressure. Those only worked correctly with the nose pointing exactly forward and within a narrow speed-band. Outside of those parameters they are not good to horrible.
This brings us to the pitot tube problem. Pitots are normally mounted on a stalk, outside the boundary layer of the adjacent skin. And then offset slightly forward of their mounting point. This is to eliminate as much turbulence from the pressure as possible and obtain a reading as true as possible. Angle of attack still has a slight effect on the measured pressure too.
Keep in mind that ram-air pressure as measured by a pitot is infact 2 distinct components, the static pressure plus the energy pressure of the still air being squashed into the fast moving pitot tube. If we were to take a tube, close the top and then drill some small holes axially on only one side, then point this out of the fuselage with the holes pointing back we can obtain a "suction pressure" that is comprised of the static pressure minus a component directly related to the speed of the craft (The negative energy pressure I was talking about) This suction pressure is not exactly as accurate as a pitot pressure, it'll be influenced by angle of attack and yaw angle more than a pitot would be.
Modern glider variometers (aka the fun gauge, indicating climb or descent, thus positive or negative fun) usually have a compensation for so called "stick-thermal". If a pilot in a glider pulls the stick back in still air (provided he has speed) the glider will climb, but in doing so lose speed. The old un-compensated vario's would indicated climb, possibly falsely leading a pilot to think he found a thermal. To eliminate this effect most gliders have exactly the arrangement I just described, with a pipe outside the boundary layer with some backwards facing holes or pipes. This suction pressure is then fed into a compensation bellows arrangement. (on mechanical vario's atleast, theres now also digital once that use solid state components) These bellows expand as the speed drops and temporarily expand the volume of the "reference volume" thermos-bottle of the vario, thus preventing the vario from showing a climb. This is called Total Energy compensation. The reason for using a negative pressure for this compensation and not the ram air pressure taken for the speed indication is because mechanically the negative pressure is easier to use for compensation. There's no reason other than the slight loss in accuracy and stability why it couldn't be used to take a speed measurement.
Now where is the "I'm putting on my cool pilot shades" icon?
I don't have a problem with carrying a baro (or IAS) sensor, at least for academic purposes, but it just doesn't make sense to me to say that you "don't need the GPS altitude", which you'll be getting anyway, and instead rely upon an additional a baro sensor which, in this case, can only infer altitude from an initial field level calibration.
I certainly accept that the GPS altitude is going to be less and less accurate as it gets higher and higher, and may not be usable at all above 59k ft, but I don't see that as a real issue. At low altitudes the GPS altitude accuracy is going to be ~30 metres but I can't see a baro sensor really being that much more accurate. Furthermore, why would we need to know, for flight control reasons, what altitude we're at? Vulture 2 isn't going to need to do a controlled landing and it's absolute (ground) speed will naturally decay as it descends into thickening air (as per our previous discussions about IAS). Like I said in an earlier post, I suspect it'll just be landed like a free-flight model which, as far as I'm aware, amounts to flying it into the ground at it's best (lowest) sink-rate.
I can't see that Vulture 2 is going to be expected to fly a nominated course, as you would in your glider when attempting a 'task'; it just needs to end up in the landing region and then stay in the vicinity of that region until it makes friends with the ground again. It won't need to follow a pattern or approach from a particular direction, so it doesn't really need to know altitude or airspeed to ensure it complies with timings, patterns or directions.
In any case, there's still going to be the potential issue of pitot freezing and unless it can be ensured that it won't happen it can't be relied upon as a primary sensor.
The background info was interesting though.
"Just as a reply to the accuracy bit, even a 1960's mechanical altimeter is going to give you better accuracy than ~30m. The resolution for that sensor I linked to is under a meter even in low accuracy power saving mode."
I'm happy to accept that a mech baro altimeter can give better than ~30m accuracy but I just don't think it's needed for the Vulture 2 aircraft.
I must confess that I didn't look at the specs of that sensor earlier but have done now and it is a nice bit of kit. The main issue with it seems to be noise but that's only going to be a problem with relatively high sampling rates, which you don't really need in a relatively low-rate altimeter. Passing the data through a digital moving-average or noise-spike filter is pretty trivial anyway.
> I figure accelerometers and gyros are pretty much a given for any autopilot.
I was thinking more along the lines of monitoring the movements of the rig rather than part of the payload's attitude control. This might be useful during any test launches to see how the rig moves when there's a mighty thrusting rocket trying to slide free from it. For example, if the release isn't as smooth as hoped for, a plot of rig movements might show whether or not it's likely to jerk around and risk catching LOHAN's rear end and knock it off balance.
I know a chap who is working on an on-going project to spot floating debris in the Pacific using UAVs and during one cruise, when the weather was too bad for any flying, he was able to use the UAV's on-board accelerometers to plot exactly how sea-sick they were all getting.
That’s a really good point — thanks, Stuart.
I agree that the reaction experienced by the truss is the principal driving factor in the relationship of the truss and the plane. This is why I have been discussing variations of piston launchers — by controlling the reaction force of the launcher we can derive stabilization. Consider F=ma; by taking the mass of the launcher and knowing its acceleration, we have a recoil reaction force getting the plane going NOW, in a degree we can control.
So consider a plane held by its wings or fuselage, with quite short but sturdy and lightweight guide trusses. By enclosing its engine(s) in designed devices, we can control that force (we need tests, of course).
Now, by taking the moment of inertia of the launcher and the moment arm of the launch force into consideration, you can design a system to get the plane moving really fast off the line, which admittedly isn’t usually the way you launch a plane.
But here we have the unique experiment of being able to design anything we like. Now, I fully appreciate there are professionals working on this who KNOW WHAT THEY ARE DOING in a way we can only dream of doing it. So I'm just shooting the moon in thinking about this, because, well, why not? (The plane was designed only on the basis of looking cool, not much actual aeronautical thought was given to it. The high-aspect wing is a good idea, I agree; think of the U-2 for example. I am a bit concerned about structural stress in a long thin wing, it could be a limiting factor in how much thrust capture we can use. It’s a design tradeoff.)
I guess my central point is, if we design for dynamic equilibrium, the launch action will happen so quickly that it won’t really matter what the launcher forces are from other sources like the balloon — it would act the same in freefall, although it could go in any direction unless gyro-stablilized now that I think about it.
Let’s think about this. suggestions are welcome.
Why not launch LOHAN straight up from the TOP of the balloon? Thusly:
....| |.... - Lohan
OOOO - Balloon
.....|..... - cable
===== - avioncs
The teathered avionics should provide enough counterweight to ensure LOHAN is pointed and launched upwards. No worries about twisting effects or icing (or weight!) from the truss/rod arrangement.
Obviously some sort of net would be needed around the balloon to support the launch platform on the vastly expanding balloon. Easily tested on the ground, I'd've thought.
Couple of problems with this idea. First, is the net going to be non-stretch or will it stretch with the balloon? If it's non-stretch then how are you going to keep it aligned so that the LOHAN remains perched on top of the balloon whilst the net is loose and baggy at low altitudes? If it's stretchy then it's really no different to the skin of the balloon (you wouldn't want the net to restrict the expansion of the balloon because it needs to expand, to maintain lift by reducing its density as the outside air pressure drops - if this didn't happen i.e. the balloon kept the same volume as it ascended it would reach a point where its density (mass per unit volume) equalled that of the thinning air outside it - result = no more lift. By letting the balloon expand, its density continues dropping, keeping it below the density of the air in which it floats).
Second is that the avionics package isn't rigidly coupled to the balloon, so with the weight of LOHAN on top, it'll still tip over. In fact , this scheme results in least stability when everything is vertically aligned and will only become stable once the balloon/LOHAN combo has tilted over.
"Aha," I hear you say "what if we mount LOHAN sideways so that as the balloon tips over it's then pointing upwards?" Well, not a bad idea, if you can guarantee which way it all tilts - it could tilt the other way so that LOHAN is pointing downwards instead of upwards.
A very long rigid pole, with the electronics package at the bottom and LOHAN on the top, and with the cluster of balloons to lift the entire ensemble being attached close to the top, would work but when you factor in the enlargement of the balloons and the need to make sure that they don't wrap around LOHAN as they expand, you're looking at a _very_ long pole (Wikipedia says that weather balloons can typically expand by a factor of 100 before they burst - it's unclear whether that refers to volume or size though).
Commercial toroidal latex weather baloon - indeed no site. Therefore out.
Fixing the payload might be more difficult, also.
However - just for the record - if you had one, the radius of the inner clearence should rather grow. Climbing up, the torus will strech uniformly in both directions of the surface, if it wasn't made from a bent cylinder.
If your initial clearance is e.g. 0,54 m, the final clearance could grow to 5,4 m (conveniently assuming 55 km height with air pressure down to 1 hPa, so that volume is increased by factor 1000).
The problem that I think we need to solve in the launch system is getting the thrust line & center of gravity CoG correct. At altitude the aerodynamic drag on Lohan will be fairly minimal but having a large wingspan for gliding will mean that you'll possibly have the wings & cog much to far forward. If it is feasable to have the wings swept back
Rockets / darts have fins at the base of to ensure that maximum drag is aft of the cog and the thrust line goes through the cog, if you dont the rocket will indeed start going into a spin or tumble.
the launch rail system is good, but it changes the dynamics of the cog of launch as the cog will be comprised of the launch rail assembly and lohan. Therefore I would suggest having two motors, one mounted on (or as close to) the rail and one on lohan. The Rail launch rocket would have very limited thrust & a very short burn time, this will mean that during the initial launch phase the thrust line will be directed to take lohan off the rail. once that is done the main motor will then be pushing through the new cog point of lohan. This has the advantage that lohan will be traveling at a higher speed exiting the rail and so will have greater vertical angular momentum to keep traveling in a vertical (ish) direction.
I don't think that there will be a problem with getting the thrust line and CoG etc. correct; I gather that the airframe is going to be computer 'printed' from powdered nylon and so will be accurate and conform to very fine tolerances.
Swept wings on a low-speed aircraft, like Vulture 2, are not a good idea; whilst swept wings bring benefits in high-speed flight, at low airspeeds the air starts to flow along the outer part of the wing instead of across it. A long, straight, high aspect-ratio wing is really what's desirable, and I trust that the soton lads will know how to arrange the internal bracing to reduce twisting.
Once again, there's no reason to suppose that the direction of thrust from the rocket motor will be so variable or that it will be mounted so badly that it will result in off-axis thrust and produce a significant turning moment. In any case, Vulture 2 will have flight control surfaces and an active autopilot/FCS.
You shouldn't be thinking in terms of combining the mass of the launch platform with the mass of the aircraft; the only binding between them will be the residual friction between the launch rod and the runners; apart from that residual friction, the force from the rocket will only be acting upon the aircraft.
(After doing some force vector diagrams and numbers I've come to the conclusion that the launch rod is the best solution; sure, before the rocket is ignited, the aircraft will be 'hanging' from the launch rod but once the rocket motor starts to burn the aircraft will actually be pushing up against the launch rod and no longer hanging from it. The wing guides will be necessary not only to prevent the aircraft from flapping about too much during any low-altitude turbulence but also to stop it from twisting around the rod due to that upwards force on the launch rod once the motor is burning).
Yup I do agree with you point that the precision of construction should allow for the correct cog to be determined. But at issue (and perhaps didn't expand on it properly) is that the cog is generally (subject to trim) around the centre of axis of turning, therefore at inital start launch point maximum drag is to far forward to allow for stable launch. At high altitude the stall speed of a wing rises (read coffin corner) it will take a much higher airspeed to allow for normal aerodynamic flight, so any automatic control of surfaces for stability would be pretty useless. Basically I think we need to give Lohan a big strap-on!
The CoG of an airframe is not dependent upon trim; it's just down to the distribution of mass around the airframe and all fixed wing aircraft are designed to have their CoG at least very slightly ahead of their center of lift, more usually called the Aerodynamic center, for stability and recovery reasons (an aft CoG makes it very difficult to recover from a stall, where you've lost aerodynamic control, because it will tend to continue to promote the pitch-up attitude whereas a forward CoG will tend to pull the nose down, reducing the AoA and allowing recovery from the stall).
I can't see that where drag applies on the airframe, at least at the sub-sonic speeds that Vulture 2 is going to be achieving, is really going to be an issue.
Taking the material and size limitations of Vulture 2 into consideration, I can't really see it doing much effective gliding at very high altitudes; a controlled descent to thicker air is probably going to be the main objective immediately following launch. I think that trying to boost the speed to achieve "normal" flight at such high altitudes is going to be incompatible with the sort of high aspect-ratio wings you'd need for gliding there.
It's worth having a look at the Perlan project though, which aims to get a glider up to ~90k ft and beat the current gliding altitude record of just over 50k ft.
Up for discussion is the two balloon scenario as mentioned on the latest LOHAN post.
Two balloons one high above the other both connected to a strain gauge type micro-switch on their tethers. When either one of the balloons burst its micro-switch fires the rocket. A timer is used to only arm the system only after a suitable delay after release to alloy the craft to enter the calmer air in the upper atmosphere.
1. Simple to build
2. Ensures maximum altitude before launch
3. Remaining balloon stabilises platform for launch
4. Fails safe, if one balloon burst early the rocket will still fire at altitude on the timer
5. Can be tested at low altitude
I can't see where it says that the two balloon option is being considered again.
But how would you tether two balloons, one above the other anyway? You can't run the tether through the lower balloon and even if you could attach the upper balloon tether directly to the top of the lower balloon you'd be adding to the stress upon the lower balloon because you'd be stretching it, which would likely lead to premature bursting. On the other hand, if you tried to run the upper balloon tether around the lower balloon's envelope it'll press into it, which would also not be a good thing.
In theory, a four-balloon set up could work. You'd have three lower balloons suspending a triangular platform, with the fourth balloon running up between them on its own tether. If you could rely upon all the balloons bursting at exactly the same altitude the upper balloon could then be used as the trigger. However, I strongly suspect that differences in manufacturing and inflation would give a margin of error of several thousand feet, which would mean that the upper balloon tether would have to be corresponding long; in practice this one is a no-go too.
The four-balloon idea is neat, if they do consider a cluster balloon again. It could be made workable by adding about 10% more helium to the top balloon.
One big problem with the two-balloon approach is that the tension waves along the support lines will destabilize the platform. I think it’s a bad idea in any case to depend on a flexible line for moment resistance — never try to push a rope! Stabilizing gyros on the platform are important in any case.
There are problems, How to tether the balloons, what happens after burst and the payload is bouncing like it's on a bungee cord for a some time. However I still have had no answer to the question:
Will the rocket add enough to the total altitude to make up for the margin for error that must be allowed to ensure that a single balloon will definitely not have burst at launch?
As you're intending to release at +27 - 30km 90,000 - 120,000ft we could use the increase in UV radiation to trigger the launch. The incidence of UVC radiation increases at about this level http://en.wikipedia.org/wiki/File:Ozone_altitude_UV_graph.svg so we need a sensor that can measure the mean UV incidence and calculate the approximate height of the ballon. I suggest we mount the sensor on the top of the ballon so it isn't shaded.
If you know the largest diameter the balloon will get to before busting, you can use this to trigger the release. A length of string fixed at the top of the balloon down one side to a pull release switch fixed at the bottom would do it. As the balloon expands it gets to a point when the tension pulling on the string around the balloon pulls it taught. You could use a grenade pull pin type setup at the bottom as a release mechanism. This way you get to the highest altitude that the balloon can get to before releasing rather than a fixed altitude.
I think you underestimate the fragility of these balloons. As I understand it you can’t even handle them without gloves, because skin oil would make a weak spot that will cause premature rupture. A string being pulled taut would cut right through the latex as it got fairly high.
Who said anything about telemetry being used? That is far from a foregone conclusion, since transmitters take a lot of power which will be exponentially more costly as the temperature drops with altitude. The only really necessary telemetry is a phone to call in its GPS coordinates upon landing, or perhaps a short while before when it’s warmed up a bit and is in range of cell towers.
Problems 1) launch altitude control, 2) Maximise launch altitude 3) Deal with problem of premature balloon burst.
Possible solution have two identical balloons with launch triggered by the angle between tethering ropes which form a 'Y' shape with the launch platform. As the balloons expand the 'Y' opens wider and at a certain level triggers launch mechanically or electrically. The launch also is triggered by one of the balloons bursting by the same method (as the burst balloon falls, the 'y' open and also triggers launch. This also allows the possibility of maximising the launch altitude as the system will not become unstable with a single balloon burst. The balloon burst also means that the tension on one of the tethers falls to zero, this could also be used to verify launch triggering condition, avoiding complex circuitry.
(1) The side-by-side configuration would be unstable. getting the two balloons to rise in lockstep is essentially impossible. (2) The loss of one balloon would set the platform swinging wildly, it would take at least a minute to settle down, and long before that (3) the stress spike on the second balloon will trigger a loss there too.
As far as I understand it the explosive self destruction of a balloon at altitude could damage anything too close and would certainly cause another balloon next to it to burst. They would need to be some distance apart, probably one above the other. Difficult but not impossible to arrange. This arrangement would induce the platform to bounce rather than swing at the loss of one balloon (or set of balloons) which may or may not be better.
If I only got 30m GPS accuracy, I would throw it in the bin.
Why are people re-inventing the wheel? GPS works perfectly well up into orbit, is cheap, light, low power, durable, flexible, and capable of being interfaced with similar cheap, light, low power (etc) systems like the Arduino, Mbed, PIC micros, in fact anything which has a serial interface.
Adafruit have a $39 unit which measures 23x35mm and weighs 8.5 grammes. It is not altitude capped, and has an internal data logging capability. It draws a maximum 20-25mA whilst actively tracking. It also supports 10Hz updates.
This is a problem that was solved a long time ago.
the problem with GPS is that in a civil application the maximum altitude a GPS is specifically restricted to & rated is 60,000 feet http://www.fas.org/spp/starwars/offdocs/itar/p121.htm#C-XV This is because the ITAR regulations conclude that anything going higher must be some kind of missile (not that the North Koreans will get to test this theory...). That leaves only a few options;
1. Altimeter based - reasonably accurate, but as presure falls the accuracy tails off, so its difficult to calibrate, this is probably the most simple to achieve. Atlimeters are normally calibrated to standard flight levels & temperatures, 90,000 feet is quite a bit higher!
2. Ballon burst - accurate, but the causal effect would lead to a very unstable launch.
3. Horizon Curvature Measurement - Good but would require a lot of work to calibrate & computing power etc.
4. RDF & Telemetry - By placing an radio beacon on Lohan and measuring the inclination from two / three ground based positions (separated by a few mile or two) we can triangulate the approximate height. But this will require good two way telemetry to remotely launch Lohan.
5. Light metering / UV radiation - simple to impliment but may be difficult to calibrate accurately.
After a post in one of the recent article forums, I've been doing some digging about home build zero pressure or superpressure balloons.
I can understand the reasoning for choosing a weather balloon. They are cheap (relatively anyway), easy to obtain, no fuss in getting them back down, lots of previous knowledge to go by.
But from what I can find, for rockoon flights the big boys (NASA and the likes) seem to be using zero-pressure or Polyethyleen film valved super-pressure envelopes. The advantages of using a zero-pressure design are big. The balloon soars to a maximum altitude and stays there until commanded to drop, thus giving you a much larger launch window and a lot less fuss about how to time the launch. (Just use a timer long enough that you can be sure the balloon reached (near) maximum altitude). There seem to be quite a few HAM radio enthusiast using homebuilt zero-pressure balloons for high-altitude flights.
For instance there is this "tutorial": http://diydrones.com/profiles/blogs/team-prometheus-how-to-make-a-zero-pressure-high-altitude-balloon (look in the comments, the author posted the tutorial amongst the comment thread, not the most readable, but it seems a good guide)
The University of Cambridge seems to have dabbled in high altitude ballooning as well, might be worth it to give them a ring. (http://youtu.be/uK80MXHQ5hA)
A variation is the valved super pressure balloon. This design maintains a slightly higher inside pressure, but limits the pressure differential through a spring loaded valve to just below burst pressure. This has the advantage of a slightly more rigid envelope, keeping its shape better in gusts of wind.
Having made a few non-rigid sky lantern style balloons out of bin bags, I think I know what kind of idea you're on about with a zero pressure balloon. I can't say it would be masses of extra effort to stick four or six big sheets of plastic together. The only real difficulty will be making sure the edges of the mylar sheets (or whatever you use) are heat-sealed properly so the helium doesn't leak out. Easy enough to test by plugging the neck section and sticking a weight on an air-inflated balloon body overnight. No significant loss == AOK for a four or six hour mission.
The rig can be attached to either a circular or #-shaped frame connected to the bottom opening of the balloon. Make the neck section long enough that a bit of swinging won't lose significant amounts of hydrog^Whelium, and the burst problem is solved.
Deflating can be via one of a number of mechanical or pyrotechnic methods people have already described here (I personally favour a few redundant lengths of fuse attached to the balloon), with the balloon being made of something photodegradable so if the worst happens, you don't have a launch pad cluttering up the upper atmosphere for months.
I haven't completely finished thinking this idea through yet, but anyway...
The basis of the idea is to use one balloon inside the other, both being partially inflated but so that the sum of their inflations equals that of a single balloon. The release is then triggered by the bursting of the outer balloon, which will be larger and therefore under more stress than the inner balloon. The inner balloon, being under less stress, should not burst and whilst having insufficient lift to maintain altitude, should result in a controlled and reduced rate of descent, at which point Vulture 2 is sent on its way.
Fabricate a short length of coaxial tubing with, at the upper balloon attachment end, the inner length of tubing being longer than the outer. The two coaxial tubes need to be brought to separate inflation feeds at the bottom inflation end of the combined coaxial tube. With both balloons deflated, attach one balloon to the inner coaxial tube and then carefully feed this balloon inside the other, which is then attached to the outer coaxial tube.
When the balloons are to be inflated, partially inflate the outer balloon first and then inflate the inner balloon, the aim being to achieve the same total volume and pressures in the combined balloons as you would in a single balloon.
The thinking behind this is that you start by imagining a single inflated balloon and then ask what would happen if there was an internal membrane separating the inner volume of the balloon from its outer volume? The pressure within the balloon was uniform within the total volume before introducing the membrane and just introducing the membrane should not change this, so the pressures inside and outside the membrane will still be equal. Where it gets more tricky is when you try to factor in the stresses on the envelopes of the two balloons when both are under positive but equal pressure and how the relationship between the pressure of the two balloons will change as they ascend and expand.
I can see two potential issues with this straight away: the risk of the shock of the outer balloon bursting triggering the inner balloon to burst as well, and detecting the bursting of the outer balloon.
I'm envisioning the two balloons being inflated to somewhere between 50:50 to 75:25 percent (inner:outer) so that there would be quite some clearance between the two balloon envelopes, which should increase in absolute terms as both balloons expand and which, I would hope, would provide sufficient safety margin against the shock of the outer balloon bursting triggering the burst of the inner.
As to detecting the bursting of the outer balloon, the best way I can think of is by some sort of shock gauge, although I suspect that this would need to be disarmed until the whole ensemble has reached relatively smooth high-altitude air, to avoid being triggered by low-altitude turbulence.
Yes, that is one of the potential problems I mentioned. However, the outer balloon won't simply collapse when it bursts because it's under positive pressure i.e. the higher pressure gas that was inside it, and in most of the slow-mo films of bursting balloons that I can remember seeing the bursting envelope seems to mostly follow its pre-burst outline and not simply collapse inwards. Here's a youtube link to a good example: http://www.youtube.com/watch?v=ejWf8iXjXZk
If there's enough separation between the two envelopes then it _might_ work, remembering that the inner balloon, being much smaller, won't be under nearly as much stress as the outer balloon.
I'm quite happy to admit that I'm not totally convinced that this scheme would definitely work but it would be a relatively easy and inexpensive experiment to try, starting with ordinary air-filled party balloons and progressing to larger balloons if the smaller experiments are encouraging.
If it did work though, then it would allow a (relatively) stable launch at optimum altitude.
Ok another very simple way of measuring the altitude, at sea level the speed of sound is 340ms at 30,000m it is 300ms. Therefor we attatch ultrasonic transducers on a plank 1m apart facing eachother and a simple arduino counter. Transducer A pings at a signal and when Transducer B recieves it it pings back to A and so on. At sea level you should get a signal of 340hz whilst at 30km it will drop down to 300hz.
I was thinking of utilising the fact that there'll be tension in the chords between the truss and the balloon just before it bursts and none just after. I was also thinking about those spring loaded tape measures that snap back when you pull out the measuring ribon. You could have some kind of spring loaded system such that there's a chord connecting the truss to the balloon at tension while the balloon is ascending, that will snap back into the truss when the balloon bursts. The end of the chord attached to the balloon could form an electricital contact for the trigger that will make contact with the other... um... contact when the chord retracts back into the truss.
How about replacing your tape measure with a simple spring scale?
Chose a spring scale with a range of 1/2 to 1/4 (guesstimate) of the total weight of the truss (incl Vulture 2) so that once the balloon's inflated and the truss is suspended the spring scale will be fully extended and at the limit of its travel, so as to avoid bouncing off the upper stop during the ascent.
Botch a new lower limit stop, at about 1/8 (another guesstimate) of the full range, so that the spring is still under some tension when it fully retracts. Then epoxy an insulated contact onto the scale, beside the new lower stop, and another to the spring scale indicator so that when the balloon bursts and the spring scale retracts it makes a firing circuit.
I think the the idea of attaching anything to the parachute canopy is risky because of the possibility of it interfering with the opening of the canopy. As well as the risks of the canopy failing to inflate quickly at high altitude and of the long release cord getting twisted around the tethers, there's also a risk of it being blown taut and into a loop (think of a fishing rod with only a top eye and no intermediates) which may or may not interfere with its intended operation.
Yep, as illustrated, I suspect the line of the trigger release mechanism will get wound round the chord/parachute/whatever, pull taught and lead to premature ejection of the rocket, given spinning seems to be such a problem. A spring-based system is likely to be a safer bet, barring jogging up and down during any violent spinning.
The concept of the parachute giving a tug to eject the rocket is so appealing, however, that I think it should be worked on further. Perhaps the swivel mechanism could be placed above the chute and the trigger chord placed inside a straw to keep it out of harm's way?
By coincidence, the ham publication QEX for May/June 2012 has an interesting article titled "A Simple Sensor Package for High Altitude Ballooning," by John Post, KA5GSQ.
He discusses how to normalize the output voltage of the Honeywell ASDXACX015PAAA5 pressure sensor at 100,000 feet, which was one of the previously discussed options.There is a lot more information that might be worth reading. You can download the article at
LOHAN MUST FIRE before the balloon bursts because the balloon and connecting cables will WRAP AROUND LOHAN before it gets off the rod....
I direct you to this video http://www.youtube.com/watch?v=kAhaIDNVyC0
at 5:20 the balloon bursts.
at 5:33 the remnants of the balloon are shown WRAPPED around the truss which held their camera...
at 5:37 you see the horizon at 90 degrees and the bag hold the control circuitry at 90 degrees to the camera... ie. the truss is tumbling...
at 1:31 you see the small chute they had on the contraption to act as a stabilizer during descent...
at 5:55 you see it open and working it's stabilizing magic... the camera is now facing upwards again...
LOHAN MUST FIRE before the ballon bursts because no chute can stop the spin of the truss in a short enough amount of time to fire LOHAN:
In this video: http://www.youtube.com/watch?v=_00eZtsuJ9M
at 2:35 ice is shown flaking off the parachute... interestingly the metal can does not have any ice... What materials on LOHAN might ice up?
While this project's chute seems to have immediately stabilized the platform relative to the horizon, from 2:35 to 2:50 we see the a pronounced wobble which surely would cause LOHAN unhappiness whilst leaving the mighty rod...
Oh and one other thing... it is spinning like crazy...
LOHAN MUST FIRE before the balloon bursts because upon burst the truss as designed will immediately fall forward (nasty gravity) and POINT DOWN allowing LOHAN to either fire straight down or just slide off:
I direct you to this video of a contraption which was heavier in front of the camera... http://www.youtube.com/watch?v=ZCAnLxRvNNc
at 4:10 the balloon bursts and the we see an immediate pitch forward because of unequal weight ratios...
The inevitable tumbling begins....
In this video http://www.youtube.com/watch?v=zf1i2qyvei8&feature=endscreen&NR=1
at 4:14 the balloon bursts and the camera which was pointed UP... by 4:17 is pointed 90 degrees to the horizon and at 4:19 is pointed DOWN....
The inevitible tumbling begins....
FORGET THE PARACHUTE DEPLOYMENT GIZMO... I SAY THIS FOR YOUR OWN GOOD
at 7:40 you see great example of your chute and balloon wrapped around your wildly tumbling near MACH 1 falling LOHAN of death...
Fire LOHAN before that balloon bursts...
Thanks... sorry bout that.... eh hem...
At 7:40 you see the balloon and parachute wrapped around the truss carrying the camera, etc etc...
I know the balloon in going to spin and that wind will also cause LOHAN to impart more spin....
But i have been thinking about the spin since watching a number of the balloon videos available.
Would it be worth it to use some form of rotation resistent rope and/or a swivel between the balloon and platform.
If you do just go with a standard twine or climbing rope, it might do a bit of good to pre-untwist it by allowing LOHAN to hang from it overnight prior to launch....
Any sort of "rope" will be far too heavy, and a 'climbing' quality rope unnecessarily expensive as well.
I've previously suggested Dacron Big-game fishing line - a quick check on e-bay shows a spool of 180lb x 100ft braided Dacron line for £5.11.
Can't you just measure the weight of the truss + the plane maybe once every tenth of a second. If that weight is less than say 10% of the actual weight it would mean that the balloon has burst and the assembly is falling. If the weight stays below 10% for three (or more) consecutive measurements it's probably not just a wind gust but an actual balloon burst. Use the altimeter to only enable this system above most of the weather to ignore a bumpy start.
If that is not enough use the above to prime the system and then wait for the jerk in the tether line when the parachute deploys using the same weight measurement system and then fire.
Use a longer tether line from the parachute than the height of the balloon to prevent it from covering the plane...
..if the folks on it could be persuaded to try to 'snap' a couple of piccies. Your target height is 100 kilometres or so, which is about 300 Km below it, and as it does fly over Spain (I think)...Jut a thought...If they have something bettter than a couple of Kodak Instamatics, with accurate timing, they might get a decent shot...
Just reading the roundup article posted to the Reg. If the heater foil for the motor does need power during the ascent, could it be connected using something like the Magsafe connectors to ensure separation at launch? Article on a DIY setup (for a Thinpad as it happens):
Four slivers of magnet (bits of speaker magnet work well; not Neodymium!), two on the body, two on the ends of the wires from the heater.
Test the pull-away forces with small weights. If the pull-away force is too high, either reduce the cross-section of the slivers; or place one or more pieces of aluminium foil between each pair to reduce it.
Use a little Jena (http://www.jenalabs.com/contact-fluid/contact-fluid.html) or similar to ensure good electrical contact.
It is possible to get the pull-away forces down to a few 10s of grams whilst retaining good vibration resistance of the connection.
(Still not sure why the launch rod has to be titanium -- or any metal? A carbon fibre rod would be lighter, stiffer and less prone to icing.)
A design issue to consider:
If I understand correctly, there's an accelerometer on board that is supposed to detect balloon burst - I guess by detecting a switch to zero-g. actually, detecting loss of lift.
In case this detection is used to fire the rocket motor, then careful false-positive prevention is in place - we don't want anyone firing off prematurely..
I can imagine false balloon burst detection due to atmospheric instability, running into a cloud for example, which could play havoc on acceleration signals.
Being unfamiliar with the accelerometer and computing power on disposal, this is as far as I can go.
What about using a laser rangefinder of some sorts? Stuff the device in the neck of the balloon and it can monitor for when it's pointing at open space.
If the sudden pressure drop when the balloon bursts can be measured and acted on, there's now three ways of detecting a balloon burst. How about if LOHAN has all three methods incorporated and the onboard computer hits the go button when 2/3 methods report a burst?
What about a good old fashioned nylon thread looped around a ceramic resistor?
This has the advantage of working 100% once fired if a thyristor is used.
Also an interesting idea to add to the balloon is a high altitude UV radiation monitor made from a clear epoxy encased Bluray burner diode (LED'd ones work fine) connected to a small op-amp and voltage to frequency converter such as a 555.
These are pretty accurate and sensitive to wavelengths between 395 and 375nm (UV-A) so could be used as a primitive ozone depletion sensor.
Could be attached to either the primary or secondary payload and useful science done with it as long as it has been temperature/intensity calibrated on the ground.
Also worth considering, a VCSEL based (0.4mA) atmospheric dust sensor.
Adding solar cells to boost the onboard power might be feasible as the wafer type aren't that heavy and provide about 150mA at 1V per 20cm on a side piece.
Adding a WiFi tracker would also be useful in case the main transmitter breaks.
Use silver demister paint sprayed onto a prepared pizza base :-) use clingfilm held on the surface with PVA glue, or use thin copper tape.
This can double as a 10.250 GHz antenna as the wavelengths are compatible with both systems.
Its also feasible to make a small "blob" of low melting point gallium/indium alloy which melts at a relatively low temperature.
This is an order of magnitude more sensitive than the loop of nylon but has a breaking strain in the multiple kilos.
Also two electrically separate resistors, thyristors and batteries connected to the "base" means if one independent firing circuit breaks during ascent then the other should fire.
Use optical LM567 based triggering so no wire is needed, with the correct frequency being needed for it to fire.
As a second backup, have an altitude based trip which severs the connection to the balloon if the internal pressure exceeds a set limit, with three sets of rotors and a tilt sensor to keep the payload upright during descent so the rocket can fire.
This post has been deleted by its author
Re Heater connection wires
The lowest possible tech solution is to use two paper clips superglued to the motor with bare wires threaded through them. The disconnection force is minimal and there will be no trailing wires, The extra weight on the rocket will also be minimal.
The idea also has the advantage that it will be easy to test using a variety of paper clips as found in almost any office as no construction work is necessary.
A quick thought inspired by the base plate suggestion.
Why not take the Scaletrix approach, run two "tracks" from the electronics enclosure parallel with or attached to the titanium rod (insulated of course) between the aluminium plates, then the vulture 2 just needs a pair of metal tags to make contact with the tracks.
An alternative could be to use the same approach but deliver the power over sprung connectors from the tracks to a pair of contacts on vulture 2.
Having just read the latest LOHAN blurb about power connection for the heater. Would you consider:
1. Conductive rubber for the rubber pad on the titanium rod. Build the pad in two halves around the rod with insulation between the two halves and the rod, and put contacts on the teflon insert support.
2. Conductive "rubbing strips" on each wing toward the trailing edge, in line with the teflon strips, and conductive "leaves" (fingers?), carbon brushes or rollers on the rearward ends of the teflon strips. Make the assemblies springing stiff enough and they would also deter any "swinging" on the ascent.
Well that's my two pence worth.
Another way to connect balloon burst to electrical switching would be to put a band around the lower part of the balloon containing a pressure switch; closed due to the pressure of the balloon acting on it.
At balloon burst the switch would open, providing the required signal to the electronics to fire.
1) Firing during free fall is probably a bad idea as it could be pointing anywhere.
2) Firing at balloon burst might be OK if quick enough to get clear before the possibility of tumbling.
3) Instead of that pin against a spring (which will jam) how about a clothes-peg with a couple of drawing-pins embedded but held apart by piece of card. The back of the drawing-pins are soldered to firing circuit. The card is tied to the fail-safe line as in your diagram. I spent ages on some ASCII art but this forum thing ruined it!
4) I don't like the idea of tying the fail-safe line to the parachute but I can't think of anything better mechanically.
5) I thought that a straw full of salt water (sealed in) might be useful in preventing premature triggering as the straw would remain flexible until a decent altitude when the water would have frozen solid. Perhaps someone can think of how this might be useful.
Have you checked your GPS receiver for accuracy at altitude? The last time I used a Garmin through the window of an international flight it was under-reporting altitude by almost 10% as compared to the jet's navi display. I don't think they anticipated their gear being used at 30,000'.
Hmm, that would depend on where the display in the passenger cabin is getting its feed from. If it's coming from the barometric altimeter, your Garmin may have been more accurate.
Assuming that the plane was right and the Garmin wrong for a moment, there's another factor here. The signal view through the plane's window will only get a limited subset of the sat constellation, as the metal body of the aircraft will act as a rather efficient obstruction to others, which is a bit of a downer for good triangulation. For a fix in 3D, you need a minimum of four(?) sats locked and the fix improves with more. A GPS on a balloon with a good signal view of the entire sky should get a far more accurate 3D fix than the rather one-sided bias from the plane's window.
The cabin display will use the barometric altimeter - or at least, the flight crew will when in uncontrolled airspace. 'Flight level' altitude moves up and down with ground pressure and temperature, but if everybody uses it, all the planes stay at their nominal flight levels and go up and down together - this helps stop them bumping into each other.
I did some testing with the GPS we'll be using; on Long Mountain in Shropshire it logged heights about thirty meters greater than the OS positions, but (a) from inside a car and (b) GPS and the OS don't use the same geode so there's a potential offset. On the bench in the house, the device usually reports the table at 170 metres - the OS think it's at about 120 and simply by moving my hand over the receiver that height drops 50 metres... the software filters the altitude data to smooth out changes like that, but we will be using a number of large differences as triggers to launch.
was watching something on youtube about some Russian scientists who talked about balloons only usually getting around 22km height. However with a little prep they said 30km height was possible. http://youtu.be/MUAAtSyuBEc?t=17m10s around the 17:10min mark. They said they mixed the balloon in kerosene and benzine.
Trying to direct/control a rocket with nothing more than a balloon, string, and rod sounds like the beginning of a bad joke: This balloon, string, and rod walked into a bar...lol
The rocket and launching rig should make a clean separation. The rod is likely to cause more issues than solve.
The rocket needs to be inherently stable during acceleration regardless of how you launch it. This means the CG needs to be aft of the thrust. Put a long rod on the tail. Drop the rod for the glide home.
The article: "Battle continues over LOHAN's mighty rod" Published May of 2012, mentions something about a test launch of the rod system? Did that happen?
Below is a weight shedding scheme for the suggestion in my previous post but, first, for clarification of further discussion, my view of acceleration by a rocket:
1) In the initial portion of the launch the rocket's acceleration vector will be entirely determined by the relationship between the center of effort (CE) and the center of gravity (CG). This is the period where ground based rockets rely on a launch rod or rail. If the rod is not stable it will create wild asymmetry between CE and CG. Even a very small amount of friction at the rod-tube will (albeit briefly) mechanically couple the launch rig to the rocket - meaning their combined moment of inertia will determine the rocket's trajectory. Friction is a huge question mark; far too many parameters there for predictable/repeatable events.
2) In the second portion of the launch the direction of flight will be determined by the relationship between the rocket's CE and the center of aerodynamic resistance (CR). Ground based rockets have a thick atmosphere to work with, a lot of speed, and fancy vectored thrust to be able to enter this portion of the launch shortly after leaving the rod/rail. LOHAN does not - a variation from a ground based launch is a necessity.
3) This last portion aerodynamic stability, may never be attained (as has been previously discussed) due to the thin atmosphere and relatively small control surfaces - alas, I am not an aeronautical engineer specializing in high altitudes at mach speeds, so I can't answer that one!
Scheme for each of the above mentioned portions of launch:
1) Use a long weighted boom at the tail so that the rocket will literally stand on its motor. That is, if you supported the rocket at the nozzle does the entire aircraft point vertical? This is absolutely critical because of the lack of stabilizing rod and active stabilization.
2) Drop the weighted portion of the boom yet, leave enough boom length to keep the CR aft of the CE, which will be necessary for stable flight. perhaps as the rocket accelerates and the stabilizing effect of the rod increases, several potions of the boom may be dropped.
3) Drop the entire boom for aerodynamic flight, both during the burn and after.
Ahh, then the real trick right: Making it work.
Make the boom hollow with a long shaft running down the center. The shaft will be cut into portions coupled to each other with sockets. A servo rotates the shaft assembly which has several sets of unequal-length threads cut into each end. The first portion of the boom is bolted on by the shortest threads. The second portion and any subsequent portions have progressively longer sets of threads. Servo unscrews only enough threads to drop the intended portion.
This one is in my neck of the woods as I am an electrical engineer specializing in electrodynamics.
Use the magnets to press the electrical contacts together. That is a great idea.
I would further recommend supporting the entire rocket by them in lieu of the rod (see above :)
Making it work:
What are the electrical tails on the magnets for? Are they to reverse-saturate the magnets? This would effectively 'turn the magnet off.'
If that does not work then try:
Couple the magnets via iron cores that remain on the launcher. Wrap the cores with coil-wire. When it comes time for separation use a large pulse of electrical flow to induce a magnetic field of opposite polarity of the permanent magnets this will literally push the rocket away from the launcher. Make sure the coils are in series to ensure a synchronized release.
I thought that I may be able to help with suggestions, from an engineer and pilot with high-power rocket experience. I have successfully flown a two stage "L" powered rocket with radio second stage ignition and radio chute ejection.
Years ago, motor ignition was a constant problem. The solution was to use a reliable electric match (or two) and epoxy it into a plug of actual propellant. The propellant will not ignite while cutting with a sharp knife. Cut a piece about 1" long and trim the sides to fit into the motor nozzle. Use a drill bit, in your hand, (no drill needed) to make a hole for the electric match. Use some quick set epoxy to glue the match inside the hole. When ready, epoxy the whole assembly inside the top end of the rocket motor. When that electric match is fired, that motor WILL burn. Never had a failure that way. Igniters that pop, like a firecracker, will usually not light a motor.
If you are worried about the cold, you can test fire it after being wrapped in dry ice for an hour.
1 - These motor casings are quite hot after a burn. Even though it is cold up high, can the motor soften or melt the fuselage?
2 - As mentioned in a previous post, stability in the thin air may be a problem. Basic rocketry tells us that the aerodynamic center of pressure (CP) must be behind the center of gravity (CG). The more the better. You have a unique situation. There isn't much aerodynamic anything at 100,000', so, until you build a lot of speed, you will rely on the center of thrust being aligned with the center of gravity. If it isn't, you end up with a corkscrew flight pattern. This stability is easy to achieve, if the vehicle is symmetrical and all is aligned with the central axis. If not, you really need to make sure that the thrust is aligned with the CG.
3 - In the thin air, this may not be an issue, but I have had 1/4" plywood fins completely break off an "L" powered rocket. It was due to turbulence/instability from a mirror shroud for a camera. It would be very difficult to break those by hand, but the aerodynamic forces snapped them clean off.
The wings on Vulture 2 look much more fragile. It might not survive a J motor flight at sea level, but maybe ok at 100,000'. Since there is little drag up high, your acceleration will be based on weight and motor impulse. If Vulture 2 weight is 4 pounds, final speed in thin air might be about 400mph on a "I" motor and 600mph on a "J" motor. Maybe the aerodynamic forces would be equivalent to less than 100mph at sea level. In that case you should be ok.
Wish you the best for a great flight!
A suggestion for attaching electrical connections between the plane and the truss.
For each connection a short length of backward facing copper tubing would be fixed to the plane (or even inside the body with the tail end of the pipe outside). The front of the copper pipe is soldered to the internal wire.
On the truss a cable hangs down to the plane with a small piece of steel wool soldered to the end. To make the electrical connection the steel wool is then pushed into the copper tube on the plane. The springyness of the steel wool will hold it in place but should slide out of the tube as the plane launches.
The amount of grip needed inside the tube can be varied by soldering more or less steel wool to the end of the wire. Install one tube/wire combo for each required connection.
Alternatively, steel corset boning against a u-shaped (conductive) profile could be used.
It's nothing more than a flattened interwoven steel-spring double spiral, which has the lovely property to be pretty bendy in the flat plane, and not really bendy at all to the sides. They also weigh next to nothing to boot. Short enough, or doubled up they may even help stabilise the plane on the truss in-flight. It'd make a bit of a "noisy" connection, but afaik all the stuff on the plane is on/off so that shouldn't matter much.
( and for those who wonder.... The boning in the pic is actually meant for male clothing: holding your cuffs nice and round in black or white tie attire... :P )
Geodetic construction loses a significant amount of it's strength and strength to weight ratio when split lengthwise and then the skins edge glued together.
I don't see the foam airframe with those high aspect ratio flying surfaces surviving the impulse of a rocket motor, even in low density air..
I have come late to the party and may have missed a whole bunch of technical solutions - has there been a test firing of a prototype airframe for structural integrity?
While the idea of a 3D printer produced airframe might be exciting, the glider would be far stronger for roughly the same weight as a monocoque structure made of carbon fiber and epoxy. Here the inner mold for the airframe could be a 3D foam plug that would be dissolved away by solvent after the exterior monocoque skin is formed over it.
At the very least carbon fiber could be inlet into the foam in the form of spars, ribs, and longitudinals.
Or the carbon fiber can be inlet into the foam in the form of geodetic spirals, then the foam dissolved, and the spiral framework left behind covered with monocoat.
You might benefit from collaboration with the hand launch RC glider community, or the indoor microlight airplane folks, etc.
The planform as shown for the airframe will tumble wildly out of control the instant the motor in it's tail fires, and likely rip the wings off - which is why I suspect there has not been a test firing of an airframe.
I'm thinking that a very small motor to just get the airframe off the launcher, and a single RC channel (or GPS/gyro) rudder control to bring it home will give LOHAN a better chance of a successful flight. KISS principle.
On the idea of copper tubing with metal wool inside for electrical disconnects for the motor igniter - it is quite original and I admire the author. (aluminium weighs less) The tubes however should remain behind on the launcher, not go with the airframe, for weight reasons.
If the group is fixed on having a significant climb in altitude from the motor firing, then I see it as having to be a rocket carrier with a piggyback airframe. The rocket motor will be at the forward end of a carbon stick. The airframe will be mounted at the rear end of the stick (like the tail feathers on an arrow) putting the center of thrust well ahead of both the Center of Gravity and the Center of Pressure.. This should give better stability.
Think of the motor and stick functioning like a bow and arrow - or the throwing stick of a spear.
As long as the bowstring (the stick) is pulling, the arrow (the airframe) is held by inertia against the string via the nock on the arrow. - By fingers in slots in the case of our airframe.
When the motor abruptly stops (most of it's mass has been thrown away) and the greater mass and inertia of the airframe makes it fly on ahead of the slowing motor/stick thus separating the airframe from it's launcher.
Anyway, those are thoughts as I skimmed through the comments.
Cheers, eh wot.
Some thoughts upon reading your post.
I'm assuming the boffins in training who design the LOHAN airframe did some calculations on the stability under thrust and stability in flight. I have no doubt this airframe will fly in normal atmosphere. How it'll fly in the rare atmosphere at altitude is anyones guess.
A test flight of LOHAN is still on the to-do list at this point. In terms of load on the airframe I doubt a test in atmosphere of a firing in atmosphere is going to be very instructive as things like aerodynamic flutter and high lift loads start rearing their ugly head.
The stick idea is a possibility but how do you keep the rod on the plane during the ascent? This creates a whole new slew of problems to overcome.
That the thermal expansion/contraction of your your copper-wire (tinned?) electrical contacts and the tubing it slides into has to match. It is _slightly_ warmer at ground level than at altitude underneath the balloon, and it could be *embarrassing* were those wires to hang up on ignition.
Having just read the article showing the sliding wires and tubes arrangement for connecting the motor heater, I think the bent tubes protruding out of the plane are bent the wrong way in the pics. Should they be bent toward th e exhaust, so the wires pull out cleanly as the plane launches forward from the truss? Or am I mis-viewing the pictures?
In this type of unprotected disconnect it may help if the connections are offset. Neither the copper tubes nor the connecting wires can make contact with its opposite polarity. For example, if the length of the copper tube is 6mm, cut the positive lead 8mm shorter than the ground lead and move the positive copper tube 8mm forward. This also serves to prevent miswiring.
And if so, just reading through and looking at burst detection ideas, one came to mind.
Part of the hook replace with 2 contacts & a rubber band / spring or load sensor, when all lift is removed e.g. balloon no longer lifting due to sudden loss of contents the contacts will meet or the load sensor is triggered. thereby informing the pilot it is launch time.
Biting the hand that feeds IT © 1998–2020