LOHAN ideas..

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...

[Poor Coco]

Re: Launch at balloon burst

That would require a box containing the plane which, being large and able to withstand a significant air-pressure difference, would weigh a hell of a lot.

[Magnus_Pym]

Re: Launch at balloon burst

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?

[Poor Coco]

Re: Launch at balloon burst

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.

[Magnus_Pym]

High altitude control

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.

[Magnus_Pym]

Another alternative launch idea

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.

[Anonymous Coward]

Re: Another alternative launch idea

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.

[Magnus_Pym]

Re: Another alternative launch idea

I was thinking that the rocket is unlikely to maintain a straight line course after launch as it has no viable control mechanism. I make it that at a range of 100m a difference of only 0.002 degrees will miss a 10m wide balloon

[Poor Coco]

Re: Another alternative launch idea

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.

[Magnus_Pym]

Re: Another alternative launch idea

"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.

[Poor Coco]

Re: Another alternative launch idea

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.

[Magnus_Pym]

The real question is...

... 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.

[Anonymous Coward]

I don't think it will work.

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.

[Anonymous Coward]

Sorry, meant clockwise in previous post, not counterclockwise... (N/T)

EOM

[Peter Fox]

Bent coathanger does it all.

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.

[MadX]

Ice issues on the launch Rod ...

What about using a copper pipe ? The pipe is heated from the inside with a basic NiChrome coil. Power source for heating can be housed on the main truss. Might be overkill, but you are guaranteed that the pipe will not hold the rocket back ...

[Poor Coco]

Re: Ice issues on the launch Rod ...

A copper pipe with heating wires would probably weigh as much as the airplane.

[Ron B]

Re: Ice issues on the launch Rod ...

Don't forget the batteries. LiPo has a fantastic weight/energy ratio, but they might set your truss on fire if the current limiting isn't worked out right.

[Ron B]

Rods are out. Rails are in.

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

[Magnus_Pym]

Re: Rods are out. Rails are in.

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.

[Ron B]

Re: Rods are out. Rails are in.

Too much spin or side wind on the wings in the lower altitudes will batter the craft and try to rip the launch bearings off of their mounts. I think it would be worth an ounce or two of paper streamer to keep everything pointed into the wind.

[kleinman]

Re: Rods are out. Rails are in.

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.

[Poor Coco]

Re: Rods are out. Rails are in.

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.

[imanidiot]

Re: Rods are out. Rails are in.

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.

[Anonymous Coward]

I can't believe so many of you are reluctant to see LOHAN on a long, hard rod.

[Anonymous Coward]

Launch rail (rod) and backplate issues (reprise)

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.

[Poor Coco]

Re: Launch rail (rod) and backplate issues (reprise)

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.

[sisk]

Re: LeeE

The icing and dust problems could both be solved by filling the hole with the lube mentioned in the article before sliding the shaft in. With the tube thus filled with both slippery lube and a firm rod there will be no way for dust to blow in.

[Anonymous Coward]

Tether considerations

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.

[Joeman]

What about some kind of disposable "Sheath" covering the rod that gets pushed off by the movement of the plane along the rod.

The Sheath would protect against the elements and help prevent icing, but you would need to teather it somehow so it doent drop on anyones head!!

[GreenOgre]

Stiff rod with lube should do it ....

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.

[Cliff]

Backplate - if it was neatly parabolic (or had a curved bit of it to channel off part of the gases at least), it could spray nice hot exhaust gases over the titanium rod, making sure icing isn't a problem for long.

[Mike Moyle]

Backplate/Rubber Bumper

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.

[saif]

Launch condition optimisations

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.

[Mike Moyle]

Re: Launch condition optimisations

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.

[Magnus_Pym]

Re: Launch condition optimisations

At launch altitude there is very little air and very little air movement. It's getting past the turbulence on the way up that is a problem.

[Anonymous Coward]

Re: Launch condition optimisations

I like this idea.

[imanidiot]

Not sure if I mentioned this before

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? ;)

[Magnus_Pym]

Launch-smaunch

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.

[Joeman]

Re: Launch-smaunch

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..

[imanidiot]

Re: Launch-smaunch

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.

[Magnus_Pym]

Re: Launch-smaunch

@ iamanidiot

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.

[imanidiot]

Re: Launch-smaunch

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

[Magnus_Pym]

Re: Launch-smaunch

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.

[Poor Coco]

Re: Launch-smaunch

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.

[imanidiot]

Re: Launch-smaunch

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

[Magnus_Pym]

Re: Launch-smaunch

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?

[Poor Coco]

Re: Launch-smaunch

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!

[Poor Coco]

Re: Launch-smaunch

“…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.

[imanidiot]

Re: Launch-smaunch

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.

[Poor Coco]

Re: Launch-smaunch

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.