Did you look at what other people are doing?
It's fair to say that the question of just how we fire the Low Orbit Helium Assisted Navigator (LOHAN) Vulture 2 spaceplane rocket motor is a touch thorny. Click here for a bigger version of the LOHAN graphic We've been mulling the possibility of using a barometric-pressure-operated setup, and yesterday threw the problem over …
Given the existing discussions about the accuracy of barometric height measurement, I call BS on: "The altimeter uses a custom absolute pressure device to precisely measure altitude values up to 25,000 (or 60,000) feet in one-foot increments."
BUT: "Note: the latest version of the ALTS25 actually measures and reports altitudes to 100,000 feet ASL. However, above the normal specified ranges, accuracy is not guaranteed. Apogee detection functions to at least 60,000 feet for the Standard ALTS25, and to 100,000 feet for the extended ALTS25-60K version."
If all you are intereseted in is apogee detection then the accuracy is not too important.
PS - if you are going to put in a balloon-burst failsafe, then why not use that as the primary? At the bottom of a fifty-foot tether there is not going to be much shrapnel to worry about.
About the barometric pressure... couldn't you reset a sensor in a pressurised vessel to zero so that by the time the balloon hits 60,000 feet, the sensor believes that it is at 40,000, and gives you an additional 20,000 feet?
Just a thought... I'm sure you could test that at Qinetiq?
With a known distance from the object and a fixed focal point, vision systems are actually pretty damn good at determining size. Only problem is going to be you're looking up at a balloon 40 meters or more above you, while dangling from a flexible rope, being swing around like mad. As long as most of the balloon is inside the camera frame you can still determine a radius (using a Hough transform or similar) But if the camera loses sight completely, it COULD screw you over quite badly
I can see possibly this one is not going to work but I'll float it anyway.
You want to launch before the balloon bursts and presumably burst occurs because the balloon has ceased to be stretchy enough.... it has become stiff.
Measure the differential pressure, internal to the balloon and external, and by looking at how it is changing over time you might/should/won't get an indicator as to how close the balloon is to bursting.
Assuming there is anything to the idea then it should be possible to run a set of destructive tests at ground level to gather data and provide for calibration... although temperature is likely to have a marked effect on the elasticity of the balloon.
Anyway.. It's an idea.
Here's another one...
Mount a 'speaker' and 'microphone' inside the balloon. Drive the speaker with a 'chirp' and do an FFT, signal processing, on the result from the 'microphone'. Analyse the output for peaks. This will indicate how 'stressed' the balloon is overall but will also, possibly, pick up on localised 'stresses' whereby although it looks like the balloon is OK there is part of it that is going to 'give up' prematurely..... you start to get high frequency returns.
If you use a small PIC such as Arduino Nano (small and lightweight) teamed with a barometric pressure sensor (http://www.ladyada.net/learn/sensors/bmp085.html) then you can have the PIC monitor then trigger a solenoid/valve at the right time.
Additionally you can use the PIC to provide other services such as guidance, GPS etc in one small unit.
Hope that helps.
Part of the issue with the original release mechanism was the friction from the payload release. If all it needs to do is make electrical contact (or perhaps activate an optical switch) then it will be much more predictable.
And rather than adjusting the quantity of air, use a nominal amount and have a calibration run in the chamber. Put a scale alongside the pushrod, turn up the altitude, and then see where the rod gets to - that's where you fix the trigger switch.
Or, how about this for a DIY low pressure switch ?
You need a small chamber, across the end of which is a porous plate and a sheet of electrically conductive film. You suck down the chamber pressure to that at which you want it to trigger - and seal it. The film will be sucked hard down onto the porous plate - but because it's supported over all it's area, it won't stretch or burst.
Once the external pressure drops below that in the chamber, the film will lift off the support plate and start to ballon - which means it'll touch the nearby contact and trigger the circuit. Or you could trigger optically by various means.
As you've already built a very small hypobaric chamber (REHAB - if your experiments don't destroy it !), you'll be able to check the operating pressure before launch without needing to call in favours at the full sized chamber.
> Or, how about this for a DIY low pressure switch ?
Here's a variant of that idea that might not need so much setup:
Put a condom in a plastic tube. Put a microswitch (or similar) at the end.
Seal the end of the condom.
As the pressure drops, the condom grows. By correctly calibrating the amount of gas in the condom / length of the tube, a firing pressure will be attained.
Make sure to debounce the switch heavily...
I agree - the video clearly shows step-transitions due to friction in the rod/case/guide. However, using an expanding membrane (balloon/condom/higher-tech material) acting directly on a switch has to be worth considering. It is cheap, simple, light-weight, and you will (hopefully) have a way of testing/calibrating it yourselves (and surely condom use is compulsory in REHAB?). From my point of view it is worth trying just because it fits the "garden-shed" ethos better than GPS>controller>actuator mechanisms.
Maybe this is going to offend the technophiles among you, but why not launch a (small) series of test baloons and see how long it takes to get up to launch (or burst) altitude.
You can then have a very simple timer set to go off at a few thousand feet below burst height to launch LOHAN.
It saves on having a relatively heavy pressure measuring instrument and use a cheap gold plated watch as the timing mechanism :)
And since i'm no boffin i have to make some assumptions.
You want to ignite the rocket at or around a pre-determined height and you want to launch before the balloon bursts. You already know the temperature and the pressure and i assume you must also know the time. If thats true then you only need a timer. If its not true then a maths expert could surely work out a rise time given various wind speeds and balloon trajectory and that would get you a "time" window where you are guaranteed to ignite the rocket before the balloon bursts and at the height you want.
Couple of quid all in including the timer chip?
Sorry guys, speaking from experience, time to altitude is very variable and depends on a whole host of things.
As a suggestion for a launch switch, turning things on their mechanical heads, how about an aneroid capsule? This would still expand with height, but will be reuseable and relatively robust. These were used in older radiosondes to measure pressure. Get hold of one of these and you could use it to operate a micro-switch or similar. You could also calibrate it with your hypobaric chamber.
Helps the GPS device get a fix, not, as many people seem to think, make it work wilthout satellites.
Assisted GPS, generally abbreviated as A-GPS or aGPS, is a system which can, under certain conditions, improve the startup performance, or time-to-first-fix (TTFF) of a GPS satellite-based positioning system.
Many mobile phones combine A-GPS and other location services including Wi-Fi Positioning System and cell-site triangulation and sometimes a hybrid positioning system.
// sorry, pet peeve. ;-)
I don't think that using a pressure sensor represents the problem that some people have made out. The standard sensor used on radiosondes (http://www.eol.ucar.edu/instrumentation/sounding/gaus/eldora-specifications ) goes down to 3 hPa (0.3 kPa) with a sensitivity of 0.1 hPa (0.01kPa). An entire radiosonde is only a couple of hundred quid and most of that isn't the pressure sensor. Sondes have temperature sensors as well, but I would avoid that as an idea because of the necessity of working out whether you are above or below the tropopause.
(I note that a quick trawl of the web suggests that there are various suppliers of industrial pressure sensors for vacuum equipment but goodness knows what those cost.)
Actually, modern radiosondes have a GPS unit in as well as the meteorological sensors. So that tells you that GPS will definitely work at radiosonde altitudes and speeds. And ordinary, un-assisted GPS is entirely adequate: it has an accuracy of about 20m in the vertical. I would avoid all of the various Heath-Robinson suggestions and concentrate on which out of a pressure sensor or a GPS unit can be obtained for the least money and at the smallest mass.
At that height you will be well above the Tropopause. You will have also well passed the Stratopause. The one to look to at that height is actually the Mesopause.
This gives a decent temperature/height profile, http://apollo.lsc.vsc.edu/classes/met130/notes/chapter1/vert_temp_all.html but the pressures at that height are very variable.
The GPS units in 'sondes are pretty good, but unless they've upgraded them since I last launched one (about 7 years ago) they can be a bitch to get the initial lock. You also need to feed in the surface pressure in order for the ground station computer to work out the altitude, as the GPS is only used to calculate wind speed and direction in the horizontal plane, rather than altitude.
Apologies in advance for this one but has the effect of the rocket firing been considered in relation to the tethered balloon above and its escape trajectory?
What I thought was:
Rockets are usually fired from a platform that is either so large i.e. the ground or from a device that has significant mass (a plane launching a rocket) and momentum to ensure that any effect of the thrust of the rocket, whilst still attached to the platform is negligible.
If the launch platform is attached to the balloon by a flexible medium would the rocket thrust alter the angle of the platform in respect to the balloon and potentially cause the rocket to impact with the inflated body?
If the balloon was attached by a more rigid medium would the thrust of the rocket not cause the combined mass of the platform and balloon to rotate and alter the angle of trajectory?
(The latter not being so much of an issue as the thrust induced rotation may change the angle from 20 degrees from the vertical axes to almost 0 by the time the rocket was release)
Really I just like a thrust filled post.
This shouldn’t really be much of an issue. There are fundamentally two options for the motor when mounted in the launcher; one, an open-air firing, the other, an enclosed firing container that works to some extent as a piston launcher to ensure positive separation and dynamic stability through recoil action.
If the engine is not enclosed, then the firing of the motor itself will have little effect. The thrust reaction works directly on the aircraft’s fuselage. Essentially the only change as far as the balloon is concerned is a sudden drop in payload mass — which will cause it to rise, but not nearly as fast as LOHAN. The payload mass will swing, but that shouldn’t horizontally displace the balloon much — and at launch it will pull the balloon AWAY from the launch trajectory.
Using my supernatural powers of estimation and the geometry sketched by Lester, I figure the plane will be clear of the balloon between 1 and 1.5 seconds after launch, with a speed somewhere on the order of 100 km/h at that time, assuming no piston launcher.
If there is a piston launcher, then the effect of the engine firing will be the balloon rupturing if it hasn’t already (the launch pad will jerk hard on the tether and will surely push the tired balloon over the edge) so there really isn’t much to think about. The balloon will consist of latex ribbons at that point, and not be a substantial obstacle.
if you can get a Barometric measuring stick attached ??
are you going to be able to receive live remote telemetry ?
if so ... cutting out as many middlemen as possible would be a remote electronic trigger ?
it worked real well for the guys attempting to blow up a big rock hurtling to earth in a documentry I saw a while back ! I think Bruce Willis was the presenter on it .. if memory serves !
I like that idea; after all it's less than 50km and total line of sight, too.
And then I thought of this:
You (that means the SPB Team) could use the signal travel time of a radio repeater beacon on board to determine the height.
Have a 'peep-repeater'. Send a signal to the balloon that just gets returned. Measure the time.
Electromagnetic waves travel at ~300,000 km/s. That would make about 6 microseconds for a 1km ( 3300 feet) answer signal (3µs up and 3µs down). At your targeted height of 100,000 feet that will grow to 180 µs turnaround time. If the returning signal takes longer than that, send the trigger signal.
You could actually sit there with a Big Red Button and watch the numbers growing and then press it at the desired height! Come on, isn't that epic?
Power consumption is the bugaboo. The temperature, about –60°C, will suck the life out of any batteries as the power requirements increase due to increasing range. To make this work you would need a helluva lot of batteries, even if some of them were used purely to heat the others.
They're not staying there for ages, the temperature does not drop to –60°C immediately after launch, they don't need a lot of transmitting power (2x 0.5s answer-beeps at 3W per minute would nicely do, which will roughly give you a consumption of less than 2Wh* even if your transmitter is not all that efficient), a bit of insulation might do some good, non-rechargeable alkaline batteries don't suffer from low temperatures as much as rechargeable Lithium cells, if you feel too cold, you can include a chemical handwarmer...
They have been up there before and shot video; I can't remember any battery-powered battery heaters from PARIS and they surely didn't use any magic then.
*~ the capacity of a single AA cell.
I like this one - Big Red Buttons must be considered! Seriously, monitoring height/pressure by either a small camera watching a barometer (camera already in the payload, so no great increase in complexity/weight), or the "ping" suggestion from another poster. There is also the reduced need for a back-up system - if things go wrong, the BRB can be pressed to achieve a launch anyway.
All pressure or altitude based firing systems rely on firing the rocket before the balloon bursts, or to put it another way before maximum altitude is reached. Balloon bursting is unpredictable to say the least and pressure or altitude measurement may not be exact so some large margin for error must be allowed. This margin for error is likely to be larger than the distance the rocket will travel. So in effect LOHAN will be a lot more trouble but may achieve less than PARIS.
If LOHAN is going to be transmitting in some way, how about having 3 well spaced ground stations with some old Radio Direction Finding kit (as seen in all the old films where the Gestapo are trying to find the Resistance radio), a bit of quick 3d trigonometry and we know the altitude - then at the appropriate height just send a signal that triggers the release?
I you want to keep things simple, don't complicate the situation with a bursting balloon!
With the current system you have to worry about launching before the balloon bursts. A conventional "zero pressure" high altitude balloon is not sealed. It will achieve a higher altitude than a super pressure balloon* and maintain that altitude for long enough for a very relaxed launch window. If you prefer a sealed zero pressure high altitude balloon then, out of politeness, you may add a burster to deflate the balloon after launch.
*sealed latex weather balloons are "super pressure" balloons, because the internal pressure is higher ("super") than the external pressure. Zero pressure balloons have zero pressure differential.
A sealed zero pressure balloon is an envelope filled with enough gas to allow it to rise, but not enough to burst the envelope at altitude, but that means you need to fill it with a measured volume of gas. It's easier to partially fill a balloon with an escape vent at the bottom.
Welding a big polythene balloon is a reasonably simple d-i-y job, if you have access to a working area large enough to lay out your balloon segments.
Supposedly, some GPS manufacturers have either misinterpreted the altitude/speed limits, have elected to be more conservative, or have simply had a programming error in their code (e.g., "||" rather than "&&"), such that the GPS stops working above a certain altitude, regardless of speed. Somewhere, there has been a list compiled of GPSes which exhibit this problem, versus ones which will work fine up to 120+K feet. Some research may be required to find it, or some communications with some of the left-pondian (e.g., USA) groups who are into high altitude ballooning may save you some time and expense. Here's some (dated?) information. Click on "The Science" and then either "GPS Pass" or "GPS Fail":
Note that your control system should have a way of preventing the ignition of the rocket motor under certain conditions. For example, it shouldn't fire the motor if the altitude is below 10,000 feet (which will prevent the motor from being fired while the device is being launched, or in the event that a premature balloon burst results in the rocket reaching ground before being fired; you don't really want it to fire on the ground for safety/fire/insertion reasons.). You may also want to disarm the firing circuit based on elapsed time (e.g., Don't arm it until after 3 minutes after launch, and disarm it after 3 hours after launch, to account for a pinhole leak turning the balloon into a "floater" that may carry it into another country's airspace.).
Oh, the current record for an amateur floater was a launch from California that dropped the payload into the Mediterranean! (Yes, transcontinental, and transoceanic, too!).
As for launch angle, have you considered launching horizontal, with enough control capability on the rocket to change to a vertical orientation after a short amount of flight? That would minimize the chance of the rocket hitting the balloon (or shards thereof), or the parachute, etc., even in the event of a wildly swinging launch platform due to the extreme winds at altitude.
As for rocket motor ignition at altitude, it may be well to research the work down by Bill Brown and his Huntsville, Alabama team, while launching their HALO (High Altitude Lift Off) project:
"It should be noted that it is no small task to launch a rocket from a balloon. The first thing that one must understand is that solid rocket fuel will not ignite and burn in a vacuum. You can make it all ammonium perchlorate. The amount of oxidizer makes no difference. In a vacuum your motor will not burn. The secret is that the fuel must be under pressure. The military knows this lesson well. All high altitude motors have to be specially designed to ignite at altitude."
I've worked with Bill before (even recovering one of his wayward HAB payloads), and he's a great guy. You might look him up and drop him a note to ask for more information from him.
In any case, good luck with your flight/launch, and we'll be anxiously awaiting the report. Oh, and have a pint for me (It's almost lunch time on this side of the pond.). :-)
Perhaps you've misunderstood how the RDAS (and other accelerometer based altimeters) work - or maybe I have.
The altimeter measures the acceleration while going up and detects the apogee and fires a channel (usually the drogue). On the way down, it measures the barometric pressure until it gets to a pre-determined (sometimes programmable) altitude and then fires another channel. So upwards, which is all you are interested in, is done by accelerometer so who cares about air pressure. Worth getting this checked by AED as its been a while since I read up on this.
also worth contacting this guy.
He's very knowledgable and could probably make you a bespoke solution. The Raven looks to be more programmable too and check out the Baro range
Such a long piece of strong between baloon and rocket sounds to me like it'll be relatively unstable. A lot of pendulum action could be going on there.
I'm sure this has been discussed and dismissed but why not have a shorter rope with a weight on it and attach the rocket + launcher to the top of the baloon?
Is it just that the extra weight of the weight will reduce your max altitude?
Here's an off-the-wall idea:
Launch before dawn. When the balloon rises sufficiently far above the surface, it'll see the sun over the curve of the Earth. (Or a satellite, or the moon, or some other easily detected object.) You'll have to compensate for the Earth's rotation as the balloon takes time to ascend, but the calculations needed to determine at what altitude the sun becomes visible are pretty well understood.
The disadvantage is that you won't get a good view during ascent (because it's dark). But you might get some really good views during launch in the dawn light.
I have no idea if that would work (can't see why it wouldn't), but that was such a beautifully simple solution that I have to say: genius.
You'd be above most of the clouds, you're highly unlikely to be moving faster than the Earth is rotating and best of all, you'll enjoy an amazing sunrise from the camera.
Might you also have to compensate for the refractive effects of the atmosphere? At sunrise, you'd have solar rays entering the atmosphere at grazing incidence, dipping into the lower atmosphere down to ground level and then back up again to LOHAN. Atmospheric density varies along the entire path.
Why not just use a radio trigger? Have 2 groups, a few miles apart, with telescopes and cell phones. Each reports the azimuth of the balloon, as recorded by the telescopes. A bit of math, you have altitude. At desired altitude, send the launch command. You could likely use a simple model RC controller in the 2 meter amateur band (with a licensed control operator, naturally), and if needed an amp to get the needed power levels to make the trip (I don't know the regs in the UK, but in the US amateurs are allowed up to 1W for such operations (Part 97.215) - and 20 miles line of sight shouldn't be an issue at 1W.
First, I thought LOHAN did indeed have GPS onboard. If it *can* work at altitude, naturally that's ideal.
Second, it depends what kind of fuse the -REDACTED- new motor has. If it's of the "light blue touchpaper and retire' type, forget the rest of this rambling. If however it's of the type where a spark gets the thing going instantly, aided perhaps by a bit of picric acid*, or similar primary explosive up it's jaxy , then perhaps an audio sensor's a possibility.
Imagine a simple crystal microphone were attached to the balloon, nice (biased) op-amp/comparator solution to trigger on balloon burst**. Now, if LOHAN can be persuaded to sod off sharpish, then the inertia of the truss would prevent any instability affecting launch within - oh, guess - 200mS. If plastic vanes were stuck to the truss, that might give even more time
*On second thoughts, Picric acid's nasty stuff. from my *ahem* friends' copy of the Improvised Munitions Handbook, they had some illustration of a bloke making it from (?) aspirin in a stream. Sadly, "he" lent it to his mate Abdullah who hasn't been seen since...
** Assuming the balloons go 'pop' not 'pffffttt'
One problem with the night launch is that it is, by definition, a "night" launch, and, as such, the legalities may be a LOT more complex (Massive strobe lights, ATC notification, prohibitions, waivers, etc.).
Plus, your ascent time may be on the order of an hour or more, although this is somewhat variable, depending upon the free-lift of the balloon, the payload mass, the balloon size, the temperature, the humidity, the air pressure, the winds, etc. Somewhere, there is a balloon ascent calculator that's not too inaccurate. But, I wouldn't plan on anything closer than about 15 minutes at the best.
I've just come across a potential error with the radio altimeter idea from earlier. If the balloon does not go up straight, the distance will, of course, be incorrect. Triangulation with two or three ground stations could solve this (like in David D. Hagood's telescope idea [clouds? loosing track?] above) if it gets out of control.
BTW; Sorry to Vordicae for misspelling the name...
Before GPS we used to use a whole variety of hyperbolic positioning systems. One that ought to work well here is a range-range or pinground system. Have the platform send a radio ping to a repeater on the ground and wait for the (frequency offset) return. the round trip time will define a sphere around the ground station, and hence be a good approximation to altitude. I am sure baloons go more up than sideways.
Back it up with the baloon straingauge and a timer, mind.
Whatever method you choose, presumably the optimal altitude is where the baloon runs out of lift, at which point the rate of climb will be tending to zero.
Such a system would overcome the downrange problem with the spherical methods - downrange speed would be roughly constant, so rate of change would be zero (for all practical solutions - if the slant range was > 45 degrees it would be wrong, but I am guessing the slant range will be no more than 2 degrees off zenith).
Rate-of-change is far more accurate than absolute if you are using accelerometers.
Unfortunately MEMs accelerometers (basically the only kind you can afford) are incredibly noisy.
This does mean that an accelerometer may be too noisy to monitor a balloon launch, as the linear accelerations are very low post launch. It may be worth looking at this though.
- A lot of people consider trying to use MEMs accelerometers to determine altitude of a platform but double-integrating tends to amplify the noise beyond any sense of usefulness.
I had a sales manager once who was a firm adherent of the above principle, however it all depends on the ignition lag.
Yesterday ( the 17th ) it was suggested that it might take up to 5 seconds of triggering to get the rocket motor to fully ignite. I am sure I have seen another estimate of approx. 200mSec..
If the latter is nearer the case, then firing on balloon burst becomes practical. Merely mount the main lifting cable on a point designed as a spring loaded switch, open under lift. When the balloon bursts, the spring relaxes, the switch makes, and the motor fires. A back up timer would be a good safety measure.
A night launch is almost certainly out of the question - has anybody seen the regulations for flying a kite at night ?
Simkin, above, makes a good point. Rocket motors are recoiless, upto a point. There is a considerable Sci-Fi literature considering using RPGs etc. in space combat - messy !
I still drink to your success !
Username/Password prompt will probably pop up - just cancel it and the page will load anyway.
You'd need a PIC, Arduinoor other micro-controller between it and the rocket firing mechanism. But weight should be less of an issue on the truss than on the plane?
And once you've got a microcontroller on board the truss there's lots of possibilities. Deploying the parachute shortly after launch, triggering a camera, more sensors for detailed telemetry, etc etc.
Altimeter Absolute Pressure Sensor MS5561
The MS5561 is a SMD-hybrid device including a precision piezoresistive pressure sensor and an ADC-Interface IC. It uses a 3-wire serial interface for communication. The module dimensions of 4.75 mm X 4.25 mm and a height of only 1.6 mm allows for up-to-date SMD design. It provides a 16 bit data word from a pressure and temperature dependent voltage. The MS5561 is a low power, low voltage decide with automatic power down (ON/OFF) switching. A 3-wire interface is used for all communications with a micro-controller.
Pressure resolution 0.1 mbar
Operating temperature -40°C to +85°C
Supply voltage 2.2V to 3.6V
Low supply current typically 4 µA and standby current <0.1 a="" li="">
Calibrated temperature and pressure sensor for 2nd order compensation
ESD protected, HBM 4 kV
Altimeter applications <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
Personal navigation devices
Digital cameras with altimeter function
I've used Intersema parts before, Simon, and they're tricky to talk to but excellent once they're running; they compensate for temperature and manufacturing differences and give very accurate results.
I haven't used this particular part, but you've reminded me I do have an otherwise unoccupied experimental altimeter/variometer using the MS5534 which claims to have resolution down to 1kpa. I intended it for use under 15k feet, so I didn't much care about such rarefied air... I'm quite happy to dig it out and donate to the cause, if the pit crew could use it. Conveniently it has a detachable display on a circuit board a couple of inches square, and some convenient output triggers. It would certainly need testing beforehand, but you'll be doing hypobaric tests anyway.
Drop me a line if you can use it and I'll dig it out, guys. It will probably need some software changes to display that altitude, but shouldn't be impossible!
SImple. You send a signal that the balloon has burst to CERN, they rig that to one of their FTL neutrino thingamajigs, and presto, you have a trigger signal back at LOHAN a wee little time earlier.
Maybe they can even send some of that lead they have flying around at high speeds right up LOHAN's, err, exhaust, for some extra impulse.
Why not just look at the previous footage and see how long it took to get to it's limit subtract 5% for tolerance and use a timer?
It gives a fairly accurate idea of altitude (I dare say better than pressure sensors)., with a simple time/speed calculation :)
I expect I'm missing something obvious.
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If the balloon manufacturer issues a chart of load vs desired height --> amount of helium required, then a timer becomes feasable. Let the balloon reach the desired ceiling, hover about a bit at that height(drift calculated from weather charts) and fire the bugger a few minutes after it gets there.
Meanwhile, the balloon, freed of its constraining burden (I'm assuming LOHAN is a large part of the weight of the whole assembly) is now free to hurtle upwards, and pop at its leisure, returning the truss, etc. by parachute as intended.
cosmic ray counts peak around 15km then start to decrease. You can use this fact to detect when you've passed 15km, and then (assuming you're still going up) you can continue to use counts/min as an altimeter. see http://hyperphysics.phy-astr.gsu.edu/hbase/astro/cosmic.html
Three balloons on a ¥ frame.
Rocket underslung on the 'l' , nose down a few degrees.
Q O O
\ l. /
Inflate the balloon (Q) on this leg more than the other 2, also counters the extra payload mass on this leg.
This should cause this one to fail first, at which point the frame should swing around the center point, bringing the rocket (=) up into launch position...
The balloon ties can be of different lengths to allow clearance to be maintained with a smaller frame.
KISS principle, no altimeter, no GPS, just a tilt switch to ignite the motor.
Apologies if the ASCII artwork is poor.
There was a comment earlier that requires reiteration.
Two balloons, one above the other.
The top balloon is for primary lift and is inflated to the appropriate degree. A pressure sensor in that balloon is set to detect a rapid DROP in pressure and when detected to trigger the launch.
The bottom balloon is filled to only 75% of the amount the top balloon is filled to. It should NOT burst at the same time as the main balloon, but should provide enough lift to stabilize the platform during launch.
So, the top balloon bursts, triggering the launch of LOHAN.
The platform is kept stable and in the appropriate attitude by the second balloon.
The launch platform will continue to rise until the second balloon bursts.
The advantages of this system:
1. A simple, physical switch to effect the launch of LOHAN.
2. No need to measure altitude. The launch will occur at the maximum altitude a single balloon system would have achieved.
3. Actual altitude of launch can be inferred from the on-board cameras after the payload is retrieved.
1. Two balloons. SPB wasn't really hot on two balloons.
2. Twice the labour required to hold down the balloons during launch, e.g twice the beer.
3. How will a balloon filled to less than its maximum behave? Will it still continue to rise and burst?
Measure the circumference of the balloon. A possible way to do this is:
Find the burst radius of the balloon. Subtract a safety margin to get the launch circumference. Attach a number of eyes around the circumference of the balloon then tie fishing line to the first eye. Thread the fishing line through the rest of the eyes, once you get back to the first eye take the line down to the launch platform. Add extra line for the difference between the launch circumference and the current circumference. Put a big knot in the line at the launch point. Run the line through a switch arm with another eye on it. As the balloon gets bigger the line gets pulled through the eye on the switch, when the knot snags in the eye the pull on the the switch arm will activate the switch. Boom! (or is that whoosh?).
An even simpler version would be to attach the line to the top of the balloon and run it straight down to the launch platform via an eye on the launch pad attachment rod/rope(s). No eyes on the balloon at all.
The excess line could be put on a reel with a light spring to keep tension on the system to prevent tangling - fishing line needs to be controlled.
This should work pretty well, the force of the expanding balloon will be quite high so you wont need to make the line system too sensitive or friction free. It also doesn't require any calibration like the condom/second balloon version would require. Also easily tested in a commercial freezer by over inflating the balloon.
Issues I see:
o Attaching the eyes to the balloon in a way that allows for the expansion.
o Making sure the line will slide freely when it get super cold.
o Making sure the storage & tensioning reel still turns freely in the cold.
o Tangling, especially in any wind, also as the line gets stiffer due to the cold.
o Keeping the line away from the launch path
o The launch platform must not rotate independently of the balloon or the fishing line may tangle with the main line.
o I have assumed that the burst circumference is known and doesn't have much deviation.
My understanding - I stand ready to be corrected - is that the helium balloons are essentially atmospheric pressure; they contain a fixed volume of gas which expands into what might be described as a 'loose fitting' container that gets bigger simply by filling out the folds. I don't think the pressure rises until the envelope is taut and starts to exert a pressure on the filling - presumably, just before it bursts.
We have a number of constraints on the firing time:
- as high as possible, please
- but with a stable platform
- so with the balloon still lifting or at least stable
- we don't want to drift out of our airspace if we don't get altitude
- we don't want to launch accidentally at ground level or close to it
- we don't know how high the balloon will go before it bursts
It seems to me that as others have said, we need a multi-layer approach. So that we launch if:
- height is above 15k feet (range safety)
- height is at or above calculated maximum lift
- time is greater than calculated lift time (we should have data on that from Paris, no?)
- balloon is about to burst
- we're pointing above the horizon
We might also consider the issue of range safety and perhaps think about destroying the launch package if we lose tension on the lift wire - i.e. the balloon's burst.
I wonder if a differential pressure sensor between the balloon and atmosphere might give a warning of 'about to burst'? If it maintains a constant pressure as it rises and only increases once the envelope's full, we should be able to detect that... I don't know how well a temperature system would work; it's hot up there (in that the air molecules are zipping around pretty actively) but there isn't a lot of air to actually be *warm*. I have no experience in this direction...
variation on range/range - (worked for years!)
use transmitter on rocket, receivers and computation gear on ground keeping track. then radio signal to manually start ignition once achieved required height. added bonus of tracking rocket once ignited and recording max height achieved.
I may have missed if someone has already suggested this, but whatever...
At ground level the speed of sound in air is about 340M/S, at 30KM or about 100,000ft the speed of sound is about 300M/s. An oscillator tuned by a resonant tube of air would fall in frequency by about 12% at this altitude. This could be measured and used as a trigger, also enabling the use of the phrase "LOHAN's Organ".
Assuming launch on a day with little cloud cover, could you not analyze the image of the ground as the balloon rises, watch known shapes determined by recognizable points, do a bit of spherical geometry and trigonometry, and calculate altitude? The kit would be quite light-weight, the mechanics fairly simple, but the programming would need to be somewhat sophisticated (but shouldn't be too hard for resources available to El Reg's boffinry).
(Paris because you don't have a Lindsay icon)
High(ish) power ground transmitter sent out a pseudo-random signal. At Apollo the signal is frequency shifted to a down link. Range accuracy is set by the digital data rate with all the heavyweight correlation done on the ground (IIRC Apollo did something like 1 metre at 250 000 nm while consuming <5W total with something like a 1 or a 10 mbs data rate range rate was set by the doppler shift that needed to be cancelled out to recover the signal. Minimal in this application)
But Lohan does not need this performance. Effectively if you could detect a *single* bit shift between the transmitted and returned signal (IE at a straight up distance of about 30480m). I estimate a data rate of 10 000 bps would be enough. The longer the slant range the bigger the gap. With 2 channels and some geometry precision release should be possible (IE where the 2 lines meet in the sky) , again with all the clever hardware on the ground.
Of course the classic one is carrying a radar reflector to track its range then hit a radio transmitter to fire it.
PARIS popped at just under 90,000ft. (27400m). Unless published weather balloon data can directly applied to the LOHAN flight we cannot know if this is good, bad, or about average. So either carry out a series of PARIS flights to obtain better data or use PARIS as a benchmark and make a large allowance for error. Allowance must also be made for inaccuracies in whatever system is used to measure the altitude. I think we could be looking at somewhere around 20,000 meters.
The point is that even knowing the averages LOHAN has got fire the rocket at a lower than the lowest altitude at which the balloon has a significant chance of bursting.
P.S. according to this website it is possible to get meteorological balloons that get average at 38,000 meters!
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