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 …
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
I've probably missed the answer to this but
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?
It has been discussed and dismissed
because a) a mass on top of a balloon is not stable either, b) those balloons change size quite a bit during ascent, which your attachment method has to take into account, and c) those balloons are not half as robust as required for such a setup.
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.
Re: Solar ignition!
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.
Re: Solar ignition!
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.
Re: Solar ignition!
...or launch when going dark, to get ascent footage during daylight. You could even use the same camera.
Why not a radio trigger?
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.
Definitely, I've missed a lot, but...
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'
Re: Definitely, I've missed a lot, but...
The same launch-on-burst trigger can be achieved with a switch or strain gauge detecting whether there's still pull on the suspension cord.
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.
The 'radio altimeter'
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...
GPS modules that work
The uk high altitude society has a small list of known good GPS modules that work at high altitude
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.
rate of change
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).
Re: rate of change
If it’s this hard to detect altitude, then it’ll be even harder to distinguish a cessation in change of altitude. The random motions of the balloon will bury that extremely subtle signal, so accelerometers won’t work.
Re: rate of change
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.
Re: rate of change
Looking at the PARIS data the balloons rate of change does not slow as it approaches max altitude. It just keeps on rising until it pops. At that point the rate of change suddenly becomes very large indeed.
Can you track it from the ground and fire a laser at it when you want to deploy?
Or track it with a laser and turn the laser off when you want to deploy?
Probably not, but I like thinking left field!
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
Re: Micro Altimeter?
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!
FIring just before balloon burst
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.
Why not just make some loops on the balloon, feed a string to it that has the diameter you want your balloon to be. Once the string gets tight you launch. This should be easily measurable by any simple kind of pull-chain switch.
See strain gauge mentioned previously.
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 rays peak at 15km-- use counts as altimeter
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
Back of an envelope design
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?
I agree but there is no need to have two differently filled balloons. They will be very unlikely to burst at exactly the same time. Either can trigger release. Also the payload may remain stable for some time after launch for some better shots.
Super simple and about as low tech as you can get...
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.
Re: Super simple and about as low tech as you can get...
There is no way to attach anything to the balloon without seriously compromising strength and maximum altitude. (Except ofcourse the main payload attachment)
Summarising some arguments
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...
GPS is crocked to not work over 24,000 feet to provent terrists using it in missiles
range-range & manual det?
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.
How about a really long fuse lit when the balloon is launched?
My technical expert, a Mr Wile E Coyote assures me this is guaranteed to work.
Going for a song....
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)
A low tech variation of the Apollo S Band system might work.
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.
Safe release altitude
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.
Re: Safe release altitude
Luckily PARIS is not the only weather balloon ever launched. Ring up the national weather service and kindly ask how to find their sounding diagrams. They launch hundreds of balloons across the globe each year. Shouldn't be too hard to find an average burst altitude.
Re: Safe release altitude
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!
Re: Re: Safe release altitude
The burst height depends on the weight it's carrying, though, so you can never been certain what it'll be.
Do the makers have a specification for the differential pressure at which the balloon bursts?