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 …
Did you look at what other people are doing?
I notice his GPS based altimeter stopped working at about 24,000 feet. Also the barometric altitude and GPS altitude show cosiderable difference above about 17,000 feet.
"The ALTS25 precisely captures the highest altitude obtained during a flight to a maximum of 25,000 feet above sea level (by special order, the ALTS25-60K is calibrated to altitudes up to 60,000 feet)" -- only 60,000 feet.
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?
Can you have a camera or something watch the balloon and launch when the size is right?
Interesting concept, the balloon should be fairly easy to identify, as it's a very different colour to the general sky.
However, vision is very bad at detecting size - even human vision is terrible at it - it's much better at determining shape and attitude.
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
Also, what is the right size? And what is the production tolerance on those balloons, in other words, how reliable can the size be used as an indicator that it's close to bursting?
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.
You should be able to test this at ground level as the baloon size is dependant on the differential preasure. The baloon is likely to have different elasticity at the higher temperature on the ground.
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.
The problem with this (as was pointed out in the article) is that the pressure sensors that work at that height are rather fragile at ground level. The one you linked only works to 9,000m, they are aiming for a touch over 30,000m.
Also, I think you mean uC rather than PIC.
Yes I meant uC
(insert embarrassed smiley here).
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.
A launch is preferable to a no-launch, so I suggest a simple timer override set to launch after all other measures should have launched.
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?
Andy - thats exactly what I was thinking, unless I'm missing something the acceleration of these ballons (with appropriate parachute enabled weight) ought to be pretty consistent.
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.
These are also easily available from old-school barometers, although I'd want to test one in a chamber to be sure it can survive to altitude as those might not be intended for such low pressures!
Bit low-tech, but have a ikkle trigger balloon; when it go bang, launch. Would be lightweight - fill it with some hydrogen and it'll weigh nothing.
The bang of the small balloon might destabilise the launch platform.
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.
From the graph the temperature is just starting to rise again at 80,000 feet. Why not look for a temperature rise to 5C above the minimum reached so far in the flight. That looks about the right trigger point to me.
two balloon system?
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.
Can you not do a test launch.
the balloon with some sensors to measure temp, pressure etc along with GPS etc etc a camera to measure the balloon size as it bursts
With all this data you could work out a mechanism to decide when to release the payload.
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.
100 000 feet of fishing reel weigh? Of course, you would have the devil's own job in reeling that much nylon back...
Whether the reel is made of metal or plastic, it would be several tons.
I think you meant monofilament nylon. In which case, 30,480 metres of 0.22 mm filament is 1.32 kg of nylon, assuming a density of 1.14 g/cm^3, 30,480 metres of 1.0 mm filament is 27.2 kg
Strain gauge on the side of the balloon.
The elastic properties of the balloon should be measurable, the stretch or strain on the skin should indicated the pressure differential.
As it's the differential you need to worry about the balloon bursting.
No go — the balloons are so delicate that they can’t even be touched without gloves on the ground. A strain gauge would put so much stress on the balloon that it would rupture before it got halfway up.
How about measuring the impedance or capacitance of the balloon from the mount point?
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.
Agree. And a fire-upon-burst system would also help ensure the mission isn't a complete failure even if the balloon burst earlier than expected.
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.