Original URL: http://www.theregister.co.uk/2012/05/18/lohan_trigger/
Vulture 2 trigger triggers serious head-scratching
Just how do we fire that LOHAN rocket motor?
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.
We've been mulling the possibility of using a barometric-pressure-operated setup, and yesterday threw the problem over to you lot for your expert input.
Evidently, there are as many possible ways to get a rocket motor to fire at altitude as there are to skin a cat, although depriving some poor moggie of its fur seems a much simpler proposition than ensuring LOHAN goes bang.
First up, plenty of readers have suggested that we launch at balloon burst, to maximise altitude. That's an absolute no-no, in our opinion, because we have the prospect of a tumbling launch platform.
We could concoct some form of gyroscopic stabilisation system, but that runs against our less-is-more KISS convictions.
So, we're going for a pre-burst launch at a predetermined altitude. What are our options? Neil Barnes doesn't much like the idea of the pressure sensor approach:
The problem you're going to have is that at 100,000 feet the pressure is only just over 1kPa (as you've noted) but it's getting thinner up there very slowly - at 70,000 feet it's still only 5kPa.
So you're really going to need something with a range of no more than about 10kPa to have any hope of being within even a few thousand feet of your selected altitude. MPX10 works in that range, though it will require temperature compensation - it may be simpler just to arrange a small circuit that will trigger once the pressure drops to a predetermined level. The problem is, though, that they burst at about 100kPa - i.e. ground level - so it's going to be really fragile if you launch on a high pressure day, or even in storage, with a gauge type. A two-port differential type doesn't really help you either.
It may be that you have to consider some mechanical approach, as with Paris.
Many of you agreed. Arrrggghh-otron chipped in with:
How about a small balloon (or part thereof) partially inflated at sea level that presses on a micro switch when the balloon inflates? Sounds too simple, I must be missing something...
I agree a mechanical solution would be simple and should be reliable. As an alternative to the balloon concept, what about a syringe? The air in the cylinder will expand at a predictable rate and the plunger will press a sensitive switch. It would be repeatable and testable without affecting the results, unlike a balloon whose rubber/latex may degrade and change after each use.
Here's our initial concept graphic for a mechanical Vulture 1 release mechanism...
..and here the finished product:
The principle was that the air inside the oxygen tube gradually expanded, pushing out the metal rod until a simple mechanical connection released the Vulture 1.
It was just a matter of calculating just how much air was required inside the tube at ground level to extend the rod a set distance by the target altitude. We actually used 55cc of aluminium oxide powder instead of liquid to fill the 70cc compressed tube.
Here's the release mechanism in the hypobaric chamber, at a simulated ascent rate of 1,000 feet per minute. Obviously, this footage has been speeded up a bit:
As we all know, this magnificent garden-shed-built device did indeed release the Vulture 1, although it was pretty well impossible to calibrate with any degree of accuracy.
This is why we're now considering an electronic solution. What about using GPS? Faye Berdache entered the fray with:
Any GPS system should do altitude as well as position (it's what it was designed for). Should be accurate enough. Though I'm not sure if any phones will do the job, as most are Assisted GPS, which relies on land based transmitter triangulation and the algorithms are only concerned with calculating map positions, not heights.
Tom 7 offered...
If it's just altitude I would imagine it should be possible to mod an Android phone app to give you the proper altitude - I believe paranoia prevents most sat navigation jobies working at altitude but the signals should still be there.
...which prompted Reginald Marshall to correct:
Satnav is limited on a lower level: ITAR (the same export regulations that gave us 40-bit encryption 15 years ago) limits the receivers' working range to less than 60000 ft / 1000 kt (~18 km / 1800 km/h) at the same time - the idea being that you can't easily piggyback your ballistic missile guidance system on GPS. This is usually done in firmware.
Theoretically, LOHAN should be OK, since it would reach a greater altitude, but at a lower speed. However, barometric sensors are simpler and accurate enough; there is no reason to complicate the design.
Yes, there's a commonly-held belief that GPS won't work if you're too high or going at a fast rate of knots, as quantified above. As Reginald points out, you're limited to doing both simultaneously.
LOHAN should be happy with a GPS-operated system, then, but does it offer any advantages over a barometric sensor?
We clearly need to look into this a bit more, and see what kit is readily available, or what we might be able to build ourselves. Robert Heffernan suggested:
I have assembled (in prototype form) a rocket ignition and telemetry system from parts ordered from Sparkfun; basically it was an accelerometer, barometer, GPS, SD card reader for logging, Arduino, Bluetooth module for pre-launch comms, and then some FET-based output for switching igniters, etc.
With open-source driver code and plenty of existing libraries for the hardware, writing firmware to do the job is simple. That way you don't need to hack on a commercial unit to do what you want and you can release the schematic and source as part of the project. I am sure there would be plenty of people out there willing to send you a firmware package, schematic and bill-of-materials based on the requirements of the project.
So, we've still got a bit of head-scratching to do on this one, and here's another possible line of attack, as presented by Francis Vaughan:
The temperature varies with altitude quite nicely, and it actually starts to increase again above about 80,000 feet. Around here you get about 1°C every thousand feet, which isn't difficult to track with a simple semiconductor thermometer.
Clearly there are variations, but this altitude is above pretty much all of the weather, and you should see a stable relationship with temperature to altitude as good as you see with pressure. What you might want to look at is the NRL empirical model that can give you a prediction of the temperature versus altitude taking into account current conditions and location.
Interesting. Alternatively, what about PC Paul's idea?
Could anything be done with optical sensors? Model autopilots often use two simple opto sensors to detect the horizon, the difference between above and below is easy to detect. Ideas:
- Monitor the curve of the earth with a row of sensors to judge altitude. Probably doesn't change quickly enough to give good resolution.
- put a sensor inside the balloon to spot the burst point
- watch the balloon from the gondola to spot the burst point
I notice on many balloon launches there is a fair old spin going on by the launch point. Can this be stopped, maybe with the same gyros that keep it stable after the pop?
On the other hand, SirDigalot reckons we may have missed the most obvious solution:
Can't you train the Playmonaut for this? What do you pay him for? Right now all he has to do is be inside LOHAN...
Trust us, being inside LOHAN is quite enough of a challenge for one man, without asking him to manipulate mighty thrusters as well.
Finally, now that we've made it clear we're launching before the balloon burst, you'll be be asking yourselves what happens if the mighty globe goes pop prematurely.
A very good point. Some form of failsafe triggering system is needed to cover that eventuality. Poor Coco thinks an accelerometer might work at balloon burst by sensing the launch platform's descent.
Or what about Gordon 10's cunning plan?
Just put a strain gauge or similar on the balloon string. Strain drops to zero > 1 second - fire.
Right chaps, we reckon a few inspirational pints in the pub are in order. While we sup a couple of ales and ponder LOHAN's trigger, we thought we'd show you a rough calculation we did about the Vulture 2 launch angle, seen on the right.
We knocked up a quick beer-mat sketch based on the estimated final balloon diameter, plus a safety margin, and the intended launch angle as agreed with the Southampton Uni Vulture 2 design team.
As you can see, the required tether distance between the balloon and the launch platform - our fantastical flying truss - is pretty long.
Quite how that distance affects the whole rig's behaviour in the air remains to be seen. Some kind of test is in order to see if there are any serious implications in having the balloon so far above the truss, and we'll also have to consider just where we stick the recovery parachute.
All of this should keep us busy for a while, and if you'll excuse us we'll get straight to it in the aforementioned hostelry in what's euphemistically known down at the Special Projects Bureau as a "spaceplane brainstorm". ®
Further LOHAN resources:
- New to LOHAN? Try this mission summary for enlightenment.
- You can find full LOHAN coverage right here.
- Join the expert LOHAN debate down at Reg forums.
- All the LOHAN and Paper Aircraft Released Into Space (PARIS) vids live on YouTube.
- For our SPB photo archive, proceed directly to Flickr.
- We sometimes indulge in light consensual tweeting, as you can see here.