0g-Detect for freefall protection for under a tenner. There's one fitted on the Habuino HAB board.
The Low Orbit Helium Assisted Navigator (LOHAN) team is inviting experts among our super readers to submit final proposals for a back-up ignition system for our Vulture 2 spaceplane's mighty rocket motor. Click here for a bigger version of the LOHAN graphic As SPB ballocket regulars know, the primary system designed to light …
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Amongst other options:
1) target height reached
2) current height too low below previous max height
3) (possible - still thinking about this: too long without height changing)
What about something really simple: the parachute pulls an insulator from between two springs? I can lay my hands on a little beryllium copper wire...
Try something like this;
The manufacturer will set the pressure for you, calculate it to trigger above your main system pressure (allow for some error in both systems). If there isn't one with your exact pressure, you could use a differential pressure switch and put a known pressure on the second port. Looking at the one in the you can set it between 0.25mbar and 124mbar (30,000m altitude = 14mbar)
Again, why re-invent the wheel when there is already tried/tested tech out there that does it for you :-)
this gives you your mechanical solution you're after :-)
I'm with this guy.
But I expect you only want the fail safe to go off if/after the first one doesn't or with some delay so it'd need to not fire when you first reach burst altitude, although there may be some delay for the igniter to.. uh.. ignite anyway so that'd take care of that?
Perhaps you'd be better off using a pressure switch as primary and GPS as backup as you can more easily but some delay or hysteresis into the GPS trigger?
the way you might do it would be using a watchdog on the electronics that powers a normally closed relay which is in series with the pressure switch. i.e. if the watchdog is cleared regularly it powers the relay and keeps it open, thus preventing the pressure switch from firing the rocket.
If the electronics fails to clear the watchdog, or loses power completely, the relay automatically closes (they're spring loaded and need power to hold 'em open), and allows the pressure switch to kick in at the correct altitude.
The caveat being the software needs to be well written to ensure it doesn't arbitrarily keep resetting the watchdog (a common mistake when implementing watchdog monitors)
you could also have a seperate radio system to close the relay to allow the pressure switch to fire the rocket.. all of it is extra complication of course, it just depends if you want to allow the balloon to burst without firing the rocket (for a re-try for example)
What you need to get or make is a 2 channel slip ring connector to replace the swivel.
Then a cord from the apex of the parachute with a light elastic band and a magnet.
The elastic band is attached to the swivel, then the magnet, then the cord to the apex.
When ascending the elastic is stretched say three inches, mount the reed switch two inched above the swivel.
You could have it glued to a thin plastic tube which would allow a safety pin to hold the magnet away from the reed switch until the balloon is ready to launch and the cord is taut.
The reed switch mounted on the swivel is triggered when the tension is released, as the parachute opens the magnet will move downwards to activate it.
Water expands as it freezes. You could use this expansion to close a mechanical switch or pop out a pin or something, assuming the water is sufficiently sealed in some sort of stretchy (ideally in only one direction) container. Yes, I don't know how far up it gets cold enough for water to freeze, but you could presumably just dilute some salt in there to adjust the height. I'm thinking something like get a piece of copper pipe, seal one end, pour in quite a lot of water, stuff in a cork at the other end. When it freezes, the cork should be forced out. That movement should be enough.
Yes, I don't know how far up it gets cold enough for water to freeze
You've just disqualified yourself there. It certainly freezes, and already well below the intended maximum altitude. Your proposed solution would trigger, as a rough estimate, at 15% of that intended maximum altitude, and be utterly unusable as a failsafe trigger after the balloon has burst.
There are ways to rig the second balloon so that it only starts to provide pull on its tether after the main balloon has burst, but they involve pulleys (undesirable with regards to icing) or levers (extra weight), and you need to ensure the balloons don't chafe against each other or the rigging.
How about attaching the lift line from the balloon directly (through a small hole in the top of the parachute) to the payload or through the centre of the swivel fitting, (the top of the parachute could be held up on this lift line, so that the parachute lines would be slack), when the balloon bursts the parachute lines then take the full weight and become taught, with the lift line becoming slack
the lift line could hold a switch open against a spring
or something attached directly to one of the parachuute lines would then pull the pin out.
or some other way of utilising the differential in tensions between these two items (lift line and parachute lines) would be a reliable indicator of release.
If you use a Teflon strip - which is a good idea, btw - it makes more sense to have the contacts in parallel and mounted on springs, like relay contacts. Which makes it very easy to do - get the contact set off a suitably rated relay, using hard silver contacts rather than tin for obvious reasons. The rating is not too critical as the relay won't be required to disconnect after making contact.
actually, I already suggested a absolute pressure switch, set to trigger at the correct altitude (see the link in my post).
my only worry about the differential pressure switch on the balloon is how much differential pressure there might be between the balloon and atmostphere, and how much this will vary during ascent. The balloon expands because the gas inside is trying to equalise pressure with the outside world, so I would expect this differential to be quite small..
That said, it would probably spike before the balloon reaches the limit of its stretch, so maybe you could use that.. I prefer the set pressure method though, more predictable
There isn't enough pressure to use an absolute sensor, the BMP085 usually used to get an approximate reading is only accurate to 300hPa or about 9000M. I don't think you could rely on it and they want a mechanical fail safe in addition the several electronic ones.
I wasn't talking about a sensor (i.e. converts pressure to voltage), but a pressure switch (triggers the switch at a preset pressure level).. link again;
yes it has 20% tolerance, however on the 14.92mbar unit (approx 29500m), this equates to +/- 3mbar or around +/-1500m roughly. There are two ways around this if you need it better, firstly you set your primary electronic trigger to a level below this (say 27,500m), and make sure your burst altitude is higher than 31,000m
alternatively you can talk nicely to the manufacturer and get them to pre-select a unit with the right pressure trigger point for you :-)
oh and p.s. its a mechanical switch.. i.e. its a mechanical mechanism that closes electrical contacts, much like what has already been discussed through various garden-shed arrangements of ice, bluetack, loo-rolls etc, however engineered by professionals (probably with lots of letters after their name, like Msc, Meng, Phd etc) to actually work :-)
I initially thought of using the parachute as the trigger, but at high altitude it may not open, so I came up with this idea that uses gravity as a trigger instead:
1) Take a length of parachute cord and make a double figure of eight shape, making sure the cord doesn't pass through itself and that the continuing lengths of cord 'depart' from roughly the middle of the 8.
2) Fit a small rubber band over each loop of the eight to secure them in place. These bands should not be tight, so use very small bands at low tension.
3) Now create a metal plunger wired to the launch battery + and fit it to a powerful spring. The face of the plunger should be smooth and flat, but should also be larger in diameter than our figure 8 cord is long.
4) Fit the plunger and spring into a casing that will see the plunger in contact with a copper stop plate at rest (but still under considerable tension from the spring). Connect the copper stop plate to the + launcher circuit. This is our trigger mechanism. The casing should be open and unencumbered at the bottom, so either the stop plate is mounted separately on the truss or it is fixed to the spring casing at the top only.
5) Fit the trigger mechanism to the Truss on the centre line. On the outer left strut, directly in line with where the plunger and stop plate meet should be a strong cable anchor.
6) Tie the uppermost free end of the figure 8 cord to this Anchor.
7) Tie the other free end of the figure 8 cord to the bottom of a small weight. This weight is held in a cage mounted on the right strut in line with where the plunger and stop plate meet. This cage is open at the bottom so the weight falls out easily, but has a small hole in the top. A cone shape would be ideal in a closely mated cone-shaped cage...
8) Tie the top of the weight to another length of parachute cord which is attached to the balloon tether directly - that is above the parachute mounting. This cord should be shielded from atmospheric buffeting, so running it through a drinking straw attached to the balloon tether would protect it.
9) Place the figure 8 of cord between the plunger and the stop plate so that it prevents the two plates touching. It should stay in place through the force of the plunger trapping it against the stop plate.
Note: Before launch, cut the two elastic bands to make doubly sure the figure 8 cord will 'zip' out of position.
The method of operation is pretty simple.
a) The Balloon bursts
b) Tension is released from the balloon tether and the cable holding the weight in its cage
c) Weight falls free from cage and pulls figure 8 loop from between plunger and stop plate
d) Plunger and Stop plate meet and launch circuit connection is made
e) LOHAN launches
f) Weight fall stopped by anchor.
The only issue I can see is the 'yanking' factor that may be applied by the weight being arrested in its fall by the anchor cable pulling LOHAN off course. This could be alleviated by use of pulleys or a really long anchor cable ensuring that the weight remains in free fall until LOHAN has launched successfully. However don't be tempted to add all that extra anchor cable between the plunger and stop plate because the more cable between the two, the longer it will take to close the circuit. Perhaps coiling the anchor cable up inside a small parcel of tissue paper attached to the truss would be the way to go....
Sorry, but reading that I was suddenly ten years old again.
You just managed to evoke that feeling of tension as the little metal ball rolled down the rickety staircase in "Mouse Trap Game" and I would hold my breath as it hit the hand-onna-spring thingy. Exhaling was only possible after that had fired, the big ball had dropped through the bath onto the seesaw and the diver had correctly been catapulted into the pool.
I like the idea of having a parachute attached to the payload as then the whole kit should be recoverable after balloon pop, whether you get rocket lauch or not (assuming your tracker is working). However, the string pull idea is prone to sticking due to icing/rigidity problems (remember the O rings), so I would still advocate my switch on the balloon skin as the simplest solution.
Paris as its simple and elegant!
Some phosphorus sealed until needed perhaps or sodium and water
Something like potassium permanganate and ethylene glycol, peroxide or something similar. Mix as required, and add some iron oxide and aluminium powder for good measure.
Not sure of the min ambient temps at which these occur might need a slightly friskier combination at high altitude temps
Hydrazine and an oxidiser?
ammonium nitrate, ammonium chloride, zinc dust and water perhaps?
Yeah, some of your suggestions will burn. The question, however, is getting the igniter (which is already present, and has been demonstrated to work well) to ignite at the right moment. And if you're not sure of the ambient temps, you may want to consider that there's snow all year round on the Matterhorn. This rig is going to reach altitudes at least six times as high.
And go read Ignition! for enlightenment regarding getting self-igniting stuff to burn when you want it, and not before.
There are two cables running from the truss, meeting in an inverted V with the cable to the balloon connected at the apex. While the truss is suspended from an inflated balloon, those two cables will remain taut. When the balloon bursts they'll go slack.
What if you redesign your cylindrical spring-loaded switch so that the contacts are held well apart, not by a pin but by tension from a cord at each end. That is, as long as the cords are kept pulled, the contacts remain open.
Attach those two cords between the legs of the inverted V part way along (think of the crossbar in an uppercase letter "A"). While the balloon is inflated, the legs of the A remain taut, and the crossbar (with switch) is kept under tension. As soon as the balloon bursts the cables go slack, the spring in the switch pulls the contacts closed, and we have ignition.
It would need some experimentation/calculation to get the spring tension right. Enough to ensure a good contact, not so much that turbulence could cause premature release. As a safety measure you could perhaps retain the pin as a lock, linked to ~100m or so of cable fastened to the ground, so that the system wouldn't become live until it was well into flight? Alternatively the electronic timer safety might be enough to protect against that, assuming that the rocket won't be armed until, maybe, 30 minutes into the flight?
How effective is a parachute at that altitude? The air resistance is minimal so will it even fill up?
You can make it launch on balloon-pop with nothing more than a U shaped piece of plastic and a microswitch:
Excuse the poor drawing, I'm trying to get my fat hands to work with my new Galaxy Note.
I like the connection across the shrouds to capture the chute opening event as I think that will be your most reliable signal. I'm not fond of the part where the line passes through an eye and down to the logic. If you're going to have a problem with this system, it will be binding at that point preventing the chute from opening (freezing or just friction). Better might be to have the "pin" actually between the shrouds (inside the chute when closed). You could either have two conductors on one shroud (for open->closed) or have one conductor on each side of the chute (closed->open). For the 2nd variant, it would either have to connect to the logic or have a largish coil somewhere to fire a make-and-break type igniter (would one large spark be enough?).
I see the parachute as the problem. It may not fully open or get tangled and not have enough force to pull the safety pin out.
Make it the other way round. Make the contacts spring-loaded to close but to be held open by the tension between the balloon and payload module, with the spring forcing the contacts to close when the tension disappears (i.e. when the balloon bursts).
Safe it with a pin *before* launch (use one of those "remove before flight" red streamers). As the balloon picks up the tension in the shrouds (and the fail-safe line) pull the pin out just before releasing LOHAN into the air.
The word "failsafe" doesn't mean what the author of the headline thinks it means.
"Failsafe" refers to a design that causes no damage/harm when it fails.
A lump of clay is a failsafe rocket ignitor. When it fails, it does so in a manner that doesn't cause harm. Unfortunately, it fails 100% of the time -- but something doesn't have to be reliable to be failsafe.
you're 100% right and at the same time 100% wrong.. yes that's contradictory, but please allow me to explain!
Firstly my background as self proclaimed expert (or nearest semi-expert in the vicinity), I work in medical electronics and have had formal training in safety engineering and much experience in designing stuff that is "failsafe"
you notice the exclamation marks (in real life I might have been making a funny gesture with two fingers)
you see, failsafe is all a matter of context. a rocket that fails to ignite.. has it failed safe? it depends if you're the bloke that has to approach it afterwards!
likewise, a linear accelerator that generates high energy x-rays.. it fails and continues to deliver radiation? definitely not safe! likewise a ventilator that helps a patient breath.. it fails and stops pumping gas into the patient.. is it safe? again definitely not!.
In this case, what the author means by "safe" is that the rocket is safely away from the balloon.. so if the main launch system fails, it has a failsafe backup that ensure the launch. Remember "safe" has many meanings depending on the context of the project
"a linear accelerator that generates high energy x-rays.. it fails and continues to deliver radiation?"
Huh? Then it's not fail-safe. If your high energy x-ray device is designed to be fail-safe, it doesn't continue to deliver harmful radiation when it fails. There is no context necessary. It fails in a safe manner or it doesn't and if it doesn't then it isn't fail-safe.
Likewise your ventilator may be designed to be fail-safe in which case it would have to know when it has failed and perform some appropriate action such as sounding an alarm. That is perfectly fail safe because someone would be informed that critical action was needed. Alternately it could be designed to be fail-operative, a.k.a. redundant, and would have a backup system to take over in case the primary failed.
In short your confusing redundant systems with fail-safe systems. Either a given system is allowed to fail or it isn't.
The accelerator was meant as an example of a system that only "fails safe" if it stops performing a function (i.e. continuing to deliver radiation is an example of a system which does NOT fail safe), whereas the ventilator is the opposite.
You would hope the ventilator alarms and that someone reacts quickly enough to prevent serious harm to the patient, but that situation is definitely not fail safe! Having designed the electronics for a ventilator in a previous job, I can tell you that although the designs do include redundancy for critical systems, the pneumatics is arranged such that even under failure conditions it still provides positive air pressure to the patient. This is an example of a system that continues to provide a function under failure conditions
So perhaps I wasn't clear in last nights semi-drunken post (new batch of homebrew was ready last night mmmm), but in my own mind at least I wasn't confusing redundancy with fail-safe, but hopefully making it clear that fail-safe is a term that can have many meanings depending on what it is applied to :-)
I see two things that defiantly happen when the balloon bursts:
1) the balloon line goes slack
2) the diameter of the balloon shrinks
I would reverse your spring loaded switch design so that the spring forces the switch closed. Then you could install it on the balloon line, where the tension between the balloon and the truss keeps it open. However there is the chance that turbulence during ascent could prematurely close the switch.
Alternately you could rig it around the balloon so the balloon's tension keeps it open. Although it might be tricky getting the tension right since the balloon expands as it ascends.
To minimize the chance of accidental launch due to turbulence - replace the switch with a clockwork set to close contacts in, say 15 sec, when the line is slack. This way, a momentary slackness due to turbulence will not immediately trigger ignition...
Can still use "remove before flight" band and a removable pin for safety and added coolness.
Any number of old Russian wind-up toy cars or trains - just oil them up with high-altitude lube. As far as I remember, they even had stopper mechanisms which you can tie tethers too.
But I think something like this would be particularly appropriate for LOHAN :-)
And they don't need to be resettable - that's the whole point. If it can run for, say 20-30 secs in total, it can absorb 20-30 1-second upsets before triggering anything...
OK, but then, if it didn't have any (or just a few short) runs, the whole kaboodle would be falling for those 30 seconds before firing. That's quite a distance (2..4.5km, at 20..30 secs runtime) and it will have picked up an impressive speed too (200..300m/s, not counting drag and the deployment of the parachute). I don't think that would be a suitable condition to have LOHAN in at the moment supreme.
Set up a constant force spring, you know a coiled strip of metal like inside a tape measure, so that it is extended when it feels some gravitational pull. Once the balloon pops and enters free fall the spring will be able to wind itself up into its coiled state and make the contacts.
In ascii imagine the following sequence with the screen rotated clockwise;
:_________@ Spring extended under gravity and contacts (colon) on top. Balloon pops >POP<
:_____@ Spring starts to coil up as it doesn't "feel" gravity in free fall.
:@ Fully coiled, contact, ignition, Bob's your mum's brother.
I think you might snag yourself something larger than a duck were that to happen :-D
Still, a pint (or three) will solve any problem in existance.. world peace, starvation, cold fusion, and also why bags of nuts have that silly little label on them "May contain nuts"
In the words of Benjamin Franklin "Beer is proof that God loves us and wants us to be happy"
I'll drink to that!!!
If your gravity detector is (close to) horizontal, vector math shows that there's some fierce accelerating going on, and as a result of that there's a pulling force on the spring + cord, significantly larger than the pull by gravity. If the acceleration stops, the weight will drop into a vertical position with respect to the rig again; it will not 'overtake' the rig (except for a short moment when it will not yet have started to fall, and the force of the spring will start to reel it in). Only if the turbulence is such that the rig gets actively stopped and/or pushed downwards there's a possibility that the weight can slam into the rig. But I think those are conditions you wouldn't want to launch under.
I'm thinking winglets and a horizon-sensing "autopilot" just set to make sure the launch angle is optimum as best as it can.
Hell, a compressed-air or some other compressed-gas RCS might give you a few seconds of attitude correction and stabilisation - certainly enough to launch. It doesn't need to lift the rig - just spin it the right way after the balloon bursts.
My idea is this:- you attach a weight to the balloon (hopefully not very heavy), attached to this weight is a cord (of whatever length), on the other end of which is an insulator between two sprung switch contacts. The idea is, the balloon pops, the weight falls, it reaches the limit of the cord length and snatches the insulator from between the two contacts, contact is made and the rocket fires.
This will keep constant force on the switch
It does not. As soon as the distance between balloon and rig decreases, the tension on the shock cord decreases too. Which means you may still have some tension where a normal cord would have gone slack, but constant it is not. And it may induce oscillations in the line, so it may not even keep any tension at all.
OK, let's see if I understand the requirements. We need a backup firing trigger in case the balloon bursts prematurely. The backup must fire the rocket within a second or so after the burst, must be (relatively) immune to turbulence induced false positives, and must be a mechanical system, the simpler the better. OK?
My proposed solution will take a little trial and error testing but seems sound in concept - at least to me. Take an ordinary security/ID badge reel of the sort that clips to your belt and allows you to pull the badge out on a string for normal use and then reels the badge back in by means of a spring powered reel. At the reel end, place two electrical contacts which, when shorted, activate the firing mechanism. At the badge end of the string, place a metal washer or equivalent. Place the reel on the truss in such a position that when the string is extended it is parallel to the upper tether, the part above the Y, and attach the washer at or just below where the tether attaches to the balloon.
When the balloon bursts, the reduction in tension on the main tether must be sufficient for the reel-in spring to bring the washer down to hit the contacts on the reel, firing the rocket.
The trial/error part is finding such a badge reel with just the right strength spring. If you can locate one, the time it takes for the string to reel in should be, if not right on one second, a fairly short time. An advantage of the time it takes to reel the washer down to make contact is that temporary turbulent induced reduction in main tether tension ought not to be long enough for a false activation, and when the turbulence passes the badge reel mechanism will tend to reset itself.
Very ggo! May I suggest a small modification?
Find a badge reel strong enough to hold the entire truss+rocket and replace a section of the main balloon tether with the badge reel. During ascent the reel is extended and the contact is open. During turbulence there is enough length in the reel to allow for bumpiness without the contact closing. When the balloon bursts the reel zooms back in and the contact closes, launching the rocket.
You could put your badger eel inside the launch rig, where it's warm and cosy. Run the cord up to the swivel joint and adjust everything so that when the two lines of the inverted 'V' are taut, the cord of the badger eel is just a wee bit off from maximum extension, so that it won't ever need to take the weight of the rig. Then of course the conducting disc, and a bunch of contact pairs around where the badger eel cord passes through the lauch rig fuselage.
Swap the payload swivel with a hollow one, an inward flanged tube sitting inside an outward flanged tube with ball bearings sandwiched between the flanges.
That way you can pass the safety tether through the middle of the swivel.
(A ball retaining plate like used to support the platter in a microwave oven can be used to reduce the number of ball bearings needed without having the all bunch up at one location.)
Use a length of line for your tether in a "Y" configuration between opposing shroud lines of the chute and place a small fishing line type swivel at the junction of the Y. That way, as the chute opens, the "Y" becomes a "T", pulling the safety pin.
You could rig a switch plus a tether that pulls the switch if LOHAN slides off the rod, but that would require LOHAN's batteries to be able to supply the juice for firing the igniter.
And I think the idea is to get LOHAN to fire in an upwards attitude if at all possible. Firing the motor when going down would be necessary only if there's concern about having the craft landing with an unused motor.
Use a common office badge reel (sometimes known as a "zinger") in the main balloon tether line. At the reel end, place two electrical contacts which, when shorted, activate the firing mechanism. At the other end of the string, place a metal disk which will short the contacts when the reel is fully retracted.
Find a badge reel strong enough to hold the entire truss+rocket.
During ascent the reel is extended and the contact is open.
During turbulence there is enough length in the reel to allow for bumpiness without the contact closing.
When the balloon bursts the reel zings back in and the contact closes, launching the rocket.
p.s. Thanks to Alan Esworthy for the inspiration. If I have reeled further than others etc...
Thanks, Martin. I did think about using the zinger line as the main tether line but was unsure of its tensile strength and the total weight of the truss, electronics, and spacecraft.
On further thought, running the zinger line from the truss to the Y in the tether might work even better/faster and eliminate or at least greatly reduce any chance of tangled lines interfering with the backup function if needed.
...Paris, because she's the real zinger!
Attach a suitable switch (of the pull-teflon-strip-from-between-contacts type) at one of the parachute's attachment points, and attach a cord at the opposite attachment point. Make its length so that it triggers the switch with the parachute roughly half to three-quarters open.
Less chance of binding/freezing than with the cord-through-rings-on-parachute setup; may require some experimenting to find the right switch and get the setup right; it also requires wires running down from the parachute to the launch rig.
is an indication of how much of a problem turbulence actually is under average launch conditions, at what heights it tends to occur, and whether it actually causes sufficient loss of pull to cause a false trigger when using a tension switch on the balloon tether. There's a video of some Lego craft getting up into the stratosphere, and not being shaken to pieces in the process. Or did they glue the thing together?
Because when turbulence stops being much of a problem over, say, 5000m, then you could hook a pressure switch set to the equivalent of 7500m into the circuit (parallel to the tension switch if the pressure switch opens at altitude, in series of it closes).
It's really a last resort system. We're not planning to use it unless absolutely necessary. If the SPEARS boards fails on a first flight, we'd probably bring the Vulture 2 down with the truss, and go for a second flight, when it's all or nothing. In that case, it wouldn't really matter what way the aircraft is pointing.
is to produce a mechanical trigger whic will only fire after the GPS altimeter has failed to either detect launch height or it has detected launch height but the ignition process failed. This implies that the mechanical trigger must react to an event other than GPS altitude reading, which could be either temperature (difficult to get right), r balloon burst).
A free-fall detector would be my sugestion, such as a weight on a spring enclosed in a box or tube. When free-fall is detected the spring should return to its unladen orientation. A problem with this idea, and any other free-fall detector, is turbulence during the ascent causing the mechnaical device to fire. The difficulty with any external cords or strings is the possibility of icing up and friction.
Here is my two pennies worth:
This is based upon the fact that distance "a" reduces to distance "b" after balloon pop.
I'm assuming "a" is at least 5m and "b" will be less than 3m.
A set of electrical contacts on long, self closing arms that touch when the parachute starts to open. The upper open end of the arms would be tethered to the top inside centre of the chute, and the lower end attached to the platform below (pretty much where length "b" arrow is).
The overall length of the open arms (which would be 2 x 1m = 2m) and the two bits of tether would be equal to distance "a".
During ascent, the weight of the platform and the lift of the balloon keep the arms open (and so the electrical circuit open). In the event of turbulence, the distance "c" would have to reduce by 2m before the circuit would close, hopefully dampening temporary reductions of "c" < "a" and stopping any premature firing. (I know the un-deployed chute will go squiggly as well - couldn't be bothered to draw it.)
After the balloon bursts, the arms will have freedom to close completely and so close the backup firing circuit .
Have the arms below the closed chute material (where the squiggly bit is in measurement "c") so they can't become tangled in the chute.
The firing system could also be a tube like you show in your "pin pull" system, but with the cylinder being upright, about 2m long and the electrical contact being made as the spring pulls the piston all the way in to the tube after about 2m of travel.
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