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 …
0g-Detect for freefall protection for under a tenner. There's one fitted on the Habuino HAB board.
Got to be mechanical - no electronics
And what constitutes electronics? You have a battery and and a switch, which is already electrical. Some sort of arbitrary dislike of semiconductors? The item most likely to fail is the battery, followed by mechanical devices freezing.
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...
How about a very long piece of string and a switch?
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?
I quite like it too, but: "Please note that minimum order quantities may apply for this product." I'll look into it...
Give me a shout if you don't get any luck, there are other companies that do these things. I can probably blag a sample if push comes to shove through my electronics Biz :-)
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.
Without a picture, that makes almost no sense at all (to me). But it sound very convincing!
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.
Attach a small under inflated balloon to the pin in the fail-safe device. When the main balloon bursts and your rocket starts to plummet, the second small balloon will still have lift and pull on the pin to set off your fail safe device.
Your small balloon will pull the pin out as soon as the ground crew let go of it, even if the entire assembly is already moving upwards.
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.
Roughly what I was thinking, but instead of the (balloon) lift line holding a switch open against a spring, let the parachute line (slack during ascent, taut once it deploys) pull a teflon strip from between a pair of NC contacts.
Rather than a pin, put a Teflon strip between the contacts and attach the line to it, less friction.
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.
Also need to ensure that the lines don't get twisted (as the pin design might)...
How about a differential pressure switch attached to the ballon filler? all the time there is preasure diffrence between the inside and outside the switch is off, then when the ballon pops, there is equal preasure both side so the switch is on?
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.
Didn't the cage always get hung up on the pole at the last minute? Mousetrap kinda sucked.
c) Weight falls free from cage and pulls figure 8 loop from between plunger and stop plate
c.1) because balloon lift is lost the plunger and stop plate will start falling just as fast as the weight which is supposed to pull the cord from between them.
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.
That's not in any way different from having a switch inside the rig, activated by loss of pull from the balloon cord. And it's just as sensitive to false triggering from buffeting.
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.
Sorry, you can't attach anything to the balloon, except at its neck.
That would involve electronics again, or a mechanical clockwork (likely to freeze or be otherwise impaired by the cold) that is resettable.
No electronics. Just a clockwork from a toy (they still make clockwork toys, do they?) which can be pre-"baked" to remove moisture before putting in a lightweight plastic case...