Power Source

[Nameless337]

Hey folks, I'm hoping someone here can help out with a design project for school. My team has been assigned the task of designing a device to release a payload from a high altitude balloon. After looking at a few different options we've decided to go with a small container holding a very small rocket motor that, when ignited, will burn through the string tying the payload to the balloon. Obviously to do this we need a very reliable ignition system. I'm hoping to get some of your thoughts on what igniters out there are the most reliable and what sort of electrical power requirements are associated with the igniters. The lighter we can make the device the better. We're looking to start testing on some electrical systems and I'd love to get a ballpark for what voltage/current combination is acceptable. Thanks.

 

[Derek]

that is an interesting solution to your problem.

what made you chose to burn the string rather than just disconnect it with something like a servo or electro-mechanical actuator?

as far as a power source, lithium polymer (lipo) batteries are cheap, light and have a high power density. I'd suggest looking at those, first.

 

[gdiscenza]

A better choice than a poorly purposed rocket motor would be something like the cable cutter from Archetype Rocketry.

G.D.

 

[stealth6]

Well first of all, if all you need to do is burn through a string, you may not need that small motor. Just the ignitor (that would have lit that small motor) might do on it's own. Quest Q2G2 igniters would be the obvious choice - readily available, very inexpensive, low voltage, weight practically nothing, and super reliable. You could devise are very a basic & crude, but cheap & easy system with just an ignitor, altimeter, and a bit of cotton string.

That said, a more robust "system" that is made to do pretty much exactly what you are needing would be the Archetype Cable Cutter paired with one of a number of available altimeters. It's tiny, lightweight, and very reliable. It is made to cut standard "zip ties", which are much stronger and can hold payloads better than just "string". Many altimeters would require a 9-volt battery to operate and to fire the ignitor, but these can be chunky and heavy if you are really concerned with mass in terms of grams. However, some use tiny and very lightweight LiPo batteries. Of these, the Featherweight Ravens are very tiny/light and are also ultra reliable, give you tons of data/feedback, and are highly configurable giving you a lot of options and flexibility.

s6

 

[Nameless337]

Thanks for the feedback. Our thoughts for using rocket motors was that by using chemicals to store the energy we'd be able to get by with less power and weight. Instead of driving a motor or piston we'd only have to set off a reaction. Not having any moving parts is also a bonus. LiPo's would probably work great, but there is a concern with the temperature. at the altitudes this thing will be reaching the temperature drops to around -50C. We're actually looking at supercapacitors, which handle low temperatures much better than batteries, but in order to decide what range to look at we were really hoping to get some idea of the power draw required by the igniters.

That said, your ideas have certainly given us more to consider, thank you.

 

[CarVac]

+1 on the Archetype Cable Cutter.

I'm not sure a rocketry altimeter would be particularly good for setting it off, because they tend to wait until they feel a boost (sudden, rapid rise in altitude, in the case of barometric-only altimeters) before trying to set off charges.

 

[G_T]

If something goes wrong, then your payload may be on the ground, and that little rocket motor could start a fire. Better to use a linecutter, or a tether, or some sort of solinoid release mechanism, or a servo release mechanism (pull a greased pin to release a little eyebolt, for instance).

Rocket motors are also hard to light at such altitudes.

Beware of batteries at cold temperatures. Some lose most of their capability at the ambient temperature up there. Pick the chemistry of the battery, and the voltage and capacity, carefully.

Gerald

 

[Derek]

you can wrap a lipo (or anything else) in one of those chemical hand warmers to keep them warm.

 

[CarVac]

Chemical hand warmers are touchy. They require oxygen to work, and if they get too cold then the reaction stops.

 

[CCotner]

An unmodified Archetype cutter won't work at the altitudes the OP is talking about because of the lack of oxygen available to the BP. The only successful high-altitude deployment systems I have heard of that use BP in any form have the BP not only contained, but in vacuum-tight seals, so they are in ground-pressure air when they go off. I'm sure part of the reason behind picking a motor (I'm imagining a reloadable D21 would work lovely) is the surefire ignition at altitude.

That said, there are non-pyro-based release mechanisms that would undoubtedly be more reliable and safer, using solenoids to open hooks and the like. I'd recommend you design and build something like that. PM me; I can help if you want.

 

[bandman444]

I was under the impression that high altitude balloons have the recovery system attached to the balloon train so that at apogee the parachute deploys when the balloon pops.

Perhaps you aren't trying to deploy at apogee in which case what will you be doing to initiate the motor.

I don't see a problem with a Archetype Cutter. The problem with the standard charges is that as they go off they spread the charge rather than ignite it. In the cutter the charge has no where to go. If you are really nervous about the oxygen content, than use a sliver of Blue Thunder propellant, it contains its own oxygen and would work fine.

Good luck we want to hear more.

 

[Derek]

Chemical hand warmers are touchy. They require oxygen to work, and if they get too cold then the reaction stops.

depends on the hand warmers. they are used in balloons for purposes like this quite a bit.

https://balloonnews.wordpress.com/equipment-review/hand-warmers/

 

[luke strawwalker]

An unmodified Archetype cutter won't work at the altitudes the OP is talking about because of the lack of oxygen available to the BP. The only successful high-altitude deployment systems I have heard of that use BP in any form have the BP not only contained, but in vacuum-tight seals, so they are in ground-pressure air when they go off. I'm sure part of the reason behind picking a motor (I'm imagining a reloadable D21 would work lovely) is the surefire ignition at altitude.

That said, there are non-pyro-based release mechanisms that would undoubtedly be more reliable and safer, using solenoids to open hooks and the like. I'd recommend you design and build something like that. PM me; I can help if you want.

Agree...

That's a lot of impulse (D21) to have flailing around under a balloon just to burn a tether...

If one INSISTS on going pyro with some type of activation circuit, use an ignitor, maybe with some extra pyrogen added... a Q2G2 with a second dip doesn't require much power to fire off, so big capacitors are unnecessary... (those have dangers of their own-- touching the contacts of a charged capacitor can be downright dangerous... and capacitors have been known to blow up as well...)

Later and good luck on your project! OL JR

 

[stickershock23]

Use this...

https://defyg.com/tether.html

an e-match will set it off it will hold LOTS of weight. and you can use a 9v battery to set off the e-match it also seals for High altitude.

 

[cjl]

An unmodified Archetype cutter won't work at the altitudes the OP is talking about because of the lack of oxygen available to the BP. The only successful high-altitude deployment systems I have heard of that use BP in any form have the BP not only contained, but in vacuum-tight seals, so they are in ground-pressure air when they go off. I'm sure part of the reason behind picking a motor (I'm imagining a reloadable D21 would work lovely) is the surefire ignition at altitude.

The ignition of a rocket motor - especially an apcp motor, is far from a sure thing at that altitude. The combination of the cold and the lack of air makes transferring enough heat to the grain to ignite it fairly difficult, and the burn rate of the grain in the near vacuum (if it stays lit at all) may be insufficient to pressurize the motor. Rocket motors intended to be lit in space need either a powerful gas generator as an igniter, that doesn't just provide heat to ignite the propellant, but also begins the pressurization process, or they need something like a burst disk in the throat, so they effectively aren't being lit in a vacuum.

Honestly, I think the best bet would be something like a servo-powered pin puller - it would be extremely reliable, consistent, and have far fewer failure points than a pyro-based system. The power required to operate a small servo is pretty minimal too, especially since you could design the circuit to not even send any power to the servo until time of release. It might even take less power than a couple of igniters...

 

[Reinhard]

I wouldn't use black powder actuated hobby devices either. Besides the above mentioned reasons, they introduce also legal questions (LEUP). If a thin enough thread is used, an electrical device might be used (see Chute Tamer). There are also professionally made line cutters. They differ from the Archetype cutter in the pyrotechnic composition and the sealed construction. At least one manufacturer advertises the fact that their products have been used in space applications. They are more expensive though and only single use. They cost about 100$ a piece, the last time I looked.

Regarding electromechanical solutions, look up "RC tow release" or a similar search term in Google.

For batteries, it is best to use appropriate types like Lithium thionyl chloride batteries that will work at low temperatures too, instead of heating other types. Pay attention to their maximum discharge rates.

I'm under the impression, that most balloons use purely passive systems. The parachute is simply tied to the harness. As soon as the balloon pops, the whole assembly drops and the parachute inflates. Is there a particular reason for an active system in this case?

Reinhard

 

[bobkrech]

An unmodified Archetype cutter won't work at the altitudes the OP is talking about because of the lack of oxygen available to the BP. The only successful high-altitude deployment systems I have heard of that use BP in any form have the BP not only contained, but in vacuum-tight seals, so they are in ground-pressure air when they go off. I'm sure part of the reason behind picking a motor (I'm imagining a reloadable D21 would work lovely) is the surefire ignition at altitude.

That said, there are non-pyro-based release mechanisms that would undoubtedly be more reliable and safer, using solenoids to open hooks and the like. I'd recommend you design and build something like that. PM me; I can help if you want.

As a physics major/engineering student you should be aware that BP and and other low explosive propellants contain an oxidizer and a fuel SO THEY DO NOT NEED ADDITIONAL AMBIENT OXYGEN TO FUNCTION. Low explosive propellants are self contained and once ignited function in vacuum just fine since they consist of a fuel and oxidizer in intimate contact.

The problem with a rocket motor or other pyrotechnic device ignition at high altitude is the possibility of ignition failure due to the lack of a heat transfer media between the e-match and the propellant. The simple proven solution is to hermatically seal the e-match and the propellant at 1 atmosphere so that the heat transfer from the e-match to the propellant is both convective and conductive. The Archetype cutter is not hermatically sealed so the BP within the device may not ignite due to lack of conductive heat transfer from the match to the BP grains.

Bob

 

[CarVac]

I always wondered if you can light a black powder motor in a vacuum with say a dipped Q2G2. Since the BP is pressed firmly together, there is no way it would scatter before transferring heat to its neighboring particles.

 

[stickershock23]

As a physics major/engineering student you should be aware that BP and and other low explosive propellants contain an oxidizer and a fuel SO THEY DO NOT NEED ADDITIONAL AMBIENT OXYGEN TO FUNCTION. Low explosive propellants are self contained and once ignited function in vacuum just fine since they consist of a fuel and oxidizer in intimate contact.

The problem with a rocket motor or other pyrotechnic device ignition at high altitude is the possibility of ignition failure due to the lack of a heat transfer media between the e-match and the propellant. The simple proven solution is to hermatically seal the e-match and the propellant at 1 atmosphere so that the heat transfer from the e-match to the propellant is both convective and conductive. The Archetype cutter is not hermatically sealed so the BP within the device may not ignite due to lack of conductive heat transfer from the match to the BP grains.

Bob

Quite true BUT the problem is not that the propellant wont burn the problem is getting it LIT in a vacuum.
if you used an AP motor you would need to seal the nozzle end of the motor. with the ignition system inside (igniter ) the motor. along with sme sealed atmosphere. for the best results.. but then you have the problem of Pressure increase inside the motor as altitude increases.

with a teather or similar. the explosion from a GOOD e-match is enough to fire the thing and get it separated! ask me how I know. yes I have released on with NOTHING but a good dipped e-match.

also with a teather you could use a simple o-ring to seal the combustion chamber for the "effects" of being out of the atmosphere.

another choice to think about.

use something like the CO2 ejection system. have a co2 cartridge that you can open, have that push a piston to do the work.

 

[Nameless337]

Thanks for all the input. You've certainly given us a lot more to think about. In answer to a question, there are a couple reasons our sponsor wants an active system. First off there's a concern that the remnants of the balloon after it pops are getting tangled with the parachute or the antenna and causing some issues there, although I don't think that's very significant. More importantly, the end goal for this project is for the balloon to be filled such that it reaches neutral buoyancy at a desired altitude and have it remain there for an extended period. The release signal is already being handled by the same GPS chip that provides tracking (it records latitude, longitude, and altitude). The objective at some point would be for the balloon to stay at a desired altitude and have the package release when it strays outside of a predetermined radius.

 

[bobkrech]

I always wondered if you can light a black powder motor in a vacuum with say a dipped Q2G2. Since the BP is pressed firmly together, there is no way it would scatter before transferring heat to its neighboring particles.

There was an excellent NARAM R&D report on the subject. Unfortunately I don't remember when. It showed experimentally that BP motors did not ignite reliably in a vacuum chamber above 20 Kft.

BillSpad has launched several micromax BP rocket under water by sealing the igniter plug in with wax. This method will also work in vacuum.

Quite true BUT the problem is not that the propellant wont burn the problem is getting it LIT in a vacuum.

if you used an AP motor you would need to seal the nozzle end of the motor. with the ignition system inside (igniter ) the motor. along with sme sealed atmosphere. for the best results.. but then you have the problem of Pressure increase inside the motor as altitude increases.

I stated ignition was the problem. Also the pressure inside a sealed motor remains the same as altitude increases.

with a teather or similar. the explosion from a GOOD e-match is enough to fire the thing and get it separated! ask me how I know. yes I have released on with NOTHING but a good dipped e-match.

also with a teather you could use a simple o-ring to seal the combustion chamber for the "effects" of being out of the atmosphere.

There are a number of lightweight mechanisms that wil work.

An o-ring seal on the ram in a modified Archetype cutter with a sealed e-match opening will work provide the pressurized area behind the ram is vented before the ram impacts the closed end of the tube.

A simple tube similar to a glass electronics fuse with a straight hot wire inside with a couple wraps of a plastic line
around the hot wire would also work. Simply shorting the wire to a lipo battery would heat the wire and melt the thread.

another choice to think about.

use something like the CO2 ejection system. have a co2 cartridge that you can open, have that push a piston to do the work.

Again the CD-3 mechanism is very heavy for a balloon and the auxillary CO2 is not necessary if a properly sealed cutter ram is employed.

Bob

 

[cerving]

Servos will work just fine with a 3.7v LiPo cell too... I've done it many times. The micro ones take very little current, about as much as a Q2G2, for about 1 second over full range. The flight computer kit I designed does servos natively... see www.eggtimerrocketry.com

The ignition of a rocket motor - especially an apcp motor, is far from a sure thing at that altitude. The combination of the cold and the lack of air makes transferring enough heat to the grain to ignite it fairly difficult, and the burn rate of the grain in the near vacuum (if it stays lit at all) may be insufficient to pressurize the motor. Rocket motors intended to be lit in space need either a powerful gas generator as an igniter, that doesn't just provide heat to ignite the propellant, but also begins the pressurization process, or they need something like a burst disk in the throat, so they effectively aren't being lit in a vacuum.

Honestly, I think the best bet would be something like a servo-powered pin puller - it would be extremely reliable, consistent, and have far fewer failure points than a pyro-based system. The power required to operate a small servo is pretty minimal too, especially since you could design the circuit to not even send any power to the servo until time of release. It might even take less power than a couple of igniters...

 

[T-Rex]

OK, A question I have to ask. Seems to me there are a lot of assumptions in these replies, mostly oxygen deficentcy.

Nameless, would you be willing to define 'high altitude' (ballpark is fine). I did not see a number anywhere in the thread (with exception that BP does not reliably burn greater than 20k feet.) That may help with more realistic answers to your quiry.

If I missed the reference somewhere, I apologize.

 

[CCotner]

As a physics major/engineering student you should be aware that BP and and other low explosive propellants contain an oxidizer and a fuel SO THEY DO NOT NEED ADDITIONAL AMBIENT OXYGEN TO FUNCTION. Low explosive propellants are self contained and once ignited function in vacuum just fine since they consist of a fuel and oxidizer in intimate contact.

The problem with a rocket motor or other pyrotechnic device ignition at high altitude is the possibility of ignition failure due to the lack of a heat transfer media between the e-match and the propellant. The simple proven solution is to hermatically seal the e-match and the propellant at 1 atmosphere so that the heat transfer from the e-match to the propellant is both convective and conductive. The Archetype cutter is not hermatically sealed so the BP within the device may not ignite due to lack of conductive heat transfer from the match to the BP grains.

Bob

You say that they do not need ambient oxygen to function, yet you also say that they need ambient heat-transferring fluid to successfully light (which is oxygen if you hermetically seal the charge volume, which is what I was suggesting). My understanding is that the burn rate of coarsely-mixed substances, even when they have oxidizer mixed in, is significantly accelerated in an oxygen rich environment. This derives from a basic understanding of chemical kinetics; saturating with oxidizer removes a potential bottleneck in the reaction rate as the the 'intimate contact' often isn't as intimate as we'd like, and I would think in a material with macroscopic granularity it would be rather poor actually. I do not know if these effects are so small that they are insignificant; all I know is that many charges have failed to function at high altitude, and the ones that have functioned are hermetically sealed.

As is almost always the case, the truth of the reason behind the need for hermetic sealing is probably a combination of more favorable mixing kinetics and additional convective heat transfer. The solution is not ambiguous, however. BP charges at altitude have to be sealed. That is all I meant by the comment.

 

[CarVac]

CCotner: Black powder is in fact intimately mixed: they extremely finely grind the components, mix them thoroughly, wet them to deposit the saltpeter on the carbon, squeeze them into solid cakes, and then break the cakes into particles. Each particle of BP itself contains oxidizer and fuel in the proper ratio.

The burn rate through the grain of BP is slower than the propagation between grains, except in a vacuum.

 

[CCotner]

CCotner: Black powder is in fact intimately mixed: they extremely finely grind the components, mix them thoroughly, wet them to deposit the saltpeter on the carbon, squeeze them into solid cakes, and then break the cakes into particles. Each particle of BP itself contains oxidizer and fuel in the proper ratio.

The burn rate through the grain of BP is slower than the propagation between grains, except in a vacuum.

I did not know that the individual grains were inhomogeneous, I assumed that they were homogeneous grains of nitrate and sulfur coated in charcoal powder and graphite.

The fact that the propagation through air of of the flame front is faster than through the grains seems like a reason that the grains need to be in oxygen rich environments to burn well, rather than just thermal conductivity (burning the BP in a pure-nitrogen environment, for instance). Is this correct? I should take more chemistry classes.

 

[CarVac]

If you put the BP in a nitrogen atmosphere it would probably behave largely identically to in air, because burning grains outgas violently (isn't that the whole point?), preventing any of the surrounding atmosphere from mattering.

Incredibly hot and reactive intermediate combustion products in the flame front would probably be more conducive to igniting adjacent BP particles than mildly heated oxygen gas would. However, if there is no air at all, the rapid expansion of the gas could cool it below the temperature it needs to ignite the other black powder.

 

[WillMarchant]

Take a look at the CD3 manual https://www.rouse-tech.com/manuals.htm for a discussion of BP at altitude.