SpudFiles

Alright everyone, I have a slightly abnormal concept I am working on.
I am trying do design a cannon to be put inside a rocket, to pop off the nose cone, and eject the parachute. For reference, the usual method for ejecting the parachute in a model rocket, is a bit of loosely packed black powder, that explodes, pressurizing the entire body tube, popping the nose cone off.
This brings up several constraints that are usually not an issue with most of the stuff on this forum.
First off, it needs to be VERY light. I am talking about in the range of just a few ounces max.
Second, it needs to be SMALL. The entire device must fit in the 2" diameter body tube.
Third, it must be electronically activated, by the mini altimeter/computer I am working on.

Now moving on to the actual design.
What I am currently envisioning, is using a liquefied gas (probably either propane or butane, because CO2 is far too high pressure) to save on space.
The reason why using a liquid is so desirable is because a LOT of air is going to need to be released, in order to fill the body tube, and if the gas is liquified, at a low pressure such as propane or butane, you can get that high density, without extreme pressures.

The gas would in theory be held in with a burst disk, punctured by a solenoid, because a piston or ball valve would be far to heavy, and require more complicated electronics.

I was thinking the chamber could be easily filled if I were to use butane, and simply cut the end of a bic liter, to fill through. That way we could just fill it up right before the launch, in the field, with one of those lighter re-fillers.


Comments, questions, critiques, etc. all welcome.


edit: One last thing. The entire device must not have any sort of combustion, or pyrotechnics. The idea is to do it entirely non-pyro.

With your weight limitation, I don't see any burst disk container, solenoid, electronics module that will meet your design requirements. Is there any reason a traditional motor with the chute delay charge won't do the job?

An altimeter alone won't tell you the velocity, unless you integrate the output to look for a drop in rate and trigger on a return to zero on rate.

For weight, magnetic solenoids are very heavy. May I recommend a charged capacitor discharged into a small squib? The pyrotechnic may be enough to release the chute, or you may elect to use an electric match to burn a plastic or flammable burst disk to release gas.

For energy / weight, a propane mix that is too rich may be used that will mix and finish burning in the rocket cavity to soften the pressure surge.

Look online for ways to make your own e matches or simply use an electric motor igniter.

The main issue is that I am working on this design primarily for a specific rocket contest, in which pyrotechnics are not allowed, because it is aimed at highschool students, who can't legally handle black powder, and pyrodex is questionable, and much more hazardous.
I was really wishing we could use some sort of propane combustion, because it is pretty much perfect, not requiring heavy tubing or anything, but it is against the rules, because of the combustion.

The reason why the main ejection charge is not going to be sufficient, is because I am trying to work out a dual deployment system. The main parachute will be ejected by the motor's ejection charge, but then a second one will be deployed at a time decided by the computer/altimeter, at a lower altitude, to slow it down further. I can assure you we can figure out the whole altimeter/computer deal.

I have found several designs in the past that use a hot nichrome wire to heat a plastic burst disk to cause it to fail, but those take time to warm up, and don't always do so at exactly the same rate. Ideally we would be able to control the exact moment at which the ejection occurs.

Yet other designs just use a simple spring to push the nose cone out, held back by either a servo, or a string that gets burnt through by an e match.

As for the issue with solenoids being heavy, I was thinking that if the neck were small enough, and the tip of the solenoid pointy enough, the burst disk could be very thin, so it wouldn't take very much force to puncture it, so I could get away with a small solenoid. The only issue I see is that it might just poke a tiny hole in the disk, rather than rip it open.
As for the chamber weight, I was just thinking of the bottom of a lighter, epoxied into some thin-ish walled PVC, with a threaded union glued on the other end. Because it would be using liquid butane or propane, it could probably get away with just being a few inches of 1/2" or an inch or so of 3/4" pipe. It is important to remember that unlike a spudgun, I am only trying to increase the pressure in the body tube to about 4-10psi, rather than 100-500psi, so a little gas can go a long way, if you do it right. I have seen (very expensive machine shopped) designs using a single 12g CO2 cart for ejection. http://www.rouse-tech.com/recovery.htm (but they still use a little squib to break the seal on the cartridge...)

Lentamentalisk wrote:I have found several designs in the past that use a hot nichrome wire to heat a plastic burst disk to cause it to fail, but those take time to warm up, and don't always do so at exactly the same rate.

I had made a nichrome fired burst disk a while ago, it always fired instantaneously. You need a heat sensitive material like mylar to ensure reliable function.

Another idea could be to have some sort of spring loaded valve that is held shut by some chord than can be cut by a hot wire. (edit: but it seems you already thought of this, hadn't read your above post properly )

How about a capacitor discharge into a resistor/small capacitor placed on the burst disk? It could be done with a single small flash cap and actuated by a micro solenoid.

Is a tomy timer out of the question? If your rocket is consistent it's essentially 100% reliable.

Or just a micro servo with a needle to puncture the disk?

@JSR: Question about your nichrome wire burst disk. How did you ensure the wire was touching the burst disk? Did you attach it with a bulge, so that it pressed up against it? That is definitely worth considering, especially if we set it up to dump a capacitor through the nichrome wire, or something like that, to ensure it gets hot fast.

Let me dig it up and take some pics

edit: could only find the barrel, epoxied to a male fitting. There are two fine brass tubes set into the epoxy, each connected to a wire, which the burst disk plugs into. In order to maintain contact the wire was sandwiched between two burst disks.