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Jet gun - anyone did that?

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Unread postAuthor: FiveseveN » Thu Aug 31, 2006 3:58 pm

What's with all this talk about carburetion? Just use propane or some similar gaseous fuel :wink: I'd say use acetylene because it has a very wide combustion window, but you know the drill: use metal, not PVC.
This is a very good concept, but it will pe a PAIN to troubleshoot and tweak. First of all, I would use a really long barrel (maybe about 200 calibers).
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Unread postAuthor: dongfang » Fri Sep 01, 2006 7:22 am

Why the long barrel?

I was thinking something like a truly mobile, small-chamber, large caliber orange acceleration device (respectable recoil included for *FREE*). Heavy projectiles give more backpressure and slower flow; I believe that will be good with this design. But I'm only guessing.

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Unread postAuthor: schmanman » Fri Sep 01, 2006 7:23 am

I'm guessing you are right about the back pressure. this gun will be very cool when finished. :D
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Unread postAuthor: dongfang » Fri Sep 01, 2006 11:33 am

Like this ..?
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Unread postAuthor: dongfang » Sat Sep 02, 2006 4:07 am

OK, as for acetylene:

Molar mass: 26 g

Density: 1.1 g/cm^3

Reaction:

C_2 H_2 + O_2 - - > CO_2 + H_2O

Bal.:

2 C_2 H_2 + 5 O_2 - - > 4 CO_2 + 2 H_2O

So, acetylene:air is about 2:25 moles, or about 8% vol. Other people say about 12% (http://www.gexcon.com/index.php?src=han ... Bchap6.htm, figure 6.3). Seems to react more happily and more sooty, too. A big soot cloud would look cool :)

FiveseveN, what is your idea on how the gaseous fuel should be stored, and injected into the arstream? Tank-in-tank (sounds dangerous to me with ace.), an external tank under pressure, or something else? And, for those who don't have an oxyacetylene rig (like me ;) , do you have a good idea on how to get it compressed if necessary?

I like the apparent field usage simplicity of liquid fuel. But I have no idea whether it might actually work or not ;)

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Unread postAuthor: FiveseveN » Sat Sep 02, 2006 12:23 pm

I was thinking about a long barrel because the release and ignition could become a real pain to synchronize. You have a better chance of transferring energy in case of a delayed ignition, with a longer barrel. Obviously, the best choice is to make multiple barrels and use adapters to attach them to the chamber.

It's not a good idea to pressurize acetylene. You could use a large syringe or a small pump to inject it into the chamber. Just my two cents. Of course, you could use other fuels.
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Unread postAuthor: dongfang » Sat Sep 02, 2006 9:08 pm

FireseveN, thank you for your ideas.

Yes, I am aware that there is a challenge in synchronizing release and ignition. I had on thought of it, while having nothing else to think of today ;) : If we are very lucky, a solution where the valve piston depresses a number of BBQ igniters when it hits the extreme open position, would time well. That would be elegant. If we are not lucky, a pneumatically actuated valve could still be used: Stick a magnet in the piston, and a Hall sensor w/ circuit outside. I suggested an electronic sequencer for the inital experiments, and it shouldn't be hard to build one.

As for the fuel type, I still hope for a liquid fuel solution. It could be so easy: Just fill up the tank for every few shots. I think this ether based fuel for R/C models may have some possibilities.

The idea of having a gaseous fuel compressed along with the air in the tank is a little scary: If there is some design guarantee that the two won't mix in the tank, then I'd be happy to fire it hand held. If there is not ... oh no ... another statically-fired, potentically lethal monster. That is why I like the carburettor talk, whether that is a commercially made carb, or a simple home made venturi one.

I think the first thing to be done still is to get some data on flow through jets in venturis (for example, right after a Supah-valve is preactically a venturi) in a pneumatic gun. Both for liquid and gaseous fuels. If venturis can be made to work precisely & reliably, they would be a really nice solution.

I don't have much experience with the importance of barrel length, except, of course, that a longer barrel usually increases velocity. I can see two advantages of a long barrel in the design:

- There is more time for the gas to burn at a place where is is useful for potato propulsion
- Long barrels should go well with hot gases: Hot gas is thin, and the flow resistance should mean less than with cold gas. My super-sonic compustion gun was 117 calipers long.

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Unread postAuthor: noname » Sat Sep 02, 2006 9:10 pm

You can probably use an electric sprinkler valve (not modified, using the solenoid) to vent a piston valve and somehow wire the firing of the spinkler valve into a stungun.
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Unread postAuthor: schmanman » Sat Sep 02, 2006 9:19 pm

acetylene is very sooty. put acetylene and oxygen in a balloon and light it on fire. BOOM!!
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Unread postAuthor: FiveseveN » Sun Sep 03, 2006 7:21 am

Yup, your best choice valve-wise is either an unmodded sprinkler valve or, better yet, a piston valve actuated by an unmodded sprinkler valve. A stun gun or any other electronic ignition system will be much easier to control. Just make a trigger box with variable delay for the sparker (from 0 to a few hundred ms; it all depends on the valve setup).
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Unread postAuthor: noname » Sun Sep 03, 2006 12:08 pm

Or just tape down the stungun switch and open the sprinkler or piston. :lol:
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Unread postAuthor: nabsnoabs » Sun Sep 24, 2006 1:41 pm

I was thinkin of the same exact plans for my first hybrid

the only difference is that the combustion chamber would injected with gas instead of the back chamber
an igniter would have a spark constantly running, which would not ignite the gas in this chamber the concentrations would be too high, which would eliminate the need to synchronize the spark after the valve opens
once the valve is opened, the pressurized air hits the gas and it immediately ignites

think of bein in a room and holding a lighter and sprayin aerosol over it
the room is full of air and ur sprayin fuel into it
now just imagine the opposite
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Unread postAuthor: dongfang » Sat Sep 30, 2006 1:14 pm

Oops I didn't notice you were already here.

I like your design too; certainly it will get easier to get things on fire in the compressed mix tank than in a flow-thru chamber.

The idea with storing the fuel in the combustion chamber should work too? Well I am not quite sure really. Maybe the mixture will sort of go from too rich to too lean?


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Unread postAuthor: critical_al » Thu Oct 05, 2006 3:25 pm

Your idea sounds like a good one and should certainly be possible. It sounds like you are moving towards a pulse detonation engine which can be great fun to play with, although I've never tried firing anything from one!

I don't know how much you have looked into them but a quick search on pulse-jets gives loads of info, the valveless type is most similar to what you are talking about. Obviously this isn't a straight forward solution but it is moving in the right direction.
http://aardvark.co.nz/pjet/valveless.htm

I did quite a lot of research on this sort of thing several years ago and found that the main problem was in tuning the exhaust (barrel!) to get a good flow through. I was working on a naturally aspirated cannon firing twice a second which is actually very slow for this sort of set-up. It all becomes loads simpler if you can use compressed air, then it is just a matter of getting the mix and timings right.

Relatively simple to get an adjustable timing firing circuit, if you are at all handy with a soldering iron and a circuit board.

As a first time poster I hope you don't mind me joining in but I was inspired by your great idea and would love to see it working.

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Unread postAuthor: Pete Zaria » Thu Oct 05, 2006 6:27 pm

Here's a 5 minute Visio drawing of my idea.

Image

On the left is a pressure chamber made of 4" PVC, connected to a valve (a modified sprinkler would work, a piston would be better), connected to the combustion chamber. The valve is connected to a piece of 2" pipe that passes through the 2-4" reducer into the combustion chamber. The 2" piece of pipe has lots of holes drilled for the pressurized air to escape into the combustion chamber once the valve is triggered. The combustion chamber is also 4" pvc. There are two or more spark gaps in the combustion chamber. There's a union with a burst disk at the end of the chamber, before the barrel.

The combustion chamber is filled with propane (or another flammable gas) to a level much too high for combustion, say 75% propane (using a meter system). The spark is turned on constantly, and then the valve is triggered to release the pressurized air into the chamber. This will burst the disk immediately. Then, the fuel and air mix, detonate, and force the projectile down the barrel. The burst disk simply holds the propane in, so the burst disk doesn't have to hold back more than say 10 psi.

I'm not sure how efficient this design would be. It would be fun to play with, though. To be safe, sch80 should be used and the first 20+ firings should be done remotely.

Let me know if you're going to test any of the above-mentioned designs :)

Peace,
Pete Zaria.
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