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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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Jet gun - anyone did that?

Unread postAuthor: dongfang » Tue Aug 29, 2006 6:48 am

Hi,

Having finished my supersonic combustion gun, it's time for me o move on - with something that I don't know if it will work, like last time 8)

I thought about this: A hybrid gun that works in the opposite order of the usual: Release first, then ignition. Basically a modified pneumatic gun, without burst disks.

OK, a jet engine works by putting compression, combustion and decompression at different places, but at the same time. An Otto engine (like in a car) works by keeping them all at the same place, but at different times.

A jet spud gun [tm] is a little of both. It is a pneumatic gun. Just in front of the valve is a simple carburettor, injecting fuel into the airstream. The fuel is butane or propane. The air flow through the valve, after it opens, is too fast for ignition or backfiring in it. Slightly downstream from the valve is a small combustion chamber, wider and with an airstream slow enough for combustion (there is still some pressure). There is an ignition source there (spark plug that sparks constantly).

It is first filled up with air (bleeder line closed, fuel line closed). Then, it is fueled, which will probably cause some fuel to "spill" into the air tank. This is vented with the bleeder (barrel down, butane is heavier than air), losing a tiny bit of pressure.

It would definitely be safer than the classic hybrid (detonation is prevented in the same way as in a diesel engine -- gradual injection of fuel into combustion chamber), and it would need a smaller tank. I know it won't perform any better.

Does anyone see why this would (not) be workable?

Soren
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Unread postAuthor: schmanman » Tue Aug 29, 2006 7:59 am

One word. wow ! should work, cool, different design, I doubt it's been done before (obviously!!!)
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Unread postAuthor: dongfang » Tue Aug 29, 2006 8:13 am

I'm only worried that it will be hard to get the gas to ever burn. Igniting gas at x00 MPH and @y bar pressure could be hard. Maybe one of those shelter-things around the burner, as in a real jet engine, would do good.

But this has to be tried out. It if fails, it can always just serve as a regular pneumatic.

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Unread postAuthor: noname » Tue Aug 29, 2006 10:25 am

There would be some flaws but it should work. Like, you'd have to use a ball valve or piston valve to release the air; if you use a sprinkler it will melt the diaphragm (that might actually be pretty cool to see).
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Unread postAuthor: dongfang » Tue Aug 29, 2006 11:15 am

The flame is not supposed to pass through the valve, but OK it may backfire by the time the air tank is about to run empty.

I was thinking about a piston valve (coaxial or supah).

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Unread postAuthor: noname » Tue Aug 29, 2006 11:37 am

You have to leave room for error, so we are assuming that at least someflame is going to go backwards after the cannon is fired. As for the valve, don't waste money on a supah when you can make the same thing for 40 bucks and bit of machining. Or you can make a barrel sealing piston like a coaxial which would work and only requires a drill.
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Unread postAuthor: boilingleadbath » Tue Aug 29, 2006 6:55 pm

Sounds like a good way to increase the preformance of small-chambered launchers, or to acheive faster-than-sound velocities.
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Unread postAuthor: killagorrila99 » Thu Aug 31, 2006 4:10 am

aghh nice idea man, very good thinking
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Unread postAuthor: MrCrowley » Thu Aug 31, 2006 4:18 am

never thought of that, thats a good idea though hmm i can just picture it.... :shock:
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Unread postAuthor: dongfang » Thu Aug 31, 2006 9:07 am

Further thoughts on the subject:

One could also consider two alternatives:

1) Injection instead of gas carburettor. The gaseous fuel enters the combustion chamber through a separate valve, synced with the air valve.

Pro: No fuel tank in air chamber.
Con: Mixing may become worse

2) Real carburettor, liquid fuel
A simple venturi carb is made: A small metal pipe is run through the wall of the air pipe, either right before or right after the valve (after may be easier). The pipe is cut off at a right angle, and it ends in the center of the air pipe. The other end of the pipe goes into a small tank with liquid fuel, e. g. ether or ethylene.
When the air valve is opened, the airstream passing the end of the venturi pipe will suck a out lilttle fuel out (like an air paint spray gun).
Maybe one of you guys with a pneumatic cannon can try install a venturi, and experiment with finding the right size of pipe: Use some favorite fuel, but don't ignite it. Instead, stick the other end of the venturi in a small measuring cup (0.1 millilitre precision) and check how much fuel is consumed for each liter of air. Experiment with different venturi sizes till a good mix is obtained. Publish result, with all relevant data such as fuel type, venturi length and ID/OD, diameter of pipe where venturi is placed, tank size and pressure, valve type and port size and fuel takeup per shot.

Pro: Mechnically really simple; simple to fuel, fuel dirt cheap. Fuel spills in chamber will probably just get ejected.
Cons: May need some experimentation before it runs smoothly.

If the venturi is on the air tank side of the valve, obviously its tank's vent line will have to go back to the air tank, not out in open air. On the other hand, there is no countereffect of the venturi effect, because the pressure at both ends of the venturi is about the same.

As for the type of air valve: Obviopusly there will be som problems vith having a combustion chamber on a coax gun. At least, it will be really service unfriendly. Guess a (supah or not) piston valve is the thing to use for a Potato Jet (R) (tm).

The carb should probably be positioned before any street elbow between valve and combustion chamber. At least, that will give any turbulence in the bend a chance to improve fuel/air mixing.

To improve ignition reliability, if problematic: One could have a small pre-charge of butane in the combustion chamber, and ignite that in synch with opening the air valve (an adjustable sequencer will have to be designed; air valve is opened electrically, ignition is the electronic stun gun / flyback type). Maybe some half-baffles in the combustion chamber could improve ignition reliability further.

Sören
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Unread postAuthor: schmanman » Thu Aug 31, 2006 9:23 am

use a carb off an old 70's dirt bike or Briggs and stratton or something with a gravity fed fuel tank. easier, just bolt it on, no tuning needed. put it in the middle of the PVC line leading from the air tank to the combustion chamber, so the air will mix with the fuel when you open the valve .rig up a servo to open the throttle all the way when you open the solenoid, and hit the ignition.
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Unread postAuthor: dongfang » Thu Aug 31, 2006 9:39 am

I guess that during the few milliseconds when the cannon fires, its fuel consumption will be more like a dragster than a lawnmower??

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Unread postAuthor: schmanman » Thu Aug 31, 2006 9:58 am

It was just an idea. you could adjust it to let out large ammonts of fuel in a short period of time.a dirtbike or motorcycle carb could do the job.
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Unread postAuthor: dongfang » Thu Aug 31, 2006 10:03 am

OK, I have no experience with those, but they might just work. Could be really easy that way.

I guess the throttle will come flying out of the barrel if not fully open :P (why not remove it altogether?)

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Unread postAuthor: schmanman » Thu Aug 31, 2006 10:58 am

removing the throttle would work too. It would make It really simple. carburetors are fully tunable, so I have no doubt It could be adjusted to work. you can pick up any old carb from a motorcycle or dirt bike for not much from a motorcycle savage yard.
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