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self venting combustion

A place to ask general spud cannon related questions.
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Unread postAuthor: CannonCreator » Sat Aug 18, 2007 2:50 pm

Dude thats an awsome idea! the only thing is the air would have to be of a extremly high pressure to reach the spud in time before it leaves the barrel. But with some further reasearch, and testing, I think you have created a completely new kind of cannon.
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Unread postAuthor: meatballs » Mon Aug 20, 2007 7:16 pm

thats a cool idea, but im not sure it would work. in order for the piston to open, the pressure in the combustion chamber part would have to be higher than the pressure in the air chamber part. im not sure if these numbers are accurate, but just to illutrate lets say that the combustion chamber part reaches a peak pressure of 50 psi when fired. if the air chamber is filled to less than that much pressure, say 40 psi, when the combustion chamber pressure exceeds 40psi, hot burning gasses that would otherwise push on the potato will escape into the air chamber. and as soon as the pressure in the barrel/combustion chamber drops, the valve will close, the combustion chamber will not vent, and the pressure in the air chamber will have increased a little. without a pilot, a piston valve is essentially a check valve. if the air chamber is filled to over 50 psi then the prressure behind the piston will always be greater than the pressure in front of it and it will never open. the other risk is that as the valve opens, fuel could be blown into the air chamber along with the burning mixture around it, and then burn quickly with the new greater supply of oxygen, in which case pressure could build up in the air chamber creating what is essentially a bomb. so in short, i am highly sceptical about whether this would work, and it could be potentially dangerous (more so than potato cannons already are). you're counting on gasses flowing into the air chamber to open the valve, but as soon as that flow reverses the valve is just going to close again.

now one thing you could do is have the same idea only with a pilot valve. pressurize the air reservoir to higher pressure than the combustion chamber can reach, and shoot a potato. then load another potato in and open the pilot valve, getting a second shot and venting the chamber. you then have just a coaxial pneumatic that you have given metered propane and that you firre like a combustion every other shot, which would also be less powerful than the pneumatic shots because the pressure would have to be lower and the barrel would be shorter because the combustion style shots would use some barrel for a chamber, so it would make more sense to shoot as a pneumatic all the time, but still, it would vent itself...
and maybe im wrong and your idea would work... i'll keep thinking about it...
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Unread postAuthor: thespeedycicada » Wed Aug 22, 2007 9:51 pm

it might work keep the air pressure at only 20 to 40 psi and it might work just try it.
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Unread postAuthor: singularity » Wed Aug 22, 2007 10:08 pm

hey your back meatballs its been a while, hey theres an airsoft game Friday pm me if you want in (i got a good gun for you this time). anyway back on topic i guess if i had some very very sensitive pilot valve that would open and stay open when there was a minor change in pressure (possibly a very accurate burst disc?) it could work.
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upcoming projects... finalized clip fed BBMG and ball point pen sniper
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Unread postAuthor: meatballs » Mon Aug 27, 2007 11:57 pm

that would have potential, you could maybe even have some sort of pilot valve that is a pop off pressure relief valve, or a pilot that's piloted by one for higher flow through the main pilot valve, burst discs would work too.
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