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Propane Combustion and Tank sizes

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Unread postAuthor: jagerbond » Wed Nov 25, 2009 4:12 am

Welcome to spudfiles. This is a theory that we've been toying around with on the back burner from time to time. How about something like a 2 stroke type reciprocating assembly fed with venturi and connected to a BB gun springer style plunger set up.
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Sureshot Inc. / http://www.ultimatespudgun.com
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Unread postAuthor: broken_system » Wed Nov 25, 2009 10:54 am

Definetly. I was thinking about it though, I would have to test this (maybe it has been tested) but lets just say I went with a vent on the back that was kept sealed by the combustion, then opens with a spring afterwards (i.e no pressure). The expanding gas should still be able to launch the projectile whilst maintaining the sealed tube, then only after no more pressure is needed would it allow the vacuum to open the valve and suck in some fresh air.

The way I've been trying to picture it is kind of like sucking a drink through a straw, if it's completely sealed it's harder to draw the liquid out of the straw, however if you have some breathing room its easy. I guess where I am going with this is maybe by allowing some air to enter near the end of the time the projectile exits the barrel, it would be beneficial?

I have definitely concidered what you were suggesting jagerbond, it would probably be the most reliable way to ensure a complete ventilation of the combustion chamber, just have to find a way to make sure it seals each time to prevent malfunction, especially at a high rate of fire.
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Unread postAuthor: jimmy101 » Wed Nov 25, 2009 11:50 am

Like tech said. Nice theory and perhaps fun to fiddle with but probably not going to work.

Pulse jets and the like work this way but they operate at fairly low maximum pressures, which means the valves can be very light weight. Indeed in some there is no solid valve, it's just a plug of air.

In a spud gun though you have much higher peak pressures and would need a much sturdier, hence massive, valve. But a massive valve is a slow valve and you have very little time to get that valve open. The air being sucked back into the gun via the muzzle will be moving at nearly the SOS (at whatever the temperature the suck back air is).
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