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Coaxial Combustion Cannon

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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Coaxial Combustion Cannon

Unread postAuthor: space_weazel » Mon Jan 02, 2012 10:58 pm

So some of you may have seen that I'm working on a new cannon, and I started to wonder if I could simplify the design a little by making a coaxial design combustion cannon, So I started running some numbers and here's the reults of some math in xcell.
Image
you can see in this that a 4" chamber with the barrel 1" off the back of the chamber and a chamber that is 4" in length and a barrel that is 1.5" in Diameter and 30" long gets close to a ratio of 0.7:1 ; which from the testing of 1.5" bore guns that 'jackssmirkingrevenge' posted over in my other thread, seems to yield maximum velocity.
However I could construct a more compact gun with potentially a slightly larger chamber and get close to 1:1 most anything in the green zone.

Thoughts?

Also has there been any testing on detonation doing a coaxial design in a combustion gun? If so can someone point me to the results?

Thanks
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Unread postAuthor: Technician1002 » Mon Jan 02, 2012 11:48 pm

One item often overlooked on combustion cannons is the distance from the gas mass to the nearest cold wall. Placing a coaxial pipe in the center of the chamber provides a short thermal path to a cool surface resulting in a more rapid cooling of the combustion gas. This will impact performance in addition to the CB ratio. Chamber surface area is best when it is a low value so combution heat and resulting pressure rise remain longer.

Sealed combustion chamber pressure testing has shown the thermal decay is quite rapid in a chamber after combustion.
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Unread postAuthor: space_weazel » Tue Jan 03, 2012 9:12 pm

So I would want a larger volume (i.e. more fuel) to compensate for the cooling effect?
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