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Combustion vs Pnumatic

A place to ask general spud cannon related questions.
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Combustion vs Pnumatic

Unread postAuthor: joemama » Fri May 18, 2012 1:55 pm

I am trying to make a new cannon but cant decide between the two.

Which one will produce more power? pnu/comb?

I know there are many variables like valve type/ignition, If i was to make a combustion i would use a stunt gun ignition. If I was to make a phumatic i would maybe try to make my first piston valve (i have only used modded sprinklers)

i have my chamber volume worked out to about 185.7 In^3

My barrel length has not been worked out yet. But i am expecting a Diameter of 3 inches.

I know that this design could go either way, i just would like to get some input on why to pick one or the other.

I want to shoot actuall spuds out of this cannon.

Please give me your input

Thanks :)
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Unread postAuthor: DYI » Fri May 18, 2012 2:34 pm

Assuming that by saying "combustion" (as opposed to "hybrid") you're restricting yourself to atmospheric pre-ignition pressure, the pneumatic is guaranteed to win simply because you have imposed no such restriction on it. A 20000psi helium pneumatic will exceed the performance of any gaseous fuel mix starting from atmospheric pressure.

I'm curious as to why you haven't simply simulated both designs using GGDT and HGDT to compare performance. You'll get a lot more out of that than out of asking a "how long is a piece of string?" type question on the forum.
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Unread postAuthor: Fnord » Fri May 18, 2012 6:52 pm

This question is usually asked at least once a month. Look at the wiki pages for both types of cannon and decide which is best for your situation.

Combustions are more more simplistic and more portable in large bore sizes.

Pneumatics are best suited to small bore sizes unless you have a large compressor available.

For a pvc guns you're limited to under 150 psi. This means you won't see much power difference in a pneumatic /combustion of equal size. Combustions are arguably safer when using plastic chambers, since they are only effectively pressurized for an instant... dropping one won't cause it to turn into shrapnel.

You won't find many potatoes that actually fit in a 3" barrel, by the way.
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