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Piston cannon

Post about things you have launched or thought about launching. Also post about various materials used for building cannons. No posts about explosive projectiles!
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Piston cannon

Unread postAuthor: piperchris » Tue Mar 06, 2007 11:22 pm

I'm building my first pneumatic launcher after several combustion launchers. I have two questions:
What is the recommended chamber to barrel ratio for a pneumatic launcher? I have a 10ft x 3" barrel what should the length of the chamber be if it is 4" PVC?

Second, I'm going to be making a "Barrel Sealing Tee Valve":
http://forums.spudtech.com/topic.asp?TO ... hichpage=1

but I'm unsure what to use as a piston. Any ideas? I'd like something fairly common and not too exotic. It will be made inside a 4" tee.

Any tips or advice would be greatly appreciated.

Christopher
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Unread postAuthor: Velocity » Tue Mar 06, 2007 11:24 pm

Inside a 4" tee? Ok. Make it a 3" porting piston valve then. A good piston (which fits perfectly inside of 4" SCH 40 PVC pipe is 3" fittings, such as a 3" endcap
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Unread postAuthor: Pete Zaria » Tue Mar 06, 2007 11:29 pm

@piperchris

Welcome to the forums.

Aim for at least a 1:1 ratio (by volume), 1.5:1 is better, 2:1 is best. Anything much over that is approaching the point of diminishing returns. I know at 4:1 you're wasting performance, because the barrel isn't long enough to harness the energy being released.

Handy volume calculator: http://www.online-calculators.co.uk/vol ... volume.php

As to piston materials, search around this forum and spudtech and see what other people have used in large cannons. Many people make a piston from hotglue (using a mold...), or PVC/lightweight wood with sheet rubber on the front for a sealing face. If you have access to a lathe, your work is quite cut out for you. Also check this out: http://www.spudfiles.com/forums/viewtop ... c&start=0&

Hope that helps.

Peace,
Pete Zaria.
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Unread postAuthor: piperchris » Tue Mar 06, 2007 11:41 pm

Wow, those were some quick replies!!

rmich732:
I was thinking of using a 4" tee but is there any reason I shouldn't? It's bigger so it should be less of a bottle neck (I would think) and it should require less fittings/adapters than a 3" tee.

Pete Zaria:
I have a 10ft x 4" inch piece of pipe I'm going to use for the chamber. According to my rough calculations the chamber should be about 10 ft long for a ~ 2:1 ratio. Is that correct?

Thanks again!

I'll make sure to keep everyone updated on my project.
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Unread postAuthor: Pete Zaria » Tue Mar 06, 2007 11:54 pm

@Piperchris:

No. It should be 1.5-2x the VOLUME of the chamber size.

10ft x 4" chamber = ~1508 cubic inches.

You want your chamber to be 1.5-2x of the volume of your barrel.

If you wanted to use 2" pipe for the barrel, you'd need a 2" x 20ft (754ci) barrel for a 2:1 ratio, or a 2" x 26ft (1005ci) barrel for a 1.5:1 ratio.

If you wanted to use a 3" pipe, a 2:1 ratio would require a 3" x 106" (8.8ft) barrel. A 1.5:1 ratio would require a 3" x 143" (11.9ft) barrel.

Getting the idea? The ratios are by volume, not by feet. A 10 ft x 4" chamber is 1508 cubic feet. For a 1.5:1 ratio you want a ~1000ci barrel, for a 2:1 ratio you want a ~750ci barrel. You can achieve this with different lengths of different sizes of pipe, as I showed above.

Volume calculator: http://www.online-calculators.co.uk/vol ... volume.php

Peace,
Pete Zaria.

PS: Use the GGDT (Gas Gun Design Tool) and plug in the numbers. Run simulations and see what provides the best results. Get it here: http://www.thehalls-in-bfe.com/GGDT/
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