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Pneumatic Bow

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Pneumatic Bow

Unread postAuthor: daccel » Sun Apr 13, 2008 3:55 pm

I got inspiration for the design from Brian the Brain's post in this thread: http://www.spudfiles.com/forums/arrow-ammo-t13981.html. I forgot to search until after I drew it :roll:, so I didn't realize pneumatic bows had already been discussed to some extent in goose_man's thread here: http://www.spudfiles.com/forums/pneumatic-crossbow-t11599.html. After reading that I realized I could reverse the piston direction and use a pulley with wire instead of a solid carriage that would fly forward, but didn't feel like re-drawing. Thoughts on pro's and con's of each method?

Anyway, my idea here was to maintain a conventional form factor, and be able to shoot sabot'ed shortened arrows (or any sabot'ed object that fits the magazine) almost silently (always found the idea of a silent cannon enticing for some reason), and the only way I see to do that is to trap the propellant air and release it slowly.

It's mostly standard components - piston valves and bolt action breech. The differences include separating the tubes that accelerate the carriage and the barrel that contains the sabot, and having two of these tubes so that the magazine can come up between them. In the bottom tubes (don't know what to call them?) the rods have a larger section near the pistons which will seal in the o-rings during the last 6" or so of travel which will create a gas shock to slow and stop the carriage, because this should be quieter than a spring. Unfortunately I can't see a way to automate the carriage return. Also, the piston valve pilot air and piston tube vent air are released through expansion chamber/mufflers to minimize sound.

I'm not going to build this any time soon so if someone wants to, go for it. Then I can learn from your mistakes :lol:.
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Unread postAuthor: jrrdw » Sun Apr 13, 2008 4:11 pm

I think your air deliverly is way to small and long. Needs to be short and free from restricktion.
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Unread postAuthor: daccel » Sun Apr 13, 2008 4:40 pm

I'm not sure what part you're referring to? If you mean the two middle tubes, I was thinking they would be 1" dia., which is approximately equivalent to a single 1.4" dia. barrel, which I thought was reasonable for launching an arrow. You could generate the same amount of force with a larger diameter over a shorter distance, I just thought this layout had better aesthetic proportions than short and fat, and would allow you to wrap the four, then three tubes with tape or fiberglass to create a nice finished looking cannon. The chamber pictured here is about 3x6".
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Unread postAuthor: jrrdw » Sun Apr 13, 2008 5:11 pm

The black line running from the lower part to the upper rear behind the arrow. From your drawing that looks like the air path???
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Unread postAuthor: daccel » Sun Apr 13, 2008 6:07 pm

Sorry, my ms paint skills leave something to be desired. That's a solid structure basically four rods, two in the center tubes and two in between the three small tubes, and they're all attached at the front. As the air pushes the two pistons attached to the bottom two rods forward the whole thing moves forward, and pulls the sabot forward with it, as there's two slits in the bottom of the top barrel that allow the disc attached to the back of the two top rods to be pulled forward with it.

I don't think I've explained it very clearly.. does that make sense? Here's a picture of it 3/4 of the way through firing.
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Unread postAuthor: jrrdw » Sun Apr 13, 2008 6:20 pm

So the black rod pulls the yellow ammo out like a bow string pulls a arrow???
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Unread postAuthor: daccel » Sun Apr 13, 2008 6:27 pm

Yes. And then another possible way I think would be if you put the valve on the front and had the piston somehow tow a cable or line backwards, then had a pulley on the front to reverse the direction of pull and left the top basically the same. This would eliminate the rod flying forward which could get damaged or hit something, and also make the moving parts lighter so it wouldn't waste as much energy.
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Unread postAuthor: jrrdw » Sun Apr 13, 2008 6:40 pm

It also fires a object straight at you, bad idea no matter whats inbetween you and the object. Again, bad idea!!!
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Unread postAuthor: mopherman » Sun Apr 13, 2008 6:46 pm

jrrdw wrote:It also fires a object straight at you

so does pretty much every barrel sealing piston gun.
I think it would be better to have one barrel tube with a contained sabot.
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Unread postAuthor: STHORNE » Sun Apr 13, 2008 7:05 pm

looks like a nice idea... it would look pretty cool but performance is questionable.
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Unread postAuthor: jrrdw » Sun Apr 13, 2008 7:27 pm

mopherman wrote:
jrrdw wrote:It also fires a object straight at you

so does pretty much every barrel sealing piston gun.
I think it would be better to have one barrel tube with a contained sabot.


Yea but a piston has a short travel, his idea travels the length of the barrel. It will have time to accelerate.
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Unread postAuthor: daccel » Sun Apr 13, 2008 7:38 pm

I'm not sure if it's what you meant, mopherman, but having the rod inline with the sabot is probably the simplest and most efficient design, the more I think about it. This would double the length, but that can be fixed by just making the valve and barrel diameter larger. It can still use a magazine, it would just be shifted forward.

The only difference performance wise from the same chamber, valve and barrel is the energy wasted accelerating the rod and piston, and essentially cutting your barrel length in half. To make the rod and piston as light as possible my idea was to use an aluminum tube filled with an epoxy/microsphere mix and to make the piston out of the same epoxy mix.

Any thoughts on the gas shock that is created by the wider part of the rod as it seals in the o-rings?

Edit: added picture.
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