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Pneumatic Air Cannon C:B ratio?

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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Unread postAuthor: jimmy101 » Sat Feb 28, 2009 1:19 pm

inonickname wrote:Wrong, if you have enough of an acceleration force, and the balloon is a loose enough fit it will become deformed and possibly pop. It's like getting a water balloon, and squeezing it in your hands. It pops, doesn't it?

Therefore it would be good to make the balloon a good enough fit in the sabot so it can't expand further than the rubber can manage, but can still discard the sabot. There's where your challenge is.


Doesn't matter what the acceleration is, if the balloon doesn't touch the barrel, and if the sabot doesn't have any sharp edges, the balloon won't brake. You can accelerate a water balloon at any number of G's and it won't pop the balloon.

Making the sabot the same size as the ballon is trivial, just use a plastic cup.

The only time you have problem with G forces poping a water balloon is when the resting diameter of the ballon is a tiny fraction of the barrel diameter. If you put a 1.5"D ballon in a 2" barrel there is no way it'll pop in the barrel if properly saboted. There is virtually zero force on the ballon rubber regardless of the acceleration as long as you are not talking about something like 1"D ballon in a 12"D barrel.
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Unread postAuthor: Hotwired » Sat Feb 28, 2009 5:10 pm

inonickname wrote:
Hotwired wrote:As for the water balloons, the acceleration won't destroy them but the friction between the rubber and the barrel will tear them up if they're being pushed out with enough force. If you put a layer of material between the balloon and barrel you can use any pressure.

Alright, I suppose this would be a better way to put it:

Drag your knuckles along a piece of carpet slowly, gradually gaining speed.

Now drag your knuckles along carpet as fast as you can straight away.

The first one hurts (and damages) less doesn't it. That's closer to a ball valve. The second will hurt a lot more and that's closer to a piston valve. Simplest way I can put it.

But yes, a light lubricant or a wad or sabot will help the matter.

Possibly a useful analogy if you're describing the theory to a junior school class.

A further addition I'm going to suggest is that if you now do exactly the same with appropriate shielding over your knuckles, it won't hurt at all, however fast you drag them across some carpet.

So if we have the balloon in place of the knuckles and say a paper wrapping around it to separate it from the barrel wall, the balloon is not going to be damaged on launch.
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