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The Proper 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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The Proper C:B ratio

Unread postAuthor: The_Kangsta » Wed Feb 06, 2008 11:12 am

Im building a Pneumatic gun with a 1/2 inch barrel and a 3/4 inch stock. I know that combustion launchers have a C:B ratio, but I don't know what the C:B ratio would be on a pneumatic. Would it be ok to mount a 2 inch extra chamber onto the stock for a bigger air supply?
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Unread postAuthor: Fnord » Wed Feb 06, 2008 2:51 pm

Anything over a 4:1 C:B is a waste. From 0.5:1 to 2:1 is fine if you want to keep the chamber small.
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Unread postAuthor: psycix » Wed Feb 06, 2008 4:22 pm

C:B isnt as much important to pneumatics as it is to combustions
Larger C:B ratios are more powerful, but with deminishing returns as you go up.
As he already said: more then 4:1 is overkill.
Do something like from 1:1 up to 2.5:1
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Unread postAuthor: DYI » Wed Feb 06, 2008 4:48 pm

It depends. If you have a fixed barrel size, increasing the chamber volume will increase power (however, as _Fnord said, after about 4:1 you won't be gaining much). If you have a fixed barrel size, and assuming pressure of roughly 150 psi, increasing barrel length will increase performance until you hit 0.2:1 or so, although as Jimmy pointed out, a 0.4:1 ratio achieves just slightly less performance, and requires only half the barrel length.

So it all depends on what kind of design you're working on. If I had a dollar for every time I've provided an answer nearly identical to this to exactly the same question... :roll:
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Unread postAuthor: niglch » Wed Feb 06, 2008 4:51 pm

In my opinion, your chamber size will also depend on what you are using to fill your chamber. If you are planning on some kind of manual pump, go for a more efficient (lower) chamber to barrel ratio. If you have a compressor and aren't worried about having to do any manual labor, feel free to go higher to squeeze out some more performance. Generally, like the others have said, stay between .5:1- 2.5:1. GGDT is a good tool for finding a ratio that works for you.
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Unread postAuthor: The_Kangsta » Wed Feb 06, 2008 8:29 pm

Also, can u get multiple shots out of a sprinkler valve?
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Unread postAuthor: mark.f » Thu Feb 07, 2008 8:30 am

I think the most annoying thing about this question is its complexity to answer definitely... without a little more input from the questionee.

What are you attempting to do:

1.) Keep a constant barrel length, and adjust chamber size.
2.) Keep a constant chamber length, and adjust barrel size.
3.) Keep the overall length constant.

The answers are as follows:

1.) If you have a constant barrel length, adding more chamber volume will increase performance slightly, until you hit a point of decreased return. In other words, your graph of chamber volume vs. muzzle velocity asymptoically approaches a limit as chamber volume increases to infinity. Like _Fnord said, anything over 4:1 is a waste, because your performance increase at the point is minute, (derivative is lower). The larger the chamber, the higher the average acceleration over the entire barrel, or the higher the instantaneous acceleration at a point compared to a smaller chamber.

2.) This question is answered already. Theoretically, your maximum barrel length depends on your chamber volume and pressure. You want the chamber volume to expand to the entire volume of the launcher, so you calculate that, or just use the 0.2:1 rule others have developed. This allows the projectile to be accelerating the entire barrel length, at an ever-decreasing rate, until it reaches the muzzle, then it coasts. This would be the most efficient launcher.

3.) To keep the total length constant, obey the ratio obtained above. Pretty much the same as number two. Or, you can solve a system of equations to get your chamber and barrel lengths, but it's much more simple to just follow the ratio above.
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