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Air Chamber to Barrel 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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Air Chamber to Barrel Ratio?

Unread postAuthor: nubematic0 » Fri Oct 01, 2010 10:43 pm

I have everything I need to build a pneumatic launcher with a 2 inch wide chamber and a .5 inch wide barrel. I want to make it as light and wieldy as possible so I was wondering if anyone knows of an optimum air chamber volume to barrel size ratios. I am worried about making the chamber either over or undersized. I will post picks when it's finished in the next day or so, thanks.
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Unread postAuthor: jackssmirkingrevenge » Fri Oct 01, 2010 10:48 pm

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Unread postAuthor: BeaverRat » Fri Oct 01, 2010 11:50 pm

I normally design the cannon based on either the performance I want, or the barrel length. If you want it to be wieldy but still powerful, I would recommend a barrel length of roughly 40 inches. Now for the chamber, that depends entirely on what you want. At pressures of a bike pump or air compressor, bigger is almost allways better, but it comes with diminishing returns. Just remmember that if you are hand pumping, bigger takes longer/more effort to pump up to pressure. You will allways get more power by increasing pressure vs increasing chamber size if you are close to a 1:1 C:B ratio. Set the barrel length to what you want, and start plugging in numbers on GGDT.

Read this:

http://www.spudfiles.com/forums/still-d ... 21244.html
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Unread postAuthor: nubematic0 » Sat Oct 02, 2010 6:42 pm

Thanks a bunch, I can't believe there is a program for that too bad I have a mac and will have to use a friend's computer though.
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Unread postAuthor: whoa044 » Sat Oct 02, 2010 10:42 pm

It all depends on how small the projectile is, and how heavy it is.

An optimal barrel is probably more than 1:2, since optimal length is a lot further than 1:1.5, especially with hybrids.
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Unread postAuthor: Technician1002 » Sun Oct 03, 2010 12:43 am

If you look at the acceleration of projectiles in air cannons when you get GGDT up, you will find most of the speed is reached in the early part of the launch. A shorter barrel up to a point cuts performance a small amount. Try to keep the barrel longer than 3 feet for most air cannons. Larger chambers and larger diameter barrels at higher pressure do better with longer barrels.

Small barrels such as airsoft, do fine with short barrels due to the maximum flow a given pressure will provide.
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Unread postAuthor: jackssmirkingrevenge » Sun Oct 03, 2010 12:55 am

I made this back in 2001 when chamber size was made on a whim, it has a C:B ratio of 27,941:1 :roll: which worked, of course, but was not very efficient and a chore to pump. Today I know better, you can get much more power with a smaller chamber and higher pressure, I recommend reading through the thread BeaverRat linked to ;)
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Unread postAuthor: Easter Egg » Sat Oct 23, 2010 8:46 am

If you plan using it at 100psi or something like that, 1:1 will be fine. If you use higher pressures than that then you can go with a chamber that is smaller than the barrel. If you dont mind the extra pumping you can go with bigger chambers but i just have a bike pump...
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Unread postAuthor: Technician1002 » Sat Oct 23, 2010 10:44 am

The valve and it's ability for air flow plays a large part of the barrel size. This can be more important than the chamber size for larger size chambers.

This guy built a cannon with a 2 inch barrel and 17 gallon chamber. Using his measurements and mine, my ABS cannon out flows his cannon and has higher muzzle velocity. I was exceeding his range even though he used more than 100 PSI and I used less.

http://www.flickr.com/photos/crazybuilders/2846434642/

Hmm.. His video seems to be offline at the moment.
http://www.crazybuilders.com/item.php?id=000007&type=project_section

Edit, I located the page where he covers problems with flow restrictions. Look down the page to Flow Restrictions.
http://www.crazybuilders.com/item.php?id=000002&type=project_section
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