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.1835 C:B Ratio Acceptable for Pneumatics?

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: Jabingo19 » Wed Jul 13, 2011 1:03 pm

Alright, so here are the plans for the back half of this cannon. Includes everything important I guess. The idea for changeable barrels is due to the 1.5 inch female adapter that will be sitting at the bottom of the 2.5 inch barrel, should allow anything 1.5 inch or smaller to get in (all i really wanted was my golf ball barrel to work as well)

Comments, suggestions, criticisms?

Thanks
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Unread postAuthor: Gun Freak » Wed Jul 13, 2011 1:09 pm

That looks pretty good! Not much room for flow but that comes with the low CB. I say it is fine.
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Unread postAuthor: noname » Wed Jul 13, 2011 1:25 pm

If you're planning on using different barrel sizes, you'll need a threaded coupler at the front of the cannon and threaded bushings for every barrel. Simple enough, although costly, but an important thing to take into consideration is that you'll have to carefully measure every barrel to make sure that one isn't too long or too short to accommodate your piston's travel.

Apart from that, everything looks fine.

Afterthought: Unless you're just going to have smaller barrels to fit inside the existing one? Odd, but whatever floats your boat; that's going to end up being like 4 coaxial layers of PVC at some points.

Oh, and you might want to stick a ball valve in between the pilot volume and your quick connect to prevent accidental firing.
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Unread postAuthor: Technician1002 » Wed Jul 13, 2011 1:26 pm

The narrow gap at the perimeter actually has considerable area. My Incognito golf ball bazooka and the Mouse Musket had very similar ratios of barrel vs chamber barrel gap. Only my later built QDV designs with a shorter fat chamber have more gap.

Mouse Musket breech is a PVC coaxial with a 1 inch barrel and a 2 inch chamber. On the breech of the barrel is a 1 inch coupler for holding the valve seat seal. This left that narrow gap between the barrel breech and the ID of the chamber. It still managed to toss 1 inch gumballs through a sheet of 1/2 inch plywood.
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Incognito has a 2.5 inch chamber and a 1.5 inch golfball barrel in the same configuration.

Muzzle of the cannon shows the dimensions. 2.5 inch pipe on the outside, reducer, 1.5 inch golf ball barrel. Gap between the barrel OD and chamber ID is relatively small. CB ratio is about 1:1.
Image

4 inch chamber with a 2.5 inch barrel will work fine. One of my early launchers used a 3 inch chamber and a 2 inch barrel as a coax. It worked very well.
The red one is my 3 inch chamber and 2 inch barrel coaxial.
It beat the spray and pray at only 40 PSI.
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Last edited by Technician1002 on Wed Jul 13, 2011 1:35 pm, edited 3 times in total.
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Unread postAuthor: Gun Freak » Wed Jul 13, 2011 1:27 pm

Oh yeah I missed that on the quick connect. Yes you will need a ball valve on there.
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Unread postAuthor: Jabingo19 » Wed Jul 13, 2011 1:36 pm

Yeah I missed the ball valve too, thanks for the catch. But yes Mr Vortex, I just plan on threading smaller barrels within the larger 2.5 inch barrel, not too elegant but it will work.

Also, thanks for the reassurances Technitian, I was beginning to worry a little that flow would be a problem.

But thanks for all of the positive words!

Also: I should be able to get most of the parts through my work's PVC supplier, but if I can't, is there anywhere you guys know of online I can get parts?

Thanks.
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Unread postAuthor: Mr.Tallahassee » Wed Jul 13, 2011 2:40 pm

I recalculated and found some mathematical errors. I'm not certain where your errors were. However, I got more likely results.

Actual chamber volume 103.67247
Actual Barrel volume 289.1480779716796875
~C:B ratio .359

I sat and recalculated several times with the formula V=(Pi)*r²*h and subtracting the barrel volume from the volume of the 3'' pipe. Math CAN be tricky. :thumbright:
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Unread postAuthor: Jabingo19 » Wed Jul 13, 2011 2:53 pm

You have to take into account that the OD of the 2.5 inch barrel is 2.875. But thanks for checking my math! So yeah you don't use 1.25 as the radius for the 2.5 inch, you use 1.4375. But yeah I actually did my math wrong anyways, since the chamber volume actually ended up at 27.69 cubic inches. Glad I moved to 4"!
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Unread postAuthor: Jabingo19 » Wed Jul 13, 2011 11:09 pm

Alright, I have everything constructed except for the back firing/filling area, my sprinkler valve is still in the mail. But besides that, everything seems to be working as planned. And yes Jack, I did end up needing barrel supports. I used 3 cut pieces of .5 inch PVC to center it on.
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