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another GGdt question

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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another GGdt question

Unread postAuthor: iknowmy3tables » Wed Nov 28, 2007 4:58 pm

I finally installed ggdt, because recently my dad didn't log out his xp account and I gave my self administrator privileges, he never has time to get stuff done on this computer

anyways I found that the outer diameter of my "reservoir" really affected my preformence, why is the OD of my chamber so important for the projectile preformence?
and how can I generally guess my "flow coef"?
and I keep getting barrel choking flow in my warnings


I'm trying to build a really big cannon with my 6" pipe to beat my friend who got 600fps with an apple in a 2" barrel at 80psi, his gun is about 10ft long with a 4" chammber that wraps around in a double chambered style. its embarrassing to me because every single fitting is dwv the schrader is ripped from a tire tube and it uses a pvc ball valve (but it's surprisingly easy to turn)
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Unread postAuthor: Maniac » Wed Nov 28, 2007 5:01 pm

well to beat your friend all you have to do is wait till he blows him self up with that dwv
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Unread postAuthor: f.c » Wed Nov 28, 2007 5:09 pm

just build a large sprinkler valve cannon and you would win easily.
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Unread postAuthor: iknowmy3tables » Wed Nov 28, 2007 5:23 pm

not really that 2" valve opens like a 1/2" the flow benefit of his ball valve is huge when you got the 20 ft 4" chamber (the 180 bend makes it 10ft)
I tried my hand held sprinkler valve gun it's only 300fps :x
I really need help with this ggdt stuff so I can beat him I'm aiming for a 800fps muzzle velocity (200fps greater than his)
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Unread postAuthor: jackssmirkingrevenge » Wed Nov 28, 2007 5:28 pm

anyways I found that the outer diameter of my "reservoir" really affected my preformence, why is the OD of my chamber so important for the projectile preformence?


If your chamber is 4" in diameter, then od should be 4 and id should be zero.

if you're making a 2" barrelled coaxial in a 4" chamber, then od should be 4 and id should be 2
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Unread postAuthor: iknowmy3tables » Wed Nov 28, 2007 5:42 pm

ohhhh its for coaxlies thank you jack
now my results are looking much better?
but I still have no idea how to guess the flow coeff
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Unread postAuthor: DYI » Wed Nov 28, 2007 6:47 pm

What mass does the projectile need to have? For 800fps, high pressure may be better than ridiculous volume. If you can get a 20ft section of pipe, 800 fps would be no problem. If you are limited to 12' of barrel, you'll need a better valve.

If this is only for the purpose of beating your friend, build a triggered burst disk cannon. With 4 feet of 6" chamber, a 2" porting burst disk valve, a 20ft 2" barrel, 150 psi, and a 100 gram projectile, 800 fps should be obtainable. If you upped the porting and barrel diameter to 2.5", you would only need 120 psi. 800 fps will require nearly 2x as much energy as 600 fps. If the apple is, say, 200 grams, you may need to ditch the PVC and go with steel pipe and high pressure. With a 3" diameter 24" long steel chamber, the same valve and barrel, and 500 psi nitrogen, you could get well over 800 fps with a 200 gram apple.

If you are limited to lower pressure and weak materials like PVC, I would suggest outdoing him by, instead of shooting and apple at 800 fps, shooting a full rutabaga at 600 fps. With a 1 pound rutabaga, and the PVC setup with 4" burst disk (*BOOM*... ahhh!!!! my eardrums!!!!!), you could get a 1 pound projectile to >600 fps at 120 psi.

Either way, for the honor of spudfiles, don't let him outdo you. This is most of the reason that I have the SCTBDC - no one in my town (let alone all of Canada :lol: ) can say that their homemade air cannon is more powerful than mine.[/b]
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Unread postAuthor: Fnord » Wed Nov 28, 2007 7:10 pm

If the apple is, say, 200 grams, you may need to ditch the PVC and go with steel pipe and high pressure.


If you're going to do that, why not throw in a spark gap and make it a hybrid? (less pumping, big bang, not limited by SOS)

6 oz@700 fps = 3000 ft-lbs = BIG rifle territory
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Unread postAuthor: DYI » Wed Nov 28, 2007 7:56 pm

Or, you could take the easy route: use SCH 80 steel from Mcmaster, and unregulated helium (might be a b*tch to seal) Certainly simpler than a hybrid though, if not particularly safer...
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Unread postAuthor: iknowmy3tables » Wed Nov 28, 2007 10:33 pm

well I want to use a 2" barrel and a potato to be on the same level with him, the only tools I have for high pressure stuff is Teflon tape for prethreaded and soder flux and blowtorches for copper, burst disk cons might not be as impressive to many people who still may think bigdwvballvalvegun is better, for a big low pressure cannon I have 6" pipe and an air source, plus a upcoming trip to a quality Lowes store and budget of about $150
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Unread postAuthor: EGOed » Wed Nov 28, 2007 11:27 pm

You could manifold ( 4 ) 1" modded sprinkler valves in parrallel and easily exceed the flow rate and performance of the 2" ball valve.
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Unread postAuthor: iknowmy3tables » Thu Nov 29, 2007 5:24 pm

!!4!! modded sprinkler valves, thats $80!!! minus the hose and adapters vs an $18 ball valve I'd like to avoid that I'm already doing mine more expensive by making it from safe and proper sch 40 fittings, I'm defiantly making a piston valve
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Unread postAuthor: EGOed » Fri Nov 30, 2007 3:22 pm

LOL...I forget sometimes guys are on a budget , I never let money get in the way of my projects when I am determined to get something done.....Yes your best option is a piston valve no doubt.....But the most critical part of any cannon is the valve so dont cut corners here and spend a little extra if you have to , to make it durable and easily serviced ......good luck with your cannon , I hope you beat that ball valve cannon by 200fps..
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