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Interesting valve

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: D_Hall » Fri Mar 06, 2009 1:49 pm

jackssmirkingrevenge wrote:What about the lack of air resistance in the barrel as the projectile accelerates?

Large scale laboratory guns with evacuated barrels are something I *do* have experience with. Well... I have experience with at least one such gun.

Basic info on said gun....

Bore: 40 mm
Barrel: 25' or so (don't recall exactly)
Muzzle Velocity: Mach 6+

Basically, we have pre-set loads that give us certain muzzle velocities. When we lose vacuum in the barrel, our velocities drop about (from memory) 5%.

But what's interesting is that if you look at the numbers, you find that the gun's performance is VERY constant.....

...If you look not at muzzle velocity, but rather muzzle energy.

You just need to account for the air mass as well. In other words...

ProjMass * GoodVel^2 = (ProjMass + AirMass) * BadVel^2

That relationship fits our test data almost perfectly. So, at least for high speed guns, it's very easy to predict the performance boost (or not) of pulling a vacuum on the barrel... Just calculate the mass of the air in the barrel and compare it to the projectile mass. If the air mass is negligable by comparison, then so too will be the affect of vacuum in the barrel. If the air is significant... Well, so too will be the affect of vacuum.
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Unread postAuthor: POLAND_SPUD » Fri Mar 06, 2009 2:16 pm

Also, I'm not one to push valves much (if at all) beyond their ratings. This one is rated at 125. I won't go past 150 under any circumstances.
I suppose it can handle safely a little bit more but I understand your concern - the shape of the valve is not cylindrical and it's rather big... so there would be a lot of force acting on the bolts at 300 psi...

anyway it should be a blast even at 100 psi
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Unread postAuthor: daberno123 » Fri Mar 06, 2009 6:47 pm

I have the same valve, its on a (unposted) gun that's pretty standard with a 2" barrel and 2'x3" chamber. I forget the actual muzzle velocity of it, but it's pretty powerful at only 100 psi.

Where did you get yours from? I got one of ebay a while back. So did sjog, liki, dewey-1 and unisonmind if I remember correctly. It was quite cheap at around $15 I think.

I've been throwing around the idea in my head to mod the valve and make it a piston. I could seal the equalization holes and make an air spring to hold the diaphragm closed while filling with the mixture (~3x) through the chamber. The only thing I'm afraid of is the diaphragm being damaged from the heat (and pressure). Do you think it's feasible?

Well best of luck on your gun, I hope it turns out well.
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Unread postAuthor: Fnord » Fri Mar 06, 2009 7:08 pm

I've used my asco valve at 180psi without any problems. They seem pretty well-designed.

I also have an unposted cannon for golf balls. 48" barrel, 14 oz propane tank for chamber. The whole thing weighs 6.5 lbs and shoots golf balls at around 430 fps @ 130. (my combustion out-performs it with the same barrel)

Jimmy:
The density of air is only about 1.3mg/L.


Grams? I don't think it's 1.3 milligrams per liter.

Anyway a 6ft golf ball barrel will contain 3-4 grams of air, so at most you might get a 10% increase in energy, though that is unlikely.
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Last edited by Fnord on Fri Mar 06, 2009 7:10 pm, edited 1 time in total.
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Unread postAuthor: daxspudder » Fri Mar 06, 2009 7:10 pm

I just happen to know a fire control technician master chief (USN) who built a hybrid with a similar valve... the chamber has a overpressure relief, which when it lifts(on combustion...excess of 380psi since its for a 3 stage/350psi compressor) the air it releases compresses a pneumatic trigger which vents the 1/8" opening. He uses either mapp or propane with 350psi air, but the most he told me he has ever fired it (as a hybid) with is with 200psi, and that the relief wont lift on combustion with a base pressure of less than 90psi, for which he has a T with a ball valve and a compressor fitting which he uses to fire the pneumatic trigger, or just to fire compressed air. Someone here should build one...
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Unread postAuthor: Fnord » Fri Mar 06, 2009 7:14 pm

^Could you direct him to this site please? :D
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Unread postAuthor: daxspudder » Fri Mar 06, 2009 7:39 pm

tried... He doesnt really care to share, I might try to get a pic or some video and post it here...
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Unread postAuthor: sgort87 » Fri Mar 06, 2009 7:50 pm

Hey! I got one of those! It works amazingly! Not quite as good as a piston valve, but it's one hell of a valve and in a nice tight package.

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Unread postAuthor: Ragnarok » Fri Mar 06, 2009 8:27 pm

_Fnord wrote:Grams? I don't think it's 1.3 milligrams per liter.

You're right, air is about 1.3 kg per cubic metre, and hence 1.3 grams per litre.
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Does that thing kinda look like a big cat to you?
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