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high speed (hope fully super sonic) PVC gun FAIL!!!! pics

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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high speed (hope fully super sonic) PVC gun FAIL!!!! pics

Unread postAuthor: jhalek90 » Tue Dec 08, 2009 9:10 pm

now, before i go any further... just let me state that i was not expecting this gun to reach super sonic speeds.. this was simply a low pressure prototype.

Basically it was a 99% air tight piston... direct ally AFTER the firing valve..

once the gun was fired... the piston would rush forewords... shoving the air in front of it into a smaller bore barrel, and thus hope fully compressing the air further.

EDIT:
paint diagram
Image


the piston fired perfectly at 20-45 psi. this thing did seem to provide a reliable way to "Shove" air into the smaller barrel.

Then.... after some care full testing... i took it to 120 psi.

This happened

Image
what you are looking at is the 1'' threaded adapter that screws into the sprinkler valve... the threads were blown right off. :shock:

Here is the pice that the piston slammed into....
Image
this piece is fine... :?:

can someone tell me why this happened??
Why did the part attached to the sprinkler valve break, and not the piston impact point???

I did not knotice any HUGE gains in performance before this thing exploded... but the thing that has me puzzled is that it was so loud that my ears are still ringing.

Did i actually succeed in "boosting" the pressure, and break the pvc that way??????
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Unread postAuthor: rp181 » Tue Dec 08, 2009 9:37 pm

I do not think you boosted the pressure an appreciable amount, google light gas gun, its the same concept but with solid fuels.
I think it broke due to your piston ramming it.
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Unread postAuthor: jhalek90 » Tue Dec 08, 2009 9:51 pm

This piston hit the OTHER piece. it started out near the piece that broke. that's what confuses me.

BTW< that design is kinda what i was going for.

on a side note.... the piston is made from a mid-hardness rubber..
it looks like a rubber cork. only... less chamferd
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Unread postAuthor: rp181 » Tue Dec 08, 2009 10:35 pm

perhaps it hit and pulled the other side, the connection it hit may be stronger.
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Unread postAuthor: kjjohn » Tue Dec 08, 2009 10:37 pm

What you built is a light gas gun, except without light gas, and with a much less powerful propellant. Could be perfected, but I would suggest using metal parts, and a gas such as helium in front of the piston.
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Unread postAuthor: psycix » Wed Dec 09, 2009 6:45 am

Due to the momentum of the piston, the pressure in front of it may spike up high. This requires a metal tube.
Don't fire without a projectile. The projectile will hold the gas in longer and make sure that the piston can bump onto the air, instead of slamming into the fitting.
So working with PVC was asking for trouble in the first case.

The fitting it slammed into held it, and the force was redirected along the pipe, and pulled on the threads.
Also, it may have been vibrating/bending, causing it to snap at the joint, as the maximum of the bending moment (leverage) is there.

Oh, for optimal performance, you should use a burst disk.
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Unread postAuthor: inonickname » Wed Dec 09, 2009 6:59 am

You did what is essentially dry firing a springer airgun. Springers rely on the heating pocket of air to slow the piston (and they aren't constructed of PVC either). Without the projectile to block the gas flow there is no bumper and the piston can collide with the end of the tube at projectile velocities. Bad.

I'd call it more a step up from a springer than comparing it to a light gas gun. A light gas gun will often use a (substantially higher energy propellant) than compressed low pressure air. A light gas gun will also use a rupture disk and usually pre pressurized hydrogen or helium in the cylinder.

I'm currently in the drawing stages of a 2-6mm bore light gas gun to fire very light projectiles at hopefully 600+ m/s. It may not be reachable with only 3000 psi driving the piston so it may need to be fired into a ram accelerator or similar. (oxy-hydrogen) Though firing the mixture may be difficult.

(currently designing a burst disk coupler, whether to go with a single set of threads or a plate retained by bolts).
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Unread postAuthor: jhalek90 » Wed Dec 09, 2009 7:00 pm

Do you think i was successful at all in boosting the pressure?
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Unread postAuthor: rp181 » Wed Dec 09, 2009 7:12 pm

negligible. I bet the energy lost in the piston is significant.
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Unread postAuthor: ramses » Wed Dec 09, 2009 7:23 pm

possibly. you most likely increased the speed of sound in the gas, causing the energy transfer to be more efficient. If you were going to repeat this experiment out of, say copper, you should use a burst disk immediately after the transition to the smaller barrel diameter. If this bursts, and will not burst at the pressure that you drive the piston with, you boosted the pressure.

The burst pressure of 1" sch40 PVC is around 1500PSI, so I would say that you broke the PVC via shock rather than overpressure. I have no idea why your cannon burst where it did.
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Unread postAuthor: Mr.Sandman » Wed Dec 09, 2009 7:32 pm

Well you did almost create btb silent airow system but i doubt you increased performance. I think i see what you were trying to do. When rag told you how spring piston airguns get past sos you were trying to create that same enviornmeant minus the spring. you probably should have made an air bumper of some sort. But what it really seems like is that the piston hit the barrel too hard and just broke it.
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Unread postAuthor: Ragnarok » Wed Dec 09, 2009 8:08 pm

Mr.Sandman wrote:When Rag told you how spring piston airguns get past sos you were trying to create that same environment minus the spring.

A perfectly viable option. It is after all how Light Gas Guns work.

You probably should have made an air bumper of some sort.

Definitely, really. It's bad practice to fire spring airguns without a projectile, because it can damage the piston or the end of the cylinder.

What's needed is a strong cylinder, a lightweight piston, and probably a burst disk to help build up an air cushion in front of the projectile.
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Unread postAuthor: Mr.Sandman » Wed Dec 09, 2009 8:20 pm

Ragnarok wrote:
Mr.Sandman wrote:When Rag told you how spring piston airguns get past sos you were trying to create that same environment minus the spring.

A perfectly viable option. It is after all how Light Gas Guns work.

You probably should have made an air bumper of some sort.

Definitely, really. It's bad practice to fire spring airguns without a projectile, because it can damage the piston or the end of the cylinder.

What's needed is a strong cylinder, a lightweight piston, and probably a burst disk to help build up an air cushion in front of the projectile.
Or you could use some sort of o-ring for the projectile to seal against to allow the buildup of pressure. At the very minimum a tight fitting projectile should have been used. Still though i dont think had it not failed that you would have shock heated the air to a suffecient point to increase fps. Maybe a reinforced barrel design and better planning and higher pressures would result in better performance.
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Unread postAuthor: DYI » Wed Dec 09, 2009 8:22 pm

For a two stage gun to be of any great utility, you're going to need a burst disk separating the pump tube from the barrel. Also, as you may have surmised, PVC is far from being a suitable construction material.

In short, I doubt that you gained anything other than a strange material failure from this experiment. For gains, you'll need a burst disk separating the barrel from the pump tube and a steel construction. McMaster has some nice forged steel fittings and seamless pipe which could be of use for a rudimentary two stage gas gun.
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Unread postAuthor: Mr.Sandman » Wed Dec 09, 2009 8:31 pm

DYI wrote:For a two stage gun to be of any great utility, you're going to need a burst disk separating the pump tube from the barrel.
Well it depends on what your definition of utility is. i dont think he is going for airsoft round through dime here :D Still though while a burst disk would be very effective, I dont think it would be absoluelty necessary to use at these pressures. Remember lgg's shoot at hundreds of kilometers a second he is going for 1125+ fps. On that note it would be terribly difficult to use higher pressures and still maintain durability.
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