Supersonic Aluminum Cannon- Piston Valve Help

[vakimo]

Hi, this is a cannon im developing for a school project. Although its way overkill for the actual task, i couldnt turn down an opportunity to use the schools equipment to machine and weld this It uses 1000psi via equilizing a 2000psi oxygen tank wih the same volume of compressed air (and to the safety nazi's, yes everything is rated for twice that pressure ). Everything is machined and welded already except part of the rear for easy access. My problem is that i can not get the valve to seal properly and build up pressure. Im using a very tough rubber to withstand the 11000 pounds of pressure pushing it against the barrel, but it seems its not soft enough to make the initial seal. I thought this could be fixed by adding an aluminum cone or similiar shape with a rubber coating to the piston head (as shown below) in order to easily make an initial seal on the inside of the barrel, building up pressure until the heavy rubber seal could take the load.
I realize that this is rather insane pressures to work with, so i thought id check with the spud gun community before i go any further.
Any help or advice would be greatly appreciated

ps- im trying to break the sound barrier, as its been broken at only 800psi. If i can get it working, later modifications may include propane and oxygen mixtures of low psi combusting to produce roughly 1000 psi of very heated gas to increase speed... but ill save that for the hybrid section

[Gippeto]

Don't think yer gonna break mach with that as drawn and seemingly...built.

You may consider adding a seal to slow chamber filling...this will increase initial seating force...presuming you are filling from the pilot side.

If that fails to work...something is misaligned.

[vakimo]

Thats might help, as of now i have a 0.013" gap.
and yes, i am filling and releasing from from the hole on the pilot side.

As far as the design goes, what would prevent it from breaking mach?
Also, what type of pneumatic design is typically used for breaking it?

[saefroch]

The maximum expansion rate of the gas will limit performance. Almost any piston valve can propel something beyond the speed of sound, but it requires a very high-flow valve. Without numbers I can't be sure, but it doesn't seem like that valve can open to full flow...

[vakimo]

Although it may not look like it, i crunched the numbers and its designed to maintain the same flow rate of the 2.5" barrel. The cone attachment will probably affect this however. It might need to be much shorter to accommodate.

[jackssmirkingrevenge]

Have you tried "shocking" the piston into sealing by giving it a sudden burst of air?

Instead of a cone, an o-ringed cylinder that just enters the barrel would be a better idea, and surely within your construction capabilities.

[vakimo]

No i havent, but that might just do it.

I was reluctant to use an o-ring because i thought they might not be suited for sudden contact to an edge that it has to slide under.. that was just a thought though. do i really have anything to worry about there if i smooth out that edge some?

[ramses]

Have you tried "shocking" the piston into sealing by giving it a sudden burst of air?

Instead of a cone, an o-ringed cylinder that just enters the barrel would be a better idea, and surely within your construction capabilities.

I need to "shock" my 4" piston valve to get it to seal. It's annoying, but a compression spring behind the piston might make that unnecessary.

[velocity3x]

ps- im trying to break the sound barrier, as its been broken at only 800psi.

Actually.....700psi nitrogen and a golf ball.

[vakimo]

well shit... thats intense. i was planning on using a metal filled tenis ball.. cant wait for the results
ramses, thanks for the tip, if i can find one a spring would be great.

One important question though, how hard will that piston shoot back when its fired? theres only a quarter inch ring of rubber as a shock absorber right now... should i be concerned about structural damage??

[jackssmirkingrevenge]

One important question though, how hard will that piston shoot back when its fired?

More than you expect.

There's only a quarter inch ring of rubber as a shock absorber right now... should I be concerned about structural damage?

Yes, I would at least double the rubber thickness.

[Technician1002]

My 2 cents worth.

Move the barrel centering items a little further from the breech of the barrel to reduce the turbulance in the sonic flow. They do not need to be right at the breech.

Lap the valve together to get the initial seal to happen. They must mate with close tolerances.

[ramses]

My 2 cents worth.

Move the barrel centering items a little further from the breech of the barrel to reduce the turbulance in the sonic flow. They do not need to be right at the breech.

Lap the valve together to get the initial seal to happen. They must mate with close tolerances.

Yes and yes. Just put some fine sand paper on the pison instead or rubber, and spin it with a drill. This will fix nicks in the valve seat, as well as any misalignment from the barren not being cut square. Before you do this, make sure the face of your piston is perpendicular to the piston axis.

I would suggest closing the gap between the piston and its housing. Mine is a very close fit, but still needs a burst of air. This will help keep the piston aligned as well.

Try to get a welding helium tank if you can.

[Moonbogg]

Nice cannon. Those pressure make me nervous. Be careful.

[inonickname]

Surprised noone's mentioned how unsafe this is yet. You have HEAPS of oxygen in that chamber, which can very well lead to destruction of seals, oils, etc. and can be very undesirable. 1000 PSI with ~71% O2 could equate to in excess to the equivalent of a 200x hybrid mix if the circumstances are right (oil, lubricant, plastic, seals, temperature etc). That much oxygen in your chamber at that kind of pressure can be a HUGE hazard.

I'm surprised Tech didn't mention it, as I understand he's fairly well versed in the hazards of oxygen. I'm sure he can back me up on this.

Not only that, but having that much oxygen in your chamber is bad for velocity. If you want to reach mach the heaviest prominent gas you probably want in your chamber would be N2, or a very hot mix of H2O and CO2 (hybrid).