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Air gun, high pressure pumps.

A place to ask general spud cannon related questions.
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Unread postAuthor: Novacastrian » Mon Dec 17, 2007 7:21 pm

DYI, can you explain to me why one cannot exceed 500m/s with room temp air? Is that the fastest you can get air to flow or what?
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Unread postAuthor: chartreusesnot » Mon Dec 17, 2007 8:20 pm

I did manage to find a fitting on mcmaster that would fit some of the pumps, one reason for using a small chamber is that a large chamber, to take advantage of the gas, would need a humongous barrel, which is impractical, so probably sticking with a small chamber, smallish barrel is good. Also, how could one make piston for a gun of this pressure, all the weight on the neoprene seal would just cut it right? might a better valve be a pneumatic ram ball valve? albeit an extremely fast pneumatic ram...
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Unread postAuthor: DYI » Mon Dec 17, 2007 9:59 pm

DYI, can you explain to me why one cannot exceed 500m/s with room temp air? Is that the fastest you can get air to flow or what?


I don't really know for sure. All gases won't expand much faster than their own SOS, and even that requires rather ideal conditions, like very fast valves. That's the whole reason that I'm trying to avoid the entire problem by not even surpassing the SOS in my propellant gas. I'm sure Rag could give you a very satisfactory and suitably longwinded answer :D .

@chartreusesnot: You can certainly build a piston valve to withstand 3kpsi +, you just can't use the same sealing faces and construction techniques as the more common low pressure piston valves.

I've done quite a bit of materials research for my 2.4kpsi design, and have come to the conclusion that, assuming a finite budget and access to large suppliers like McMaster-Carr, one of the most effective piston designs would be a mainly polycarbonate construction (so that any internal flaws would be visible), with a thick (.5"+) PTFE sealing face machined EXTREMELY flat on the sealing face. The bumper would be any medium hard rubber, at least .5" thick. The barrel face inside the chamber would obviously have to be extremely flat as well. The actual piston (not bumper) would need to be machined to tolerances of .001" or better for optimum performance, and avoidance of substantial gas leakage out the barrel. As long as the filling system was robust enough, hardly any gas could escape out the barrel before sealing.

Please note that my design hasn't been tested yet, so it may not work perfectly, but everything that I and Jack know indicates that it should.

Also note that there are inherent dangers associated with anything homemade at pressures like this. A damaged or deffective sealing face could result in extremely fast and expensive gas leakage. Lack of a proper bumper would eventually result in piston deformation, no matter the piston material. Gases vented from the pilot have high enough energy to cause severe skin damage, and possibly internal damage. Good luck with a high pressure design, and please don't use a ball valve.
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Unread postAuthor: Lftndbt » Tue Dec 18, 2007 12:48 am

Maniac wrote:ohh sorry I had never heard of that before i just thought he meant soda bottle. My bad.


That's cool...
For someone with 48 posts... I would probably assume they ment the same... :)
I was lucky enough to score. The 1.1kg Sodastream bottle from a customer at work...
Just about to rip that annouying double valve system out....
Double valve's suck...
I'm use to filling "saftey" gas bottle's... But the filling method to bypass the lock off valve in the sodastream is annouying at best...
Unfortunatley the double valve system in it's neck appears to have been epoxy'd into the threaded canister.... Might just take it to an engineer to get a standard valve fitted to it...

How good was the flame deflection.... ROFL "your pretty dumb"
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Unread postAuthor: Novacastrian » Tue Dec 18, 2007 6:38 pm

Question: Why can't one exceed 500m/s with compressed air?

Answer as offered by Ragnarok
Well that's not 100% true, but it's close to it.
However, I shall explain why.

Firstly, it is very hard to break the sound barrier in a gas at all. Any imperfections in the barrel wall or valve will create shock waves in the air that will slow air currents through it to subsonic levels.

However, the real problem comes from the speed of the particles in the gas. It is impossible for the gas particles to exert a force on the projectile if the projectiles speed is higher than the gas particle speed. Imagine trying to push a shopping trolley that's moving faster than you are.

In room temperature air, the average speed of a particle is 500 m/s. However, that is the average speed. Some will be faster, some will be slower. Still, for an estimation, assuming them all to have a 500 m/s speed is a good start. As we don't want to get into standard distributions, we draw the line at 500 m/s, even in a frictionless barrel with an evacuated bore.

You'll have extreme difficulty getting above 340 m/s with air in the first place anyway, so I wouldn't get too hung up on the 500 m/s limit.
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Unread postAuthor: DYI » Tue Dec 18, 2007 6:56 pm

Well, that's a great answer. I knew it was something to do with the particle speed, but I couldn't find any info on it. Most of us don't have the means to achieve a frictionless bore and valve, as well as an evacuated bore, so most homemade air powered launchers won't exceed Mach 1.
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Unread postAuthor: jackssmirkingrevenge » Tue Dec 18, 2007 10:43 pm

DYI wrote:Most of us don't have the means to achieve a frictionless bore and valve, as well as an evacuated bore, so most homemade air powered launchers won't exceed Mach 1.


PCP airguns like the AirForce Condor comfortably exceed Mach 1 using a hammer valve, they do so by brute force with several thousand psi in the chamber - therefore if your fittings are strong enough, it's not as impossible as might first seem. With my 6mm burst disk I got very high subsonic speeds at just 400 psi, with less than double that I have no doubt it would have been supersonic.
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