Atmosphere and volume - vacuum cannon limitations

  • Thread starter Limebat
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  • #1
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https://physics.csuchico.edu/~eayars/publications/AJP00961.pdf
http://www.phys.utk.edu/demoroom/MECH/The Vacuum Canon.pdf


So I get the outside atmosphere is what provides air density and such in the pdf above^.
However, why is it that most cannon demonstrations use just the air from the atmosphere? Is this to simplify demonstrations / save on costs?

Basically:

Would it be better to use a pressurized gas chamber around the broken seal area than rely on Earth's atmosphere? Or is my thinking incorrect? If it is better to use a pressurized chamber, then would there be an appreciable change to the formulas presented in the pdf provided?
 

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  • #2
sophiecentaur
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Would it be better to use a pressurized gas chamber
Definitely. That's why we tend to use chemical explosions for propelling bullets, rather than vacuum pumps. The Air Bazooka is really just a novelty. If you were to try to propel an artillery shell, you'd need a vast amount of air in a very long pipe as the performance depends on the amount of Energy stored in the 'vacuum'.
An air gun (spring operated) is legally required to have no more than 16J (12 ft lbs). That can be obtained by a modest size spring in a small compression chamber. The Cautionary Note in the first reference points out that the KE of the pingpong ball on exit is a lot higher than that - luckily it soon slows down as it has very poor aerodynamics!

A good fun project, though.
 
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Would it be better to use a pressurized gas chamber around the broken seal area than rely on Earth's atmosphere? Or is my thinking incorrect? If it is better to use a pressurized chamber, then would there be an appreciable change to the formulas presented in the pdf provided?
The maximum velocity depends on Pressure/density. A six feet vacuum canon seems to get close to that velocity already, if loaded with a single ping-pong ball. High pressure air won't help unless you can raise the pressure without raising the density by raising the temperature. Spring piston air guns do this by compressing the air, as do firearms of course. if you just have a high-pressure air reservoir, it likely will not help.
If you had a heavier projectile or a shorter barrel, using pressured air might make more sense.
 

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