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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: JohnnyBOOM » Sat Jan 10, 2009 1:25 pm

jeepkahn wrote:And my question is how much does that affect acceleration???


Well, that would be a function of the speed of the projectile.

In a normal barrel, as the projectile speeds up, the air in front of the projectile compresses more and more until the point where the projectile is nearing the speed of sound. At the speed of sound, a shock wave will form in front of the projectile. This shock wave will exert a very high pressure on the projectile, opposing your chamber pressure. This is why it is very difficult to fire a projectile at speeds greater than the speed of sound.

In the case where the barrel has a vacuum, there is no air resistance ahead of the projectile, no matter what speed it is going. It is thus free to go as fast as your chamber pressure will allow, taking into account projectile mass, wall friction, and max fluid flow through you barrel diameter.

If you are looking for equations, I'm sure I (or someone else) can locate and post them. Isn't fluid flow fun! :D
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Unread postAuthor: ALIHISGREAT » Sat Jan 10, 2009 2:20 pm

Just a thought, when the projectile exits the vacume, would it make a shockwave and become unstable, potentially ruining accuracy?
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Unread postAuthor: jeepkahn » Sat Jan 10, 2009 2:24 pm

JohnnyBOOM wrote:
jeepkahn wrote:And my question is how much does that affect acceleration???


Well, that would be a function of the speed of the projectile.

In a normal barrel, as the projectile speeds up, the air in front of the projectile compresses more and more until the point where the projectile is nearing the speed of sound. At the speed of sound, a shock wave will form in front of the projectile. This shock wave will exert a very high pressure on the projectile, opposing your chamber pressure. This is why it is very difficult to fire a projectile at speeds greater than the speed of sound.

In the case where the barrel has a vacuum, there is no air resistance ahead of the projectile, no matter what speed it is going. It is thus free to go as fast as your chamber pressure will allow, taking into account projectile mass, wall friction, and max fluid flow through you barrel diameter.

If you are looking for equations, I'm sure I (or someone else) can locate and post them. Isn't fluid flow fun! :D


Equations are exactly what I'm looking for... I understand all the principles, but I'm not sure how to express them quantitively...

I'm not a rocket scientist, I'm just a Motorcycle dealership service manager with too much time on my hands through the winter.. :D

ALIHISGREAT wrote:
Just a thought, when the projectile exits the vacume, would it make a shockwave and become unstable, potentially ruining accuracy?


It most certainly creates a shockwave, not sure how it would effect stability and accuracy though, because at mach plus speeds there's a boatload of inertia...
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Last edited by jeepkahn on Mon Jan 12, 2009 12:36 pm, edited 1 time in total.

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Unread postAuthor: psycix » Sun Jan 11, 2009 5:22 pm

In a normal barrel, as the projectile speeds up, the air in front of the projectile compresses more and more until the point where the projectile is nearing the speed of sound. At the speed of sound, a shock wave will form in front of the projectile. This shock wave will exert a very high pressure on the projectile, opposing your chamber pressure. This is why it is very difficult to fire a projectile at speeds greater than the speed of sound.

If you see it that way, the vacuumed barrel would have a very big effect.
But you could also see the column of air as part of the projectile. The projectile gets a few grams heavier because of the air, but not much.
The gun becomes just a little more powerful because of the few weight loss and the +1 bar on the pressure.

Though I think that the greatest effect is the air having friction with the barrel wall. Air doesnt like to go mach 1 or greater relative to its environment. The air in the center of the barrel can go beyond mach 1 without much trouble.
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Unread postAuthor: JohnnyBOOM » Sun Jan 11, 2009 6:50 pm

psycix wrote:Though I think that the greatest effect is the air having friction with the barrel wall. Air doesnt like to go mach 1 or greater relative to its environment. The air in the center of the barrel can go beyond mach 1 without much trouble.


That's not really true. Ask your self what a shock wave is and how it relates to the speed of sound. Since sound moves through air at a predictable speed given local temperatures, pressures, humidity, etc, as the projectile starts moving closer and closer to the speed of sound, the air ahead of the projectile compresses more and more. At Mach 1, the wave of sound (pressure) can't move faster that the projectile so the air "piles up" in front of the projectile. Alternatively, you could say that the air ahead of the projectile can't be "informed" that there is a pressure difference coming down the barrel because the projectile is physically moving as fast as the wave of pressure propagates.
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