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evacuated barrel

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: SpudFarm » Sat Jun 14, 2008 6:01 pm

it would have been funny to try that on larda and chrony the increes.

mabe it gets hyper sonic.

think of all the air a golfball has to push on front of it. if we knew how much it weighted we could have added it to a round and chronyed it.
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Unread postAuthor: D_Hall » Sat Jun 14, 2008 11:54 pm

As one who's actual honest to goodness experience using research guns with evacuated barrels(*).....

In the subsonic and low supersonics, it's not worth your time. Just use a larger chamber and/or higher pressures/mixes.

For high supersonic and hypersonics, it's actually pretty impressive how easy the effects are to calculate in advance.

1) Measure you're muzzle velocity with air in the barrel.
2) Estimate the mass of the air in the barrel ahead of the projectile.
3) Add the air mass to the projectile mass and calculate the muzzle energy of the air+projectile mass assuming the measured muzzle velocity.
4) Now, assume that you have no air in the barrel and using the muzzle energy from step 3, calculate a corresponding muzzle velocity using ONLY the projectile mass.

That little algorithm is butt simple... and stunningly accurate.

(*) The gun I've played with is capable of 9,000 fps.
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Unread postAuthor: jimmy101 » Mon Jun 16, 2008 5:19 pm


Doesn't that algorithm work reasonably well for for subsonic shots as well?

Lets see, 2"D 100g spud, 2"x6' barrel, barrel volume = 226ci = 3.7L
Assume air is 1.3g/L, the mass of the air in the barrel is 4.8g. Roughly 5% the mass of the spud.

v<sub>vac</sub>/v<sub>air</sub> = SQRT[(m<sub>air</sub>+m<sub>spud</sub>)/m<sub>spud</sub>] = SQRT(104.8/100) = 1.024

So an evacuated barrel would increase the muzzle velocity by ~2.4%. Hardly worth the effort and possibly less than the shot to shot variability.
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Unread postAuthor: psycix » Tue Jun 17, 2008 2:03 pm

jimmy101 wrote:So an evacuated barrel would increase the muzzle velocity by ~2.4%.

Hmm that is quite dissappointing I think. Though more effective on lighter projectiles and longer barrels, I hoped it became more effective the closer to the sound barrier. Which still might be the case, because the effect probably drops off when being subsonic.

jimmy101 wrote:Hardly worth the effort and possibly less than the shot to shot variability.

Yes, but what if we would use a lighter projectile or a longer barrel?

Copy of your calculation, but then with double barrel length:
Lets see, 2"D 100g spud, 2"x12' barrel, barrel volume = 452ci = 7.4L
Assume air is 1.3g/L, the mass of the air in the barrel is 9.6g. Roughly 10% the mass of the spud.

vvac/vair = SQRT[(mair+mspud)/mspud] = SQRT(109.6/100) = 1.047
~4.7%. Thats significant if you ask me. Now we even didnt sabot that 100g 2"D spud in a 4"barrel. :D :wink:

And it could give you just that few extra m/s you need to break the sound barrier.
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