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Maximum speeds

A place to ask general spud cannon related questions.
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Maximum speeds

Unread postAuthor: shockwave » Sun May 14, 2006 12:01 pm

Whats the maximum output velocity of a pneumatic powered by air (in theory, discarding all the other variables)? Allso what are the speeds for other gasses?
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Unread postAuthor: Pete Zaria » Sun May 14, 2006 4:01 pm

To quote boilingleadbath (the forum's resident math genius):

"And you basicaly got it; air is limmeted to the speed of sound, and (even with a perfect valve) the amount of force it could apply to the projectile trails off as it gets closer and closer to that speed - and thus you never quite reach it.

Note that because the speed of sound increases with gas temperature, and the propellent gas is heated by shocks and friction in the barrel, 'supersonic' - compared to the outside air, not that in the barrel - projectiles acctualy are possible with normal air.
It's just that the very slight degree by which they are supersonic isn't worth the extra hassle.

Incidentaly, the only gasses that can push a projectile past the speed of sound in air are hydrogen, helium, and possibly nitrogen.
CO2 acctualy is limmeted to providing mach .85"

Do some Google research on the other gasses.

Pete Zaria.
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Unread postAuthor: boilingleadbath » Sun May 14, 2006 5:16 pm

To expand appon my quoted words there, here's a formula you can use to calculate the speed of sound of the air. (this is for room temperature)

(max velocity)/1127 fps = (g/mol of your gas)/28.95

The only part which may be confusing to you guys is the "g/mol of your gas" bit. This is the number of grams a mole of your gas weighs; it can be found using the periodic table and the chemical formula of your gas. For instance:
CO<sub>2</sub> is 1 carbon at 12 grams per mole, and 2 oxygens at 16 g/mol. Adding it up, you get 44g/mol.
This isn't 100% accurate, but it's close enough for our purposes.
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