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a quick question ! thanks

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Unread postAuthor: DYI » Sat Jul 19, 2008 1:43 pm

My dad tells me that, in regards to gaining power (in a pneumatic launcher): With a chamber diameter of, for example, 3", one would have to have a barrel diameter of 2" or so, otherwise very little energy would be imparted to the projectile. Is this true ?

And if this is true, would a conversion from 3" down to 2.5" be enough to achieve the aforementioned benefits?


A gas is not an incompressible fluid, so there aren't any pressure multiplication effects at work here. Your ball valve (which won't even be half way open by the time the projectile leaves the barrel) will choke flow flow on most barrels, unless you get it opening a lot faster than usual. The only time that chamber diameter is a problem is the exceedingly rare case of the valve having a higher effective porting diameter than the chamber does.

It is a widely believed myth that heavier projectiles require a larger volume of propellant gas to be effective. The reality of it is actually quite the opposite - for a given barrel length and pressure, a lighter projectile will actually need a larger chamber than a heavy one will. It has to do with the maintenance of high pressure required to keep drag and SOS limitations from slowing down the light projectile.

Also, I revised the quote slightly so that it doesn't look like you wrote it in Swahili, then translated it through Mandarin, Portuguese, and French with an online translator program before putting it back into English.
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Unread postAuthor: homedepotpro » Sat Jul 19, 2008 6:52 pm

i think it should be noted that diameter does matter. A larger diameter barrel is going to give more surface area from which the pressure can act on the projectile and there for more force. for instance a 1/2'' barrel at 100psi is going to have .5in x .5in x pi x 100lb/sq in = 78.5 lbs of force where as a 2'' barrel at 100psi will have 2in x 2in x pi x 100psi = 1200 lbs of force (this is all assuming pressure is kept constant). This is no sense mean a half inch barrel is unsuitable for spud guns its just going to offer less force.

Compromise is a sabot.
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Unread postAuthor: Ragnarok » Sat Jul 19, 2008 7:35 pm

@homedepotpro: You were a factor of 4 too large calculating the cross-sectional area of the barrel, because you used the diameter, not the radius.

The 1/2" barrel could put down ~19.5 lbs of force, the 2" barrel could give 314 lbs of force.
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Unread postAuthor: starman » Sat Jul 19, 2008 9:19 pm

homedepotpro wrote:i think it should be noted that diameter does matter. A larger diameter barrel is going to give more surface area from which the

Compromise is a sabot.


Good followup info homedepot...I was trying to stay in context of his dad's conception. I alluded to heavier projectiles requiring more volume of air...a more simplistic and probably less effective way of saying what you just did.

Nice sabot pic there.
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Unread postAuthor: THUNDERLORD » Sat Jul 19, 2008 9:39 pm

...A gas is not an incompressible fluid, so there aren't any pressure multiplication effects at work here....


I've been thinking about this concept in combustion:
A volume of gas expanding in a large chamber into a smaller diameter (volume) barrel.
I'm wondering if it isn't similar to taking a plastic bag thats open at one end and slapping it. The pressure released would be a 1:1 C:B ratio.
Next if a straw was added to the opening, or the open end was restricted to say a few millimeters, And the same force slap to the bag, I am sure the air velocity would be far greater with the restriction.
So wouldn't the C:B ratio be giving the optimized barrel length, And not the optimized velocity for a given weight/ diameter whether a sabot was used or not ?
I usually dream up pneumatic concepts but I am wondering about defining this combustion one lately. :roll:
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Unread postAuthor: homedepotpro » Sat Jul 19, 2008 10:04 pm

Ragnarok wrote:@homedepotpro: You were a factor of 4 too large calculating the cross-sectional area of the barrel, because you used the diameter, not the radius.

The 1/2" barrel could put down ~19.5 lbs of force, the 2" barrel could give 314 lbs of force.


sorry and thank you very much for catching that one, that was a noob mistake. I have been in physics class for about month, i think im getting rusty.
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Unread postAuthor: Ragnarok » Sun Jul 20, 2008 7:42 am

THUNDERLORD wrote:I'm wondering if it isn't similar to taking a plastic bag thats open at one end and slapping it. The pressure released would be a 1:1 C:B ratio.
Next if a straw was added to the opening, or the open end was restricted to say a few millimeters, And the same force slap to the bag, I am sure the air velocity would be far greater with the restriction.

That's essentially Bernoulli's principle, yes.


So wouldn't the C:B ratio be giving the optimized barrel length, And not the optimized velocity for a given weight/ diameter whether a sabot was used or not ?

It does, more or less, yes - but what a sabot allows is the application of a greater force on to a smaller object, allowing you to concentrate energy into it.
Basically, a sabot is force multiplication at work, using the incompressible sabot to multiply the force that can be put into a small object, compared to firing it from a barrel of it's own calibre.

Energy is force * distance. Force is pressure * area.
So Energy is Pressure * area * distance.

Double the area, you get double the muzzle energy.
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Unread postAuthor: jimmy101 » Sun Jul 20, 2008 1:56 pm

Ragnarok wrote:It does, more or less, yes - but what a sabot allows is the application of a greater force on to a smaller object, allowing you to concentrate energy into it.
Basically, a sabot is force multiplication at work, using the incompressible sabot to multiply the force that can be put into a small object, compared to firing it from a barrel of it's own calibre.

To put it another way, a sabot "uncouples" the projectile's ballistics into two separate domains. The sabot allows you to try to optimize the two domains separately. During the internal ballistics phase (projectile in barrel) you want a large cross sectional area to get maximum force and acceleration. During the external ballistics phase (projectile has left the barrel) you want a small cross sectional area to minimize drag and maximize range.

For most projectiles you can't change the area duing the firing process, it is constant in both the acceleration and deceleration phases. For example, tennis balls are not great projectiles because their area is too large for their mass (they have a small sectional density). You can get a high velocity at the muzzle because of the high area and low mass but once the tennis ball leaves the barrel the same two parameters make the ball slow down very quickly. The net result is that tennis balls won't carry very far no matter how much energy you put into accelerating them. Golf balls are the other way around, relatively low area but high mass means they accelerate poorly in the barrel, but once they clear the barrel they maintain speed much better.

A sabot lets you have a low sectional density in the barrel (which is best) but a high sectional density after the round has left the barrel. Net result is very good acceleration in barrel and very good "carrying ability" after the round has left the sabot. You've "uncoupled" the two ballistics domains and can try to optimize them separately.
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