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Barrel length for max range? Need help!!

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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Barrel length for max range? Need help!!

Unread postAuthor: halfsquelch » Fri Nov 09, 2007 2:38 am

I have a quick question. I am making a tennis ball cannon and would like to find out how long I need to make the barrel to achieve maximum range. The cannon will need to be air powered and have a presure no greater than 150 psi, and will be fired by a solenoid valve. Also, im not sure if this info is needed but, the cannon will be made from 2 1/2 inch pvc (the diamiter of a tennis ball).

If anyone can lend me a hand on this that would be very helpfull.

Thanks!! :D
Stephen
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 09, 2007 2:51 am

There is no "maxium range" - the longer your barrel, the further it will go, simple is that (provided your ratio of barrel to chamber volume doesn't fall below 4:1 or so) - so basically, make it as long as you can afford.

Also, if the intention is to fire it as far as possible, get as large a solenoid valve as you can and and modify it for pneumatic actuation.
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Unread postAuthor: halfsquelch » Fri Nov 09, 2007 2:58 am

Thanks... I gess that answers my question. And what I meant about the optimal length of the barrel is that eventualy the tennis ball will stop in the barrel if it is to long. But I can't go much over 4 - 5 ft so that wont be a problem.

:D Thanks for the quick response!!
Stephen
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 09, 2007 3:19 am

halfsquelch wrote:I meant about the optimal length of the barrel is that eventualy the tennis ball will stop in the barrel if it is to long. But I can't go much over 4 - 5 ft so that wont be a problem.


Let's say you make the barrel out of 5 feet of 2.5" pipe. If you make the chamber out of the same pipe and have it at least 15 inches long you'll be fine. That would be the minimum though, if you're pressurising it using a compressor then go ahead and make the chamber the same volume as the barrel. Make sure all your pipe and fittings are pressure rated to stay safe.
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Unread postAuthor: benstern » Fri Nov 09, 2007 11:10 am

http://www.thehalls-in-bfe.com/GGDT
"Hall Consulting's Gas Gun Design Tool is a simulation program to aid the design of pneumatic cannons."
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Unread postAuthor: jimmy101 » Fri Nov 09, 2007 12:02 pm

jackssmirkingrevenge wrote:There is no "maxium range" - the longer your barrel, the further it will go, simple is that (provided your ratio of barrel to chamber volume doesn't fall below 4:1 or so) - so basically, make it as long as you can afford.

Almost.

There is both a maximum range and an optimal barrel length for a fixed chamber size for a pneumatic gun. (See attached GGDT output)

For a fixed barrel length there is no optimal chamber size. If everything else is equal, then the larger the chamber the higher the muzzle velocity (though the velocity versus chamber size approaches a limit asymptotically).
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Attachments
Picture2.jpg
GGDT output using all default values except for a very long barrel. Velocity plotted as a funtion of projectile position (barrel length)
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 09, 2007 1:43 pm

jimmy101 wrote:
jackssmirkingrevenge wrote:There is no "maxium range" - the longer your barrel, the further it will go, simple is that (provided your ratio of barrel to chamber volume doesn't fall below 4:1 or so) - so basically, make it as long as you can afford.

Almost.

There is both a maximum range and an optimal barrel length for a fixed chamber size for a pneumatic gun. (See attached GGDT output)


Hence why I did not specify a fixed chamber size in my example ;)
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Unread postAuthor: halfsquelch » Fri Nov 09, 2007 1:47 pm

OK one more question, would it be beneficial to have the volume of the chamber larger than the volume of the barrel?
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Unread postAuthor: williamfeldmann » Fri Nov 09, 2007 1:52 pm

One other thing to think about is the actual ammo.

Tennis balls are actually quite lightweight for their size (sectional density). This provides amazing muzzle velocities but poor overall range. Heavier projectiles go much much farther, but travel much slower.

Tennis balls have the advantage of being hollow. With a sharp utility knife you can cut a slit into the tennis ball and fill it with your choice of materials. Sand or Portland concrete powder make excellent choices for a lot of mass. Then you can either sew the hole shut or hot glue it or whatever. Keep in mind that the added mass fills the void, which in turn reduces the ability of the ball to bounce or squeeze.

Also, the heavier the object, the more muzzle energy the gun will have.
Using a 4 foot barrel, 4 foot chamber at 100 psi, a 100 gram tennis ball would have 43.1 ft lbs of muzzle energy and 112 ft per second of velocity.

200 gram ball would have 76.9 ft lbs of energy and 106 fps
300 gram ball would have 105.1 ft lbs of energy and 101 fps
454 grams (1 pound) would have 141.9 ft lbs and 96 fps

With longer barrel, more chamber volume, more pressure you can see how these numbers can get quite high. Muzzle energy converts into felt recoil. If you want to launch really heavy objects for really awesome distance, a gun that is on a launch pad or tripod, etc, might be in order.

To give you an idea of what I meant, I ran a GGDT on a plan I drew up for a potential future project. Shooting a one pound tennis ball out of a 8 foot 2.5 inch barrel with 5 foot 3 inch chamber at 160 psi with helium gas, I can expect somewhere around 3,212.2 ft lbs of muzzle energy. Thats more than a ton and a half. I couldn't even hold on to that, and I am a big guy. (But I also could get around 1900 feet of range which is a little over 6 football fields) :D
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 09, 2007 2:05 pm

halfsquelch wrote:OK one more question, would it be beneficial to have the volume of the chamber larger than the volume of the barrel?


punching in some numbers, for a theoretical launcher at 150 psi firing 60 gram tennis balls with a 5 foot barrel:

chamber volume 25% barrel - 410 feet per second

chamber volume 50% barrel - 460 feet per second

chamber volume 75% barrel - 475 feet per second

chamber volume 100% barrel - 480 feet per second

chamber volume 150% barrel - 490 feet per second

chamber volume 200% barrel - 495 feet per second

as you can see, performance does increase, but too little to be worth the extra chamber size and effort, especially if you're pumping by hand.
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Unread postAuthor: halfsquelch » Fri Nov 09, 2007 2:08 pm

OK thanks for the info, I think i will make the chamber out of 3in PVC (instead of 2 1/2in) and cut it about 4-6in shorter then the barrel.
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Unread postAuthor: halfsquelch » Fri Nov 09, 2007 4:58 pm

To give you an idea of what I meant, I ran a GGDT on a plan I drew up for a potential future project. Shooting a one pound tennis ball out of a 8 foot 2.5 inch barrel with 5 foot 3 inch chamber at 160 psi with helium gas, I can expect somewhere around 3,212.2 ft lbs of muzzle energy. Thats more than a ton and a half. I couldn't even hold on to that, and I am a big guy. (But I also could get around 1900 feet of range which is a little over 6 football fields)


dumb question... how did you figure out how far this would go?

Sry I'm having a hard time thinking right now...I never went to bed last night.
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Unread postAuthor: spanerman » Fri Nov 09, 2007 5:07 pm

the range calc on ggdt or another ballistic trajectory range calc....
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 09, 2007 5:15 pm

... though range calculators don't tend to factor in the all important air drag factor, which on projectiles with a poor ballistic coefficient (lare cross sectional are but low weight) such as a tennis ball has a more pronounced effect - meaning your result will be wildly optimistic at best.
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Unread postAuthor: DYI » Fri Nov 09, 2007 9:05 pm

3,212.2 ft lbs of muzzle energy. Thats more than a ton and a half. I couldn't even hold on to that, and I am a big guy


If I'm right, then muzzle energy isn't the only contributing factor to recoil. There are (very) big game rifles in .535 and .700 calibre that have over 11 000 ft/lbs of muzzle energy and no recoil absorption, and a skilled shooter can fire them standing up (and remain in that position afterwards with a bit of luck). In comparison, the recoil on my burst disk gun (currently delayed) has destroyed many things, and kicks its new 100 pound fixed stand back significantly on every shot. If you tried to shoulder fire that thing, you'd have no shoulder left to speak of afterwards.
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