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Vacuum cannon

A place to ask general spud cannon related questions.
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Vacuum cannon

Unread postAuthor: Molybdenum » Sun Jun 28, 2009 6:13 pm

I came accross something in the school physics lab that I found intriguing. The vacuum cannon consists of a pipe that contains a projectile, sealed on both ends by tape (kinda-sorta burst disks). A vacuum pump is then used to create a vacuum (duh) in the pipe, and the tape on the side containing the projectile is punctured, allowing air in that pushes the projectile whith it to the other end. I don't know the muzzle velocity, and have never seen it fired, but there are destroyed soda cans hanging around, and a fairly large dent in the wall from it. I would build one, but I don't have a vacuum pump.
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Unread postAuthor: twizi » Sun Jun 28, 2009 6:45 pm

youd need a fast vacum
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Unread postAuthor: jimmy101 » Sun Jun 28, 2009 9:02 pm

Vacuum guns are fun, but disappointing if you've used a combustion or pneumatic spudgun. The best vacuum gun is roughly equivalent to a pneumatic charged to just 15 PSIG. Yes it'll mash a can but it won't do much else.

You really don't need anything special for the vacuum source, anything that'll pull down to less than 1 PSIA or so will work fine. Speed isn't much of an issue since it takes so long to reload the gun anyway. A water aspirator works well and usually can be had for less than $20.

I wonder what vacuum level a generic hose end garden sprayer will pull?
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Unread postAuthor: Technician1002 » Sun Jun 28, 2009 9:21 pm

jimmy101 wrote:Vacuum guns are fun, but disappointing if you've used a combustion or pneumatic spudgun. The best vacuum gun is roughly equivalent to a pneumatic charged to just 15 PSIG. Yes it'll mash a can but it won't do much else.


True until you extend the barrel and the projectile is not plowing air while accellerating. It can reach very high speeds over a long distance.
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Unread postAuthor: mobile chernobyl » Sun Jun 28, 2009 9:24 pm

This same concept is used in the muzzle side on one of NASA's light gas guns. However on the other side of the projectile is hydrogen rammed by a piston driven by an explosion which provides most of the power lol.
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Unread postAuthor: jimmy101 » Mon Jun 29, 2009 12:25 pm

Technician1002 wrote:True until you extend the barrel and the projectile is not plowing air while accellerating. It can reach very high speeds over a long distance.

True, but eventually you're limited by how fast air can enter from the breech end. No air in front of the round to accelerate but a long barrel means there is a long column of air behind the ammo that has to be accelerated. And, that air is cooling off so the local SOS in the barrel is dropping.


On a lark I went out in the garage last night to see what kind of vacuum a hose end garden sprayer will pull. I've got two, on does about 1 PSI vacuum (takes a chamber down by just 1 PSI below atmospheric), the other about 4 PSI vacuum. So both are pretty wimpy. I suppose that's because hose end sprayers are only designed to generate enough vacuum to lift a colum of water that is a couple inches long.

A generic shop or household vacuum will usually pull a couple PSI vacuum.

The intake manifold vacuum on a car is what, 5 to 10 PSI vacuum? Splice into a vacuum line to evacuate a vacuum gun?

A decent water powered lab aspirator will pull a chamber down to about 25 Torr (0.5 PSIA, 14.2 PSI vacuum). The bank off pressure (vacuum) is related to the water's temperature.

I wonder if a ghetto aspirator made from a 1/4" brass compression T would work?
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Unread postAuthor: spudmanb3 » Mon Jun 29, 2009 12:29 pm

Why not just use the vacume pump to remove the air from a room and shoot a regular pneumatic in the room :)
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Unread postAuthor: jimmy101 » Mon Jun 29, 2009 4:23 pm

spudmanb3 wrote:Why not just use the vacume pump to remove the air from a room and shoot a regular pneumatic in the room :)

Hmm, lets see, 12x12 room with 7' walls. Evacuate to near 0 PSIA. Force on one wall is (7')(12')(144in<sup>2<sup>/ft<sup>2<sup>)(14.6 pound/in<sup>2<sup>) = 177811.2 pounds = 88.9 tons. So that doesn't appear practical.</sup></sup></sup></sup></sup></sup>

oops, messed up the HTML tags for superscript...

Force on one wall is (7')(12')(144in<sup>2</sup>/ft<sup>2</sup>)(14.7 pound/in<sup>2</sup>) = 177811.2 pounds = 88.9 tons force.
So that doesn't appear practical. :roll:

But that got me thinking, time to rip the covers off my shop compressor and see if I get lucky and have a useable air inlet fitting ... what blows can often suck. :oops:
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Unread postAuthor: mark.f » Mon Jun 29, 2009 5:01 pm

jimmy101 wrote:But that got me thinking, time to rip the covers off my shop compressor and see if I get lucky and have a useable air inlet fitting ... what blows can often suck. :oops:


Indeed.
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Unread postAuthor: Technician1002 » Mon Jun 29, 2009 7:55 pm

jimmy101 wrote:True, but eventually you're limited by how fast air can enter from the breech end. No air in front of the round to accelerate but a long barrel means there is a long column of air behind the ammo that has to be accelerated. And, that air is cooling off so the local SOS in the barrel is dropping.


Since temperature is related to molecule velocity, as a mass this is entirely true. However, the faster (hotter) molecules will travel faster than the average and the colder will lag behind. This principle is used in no moving parts compressed air refrigeration. The hot gas exits one side and the cold exits the other, separated by a cyclone effect.

Faster hot stuff is on the outside, cooler slower stuff is on the inside.
http://www.newmantools.com/vortex.htm
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Unread postAuthor: D_Hall » Mon Jun 29, 2009 8:53 pm

mobile chernobyl wrote:This same concept is used in the muzzle side on one of NASA's light gas guns. However on the other side of the projectile is hydrogen rammed by a piston driven by an explosion which provides most of the power lol.

You're totally misinterpretting the use of vacuums in the barrels of high velocity guns. It's not intended to increase the effective pressure in the breech; that's easy enough to do with a small increase in powder. Rather, such guns tend to shoot light projectiles. As such, the mass of the air in the barrel is a very significant portion of the mass the gun must accelerate. In the gun I have at the office, that number is about 10%! So, I can get a 10% increase in muzzle energy by elliminating that extra mass (ie, pulling a vacuum).
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Unread postAuthor: jimmy101 » Tue Jun 30, 2009 12:21 pm

D_Hall wrote:In the gun I have at the office, that number is about 10%! So, I can get a 10% increase in muzzle energy by elliminating that extra mass (ie, pulling a vacuum).

As always, you sure work in a fun "office". :D
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Unread postAuthor: D_Hall » Thu Jul 02, 2009 11:45 pm

9,000 fps is indeed fun.
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Unread postAuthor: maggotman » Fri Jul 03, 2009 7:24 am

fridg compressors make a good vacume ive used them when casting expoy and thay holg the cacume becuse of the internal checkvalve.
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Unread postAuthor: inonickname » Fri Jul 03, 2009 8:05 am

maggotman wrote:Fridge compressors make a good vacuum I've used them when casting epoxy, they hold the vacuum because of the internal check valve.


Yep.

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