Okay, here's how it works. On one of these vacuum cannons, you have 2 parts: the barrel, and a vacuum pump. The vacuum pump is connected to the barrel using a quick disconnect valve of some sort, coming out perpendicular to the barrel, somewhere ahead of the back end. The main differance between this cannon and a compressed air cannon is that this type does NOT require a pressure tank, the potential energy is all stored in the barrel itself. To charge the barrel, first a loose fitting cap is placed over the muzzle (it simply needs to be able to form a seal with the muzzle, but not stick) and a projectile is loaded into the breech end. Then, using some manner of tape (Tuck Tape, the type used for sealing windows and stuff seems to work best) the breech is sealed off. Now, the vacuum pump is hooked up to the quick disconnect on the barrel, and the air is pumped out. It is important that the disconnect is located ahead of the projectile in the barrel, for obvious reasons. Firing the cannon is accomplished by simply piercing the tape over the breech. The vacuum ahead of the projectile causes the projectile to be "sucked" forward, as the air rushes in the breech to fill the differance.
Now, I figure, if that quick disconnect were a valve that could be closed, you would be able to charge this unit and disconnect it from the vacuum pump, then hold onto it until needed. This would allow for a relatively compact LAW style weapon that is little more than a section of pipe, so you probably wouldn't feel too bad about discarding it until the end of the match after firing.
Rigging up safeties and firing systems would be fairly simple as well. Safety caps would be a must, using pipe end caps with some manner of locking system to hold them to the barrel in the event of a misfire, but easy enough to remove before firing. A trigger system would be little more than some sort of lever assembly to move a pierce pin into the breech.
I'm considering building one or two of these, since it looks like they wouldn't be too expensive to make. Any questions/comments? Feedback would be appreciated.
EDIT: Okay, new image which shows some stuff a little more clearly, such as the direction you point it.
Number of downloads: 104
Also, here's a link showing a lame cannon that uses the same principal. This isn't the one I got the idea from, but it's basically the same idea. It might help you understand exactly how this thing works, because it isn't obvious.
This post has been edited by Skippy: 10 February 2007 - 09:27 PM