These are the parts for my big cannon. I've been working on it little by little and have changed valve designs a few times, what I have on this page will probably be what it will have for a valve. First, the tank is an old 20 gallon compressor tank that I have welded a 4" coupler in each end. The valve is a little more complicated. The way it will work is that air pressure will be on both sides of the piston, causing it not to move and seal the barrel off with the orings on it. when pressure is dumped from the rear chamber, the piston will move backwards because the pressure on the front side will be greater, once the piston moves back all the air in the tank will exit through the tee and venture on out into the barrel, causing what ever is in the barrel to accelerate quite rapidly. I am planning on using a 3" diameter barrel most likely 20 feet long and possibly 30 feet. For that long of a barrel I'm going to have to build a hell of a barrel support structure. The cannon will probably be mounted on a trailer that I will build specificaly for the cannon, maybe even having hydraulic elevation adjustment, depending on my budget at the time, but like I said, I'm workin on this little by little....
added 8-31-00
I added another huge air tank to my collection the other day, well it's not in my possesion yet, but I've got it reserved. It's a 100 gallon air tank that was used for an air over hydraulic hoist in our shop, the seals are leaking on the cylinders and, my boss is thinking of replacing it with an above ground hoist, so when he does (which should be soon) the tank will be mine if everything goes right. Right now I'm thinking of using a 5" dia x 40' long barrel and a homemade 5" steel valve. I'll keep you all posted.
added 10-6-2000
Here is a diagram of the type of "valve" I believe I will be using on my 20 gallon tank. It would be simple to build, but at a couple of costs i'll describe later. How does it work you ask, well the rupture diaphragms will burst at a certain pressure thus releasing the pressure from the tank to the barrel. The reason for the 50% tank pressure in the secondary chamber, is so that you can trigger the burst, you have more than the burst pressure in the air tank and with 50% pressure on the other side it will support the diaphragm so it won't burst, also the 50% pressure isn't enough to burst the secondary diaphragm. When the air is released in the secondary air chamber the first diaphragm will burst because the 50% tank prressure isn't there anymore and therefore not supporting it, the second diphragm will burst because the tank pressure is there now and nothing to support it behind. This should provide an almost instanious air flow to the barrel with the barrel being the only restriction. You can use various materials for the diaphragms, plexiglass should work as well as aluminum foil or aluminum cut from pop cans, you would just have to do some testing to see at what pressure they burst at. Bad thing about it is, the time you would have to take between shots to replace the diaphragms.