[jrocket567]

not nearly advanced as your plan, but very simple--bummin around in the back lab, we took two accumulators, hooked them together, and then to the compressor- out the other end we stuck a tube into the bottom of a 1/2" (i think) diameter PVC pipe about 4 feet long-- we shoved a tootsie roll down the pipe, and with the regular bulky relief valve, turning it as fast as we could, launched it about 75 yards- you might not have to go through all of them fancy valves and stuff to make it work- KISS