Our catapult was powered by 2 8" stroke 2" diameter pistons. at the begining of the match the latch would engage and the accumulators on the cylinders were charged to 60 psi. We had a third cylinder (our latch) which was i think 3" stroke and 1" bored. we machined 5/8" shaft to thread onto the end of the piston. The latch worked by extending into a bracket on our catapult which was really just a ball bearing supported by 2 plates. When we fired we just applied a small amount of pressure to the latch and the catapult launched. the only damage we had was that the 28 mm extrusion we used for the catapult had quite a bow in it at the end of the season due to all the pressure.
Its hard to see but it is vissible in the 3rd and 3rd last pics.
http://team781.com/gallery/2008/2008waterloo/