[hrench]

In 2008 team 1108 used large pneumatic cylinders to catapult the ball by holding the catapult frame down with hard-drive (neodyniem) magnets. The pressure built in the cylinders to about 50 psi before the magnets were overcome, then the whole thing gives at once.

you could also do this by 'vacuuming' the ball down as a latch or even put a car-door-type latch on the catapult. When the cylinders are pressurized you open it.

But I think the magnet thing is easiest.