I recently saw a TV show about the most american activity ever invented ; "Punkin Chunkin." One category on it was torsion powered catapults. (some others were human powered, air cannons, centrifugal force, and so on.) I used a 2x4 for the base, and small bits of wood that the rubber bands attached to, with the help of screws. I twist the pvc around and around, then I aim it and let it go. The extra pvc pipe you see is just to stop the shaft of the catapult from hitting the ground. It fires about 15-20 feet.