Ok so instead of building a ballista we built a trebuchet (don't know how to spell that sorry) because we realized the hook would have to be another input to the device, which was not allowed. So we whipped up a trebuchet in 70 minutes (the fastest we've ever worked) and it launched the orbit ball 18.5 feet (with rolling). We hade a 22" X 26" base and had a pivot made of two PVC pipes around each other. We taped the arm to the pivot and held it up on the counter weight side with 2 X 4 (about 4 feet tall). The pnuematics then push the 2 X 4 out from under the counter weight and it worked really well. All I have to say after that is thank goodness for duct tape.
