Interesting design, looks more like a ballistic ballistae. When I think of a catapult I always picture the classic greek style with the spoon-like throwing arm.
This is painful to admit, but back when I was in the SCA, we had a catapult. It wasn't huge, it could be carried by three or four people, and only took three to and one to fire. But it could lob something the size of a couch pillow about a football field in length. We also had a, arballast, which is like a large crossbow, but smaller than a ballista. That one was on two large wheels like a cannon, so it was easier to move around. Both worked on teh principal of heavy-guage surgical tubing, so they weren't exactly "authentic". But man, they were extremely fun to play with.