If you want to build a spring powered catapult you might want to consider a section of a cross country ski, or toy bow and arrow as sources of material. Both can usually be found cheaply at many goodwill stores. We prototyped such a launcher and it can be made extremely accurate and reasonably powerful. Our students really want to do a reaction wheel, but I think that is more weight and complexity than required. Take a look at the soccer kicker that the robowranglers did a few years back for ideas. They used a simple cam to control the spring.
http://www.youtube.com/watch?v=_hTyXQUgYLE