These books, available at Amazon. The Art of the Catapult: Build Diary of alicia keys zip Ballistae, Roman Onagers, English Trebuchets, and More Ancient Artillery - Nice book that shows you how to build ten different catapult projects, moderate wood working and pvc piping kind of stuff. Good age of 7th grader
with complete instructions and materials lists. Backyard Ballistics: Build Potato Cannons, Paper Match Rockets, Cincinnati Fire Kites, Oeys Ball Oof, and More Dynamite Devices Want to get a little more creative. This book is off the hook.