How to build a catapult part 1

Bill Gurstelle is your MAKEcation counselor for the make-a-trebuchet Family Challenge. Build a trebuchet and post pictures tagged “MAKEcation” in the MAKE Flickr pool to enter to win a $100 Maker Shed gift certificate!

Catapults are wonderful, exciting examples of technology. (I use the catapult to mean any projectile hurling device of ancient descent. Such devices may use springs, counterweights, or human energy.) They are simple yet complex; delicate yet brutal. Unlike looking at say your computer or an airliner, you can pretty much look at a trebuchet or ballista and kind of understand what’s going on. On the other hand, the physics and kinematics are complex and intricate.

Tips for building a catapults:

• Purity of concept aside, it pays to use modern materials. While it might be cool to build a coiled rope torsion spring like the ancient Greeks, you’ll probably find it difficult to get enough spring power without torquing down the ropes so much that you’ll crush the frame., Experiment with the use of bungee cords if you build a torsion or tension spring powered model.
• I find hardwoods and metal frames work better than softwoods. Practically speaking though, dimensional lumber is what most people wind up using. Overbuild if you can, doubling up the wooden frame pieces. You’ll be glad you did.
• If you’re making a gravity powered trebuchet, consider forgoing the pouch on the sling and simply attach a cord to the object you’re hurling:. That way, when the projectile flies, then so does the cord. I call that “a sacrificial sling.” It’s way, way easier.
• Consider your release mechanism carefully. For bigger catapults, consider a pelican hook (it’s a peice of hardware that sailors use, and you get them at stores that cater to boating) or possible an archer’s arrow release. You get those at sporting goods stores. Their fairly cheap and they work well with smaller machines. Much more info on catapult triggers in a forthcoming post.

There’s a lot of historical information and plans for many sorts of catapults in my book, The Art of the Catapult which is on sale at the Maker Shed.

More:

• MAKEcations: deadline extended!
• Maker Workshop – Portable Trebuchet on Make: television
• MAKEcation trebuchet bundle in the Maker Shed