Here’s a basic torsion catapult. It uses one or more elastic bands to launch a U.S. penny if you print it at 1:1 scale. You probably won’t get enough torque to be impressive with less than 3 bands.
Construction is snap-together, but it wouldn’t hurt to put a drop of super glue on the joints for durability.
Feed an elastic band through each of the bores on the side braces. Tie it through the bore on each winding key. Slip the end of the throwing arm through the rubber bands. Repeat with as many rubber bands as will fit through the holes.
Twist each key 10 times and then lock it in place by inserting the locking tenons into the square holes on the side braces. The more twists, the more launching power you’ll have. Careful, pennies can sting, especially if you take an Abe to the forehead.
You can change the payload’s arc by raising the front of the catapult.
Inspiration for this catapult came from The Art of the Catapult. You won’t find a better introduction to medieval siegecraft than this book. It’s got plans to build all kinds of stuff from catapults to trebuchets. Very kid-friendly.
If you read The Art of the Catapult you’ll learn that “catapult” is more of a catch-all term for this breed of siege engine. This model would more properly be called a mangonel.
If you’re looking to build stuff with a PG rating, Whoosh Boom Splat: The Garage Warrior’s Guide to Building Projectile Shooters is a better bet.
Flagrant stagecraft alert: the clever-eyed among you will note the use of black electrical tape to keep the throwing arm cocked for the photo.
HBP: 120° C
1.8mm Black ABS
Layer Height: .25
Number of shells: 1
Feedrate: 25 mm/sec
Travel Feedrate: 55