Tabletop Onager

A few weeks ago I built a tabletop trebuchet. After playing with it my sons and I decided we needed another machine to hurl our balls back to the trebuchet. Rather than build another trebuchet I decided to build another seige engine. After reviewing the different specimens from the different categories of seige engines (counter weight, tension, and torsion) a torsion powered machine seemed like it would be neat. Since this would be my first torsion machine I decided to build an onager.
I'd never heard of an onager before either. It turns out that it's very similar to a torsion catapult with one exception, instead of using a basket to hold and release the projectile it uses a sling like a trebuchet. Since I have some recent experience with sling design the onager seemed like a good fit.
After doing a bunch of research here's what I came up with:

  • Frame: 18" long, 8" wide, 6" tall
  • Throwing arm: 8" long
  • Torsion cords: surveyors string (150" yielded 24 strands)
  • Projectiles: anything between .50 and .85 ounces (we use superballs and balls made from duct tape)
  • Effective range: 10 to 25 feet

I have posted photos with notes here.