Since posting my LEGO trebuchet, I received several requests to try and build a working LEGO ballista. For those unfamiliar with ballistas, they were ancient weapons used for probably well over 1000 years, from the time of the ancient Greeks all the way up to the middle ages. They used the energy stored in torsion springs, created by twisting loops of rope, to fire a projectile.
I finally took some time a few weeks ago to tackle the project, and here is the result. It is quite a bit smaller than my trebuchet, but still packs a punch. It can launch a 2×4 brick up to 15 feet! Check out the video to see how it works.
If you are interested in building one yourself you can find the instructions below. The frame itself is very simple to build, but threading and tensioning the strings is a bit more complicated. Hopefully the instructions are clear enough in this regard. I also briefly show how to thread and tighten the springs in the video.
It’s important that both strings be twisted to have about the same amount of tension. If they are too unbalanced, then one arm will be much ‘stronger’ than the other and it will not operate properly. If you encounter this case, just unwind the ‘weaker’ spring and re-insert the piece at the base of the arm from a different direction (front/side/back) so that you can twist the string another 1/4 or 1/2 a revolution to match the other one.