The Ballista

By MPF (originally written March 2007)

A ballista was a weapon used in ancient times that resembles a giant crossbow and is able to propel projectiles weighing several kilograms up to 500 hundred meters away. The power to launch projectiles comes from a pair of ‘arms’ on either side of the weapon that are attached using several loops of rope that are tightly twisted. The torsion of the ropes provides great force that tries to return the swinging arms back to their original position. These two arms are then connected with a rope and a projectile is placed against the rope (or some device that holds the projectiles that is connected to the rope.) When the connecting rope is pulled back and then released, the arms swing back to their original position pulling the projectile along a rail and out of the front of the ballista.

Ballistas were used in ancient times. The Greeks first used the weapon and the Romans adopted it around 400 BC. It was used up until the middle ages when it was eventually phased out and replaced with the much more powerful trebuchet.

My Design

While the ballistas used in ancient times were on average about 20 feet in length, I decided to build a smaller version for practical reasons. My design was a 5 feet long by 3 feet wide with arms that extend giving a total width of 5 feet. My main inspiration was just to study various pictures of ballistas that I found on the Internet and create my own design. I was able to find several websites dedicate exclusively to ballista construction, and was able to incorporate some those ideas into my design.

Interesting aspects

The rope bundle used in torsion is made of sisal, which is a natural fibre and probably similar to something the ancients would have used. It does have its drawback though in that it isn’t extremely strong is prone to fraying. On the plus side, however, it is cheap, resistant to damage from sunlight and actually contracts when exposed to moisture, so if extra power is needed, the ropes can be wet.

A gear/ratchet system that allows only one direction of rotation makes the rope that connects to the arms pull back to be wound and locked. When the ballista is fired by pulling a firing pin, the rope can then be unwound by pulling down on the gear locking mechanism.


While based on an ancient design, my ballista contains many modern ‘improvements’ and modifications both for practical purposes (ie, finding material that’s readily available and inexpensive) and for safety reasons. I used modern tools to construct most of the pieces and fastened the wood together with screws rather than nails. I also used plywood for several pieces, something that the ancients wouldn’t have used. I also used a modern nylon rope for one section.

While twisting the torsion ropes for testing, I discovered that there was a lot more bending on the front of the ballista than I had expected. As a result, the front had to be reinforced with several blocks of wood in order to prevent a structural failure.

While testing the launching mechanism, it was discovered that rubbing candle wax along the rail would greatly reduce friction and thus increase the velocity of the projectile.

The main problem, however, was that with the sheer size of the ballista, transporting it to a field to conduct a full-scale test was a problem. However, at a minimum power setting, the ballista was capable of making a 2-inch wide piece of dowel punch a hole in my garage’s wall.

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