Torsion Catapults - Early Greek Siege Engines
Torsion catapults are interesting to study and to build. The Greeks discovered this around the third century B.C. when they started experimenting with different types of springs to launch projectiles.
Previously having relied on tension, the Greek engineers came up with the idea of using rope strung into tight coils as a power source. One benefit of this "rope power source" was the spring could be made very large and powerful allowing the catapult to hurl relatively heavy objects a considerable distance.
A spring created by coiling ropes is called a torsion spring. Catapults powered by these torsion springs are referred to as torsion-powered catapults or simple torsion catapults. Some of the other names for these catapults are arcuballista, chiroballista, lithobolas, mangonel, onager, scorpion, and stone throwers.
The torsion catapult consisted of a winch, one or two throwing arms, wooden washers, a stand or base, and the rope spring. Earlier designs of this catapult also employed a drawstring similiar to that found on a bow.
Torsion catapults were relatively easy to produce and capable of inflicting a lot of damage. Think of how much it hurts to be hit by a stone that someone throws at you, now imagine being hit by a rock at least the size of your head that is traveling at 100 miles per hour or faster.
One major weakness of torsion catapults was found in the torsion spring itself. If the rope became wet the spring failed to work rendering the catapult useless. Another problem with the torsion catapult was the accuracy. This issue was also caused by the rope spring. Because it was nearly impossible to produce the exact same amount of tension each time the rope spring was coiled due to slippage or where the rope landed while being wound hitting the same spot on a target repeately was a very difficult task.
Despite these challenges, torsion catapults evolved for mancy decades and became formidable weapons in medieval warfare.