Onager

The onager was a Roman torsion-powered siege engine. It's usually portrayed as a catapult with a bowl, bucket, or sling at the end of its throwing arm. Ammianus Marcellinus first referenced the onager in 353 CE, describing it as similar to a scorpion. The onager is frequently mistaken with the mangonel, a later "traction trebuchet" that supplanted torsion-powered siege engines in the sixth century CE.

The onager was made out of a massive frame set on the ground to which a vertical frame of strong timber was tightly attached. A cup, bucket, or sling was tied to a vertical spoke that went through a rope bundle fastened to the frame. To fire it, a windlass was used to pull the spoke or arm down against the tension of twisted ropes or other springs, and then suddenly release it. As the sling swung out, one end would release, similar to a staff sling, and the projectile would be launched forward. When the arm was trapped by a padded beam or bed, it could be winched back. According to Ammianus Marcellinus, it took eight men to coil down the arm of a single-armed onager. When it fired, the recoil was so powerful that the onager could not be placed on stone walls because the stones would be displaced. This was validated by a recreated onager that was significantly smaller than the ones described in the sources but still caused significant recoil. Its shot weighed between 3-4kg.

The Roman Empire's onagers were mostly utilized to besiege forts or villages. They were frequently loaded with large stones or pebbles that could be coated in a combustible material and set on fire. In the Middle Ages (about 1200 A.D.), a less powerful variant of the onager was used, which used a stationary bowl rather than a sling, allowing many little projectiles to be thrown rather than a single massive one. This engine was commonly referred to as a mangonel, though the term might refer to a number of siege engines.