Brief Description:  The onager was a powerful Roman torsion-powered siege catapult. It featured a sturdy wooden frame, a single vertical arm ending in a sling, and twisted ropes (skeins) made of animal sinew or hair. When fired, the sudden recoil caused the machine to kick, earning it the name "wild ass".

Key Features and Design:

Torsion Power: Instead of using counterweights or bent wood, power was generated by twisting thick skeins of hair or sinew to create massive torque.

The "Donkey Kick": The name onager comes from the Latin word for a wild ass. Upon release, the throwing arm struck a padded wooden crossbeam, stopping it abruptly and causing the entire heavy frame to kick backward.

The Sling: Rather than a rigid bucket, the throwing arm utilized a rope sling which greatly increased the lever length, allowing it to hurl payloads further.

Ammunition: Capable of throwing massive stone boulders (up to 100-200 pounds), clay pots filled with burning pitch or oil to ignite structures, and even corpses to spread disease.