Onager (siege weapon)

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Sketch of an Onager, from Antique technology by Diels.

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The onager was a post-classical Roman siege engine, which derived its name from the kicking action of the machine, similar to that of an onager (wild ass). It is a type of catapult that uses torsional pressure, generally from twisted rope, to store energy for the shot.

The onager consisted of a large frame placed on the ground to whose front end a vertical frame of solid timber was rigidly fixed; through the vertical frame ran an axle, which had a single stout spoke. On the extremity of the spoke was a sling used to launch a projectile.

_{"The onager's framework is made out of two beams from oak, which curve into humps. In the middle they have quite large holes in them, in which strong sinew ropes are stretched and twisted. A long arm is then inserted between the bundle of rope, at its end it has a pin and a pouch. It strikes on a huge buffer with a sack stuffed with fine chaff and secured by tight binding. When it comes to combat, a round stone (often clay balls with Greek fire in them, which explode on impact and burst into flames) is put in the pouch and the arm is winched down. Then, the master artilleryman strikes the pin with a hammer, and with a big blow, the stone is launched towards its target." -Ammianus Marcellinus, a Roman writer.}

In action the spoke was forced down, against the tension of twisted ropes or other springs, by a windlass, and then suddenly released. The spoke thus kicked the crosspiece of the vertical frame, and the projectile at its extreme end was shot forward.

The onagers of the Roman Empire were mainly used for besieging forts or settlements. They would often be armed with huge stones or rocks that could be covered with a combustible substance and set alight.

In the Middle Ages (recorded from around 1200) a less powerful version of the onager was used that held the projectile in a fixed bowl instead of a sling. This was so that many small projectiles could be thrown, as opposed to one large one. This engine was sometimes called the mangonel, although the same name may have been used for a variety of siege engines.

[edit] Models

A commercial model of a mangonel, a weapon very similar to the onager

Models and kits of Onagers are available on-line and from some toy companies. The more serious kits are often used for school science and history fairs and competitions. These can be purchased from several online stores. Truly functional Onager models use twisted ropes (torsion skeins) for power, but most of the toy onagers use rubber bands or metal springs for power.