Onager

From Slinging.org Wiki

The onager is a type of catapult which was invented in ancient Roman times to sling a single heavy projectile a long distance. Onagers were primarily a siege engine, used to destroy fortifications, although they were also occassionally used against soldiers. (A variation of the siege engine, the mangonel, was more often employed on the battlefield. )

The onager works on the same principles as a staff sling, of which it is basically a scaled-up version, mounted in a wooden frame and powered by torsional force provided by a skein of tightly twisted rope, hair or sinew. The lower end of the staff, or arm, is thrust through the twisted skein, which is stretched between the sides of the frame near the base of the onager. In operation, the other end of the arm - to which the sling is attached - is pulled down to the back of the frame using a winch or windlass. When released, the tension in the rope forces the arm up at high speed, through roughly 90 degrees, to its original position. The arm is stopped by a padded horisontal cross-bar and the sling whips up, releasing the projectile in a trajectory roughly parallel to the ground.

The onager is named after a type of wild ass, because of the kicking action of the machine when the arm is released.

Related siege engines

The staff-sling action of the arm in an onager is similar to that in a trebuchet, another (larger and more powerful) type of siege engine which in medieval times partly replaced torsional engines . The main difference was the means of storing energy to move the arm - trebuchets used a large counterweight rather than twisted rope. A variation of the onager, the mangonel, used a bucket or bowl in place of a sling. Although less powerful than onagers (due to the absence of a sling), a mangonel could fire several smaller projectiles at once. Mangonels were more useful on a battlefield against soldiers, or when covering an entrance to a fortification.