Onager (weapon)

Etymology

According to two authors of the later Roman Empire who wrote on military affairs, the onager derived its name from the kicking action of the machine that threw stones into the air, as did the hooves of the wild ass, the onager, which was native to the eastern part of the empire.[2]

Design

The onager consisted of a large frame placed on the ground to whose front end a vertical frame of solid timber was rigidly fixed. A vertical spoke that passed through a rope bundle fastened to the frame had a sling attached which contained a projectile. To fire it, the spoke or arm was forced down, against the tension of twisted ropes or other springs, by a windlass, and then suddenly released. As the sling swung outwards, one end would release, as with a staff-sling, and the projectile would be hurled forward. The arm would then be caught by a padded beam or bed, when it could be winched back again.[3]

By the 4th century, its place as a torsion-powered stone thrower had been taken by the onager, a rather simpler version operating on the same principle. This time, inside a wooden frame that had to be of massive proportions, a single arm was held in a twisted skein of sinew or horsehair. It was loaded by pulling down the arm and placing the missile in the cup at the end, and, on release, the arm flew up to send the missile on its way. The arm was stopped when it hit the necessarily strong crossbeam. Its optimum range was estimated at about 130 metres. Although it might reach much further, by then the force of the impact would have been much reduced. The 2002 reconstruction managed to throw a 26 kg limestone ball 90 yards before the timber of the weapon disintegrated after its second shot.[4]

— Peter Purton

History

The onager was used from the 4th century AD until the 6th century AD. The late-fourth century author Ammianus Marcellinus describes 'onager' as a neologism for scorpions and relates various incidents in which the engines fire both rocks and arrow-shaped missiles.[5] According to Ammianus, the onager was a single-armed torsion engine unlike the twin-armed ballista before it. It needed eight men just to wind down the arm and could not be placed on fortifications because of its great recoil. It had very low mobility and was difficult to aim. In 378, the onager was used against the Goths at Adrianople and although it did not cause any casualties, its large stone projectile was incredibly frightening to the Goths. The late-fourth or early-fifth century military writer Vegetius stipulates that a legion ought to field ten onagers, one for each cohort. These he says should be transported fully assembled on ox carts to ensure readiness in case of sudden attack, in which case the onagers could be used for defence immediately. For Vegetius, the onagers were stone throwing machines.[6]

In the late 6th century the Avars brought the Chinese traction trebuchet, otherwise known as the mangonel, to the Mediterranean, where it soon replaced the slower and more complex torsion powered engines.[7]

The traction trebuchet displaced classical, torsion-powered artillery because it was simpler and required less competence to build, while maintaining comparable range and power, and it had far higher rates of firing and accuracy (when operated by a trained crew). Furthermore, it was probably safer to operate than tension weapons, whose bundles of taut sinews stored up huge amounts of energy even in resting state and were prone to catastrophic failure when in use.[8]

See also

References
  1. Oxford English Dictionary.
  2. Vegetius, De re militari, IV:22; Ammianus Marcellinus, Roman History XXIII:4, 4; XXXI:15, 12.
  3. Denny, Mark "The Physics Teacher" vol 47, p 574-578, December 2009
  4. Purton 2006, p. 81.
  5. Ammianus Marcellinus, Roman History, XIX:2 & 7; XX:7; XXIII:4; XXIV: 4; XXXI:15.
  6. Vegetius, De re militari, IV:22
  7. Purton, Peter (2009), A History of the Early Medieval Siege c.450–1200, The Boydell Press, p. 364, ISBN 978-1-84383-448-9
  8. Petersen, Leif Inge Ree (2013), Siege Warfare and Military Organization in the Successor States, Brill, p. 409, ISBN 978-90-04-25199-1

Bibliography
  • Purton, Peter (2006), The myth of the mangonel: torsion artillery in the Middle Ages
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