Roman Catapult: Siege Engineering in the Ancient World

Named after a wild donkey for its powerful kicking action, the onager was a type of catapult that utilised a single large arm to hurl projectiles from a bucket or sling at the enemy. The robust construction and formidable destructive capability of this ancient catapult made it a vital tool in the Romans’ siege arsenal, significantly altering the landscape of warfare during its era.

Let’s take a trip back to the heady days of the Roman Empire to discover the story of the onager, how it works, its innovation, and why its mechanics remain as important to modern engineering concepts as they did to ancient battlefield dynamics.

The onager was an evolution from earlier, less sophisticated torsion-powered weapons like the ballista, which was more akin to a giant crossbow. Its innovation is attributed to Roman engineers, who developed existing Greek technologies to suit the specific needs of Roman warfare.

This Roman catapult is believed to have been named after a subspecies of Asiatic wild donkey native to the Eastern outposts of the Roman Empire known as the Syrian onager. The rear end of the machine would often kick up when a projectile was launched and this action resembled the kicking action of the animal.

The onager was a torsion-powered machine consisting of a sturdy wooden frame and a large, horizontal beam – acting as the throwing arm – mounted with twisted ropes or sinew at one end.

The other end of the arm was pulled down to the ground, where it was loaded with a projectile – typically a stone or incendiary material such as a burning bowl of oil – held in a sling or bucket.

When released, the torsion – also called torque – generated by the twisted ropes rapidly snapped the arm upward, propelling the projectile forward and high into the air toward the target. The mechanism’s design allowed it to store potential energy, which, when released in a sudden, forceful motion, converts into kinetic energy. This method of siege warfare enabled the onager to deliver heavy payloads from a safe distance over walls, into enemy fortifications, or in open battlefield confrontations.

The Romans, renowned for their strategic military advancements, used this ancient catapult across their vast empire. Its effectiveness was not just in its destructive capacity but also in its psychological impact on the besieged, demoralising enemies with each strike.

While it’s safe to assume that onagers were commonly used by the Roman army, due to the general lack of detailed records about specific battles, there are only a few direct mentions of them being used in individual military engagements. One referenced use of this Roman catapult was in 378 AD against the Goths at the Battle of Adrianople, though it did little to assist in what was one of the Empire’s most catastrophic defeats. It was also used, according to Roman historian Ammianus Marcellinus, during an Alemanni incursion into Gaul. Yet despite the lack of detailed accounts, it’s a fair assumption that the onager remained a crucial weapon in the Roman siege arsenal, and was widely used from the fourth century AD through to at least the sixth century AD.

Despite the fall of the Western Roman Empire, in later years the onager may well have remained in use with Byzantine armies, and possibly Arab forces into the early Middle Ages. However there’s no concrete evidence to suggest its use went further than the early seventh century, and while its continuation into later periods is speculated, it’s not well documented.

Eventually, the onager catapult was superseded by more advanced mediaeval siege engines such as the trebuchet, which offered greater range and accuracy, reflecting the continual advancement in military technology.

During the Renaissance, a period marked by the revival of classical knowledge, the onager became a subject of fascination among the great minds of the age, indeed of any age.

Figures like Leonardo da Vinci, Niccolò Fontana Tartaglia, Guidobaldo del Monte, and Francesco di Giorgio Martini studied the mechanics of Roman siege weapons, including the onager catapult, integrating their principles into broader explorations of physics and mechanics. The study of the onager helped bridge the gap between ancient technologies and Renaissance innovations, as engineers sought to understand the underlying principles of torsion and leverage. This interest contributed to the development of more sophisticated mechanical devices and machines, drawing directly from the lessons learned from the ancient catapult and other siege technology.

In modern engineering, the principles demonstrated by the onager remain relevant, particularly in fields that involve the dynamics of torsion and catapult-like mechanisms. For instance, the basic mechanical principles of this Roman siege weapon can be seen in the design of modern launch systems for roller coasters, and catapults on the world’s biggest aircraft carriers, where controlled and powerful propulsion is required.

The Roman onager epitomises the design and strategic acumen of ancient Roman military engineering. As a formidable Roman siege weapon, it not only dominated the battlefields of the Empire and beyond, but also left a lasting imprint on the evolution of siege warfare and mechanical design.

Its influence and legacy extended through the Renaissance and into modern engineering, demonstrating the enduring relevance and impact of ancient technology in modern times.