The Classic Seige Machine
In the medieval world, warfare often involved beseiging, or surrounding and attacking, a fortified location like a city or castle. This was before guns or cannons, so you can imagine the challenge of getting through walls. Many of the weapons available were the same ones used by ancient armies like the Greeks and Romans. One of those weapons was the catapult.
A catapult is a device that uses the sudden release of potential energy to launch an object a great distance. The light onager weighed about 1000 pounds and could hurl a 3- or 4-pound stone about 100 yards. The heavy onager weighed twice as much, but could hit a target 500 yards away with an 8-pound stone.
Catapults were sometimes used to knock down walls. Sometimes they were used to hurl rocks, burning coals, and even parts of dead bodies. The goal was to destroy property and injure or demoralize the enemy. If you've ever watched a movie with a battle scene where one army attacked a fortress, you've probably seen a catapult of these in action.
The onager stored potential energy in twisted ropes or springs, and in the wooden "throwing" arm of the machine. Soldiers would use ropes to pull the throwing arm down, creating tension in twisted ropes or springs. The arm would be secured while a projectile loaded into the bucket and then released. The tension (called torsion because the energy is stored in something that is twisted) would fling the arm forward, launching the payload into air.
There is another kind of catapult called the trebuchet that works differently. Instead of pulling down the arm and storing the energy through torsion, soldiers using a trebuchet would use pulleys to lift a heavy weight called the counterweight.
The counterweight would be attached to one end of a beam that pivoted on the machine's frame. The other end of the beam would be attached to a sling that would contain the payload. When fired, the trebuchet's counterweight would be released. As the weight falls, the opposite end beam springs upward, pulling the sling with the payload in an arc over the machine.
Because of its design, the trebuchet could could hurl heavier objects with greater speed than the traditional torsion catapult. Reproduction trebuchets at the medieval fortress of Château de Castelnaud in southern France. Photo by Luc Viatour.
Catapults fell out of use as weapons over time. We developed cannons and missiles, which have much greater range and accuracy and required less manpower to fire. The idea of using the sudden release of stored to launch something, however, is still useful.
Today, a catapult system is used to help jets launch from aircraft carriers. This system currently uses hydraulic pressure to push carriage along a long rail at very high speed. A plane, attached to the carriage, is pushed forward and the additional energy from the catapult helps it reach the speed necessary for take-off.
Catapults are still around today, although they typically designed and built for chucking pumpkins or other recreational pursuits.