Lecture 21: Machines at War – Evolution of the Catapult
Understanding Greek and Roman Technology: From the Catapult to the Pantheon
Dr Stephen Ressler (2013)
Film Review
According to Ressler the catapult provides an excellent case study of 700 years of engineering development. In 393 BC Dyonysius of Syracuse created the first military think tank to design and build weapons for his campaign against Carthage (see Military Technology in Ancient Greece and Rome ). The catapult grew out of a need to overcome limitations of the bow and arrow.
In engineering terms, a bow is a beam that operates by bending. Compression energy is created on convex side of the bow, while tension energy is created on the concave side. This stored elastic energy is released as kinetic energy (propelling the arrow).
According to Ressler, there are three ways to increase this kinetic energy
- Increasing the length of the draw.
- Shortening the bow string to increase the bow tension
- Making the bow stiffer, requiring more energy to bend it (released when the string is released).
In the 2nd millennium BC, steppes nomads learned how to make a stiffer, composite (enabling heavier arrows to travel further) bow by combining wood, animal horn and animal tendons.
Greek Catapults
The next improvement was the gastrophetes, created by ancient Greeks in 399 BC: It attached a wooden slider to the bow with an iron trigger mechanism. The bowsman cocked it by applying his full weight to the belly brace. Not only did it have a longer range, but it could penetrate armor.
As new versions of the gastrophetes became heavier and more difficult to cock, a windlass was added to make a primitive catapult and it was fitted on a base.The oxybetes, invented in 375 BC, was larger than the gastrophetes and could shoot both large iron tipped bolts and 50 pound rocks.
When the bow couldn’t be lengthened any further, it was replaced by two torsion springs, made of twisted bundles of animal tendon.
The ancient Greeks classified artillery pieces into two categories: arrow shooting euthytones and stone throwing palintones. A full size palintone could throw 200 pound rocks 400 years and 50 pound rocks 700 years.
euthytone
palintone
In his 275 BC De Architechtura, Vetruvius cites the scientific formula used to calculate the ideal diameter of the torsion springs:
D = 1.1 x ∛100 M
M equals the weight of the projectile (in minas) and D equals the diameter of the torsion springs (in dactyls).
Although the ancient Greeks and Romans didn’t have a mathematical method of calculating cube roots, Eratosphenos inventing a process of determining their value by graphing them.*
Late model Greek catapults replaced the rope torsion springs with bronze leaf springs and pneumatic sliders.
The Scorpio increased the catapult’s power with forward moving arms.
Roman Catapults
The Hatra Ballista was a Roman siege catapult powered by torsion springs consisting of rope bundles made from animal sinew. This machine was particularly innovative, with the two wooden throwing arms rotated inward, rather than outward as with conventional catapults. The inward swing allowed for a greater angle of rotation, which increased the elastic energy stored in the springs.
During strife-ridden late empire, the Romans reverted to using the one armed onager, which was less complex and easier to build.
*See https://demonstrations.wolfram.com/TheEratosthenesMachineForFindingTheCubeRootOfTwo/
Film can be viewed free with a library card on Kanopy.
https://www.kanopy.com/en/pukeariki/watch/video/146678/146720
The Most Revolutionary Act 