The Trebuchet: A General Idea
The concept of the trebuchet is a simple one, almost entirely based on simple machines. A sling is attached to one end of a lever, with force applied to the other end and a pivot in the middle. A lever allows forces to be multiplied by altering the distance that force is applied through, so a small force applied over a large distance can replace a large force. Alternatively, a large force applied through a small distance can be converted into a small force applied over a large distance. It is this latter case that a trebuchet makes use of, using a large force to move a small object through a large distance (and at high velocity).
This general concept was consistent throughout the trebuchet’s history. The main point of development was the process by which the large force was applied to one end of the lever to produce the rapid motion of the other end. The most powerful version of this is the counterweight trebuchet, where potential energy is stored in a large mass, raised off the ground. When this mass is allowed to swing, its potential energy is transferred to the projectile as kinetic energy.
The original trebuchet was what is now known as the “traction trebuchet.” This trebuchet used people hanging from ropes as a counterweight. However, as there are only so many people you can fit under a trebuchet, this setup had natural limitations in the size and speed at which it could launch a projectile. By introducing a self-contained counterweight, significantly more force could be applied to the lever, at the sacrifice of some firing speed (it takes longer to reload a counterweight trebuchet than it does for a large group of people to stand up). This allowed for a more efficient system in several other ways, most notably positioning the projectile in the sling closer to the pivot for a more efficient motion, as well as other innovations such as the swinging counterweight.
In trying to understand the value of the trebuchet, it is helpful to compare it with another well-known siege weapon: the catapult. The primary goal of both is the same: to launch heavy and deadly projectiles at structures/troops or over fortifications. However, the similarities basically end there. While the trebuchet stores the projectile’s energy by means of raising a large mass, a catapult is powered by torsion—energy stored by the deforming of rigid objects. This is usually done in the form of bending wood, twisting ropes or sinews, or both.
There are some natural advantages to the trebuchet. It is much simpler, mechanically, making it far easier to build, and its motion puts much less strain on the pieces of the engine, allowing for larger amounts of energy to be stored than a catapult can handle, and, as a result, larger projectiles can be launched. It is also far less susceptible to rain, humidity, or damage to small elements of the system causing the whole thing to fly apart violently. There are a few downsides, of course, such as the size of the trebuchet making it an easier target, as well as its general immobility, but in warfare the amount of damage a trebuchet is capable of above and beyond what a catapult can achieve certainly makes up for it.
I tried to focus on what a full-scale trebuchet would need in building mine. The launch is controlled by a small “finger” at the end of the throwing arm, at an angle to the arm. When the sling reaches a certain angle with this finger, the finger will no longer be able to hold the sling, releasing the sling and its payload. The pully system (image 2 below) for drawing the arm back down would be attached to a team of horses on a real trebuchet, who would pull the arm down to where it could be locked into place (image 3), at which point they would be unhitched from the system, the sling would be loaded, and the trebuchet would fire.
- Chevedden, Paul E., Les Eigenbrod, Vernard Foley, and Werner Soedel. “The Trebuchet.” Scientific American273, no. 1 (1995): 66-71. http://www.jstor.org/stable/24981453.
- Hacker, Barton C. “Greek Catapults and Catapult Technology: Science, Technology, and War in the Ancient World.” Technology and Culture9, no. 1 (1968): 34-50. doi:10.2307/3102042.
- Chevedden, Paul E. “The Invention of the Counterweight Trebuchet: A Study in Cultural Diffusion.” Dumbarton Oaks Papers54 (2000): 71-116. doi:10.2307/1291833.
- Renatus, Publius F. V. De re militari. c. 400 A.D.
- “The Lever.” Hyperphysics. http://hyperphysics.phy-astr.gsu.edu/hbase/Mechanics/lever.html (December 1, 2018).
- “Trebuchet.” Wikipedia. https://en.wikipedia.org/wiki/Trebuchet (December 1, 2018).
- Άγνωστος/Unknown – Χρονικό Ι. Σκυλίτζη, Μαδρίτη, Εθνικό Μουσείο/Skylitzes chronicle, Madrid, National library., Public Domain, https://commons.wikimedia.org/w/index.php?curid=13300143 (December 10, 2018).
- Tschanz, David W. “History’s Hinge: ‘Ain Jalut.” Saudi Aramco World. https://web.archive.org/web/20070912161811/http://www.saudiaramcoworld.com:80/issue/200704/history.s.hinge.ain.jalut.htm (December 13, 2018).