The Trebuchet Recent reconstructions and computer simulations reveal the operating principles of the most powerful weapon of its time by Paul E. Chevedden, Les Eigenbrod, Vernard Foley and Werner Soedel enturies before the development of eÝective cannons, huge artillery pieces were demolishing castle walls with projectiles the weight of an Cupright piano. The trebuchet, invented in China between the Þfth and third centuries B.C.E., reached the Mediterranean by the sixth century C.E. It displaced other forms of artillery and held its own until well after the coming of gunpowder. The trebuchet was instrumental in the rapid expansion of both the Islamic and the Mongol empires. It also played a part in the transmis- sion of the Black Death, the epidemic of plague that swept Eurasia and North Africa during the 14th century. Along the way it seems to have inßuenced both the development of clockwork and theoretical analyses of motion. The trebuchet succeeded the catapult, which in turn was a mechanization of the bow [see ÒAncient Catapults,Ó by Werner Soedel and Vernard Foley; SCIENTIFIC AMERICAN, March 1979]. Catapults drew their energy from the elastic deformation of twisted ropes or sinews, whereas trebuchets relied on gravity or direct human power, which proved vastly more eÝective. The average catapult launched a missile weighing between 13 and 18 kilo- 66 SCIENTIFIC AMERICAN July 1995 Copyright 1995 Scientific American, Inc. manajaniq (An Elegant Book on Treb- uchets), written in 1462 C.E. by Yusuf ibn Urunbugha al-Zaradkash. One of the most profusely illustrated Arabic manuscripts ever produced, it provides detailed construction and operating in- formation. These writings are particu- larly signiÞcant because they oÝer a unique insight into the applied mechan- ics of premodern societies. We have made scale models and com- puter simulations that have taught us a great deal about the trebuchetÕs opera- tion. As a result, we believe we have un- powerful trebuchets, in contrast, could covered design principles essentially launch missiles weighing a ton or more. lost since the Middle Ages. In addition, Furthermore, their maximum range we have found historical materials that could exceed that of ancient artillery. push back the date of the trebuchetÕs spread and reveal its crucial role in me- grams, and the most commonly used Recovering Lost Knowledge dieval warfare. heavy catapults had a capacity of 27 Historians had previously assumed kilograms. According to Philo of Byzan- e have only recently begun to re- that the diÝusion of trebuchets west- tium, however, even these machines Wconstruct the history and operat- ward from China occurred too late to could not inßict much damage on walls ing principles of the trebuchet. Schol- aÝect the initial phase of the Islamic at a distance of 160 meters. The most ars as yet have made no comprehen- conquests, from 624 to 656. Recent sive eÝort to examine all the available work by one of us (Chevedden), howev- evidence. In particular, Islamic technical er, shows that trebuchets reached the literature has been neglected. The most eastern Mediterranean by the late 500s, 1 important surviving technical treatise were known in Arabia and were used on these machines is Kitab aniq Þ al- with great eÝect by Islamic armies. The MODERN RECONSTRUCTION of a medieval siege engine hurls a piano across the English countryside. This trebuchet, patterned after ones used in Europe and the Middle East, has lofted objects weighing as much as 500 kilograms. Its largest fore- bears could toss more than a ton. (The 60-foot-high reconstruction was built in Shropshire by Hew Kennedy and Richard Barr.) 2 7 3 4 6 VIN GUGLIELMO GAL Copyright 1995 Scientific American, Inc. technological sophistication for which logical warfare had perhaps its most neers thickened walls to withstand the Islam later became known was already devastating impact. As Mongol forces new artillery and redesigned fortiÞca- manifest. besieged this Genoese outpost on the tions to employ trebuchets against at- The Mongol conquests, the largest in Crimean peninsula, the Black Death tackers. Architects working under al- human history, also owed something swept through their ranks. Diseased Adil (1196Ð1218), SaladinÕs brother and to this weapon. As a cavalry nation, the corpses were then hurled into the city, successor, introduced a defensive sys- Mongols employed Chinese and Mus- and from KaÝa the Black Death spread tem that used gravity-powered trebu- lim engineers to build and operate treb- to the Mediterranean ports of Europe chets mounted on the platforms of tow- uchets for their sieges. At the invest- via Genoese merchants. ers to prevent enemy artillery from com- ment of KaÝa in the Crimea in 1345Ð The trebuchet came to shape defen- ing within eÝective range. These towers, 46, the trebuchetÕs contribution to bio- sive as well as oÝensive tactics. Engi- designed primarily as artillery emplace- Sling was attached firmly to the beam at one end The Physics and looped over a metal prong at the other. When it of the Trebuchet reached the proper angle in its arc, the loop would fly free, releasing the projectile. Proper adjustment of the prong and the overall length of the sling were he motion of the trebuchet is crucial to achieving maximum range. T simple enough in its essentials to have inspired medieval studies of motion, but its details are subtle and require computer simulations to interpret accurately. Only recently have we come to understand how the rotation of the counterweight plays a crucial role in transferring energy to the beam and thence to the sling and projectile. FIXED COUNTERWEIGHT TRACTION Earliest trebuchets were powered by crews Addition of counterweights increased the power of the treb- pulling on ropes rather than by counterweights. uchet. The elimination of the pulling ropes made possible anoth- Crews of as many as 250 men pulled to send er innovation: by placing a trough under the trebuchet beam to projectiles 100 meters or more. In this example of hold the projectile, engineers could lengthen the sling and in- a small traction machine, the sling-holder’s weight crease the range even further. The sling rotates faster after the flexed the beam and increased the range. shot is airborne, so its length controls the launch angle. 68 SCIENTIFIC AMERICAN July 1995 Copyright 1995 Scientific American, Inc. ments, took on enormous proportions long end swung upward, hurling the projectile on the other end of the arm to accommodate the larger trebuchets, missile from the sling. might weigh between 200 and 300 kilo- and castles were transformed from Three major forms developed: trac- grams, and a few trebuchets reportedly walled enclosures with a few small tow- tion machines, powered by crews pull- threw stones weighing between 900 and ers into clusters of large towers joined ing on ropes; counterweight machines, 1,360 kilograms. With such increased by short stretches of curtain walls. The activated by the fall of large masses; capability, even dead horses or bundled towers on the citadels of Damascus, and hybrid forms that employed both humans could be ßung. A modern re- Cairo and Bosra are massive structures, gravity and human power. When trac- construction made in England has as large as 30 meters square. tion machines Þrst appeared in the tossed a compact car (476 kilograms Mediterranean world at the end of the without its engine) 80 meters using a Simple but Devastating sixth century, their capabilities were so 30-ton counterweight. far superior to those of earlier artillery During their heyday, trebuchets re- he principle of the trebuchet was that they were said to hurl Òmountains ceived much attention from engineersÑ T straightforward. The weapon con- and hills.Ó The most powerful hybrid indeed, the very word ÒengineeringÓ is sisted of a beam that pivoted around machines could launch shot about three intimately related to them. In Latin and an axle that divided the beam into a to six times as heavy as that of the most the European vernaculars, a common long and short arm. The longer arm ter- commonly used large catapults. In ad- term for trebuchet was ÒengineÓ (from minated in a cup or sling for hurling the dition, they could discharge signiÞcant- ingenium, Òan ingenious contrivanceÓ), missile, and the shorter one in an at- ly more missiles in a given time. and those who designed, made and tachment for pulling ropes or a coun- Counterweight machines went much used them were called ingeniators. terweight. When the device was posi- further. The box for the weight might Engineers modiÞed the early designs tioned for launch, the short arm was be the size of a peasantÕs hut and con- to increase range by extracting the most aloft; when the beam was released, the tain tens of thousands of kilograms. The possible energy from the falling coun- terweight and to increase accuracy by minimizing recoil. The Þrst diÝerence between counterweight machines and their traction forebears is that the sling Hinged counterweight HINGED on the end of the arm is much longer. COUNTERWEIGHT machines added yet another This change aÝects performance dra- increment to the range by im- matically by increasing the eÝective proving the efficiency with length of the throwing arm. It also which the trebuchet converted opens the way for a series of additional gravitational energy to projec- improvements by making the angle at tile motion. The center of grav- which the missile is released largely in- ity of the weight fell straight dependent of the angle of the arm. By down during the first phase of varying the length of the sling ropes, acceleration; as the hinge engineers could ensure that shot left straightened, the rotation of the machine at an angle of about 45 the weight around its center of degrees to the vertical, which produces gravity added to the energy the longest trajectory.