Through our research, the HES decided the most desirable reproduction to build would be based on the trebuchet known as the “Conrad Kyeser of Eichstatt” which is depicted in the fifteenth century manuscript titled, Bellifortis. This manuscript, referenced as Codicil MS. philos. 63, and attributed to the year 1405 c.e., is now held by the Niedersächsische Staats- und Universitätsbibliothek, Göttingen (Germany). On folio 30r of the manuscript, a detailed drawing of the Conrad Kyeser trebuchet is depicted, and presented below.
The HES selected the Conrad Kyeser Trebuchet because the medieval depictions of the machine including critical structural measurements for the machine. As Paul E. Chevedden noted in his article, “The Invention of the Counterweight Trebuchet: A Study in Cultural Diffusion,” the manuscript reported important, but incomplete information. On these issues, Chevedden stated:
The main beam measures 54 “workfeet,” or 15.55 m, with a throwing arm of 46 “workfeet,” or 13.248 m. The distance from the axle of the beam to the axle of the hinged counterweight box is 8 “workfeet,” or 2.304 m, dividing the beam in the ratio 5.75:1. A ratio of 6:1 is designated for the beam of a trebuchet in the Innsbruck manuscript of Bellifortis (Fig. 4), indicating that the dimensions given here may have been miscalculated. The trestle frame is composed of two linked supporting trusses, each forming an equilateral triangle with base and sides measuring 46 “workfeet,” or 13.248 m. The main axle is placed at the apex of the trusses 11.47 m above the ground.
With the benefit of understanding the dimensions of key machine members in relationship to each other, specifically the throwing arm, main beam, and frame, the HES believed it could reverse engineer the entire design of the Conrad Kyeser Trebuchet.
After completing its initial historical research tasks, this spring 2012 the team aggressively pursued the design process for the building the trebuchet. These key efforts include:
- Converting the initial concept sketches into detailed computer-assisted design (CAD) drawings and identifying the building materials for the trebuchet.
- Conducting engineering stress analyses to ensure the trebuchet would function in a safe manner, and
- Securing a secure work site for constructing the trebuchet.
Working as a design team led by Mr. Putnam and Mr. Ohnaka, the HES labored to develop and consider all of the major components of the reproduction trebuchet. Mr. Putnam crated and revised all of our intricate CAD drawings and Mr. Ohnaka conducted computer-based stress analyses of the designs. Mr. Sandoval formed relationships with local lumber mills so as to determine who might provide the best materials for constructing the trebuchet, as well as locating a securing location on campus to build the device. The final HES design is depicted below.
Final HES Trebuchet Design
The key measurements of the reproduction trebuchet, which is a 20% scale design of the original Conrad Kyeser Trebuchet, are:
- the throwing arm is 12 feet in length,
- the A-frame trestle support is 7 feet in height, and
- the counterweight box is can accommodate between 200 and 400 pounds of weight.
It is anticipated that our reproduction trebuchet will initially launch five-pound projectiles.
 Chevedden, Paul E. “The Invention of the Counterweight Trebuchet: A Study in Cultural Diffusion,” Dumbarton Oaks Papers, Vol. 54 (2000), pp. 74-75.