"Really Throwing It", ME Online Web Exclusive, Dec. 6, 2005

for 12/06/05

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Really Throwing It

A model of a medieval weapon sets a school record.

by Kirk D. Hagen

Ever since a wealthy Englishman started throwing small cars and grand pianos from his replica of an ancient war machine, you could say the trebuchet has made a comeback.

The Englishman, Hew Kennedy, built a 30-ton trebuchet from which he has hurled dead pigs and cows, too, in the fields near his home in Shropshire, England.

His enthusiasm spread rapidly. Now, a Google search for the word trebuchet can yield more than 1.3 million hits. Many of them will turn up pictures of Kennedy's trebuchet experiments.
During the last 10 years or so, numerous colleges and universities, including the U.S. Military Academy at West Point, South Dakota School of Mines and Technology, and Wright State University, have used the trebuchet in engineering design courses, physics projects, and recruiting events to attract students to engineering and science programs.

The trebuchet (pictured above) is different from the catapult in that the trebuchet is powered by the potential energy in a raised counterweight. The catapult, on the other hand, is powered by the potential energy in a twisted bundle of ropes, horsehair or other sinews.

Working from copies of centuries-old woodcuts and other drawings of ancient siege engines shown in books and magazines, and on various Web sites, students usually build trebuchets from scratch. Students have built trebuchets on scales that heave everything from peas to pumpkins. In latter years, kits designed for hurling specific projectiles have become available from a number of online suppliers.

Playing with trebuchets is downright fun, and designing and testing them is an excellent way to learn mechanical engineering design and Newtonian mechanics. By designing trebuchets, students learn how to select materials and basic parts for performing specific mechanical tasks. They also learn how to design basic static and dynamic structures for the frame and throwing arm, respectively.

Students discover how the mass of the counterweight and the length of the throwing arm affect the velocity and range of the projectile. In making adjustments to the angle of launch, students learn firsthand the physics of projectile motion. Through trial-and-error testing (and, if they have sufficient knowledge of mathematics and analytical mechanics), students are able to optimize a trebuchet for maximum hurls, or hurls that aim for a target a known distance away.

A trebuchet is an ancient military engine for hurling heavy missiles (usually rocks) at an enemy. A trebuchet is powered by the potential energy in a raised counterweight, unlike a catapult, which is powered by the potential energy in a twisted bundle of ropes, horsehair, gut, or other sinews.

Why is the trebuchet so popular, particularly with students? Perhaps people take an instinctive pleasure in throwing things. Small children have an innate knack for throwing food, toys, and whatever else they can get their hands on. Perhaps students take to trebuchets because of leftover mischief from their childhood.

Part of the trebuchet's popularity lies in its colorful history. Despite their daily immersion in the modern technology of computers, cell phones, MP3 players, iPods, and other electronic gadgets, students are captivated by ancient technology.

Many scholars believe that Chinese engineers designed a form of the trebuchet as early as the seventh century, but most of what we know about the device comes from medieval Europe.

An early version of the war engine called the traction trebuchet, because it was operated solely by manpower, was difficult to handle. Using a traction trebuchet, Scottish soldiers besieging Wark Castle in Northumberland in 1174 barely managed to propel a missile out of its sling. The missile landed on the head of one of their own men.

Problems inherent in the traction trebuchet were solved with the advent of the counterweight trebuchet, which became the medieval world's most intimidating weapon.

Besiegers were as creative as they were brutal with this engine of war, which some historical documents identify as the "malvoisin" or "bad neighbor." Warriors might strap a captive soldier or an unwelcome messenger to the throwing sling of the trebuchet and then hurl him, or his body parts, back to the enemy.

When King Edward I (Longshanks in the movie Braveheart) mounted a siege on Stirling Castle in 1304, he ordered the construction of a trebuchet, which was given the name Warwolf. When the Scottish defenders spotted the colossal trebuchet, they immediately offered their surrender, but the ruthless king proceeded with the assault anyway, just to see how his weapon would perform.

The trebuchet was even used in the practice of biological warfare. At Carolstein in 1422, Lithuanian attackers flung the bodies of dead soldiers into the castle along with 2,000 cartloads of manure in a successful attempt to spread infection.

At Weber State University there is a trebuchet tradition. We have used the device to teach the fundamentals of mechanical engineering design and technology since 1993. Every fall semester, freshmen in the Introduction to Mechanical Engineering Technology course divide into teams of three or four to design, build, test, and document a model trebuchet. The objective is simple: hurl a standard 6-ounce, 3.8-inch-diameter softball as far as possible.

The principal design constraints are size and energy source: The entire trebuchet, in the cocked and ready-to-launch position, must fit within a cubical space measuring 24 inches on a side, and the sole source of energy must be a raised counterweight. No other energy sources, such as springs, compressed gases, electromagnetic devices, or so on, may be used. There is no weight limit for the counterweight or the trebuchet itself, and the throwing arm cannot be articulated. Any engineering materials may be used as long as they are safe.

In the fall semester of 2004, a team of four students-Josh Lafleur, Sean Trumble, Mark Boivin, and Richard Bell-set the current Weber State record with a hurl of 184.0 feet, or about 56 meters. The previous record, set in 1995, was 174.9 feet, or just over 53 meters. Since the inception of the trebuchet contest in 1993, most hurls have fallen in the 40- to 100-foot range.

The record-holding trebuchet consists of a welded steel frame with a pair of bearings that support a horizontal steel shaft attached to the throwing arm, which is made from an old composite hockey stick. A sling consisting of a cloth pouch at the end of a pair of heavy-duty strings is attached to the end of the throwing arm. The counterweight consists of two large, freely swinging blocks of steel weighing about 50 pounds each.

We are not aware of any schools holding trebuchet contests on a regular basis under the design constraints and rules we follow at Weber State University. We propose that other schools design and build trebuchets according to the conditions stated here. If we standardize the rules, the next step would be to start a new intercollegiate sports league for trebuchet competitions.

Responses to this proposal are invited.

Editor's note: For those of you who keep issues going back that far, in January 1994, Mechanical Engineering published an article, "Building a Better Trebuchet," that chronicled some early efforts, including those at West Point and the University of Texas, to recreate trebuchets.

Kirk D. Hagen is a professor of mechanical engineering technology at Weber State University in Ogden, Utah. Trebuchet contenders can reach him by e-mail at khagen@weber.edu.