Big Hurl

A trebuchet hurls its load with a dynamic grace unmatched by other ancient machines of war. This ingenious device reigned supreme as the ultimate weapon for 1000 years, proving the value of a good engineering concept.

A trebuchet comprises three main components: a source of stored energy, a rigid beam acting as a mechanical impedance converter, and a lightweight sling—radically increasing efficiency and launch velocity.

My daughter Allie recently built one (Figure 1). She powered her tennis-ball-throwing machine with thick rubber straps, forming a substantial spring. To prepare the device for firing, you pull down on the beam, tensioning the straps (not in the photograph). Figure 2 shows the beam in firing position, with the sling feeding horizontally back under the mechanism to Point A. Here, the ball begins its flight nestled in a wire basket secured by a steel trigger. Release the trigger, and the rubber spring at Point B promptly hauls down the short end of the beam at Point C, raising the opposite end and propelling the sling, Point D, through a beautiful arc. At Point E, one side of the sling slips free from a strategically placed pin, releasing the load. The whole action, from trigger to release, takes 170 msec.

Figure 2—During competition Allie's ball left the sling
at 228 mph, traveling 235 feet into a slight headwind.

The blue stop-action markings show the beam accelerating wickedly at first, then slowing as the centrifugal force of the ball and sling, whipping upward at fearsome velocity, pulls back on the mechanism. This slowing action is the hallmark of a properly designed trebuchet. The spring or counterweight first pours energy into the beam, increasing its angular momentum. Then, the whipping action deducts the same angular momentum from the beam, bringing it to a standstill, pointing straight up, devoid of energy at the end of the cycle. The only things moving after the projectile cuts free are the projectile itself and portions of the sling. Nothing else harbors residual energy. The whipping action transfers almost all of the stored energy into the projectile.

A full-sized medieval trebuchet stores about 140,000 ft/lbs of energy, representing a massive amount of hand cranking. It can deliver nearly 70 percent of that energy into a 300-pound projectile, wasting precious little time or motion among the artillery crew. In contrast, the more primitive mangonel-style catapult cradles its projectile in a basket affixed to the end of a heavy beam. The mangonel wastes most of its stored energy accelerating its heavy beam only to see that energy lost when the beam smashes into the end-stops at the point of release.

If you have studied resonant-mode switching power supplies, this energy-transfer story may sound hauntingly familiar. A good switcher first connects a source of power to an inductor, charging it with current. The switcher later transfers that same energy neatly into the load at the end of each cycle. Do it right, and the inductor ends each cycle devoid of energy—nothing wasted.

Transmission-line networks involve a related process. A transmitter interacts with the line; the line conveys its energy to the load. If all goes according to plan, nothing bounces back, and nothing gets lost.

Engineers enjoy a long tradition of experience with dynamic processes. Over the centuries, they have developed many diverse means of dealing with them. It is my pleasure to help pass along this tradition to a new generation. Allie, congratulations on winning the 2005 Methow Valley Junior High Tennis Ball Catapult Fling.