Gravity-only trebuchet breaks the sound barrier at 776 mph

YouTube engineer Tom Stanton has built a trebuchet that flings a projectile past the speed of sound using nothing but a falling weight — no springs, motors, or explosives. On a UK field, a 40 kg counterweight dropped roughly 1.9 meters, spun a carbon-fiber arm to about 2,340 rpm, and launched a 4-gram projectile at 776 mph, edging nine miles per hour past Mach 1 and producing an audible sonic boom.

The core obstacle is that free fall caps the counterweight’s speed regardless of mass, and a conventional trebuchet arm behaves like a low gear — plenty of torque but limited final velocity. Stanton’s workaround was a tapered pulley drum with a 3:1 ratio: the wide diameter delivers force to get the arm moving, while the narrow end trades that force for a sharp jump in rotational speed as the drop completes. The arm itself had to be extremely light yet rigid, so he CNC-milled it from carbon fiber down to 116 grams, iterating past buckling failures by shaving material from the tensioned side and reinforcing the compressed side. A spring-loaded catch triggered by rope rotations gave a release window only a few milliseconds wide.

Getting there was incremental. A 10 kg counterweight produced 394 mph at 43.7% efficiency; 40 kg reached 716 mph before a 50 kg attempt shattered the clasp. After lightening the projectile to about 4 grams and re-tuning the drum taper, the 40 kg setup finally crossed the barrier — high-speed footage clocked the projectile covering 1.94 meters in 5.6 milliseconds. It’s a striking demonstration of amateur engineering: careful mechanical design and materials work extracting supersonic performance from a medieval siege concept powered by gravity alone.