Supersonic Trebuchet [video]

# Supersonic Trebuchet: Building a Gravity-Driven Trebuchet to Break the Sound Barrier **TL;DR:** A hobbyist engineer designed and built a unique, gravity-driven trebuchet using a pulley system, a carbon fiber arm, and aerodynamic optimizations to achieve projectile speeds approaching the speed of sound (767 mph), peaking at 716 mph with a 40 kg counterweight. ## The Dream of a Mechanical Supersonic Trebuchet The project began with a long-held dream: to launch a projectile to supersonic speeds using only a purely mechanical, gravity-driven trebuchet. The creator acknowledges that traditional trebuchet designs have an inherent speed limit due to gravity, as the counterweight's acceleration cannot exceed 9.81 m/s². To bypass this limitation, alternative methods like elastic bands have been used, but the goal here was to use a falling weight. The core innovation is a new design that decouples the counterweight from the main arm, using a pulley system to create a mechanical advantage. This is likened to "shifting gears," converting the slow but forceful drop of the weight into high-speed arm rotation. ## Physics and Design Innovations ### The Pulley System and Gear Ratio The system uses a **3:1 pulley ratio**. This means the rope connected to the counterweight moves three times faster than the weight itself. This rope is used to rotate the trebuchet arm, effectively multiplying the speed. However, this also means the arm must rotate multiple times (around 15 revolutions) to reach firing speed with a single drop. ### Arm and Sling Design To minimize the energy required to accelerate the arm, a **short, lightweight arm** made from carbon fiber was designed. An unusually long sling (twice the arm length) is used. The sling is wound around a curved section of the arm, allowing the projectile to stay close to the axis during the initial spin and then whip out to its maximum radius at the perfect angle for release. This design keeps the arm tip speed lower relative to the projectile, reducing aerodynamic drag. The arm itself is constructed from **carbon fiber** for its high rigidity and low weight (only 116 grams). Safety precautions, including a HEPA filter and constant water spraying, were taken during the dangerous CNC milling process to avoid inhaling harmful carbon fiber dust. A custom **aluminum hub** was machined to connect the main arm and counterweight arm securely. ### The Power Transfer: Conical Spool A **3D-printed, conical spool** was engineered to optimize the torque delivery. It starts at a large diameter for high initial torque and gradually tapers to a smaller diameter for higher final speed, analogous to shifting gears on a bicycle. The rope used is **4 mm Dyneema**, an ultra-high-molecular-weight polyethylene with a 1.5-tonne strength. ### Mechanical Release Mechanism The release is fully mechanical and synchronized with the rope unwinding. A hinged flap on the spool, connected via an internal rope to a spring-loaded ball detent on the hub, automatically releases the projectile at the exact moment the drive rope finishes unwinding. ### Projectile and Sling Refinements Early tests used 10 mm steel balls, but for safety, **3D-printed biodegradable spheres** were created instead. This allowed precise control over weight and diameter. The sling itself was made from **1 mm Dyneema rope** with a rated strength of 130 kg, braided into a specific zig-zag pouch pattern to securely hold the projectile. ## Iterative Testing and Optimization ### Initial Tests and Efficiency Analysis The first successful launch with a 10 kg weight propelled the projectile to **328 mph** (a personal record). However, energy analysis revealed a major efficiency problem. The system started with 192 J of gravitational potential energy, but only 65 J was transferred to the projectile, resulting in an overall efficiency of just **34%**. Nearly 43% of the energy was lost during the arm's rotation. ### Aerodynamic Optimization To combat energy loss from air resistance, the arm underwent a series of aerodynamic upgrades: * A 3D-printed **sharp leading edge** was added. * The entire arm was covered with **model aircraft film**. * The bulky sling connector was replaced with a sleek, aerodynamic version. * The sling guide was redesigned with vents to prevent air compression. A side-by-side spin test showed the aerodynamic arm achieved a **17.8% increase in rotation speed**. ### Weight Reduction and Structural Testing The projectile end of the arm was further lightened by **40%** with a fully 3D-printed sling seat. A test was devised using the trebuchet's own 3:1 pulley system to simulate the high forces (up to 800 N) on a standard 500 N scale, confirming the printed part could withstand supersonic launch stresses. ### Final Tuning The conical spool was redesigned to maximize the drop height of the counterweight to 1.9 meters, storing an additional 12% of potential energy. This new setup theoretically required an efficiency of only 46% to break the sound barrier. ## Final Tests: Approaching the Speed of Sound With a **10 kg** weight, the optimized trebuchet launched at **394 mph** with an improved efficiency of **43.7%**. Increasing the weight to **20 kg** and then **30 kg** showed promising, incremental gains without failure. The **40 kg** test was the closest attempt. The arm spun up to an incredible **2336 rpm**, launching the projectile at **716 mph**—just **51 mph short of the speed of sound**. The total efficiency was 39%. The release timing was slightly off, causing the projectile to hit the ground. A final, risky test with **50 kg** (exceeding the machine's design limit) was attempted. It resulted in the Dyneema sling snapping at the moment of peak force, proving the extreme stresses involved and concluding the testing phase. ## Conclusion and Learnings The project successfully demonstrated a novel, gravity-driven mechanical design capable of achieving near-supersonic projectile speeds. While it did not definitively break the sound barrier in the recorded tests, it came remarkably close at 716 mph. The journey highlighted the critical challenges of energy efficiency in such a system and the profound impact of aerodynamic drag at high rotational speeds. The structural failure at 50 kg underscores the immense forces at play and the precise engineering required for success. Source: [Supersonic Trebuchet - CharlesW](https://www.youtube.com/watch?v=Co57SfcT-h0)