Island engineer designs supersonic siege engine

Engineer David Eade with his siege engine capable of exceeding Mach 1.

Faster than the speed of sound!

LITTLE CURRENT – Mechanical engineer David Eade of Little Current has crafted a modern take on a traditional siege weapon that lies somewhere between a trebuchet and a catapult, but his modern-day recreation is capable of firing small projectiles at Mach one—the speed of sound—at distances of more than a kilometre in open spaces.

“A supersonic catapult has never been done before,” Mr. Eade told The Expositor. “I learned a lot about machine design from working on these that I’ve applied elsewhere since. To (make a catapult) is very easy, but to do it well requires a fair range of engineering and machine design skills.”

Mr. Eade built his first catapult in the fall of 2017 when a neighbour was planning to enter the Township of Assiginack’s pumpkin catapult competition. The neighbour enlisted him for his engineering expertise to build an unbeatable machine.

“The 2017 competition turned out to be a real rich engineering challenge. It was the perfect balance between something I could do deep, intense optimization studies with, while being complicated enough to hold my interest,” he said.

Mr. Eade’s mechanical engineering work normally involves creating custom robotics for the mining, agricultural and municipal sectors. That’s his day job with a Sudbury research and development firm.

His trebuchet operates using a double pendulum on the release arm. Pendulums are weights hung from an object that can swing back and forth (think of a grandfather clock); a double pendulum is one pendulum hanging from the end of another pendulum.

Double pendulums are an example of dynamical chaos, meaning it is impossible to predict the object’s path when given a starting point, even though the pendulum’s current state influences its immediate next position.

“The idea of doing something in the engineering sense with a chaotic system like this was a big enticement to keep at it,” he said. “But wondering what Mach one would sound like was my real motivation.” 

Mr. Eade had previously built a few smaller models to work out some basic engineering issues but only had one testing element in the final build, as a way of testing air resistance—it was faster to test than to calculate that value.

The entire build process took a couple of months, with development starting in November 2019 and the machine being ready in January 2020.

He completed a detailed series of plans for the machine as he went along and also built a computer simulation tool to model the dynamics of his creation, a task that taught him user interface design skills. A group of siege weapon enthusiasts from Texas has expressed interest in following his plans.

Using a high-speed camera to record his completed machine, he calculated the launch speed at 490 metres per second, or close to one-and-a-half times the speed of sound.

When fired, the siege engine emits a hearty crack as the projectile shatters the sound barrier. Mr. Eade described the noise as somewhere between a .22 rifle shot and the sound of a whip cracking.

That noise is entirely from the shock wave, evidenced by a video recording of a lower-powered shot that is barely audible.

The machine won’t be launching pumpkins at the speed of sound any time soon. He built it from the ground up to only handle small, 3.5 gram projectiles and it is finely tuned for that purpose. 

As for a specific distance range, Mr. Eade said he would not be able to calculate the backspin of the projectile without expensive computational analysis, but figured it would launch about a kilometre in an open space.

That range means the only places he can safely launch his machine are patches of open water with a direct line of sight for several kilometres.

Catapults and trebuchets are not covered by Canadian weapons laws, Mr. Eade said, because those statutes cover weapons with barrels like guns and cannons.

“There’s nothing else to cover launching things at high speeds, aside from the obvious public endangerment issues, which is why we’re shooting into several kilometres of empty lakes. There’s not much risk of any harm there,” he said.

The grand total project cost for this machine was about $200. High-powered machines that are large enough to launch pumpkins at similar speeds might cost upwards of $50,000, he estimated, but those will only reach about two-thirds the speed of sound.

Just like many fellow engineers with active imaginations, Mr. Eade has already moved onto his next challenges, which he says will likely focus on hobby electronics projects.

“Although, if someone were to come to me with a wheelbarrow full of money and ask me to build (another catapult), I wouldn’t say no,” he said with a laugh.

Guinness World Records tracks a category for the farthest distance fired by a trebuchet in three projectile weight division. So far, there are no entries in any of the three groups

A video of Mr. Eade describing his creation.