Trebuchet

My current project is one that I certainly not feasibly do here in the city. I’m currently constructing a 25’ tall trebuchet designed to throw pumpkins for my town.

For a bit of background, my local community’s claim to fame is a yearly event called Pumpkinfest, featuring a parade, a pumpkin pyramid, and until about 25 years ago, a fleet of weapons designed to throw pumpkins for the entertainment of all.

This is my first large-scale design project I have attempted and I’m currently splitting construction between my driveway and a field about ten minutes away.

I started playing around with this idea in 2023, and me and my friend, in an afternoon, built a miniature trebuchet we used to launch tennis balls across my backyard. This was a slightly different design called a whipper trebuchet, where the counterweight starts right above the axel and the arm does a full rotation before ‘whipping’ the projectile away. There was no design that really went into this one, more a fun project to do on a boring day.

Fast forward to July 2025, and I revisit this for my grade 12 physics final project. I spent way too many hours calculating all the forces and angles to try to determine the amount of torque and energy put into the projectile with the idea of keeping the parameters open so I could ideally optimise the design. I made this with the ‘whipper’ design in mind but made another variation to hopefully model a more traditional trebuchet. The python program I put it in is not my finest work.

Here’s the same equation that takes the parameters and graphs angular acceleration as a function of the angle of the throwing arm throughout the process of throwing.

Around this time I met this man walking around town and we got chatting and he told me about the old Pumpkinfest trebuchet. Since then, my goal has been to bring back this amazing tradition. (I did spend a day in the archives going through newspapers, and as far as I can tell, nobody ever got hurt or anything, they just stopped when they built a new subdivision on the field they used).

Inspired once again, I whipped up (pun intended) a CAD model of a scaled up and more refined trebuchet. A lot of the limiting factors of this project are the strength of the wood I’m using. I can always laminate more boards together, but it just made more sense to simplify my design.

My trebuchet is constructed out of 4×4 beams and other than that one detail, metric. I wanted to be able to take it apart and transport it around as easily as I reasonably could so all the pieces are designed to notch together and then plated get bolted on each side of each joint to hold everything together. The main axel is a 1” (keeping things metric is hard, ok?) steel pipe and the throwing arm has a hole drilled into it. The arm does not actually need to rotate very quickly or far at all so friction isn’t really a problem here. The speed of the projectile really comes from the sling whipping around and releasing exactly where I set it. The counterweight is a custom wooden bucket that holds approximately 150kg of sand. I went with the sand versus a more compact weight system because It gives me a lot of freedom to experiment with the mass, and in the event that something gets jammed or I need to disarm the trebuchet safely I could simply open a hatch and let the sand spill out. The sling is designed to be just as long as the throwing arm and drag along the ground until it is lifted into the air and swung around at a rapid velocity. As soon as I have a weekend I will finish that up and be able to start the final tests before hopefully operating it for the joy and pumpkin-themed holiday spirit of my community.