For the past couple of weeks I've been working on a team of mechanical engineers to design and build a trebuchet for the UVU Pumpkin Shenanigans event. I initially signed up for this project after seeing a flyer on the wall of a UVU hallway saying engineers wanted. I was in search of projects at that time so I texted the number on the flyer and received the unfortunate news that all of the positions had been filled. I was pretty bummed out but life went on.
About a week later I got a text back from the same number saying that someone had dropped out and they were looking for someone to fill the position designing the sling, release mechanism, and firing mechanism for a trebuchet. I immediately said yes, even though I had no idea how a trebuchet worked. I was thrown into a group chat with the other engineers and I was very excited because this was my first actual engineering project. Since I hadn't taken statics or dynamics, I really had no idea what I was doing and I was definitely "faking it til I made it".
The plan
During the first couple of weeks of the project, the group and I focused on research and planning out the many constraints of the project. I don't think any of us had any idea of what we were getting ourselves into. We initially decided we were going to have a 10ft arm and an 800lb counterweight in order to launch pumpkins several hundred feet.
I was pretty content to be put on the sling team because it seemed like it required the least amount of engineering experience and I didn't want to mess anything up. I spent a lot of time watching YouTube videos of other people who had designed pumpkin launching trebuchets. It was really fun.
Our initial CAD design
Day one
Eventually we settled on a design and it was time to start building. The first day was just me and another kid named Isaac. Isaac is a freshman and I don't really know what I am. Somewhere between a sophomore and a junior I guess. The point is, neither of us had really ever engineered something before so we were a little bit clueless.
We started off going to Home Depot to get some 4x4s (which I found out are actually around 3.5x3.5 inches. Why is that still a thing?), wood screws, and brackets to hold the thing together. Our total cost for all the wood was around $270 which I'm told was not that bad, but I honestly had never bought wood before and it seemed a little bit more than I expected.
Loading my truck full of wood on the first day of construction. I was driving real slow on the way to the construction site.
Oh and also halfway through building we decided to cut down on our weight to ~300lbs because of budget cuts so that was a bummer. What are you gonna do.
We ended up dumping all of our stuff at Isaac's neighbor Hiro's house (who we affectionately called "Uncle Hiro" for the rest of the project), because he had all the power tools and space to drop our junk. Because we were construction beginners, it took us about 6 hours to buy the wood and cut all the right angles and lengths. We found a flashlight and we worked until a couple hours past dark. I was having a lot of fun learning how to use the power tools and now I've found the desire to build a collection of my own.
Isaac and I posing in front of our work after the first day (and night) of construction
Assembly
While assembling the trebuchet after deciding we were dropping the weight by nearly 500lbs we came to regret going with 4x4s instead of 2x4s. It was definitely overkill and they were a pain to work with. Drilling holes with our old drills didn't work very well and even with 5 inch wood screws it didn't feel like we were getting a very good bite between beams. We also used a lot of 90 degree brackets to hold everything together.
Isaac and Nathan testing out the freshly assembled trusses
In the end, the thing took a couple of days and a few unfortunate rainstorms to assemble with no major hitches in the assembly. We put it on a trailer and brought it out to the event field for testing.
The trebuchet in 'Uncle Hiro's' backyard after we finished putting it together.
Testing
Initial testing was subpar. We failed to realize the impact that friction would have between the axle and the arm so our tests were not going great. We also didn't account for the arm moving left and right after launches and it was extremely difficult to get back centered. Our eventual solution was to just wrap some thick rope around the axle which pinched the arm in the center good enough.
We were also having major issues with our sling. We thought we could just sort of weave it with a thick rope but that did a terrible job of releasing consistently. We really had no control whether the thing shot backwards or forwards. We finally settled on a pouch approach which left no room for pumpkins to slip between the cracks.
We also were having issues with our release angle. We first thought drilling a thin 90 degree bracket onto the end of the arm would be a great idea, but upon testing we found that the bracket bent far too easily which caused the launch angle to vary between every launch. We fixed this by hammering a nail right behind the 90 degree bracket to give it some extra support and then we used some bolt cutters to trim off the head of the nail so it wouldn't catch on the rope. This ended up working great.
After testing and fixing all the major issues we were excited and ready for the big event the next day.
Event day
We showed up about an hour before the event started just to get some more test fires in with the actual pumpkins from the pumpkin patch we ordered. We went to fire the first one and quite unfortunately the weight dropped and snapped the holes in the 2x4s carrying the weights clean through. The weights dropped straight to the ground right with our jaws. The first thing I thought was "at least this will make a good blog post".
Luckily a non-profit who specializes in teaching kids history lessons agreed to let us use one of their 15ft trebuchets a few weeks before so the event wasn't completely off, but we had a big dilemma on our hands. The event was supposed to start in 45 minutes and we had a broken trebuchet that cost a whole lot of money.
We could've given up and stayed content with just the other trebuchet or we could've tried to fix it. We ended up driving to the nearest hardware store as quick as we could, sprinting through the aisle to find more 2x4s and screws. We sped home to grab our tools and got back to the event field right as the TV reporters at Fox News were showing up to gather info on a story about us. We whipped out our drills as quick as we could and doubled up on the 2x4s that had just broke and by some miracle we got it back together in time. It truly felt like I was in some action movie.
We had some issues firing the day before so we were all on edge hoping it would just work. We loaded our first pumpkin and I got to be the one to pull the pin and by some miracle with Fox News and a crowd of over 100, it fired perfectly. Better than it ever had in the past. I couldn't believe it. You can read the article about it yourself. I genuinely had no faith that it would work but it did.
The rest of the night went extremely well and we had a ton of fun doing over 30 launches. I had so much fun working on this project and I feel like I'm on another level as an engineer now.
The team posing by our trebuchet after the successful event.
Our beautiful creation firing off into the sunset