Lego trebuchet part I or, What I’ve been doing with my summer vacation so far

Summer vacation for graduate students is generally not much of a vacation. Things need to get done. Things like theses. Things like moving. So as you can imagine, I’ve been pretty busy building a Lego trebuchet.

Now I know this isn’t really related to paleontology, paleoanthropology, or pop culture by any real stretch of the imagination. But it’s pretty cool and I figure it’s of some interest to geeks.

The genesis of this idea came a few weeks ago during a party at my house. And as my friends are pretty cool, conversation turned to siege engines. So my friend Chris made an attempt to build a Lego trebuchet. He ended up making a mangonel or onager of some sort (this was a party and there was a lot of drinking) that fired a projectile pretty far (it used tension from rubber bands as propulsion instead of a counterweight). I decided to take his basic plan and modify it into a working counterweight trebuchet, even though I really don’t have a clue how they work aside from playing a lot of Medieval: Total War. Turns out it’s challenging.

First I made the base structure larger and higher than his original. A lot of this was constructed with the intention of making a winch mechanism so any theoretical Lego people could lift the counterweight back into place after firing. I tried to make a proper differential to more efficiently distribute the forces involved, but it ended up not really working.

The second challenge was the trigger mechanism. I wasn’t exactly sure how to keep the big arm in place while still being able to release it at the push of a button. Or, brick, in this case. I settled on a design where a rubber band keeps tension on a right-angled brick that latches on to a little axle on the arm. If you push down on the thin 4-holed arm in the foreground of the picture, the right angle rotates back and releases the arm. It works pretty well.

Now the weight. This is the key to the whole operation. In most of these pictures I’m using a squished-up fossil rugose coral. It’s the heaviest thing I had on me that would fit in my little net. I think I’ve settled on a piece of basalt from Mt. Etna (thanks Mom!) though. It’s lighter, but it’s also a tad smaller and tends not to get stuck in the structure itself.As of right now, it doesn’t fire spectacularly. Maybe a foot or so. I took it over to Chris’ and he made a bunch of suggestions that I plan on putting into action. First, we need to raise the structure just a little bit to give the weight room to swing. Second, the structure and main axis of rotation needs to be stabilized. Third, and perhaps most important, the projectiles need to be in a long sling that can go taut but also releases at a very specific time (instead of a bowl that simply flings a projectile, like a catapult). This essentially increases the length of the arm, swinging the projectile in a much larger radius. The analogy that springs to my mind immediately is an atlatl.

Anyway, so that’s the trebuchet so far. Changes will be made, things will be fired, Lego castles will be destroyed.