Need a castle assaulted?
Is that annoying neighbor acting up again?
Or maybe you just like throwing things?
Our world-class trebuchet engineers here at MTEC are waiting to craft for you the trebuchet of your dreams!
We also offer specially trained trebuchet crews, perfect for any
occasion.
Catapult crews come with a certified missile-retrieval engineer to retrieve the missiles after test-firing. Be sure to not release the engineer until the missile has landed.
Be on the lookout for our new HurlMaster-2000, coming this summer to a battle near you!
Enough people asked, that I decided to write this thing up.
A trebuchet is an ancient siege engine, similar to a catapult, invented by the Chinese in roughly 2000 BCE. Unlike a catapult, which uses a whippy throwing arm to propel the missile, the trebuchet uses a relatively stiff arm mounted on a pivot. Impetus is provided by a heavy counterweight attached to the other end of the throwing arm.
We built this trebuchet for a contest, and the rules specified that we couldn't use any "modern" materials (basically, anything invented in the past two millennia), and that the base had to fit into a 1-meter cube.
The square base was one meter on a side, and made out of 2x4's bolted together underneath using metal right-angle brackets. The bolts stuck up through the bottom and fitted into holes in the base of the vertical supports. The blocks at the corners are to provide additional support to keep the supports from slipping forward or back.
The supports on each side are bolted at the top to a block of wood, which has a hole in it through which the pivot is passed. The pivot is an iron rod, about twenty centimeters long and 5 millimeters thick. The arm is approximately 80 cm long on the short side, and around 2 meters on the long side. It is bolted to a small block of hardwood, through which there is a hole for the pivot. This keeps the pivot-hole from weakening the throwing arm. The counterweight masses approximately 15 kg and is made of old lead weights from a diving belt.
One way of firing is to simply lift the counterweight, place the missile on the end of the arm, then let go. Using this method, we were able to throw the half-kilo missile approximately ten meters.
A better way is to loop a string over the protrusion on the end of the throwing arm, and tie the other end to the missile. To fire, you lift the counterweight, loop the string over the protrusion, place the missile on the ground as close to the pivot as possible, then let go. The missile initially swings back, then up, and then is whipped around up and over the top of the now-vertical throwing arm. Using this method increases the effective length of your throwing arm by 30% or so, improves the release angle (45 degrees is the ideal angle), and looks a lot cooler. Using this method we were able to approximately double our range, up to around twenty meters.
The length of the string is critical: up to a point the range gradually increases as the length of the string increases, but after that point the range falls off sharply. (The optimum string length for us was approximately 170 cm.) Using too short a string can cause the missile to fly straight up or even backwards!
The buzzwords here are a longer throwing arm, a lighter throwing arm, a longer counterweight arm, and more counterweight. Ideally you want the weight to accelerate downward as close to 9.8 m/s/s as possible.
Having a taller base would help a lot, since you could increase the length of the counterweight arm. We were limited by the one meter size limit, but we should be able to increase the height by making the base narrower and fitting the trebuchet diagonally into the cube. We weren't this clever the first time around.
The weak point of the whole thing was the pivot and the slanted supports. We used an iron rod about 5 mm thick which went through a hole in a block which was bolted onto the throwing arm. (We didn't run it directly through the throwing arm because we were worried about it cracking.) When you put weight on the arm, the rod would bend a little as the supports moved slightly inwards. The only thing keeping the supports from collapsing inward was the iron pivot; I think that given enough use the pivot would crack the top of the supports.
We used a wide base with plenty of sideways clearance to keep the arms clear of the 15 kg counterweight. After firing it a few times, we realized that this wasn't so critical. You could decrease the angle with the vertical made by the supports by making the base narrower. You could even use vertical supports and a longer pivot rod, but you would have to be careful about the pivot rod bending, breaking, or binding against its mountings and decreasing efficiency. Since the throwing operation is so smooth, you might be able to get away with resting the pivot in grooves on top of the supports, or just using large holes; this should reduce binding. You could also use a thicker rod, but that would mean a thicker hole in the throwing arm which is the weakest part of the whole thing.
You could experiment with a tapered throwing arm, that would be strong near the pivot, and light in weight out near the end. You could use different materials in the throwing arm: hardwood down close to the pivot, and then switch to light, strong bamboo out near the end. (We didn't have any bamboo for this one.)
