My desktop trebuchet experiment

After watching the NOVA machines perform I just had to build my own trebuchet. A bit of digging on the world wide waste of time produced some sizeable amount of reading on the matter, including several sites that had pictures and plans of small trebuchets. I tried to get a desktop trebuchet kit from America, but the shipping charges were crippling.

Undaunted I continued to search for small trebuchet plans until I found a couple of sites that did the trick. On one site I found plans for a trebuchet with a 20" throwing arm, and someone else had already used the plans to build a trebuchet actual size (the plans printed on 1 sheet of A4 paper) using lollipop sticks. This trebuchet would throw a pea six feet. On another site I found some very useful guidance on the general principles of a trebuchet's form. Using the information I had gleaned from a variety of sources, I decided to start playing with my own lollipop stick trebuchet.

The crucial pieces of information that led to the shape of the tiny trebuchet were these:-

counter-weight should be 80 to 100 times heavier than the projectile
the angle of the throwing arm should be 45 degrees when cocked
the upper part of the throwing arm should be 3 to 4½ times longer than the lower end (I chose 4 times)
the sling should be three quarters the length of the upper part of the throwing arm (for starters, that is).

I had decided that the upper end of the throwing arm would be 1 lollipop stick long, and that I wanted to be able to throw something just a little heavier than a pea. I hazarded a guess that I would be throwing no more than 1 gram, so that meant I needed a counter-weight of around 100 grams. After a bit of sketching using the above angles and ratios I quickly came up with a basic design.

After a couple of evenings and a weekend I got to the test firings. After a little tinkering with the size of the sling pouch the tiny trebuchet would throw a marble some 5 feet, a dice 7 to 8 feet, and a peanut went into (admittedly low) orbit. It's certainly got enough power to have my immediate neighbours in the office worried. The arm rocks after firing, indicating that not all the energy from the weight has gone into propelling the projectile. I think that if the sling was made longer the projectile it would take more energy from the arm, and thus throw farther. However, I've yet to try this. I've got distracted by building the floating arm trebuchets.

For those of you interested in building your own desktop trebuchet, I've now got some plans ready. If you do have a go, how about mailing me and letting me know how you got on. Karen did just that, see the links below for her page.

(treb01treb1.jpg 44kb)	Here is the unloaded tiny trebuchet at rest, it stands at just over 8 inches
(treb01treb2.jpg 54kb)	Here is the trebuchet broken into two pieces. The throwing arm is detachable from the frame, and the counter-weight and sling are detachable from the throwing arm. I had this in mind when designing it since I didn't know how much trial and error there would be and I really didn't want to glue everything in place irretrievably.
(treb01treb3.jpg 28kb)	Here is a close-up of the frame, you can see the slots that the throwing arm rests in, and the blocks glued to the outside of the frame to prevent the arm from travelling sideways. This allows me the luxury of changing the throwing arm by lifting it up and sliding it sideways.