This laser cut has been kicking around for some time so I thought it would be a good idea to get it to a functional state. It's not perfect so expect another revision in the future.
Conventional ballistas use twisted rope as a power source at the base of each arm. I decided to use elastic bands, but when they twist they don't supply as much power (insert questionable physics). So I kept the bands and arms slightly off centre this way all the power comes from stretching the bands rather than twisting them. It seems to give a fairly good kick, launching that pencil 10 metres or so. Round the corners of the wood off though as my elastic bands rub and snap quick frequently. (svg here)