We are building a Torsion powered bow (Ballista) for our siege engine project. It is set up as a bullet crossbow, with the option of firing bolts with the use of a removable track. Our plan is to build it to fit within the specifications of our 18x18x18 size limit out of wood and string or modern elastic materials using the mill machine and laser. It will be assembled using a pin and glue system instead of bolts due to the historical nature of our project.

The Ballista is a Torsion weapon invented by the Greeks and widely used throughout the roman empire as a siege weapon. It survived through to the middle ages and was used along side Medieval siege weapons such as catapults and trebuchets. Its design is rather simple; It is two torsion springs made out of rope or sinew with the prods of the bow arm placed within them. Beyond this its mechanism is similar to the crossbow. The ballista uses a windlass to cock the mechanism and a quick release mechanism to fire the bow. It can be used to fire bolts or round shot.

Step 1: Materials

24"X48" "Handy panel" of MDF Particle Board
1/8" Braided nylon cord (48') (for torsion engines)
thin nylon cord (for bow string)
1/4" steel rod (36") X2
wood glue
1" diameter PVC Pipe
small piece of leather (for bullet pocket)

The material cost for this project is surprisingly low, hovering around the $25 mark for all building materials.

Mill machine with travel of around 24"X48" 
Shop vacuum 
Solidworks 3D design software
Mastercam mill path/design software
set of hand files 
hack saw with metal blade

Safety Gear:

I'm loving these SPHS engineering projects. Keep up the great work, Paladins.
<p>Smaller than expected.. make a big one!! lol. </p>
Yowsa, pretty cool. Was about to say i want to see video (used to the normal youtube link) then noticed the mp4's - very nice!!!
Thanks for your kind words, we are at a high school that does not allow YouTube or vimeo, so this was the best way to put vids up.
Dang, I need to revamp mine. It was twice as large with a quarter the range.

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