A month or two ago I was reading a book on Greek and Roman engineering and was inspired to make a ballista. It was pretty simple and worked reasonably well for the nonexistent amount of planning that went in to it. Pictures can be seen here. The only engineering hurdles it didn’t accomplish were adequately maintaining and adjusting tension in the springs, and a trigger mechanism to hold the string until it’s ready to fire.
I just finished a design for a trigger that uses only parts that were available in ancient times (maybe not quite, but if this were the size of real ones it could be).
The only thing that worries me is the placement of the rod in the slider. Since the position in which it holds the most tension is where the rod tries to slide forward, I’m afraid it will be easier for it to slip slightly out of position without having to move the slider down. Regardless, this will just be minor slippage so it shouldn’t be a problem. If it works well I might make two of these, one from wood for the actual weapon, and a second from plastic for demonstrating how it operates.
Speaking of ballistae, does the maker faire catapult competition sound awesome to anyone else? I would have gone just for this had they made it known with more time before the event, but alas they didn’t. Is anyone interested in doing something similar in Chicago? I bet there’s enough nerds in Chicago interested in making weird stuff to have a decent competition.