Tag Archives: Ballista! Ballista!

Every boy needs a ballista

There is a special bond that a father and son sometimes get to share…when building a siege weapon, for instance.  In one of Isaac’s classes, the final project was to build a siege weapon of some sort to throw a water balloon as far as possible.  Wen Isaac mentioned the project, I immediately though of building a catapult or a trebuchet.

Building the ballista

Building the ballista

Isaac and members of the class had done research though so there wee a number of other weapons that he learned about which are more unusual (to my mind) and a lot more fun for their uniqueness.  Isaac decided he wanted to build a ballista.     A ballista is basically a crossbow on steroids.  That’s perfect!!!

Dry-run testing the ballista
Dry-run testing the ballista
For Pride...for honor...for goodness sakes
For Pride…for honor…for goodness sakes

Original Greek ballistas used two arms that were wrapped with twisted sinew, leather, who knows what to produce tension.  Regular bows use flex in the arms to produce tension that ultimately dives the projectile forward.  In the original ballistae, the torsion in the sinew generated tension to launch the projectile.

The ballista
From behind
This doesn't look safe now that I look back on it
This doesn’t look safe now that I look back on it

We decided to modernize/pervert the old design to use flex  in the bow arms to provide tension to launch our projectile.  PVC pipe to the rescue!  Isaac and I cut, hammered and cussed this thing into existence this week and launched a tennis ball several times…our max shot was 20 yards.  We had one somewhat serious design flaw for which we had no time to remedy.  We ran a single rope from bow-end to bow-end passing through the projectile cup in the middle.  The rope went through the back of the cup so when we released the tension, the rope had to push the cup which often caused it to flip as it tried to push so hard and fast.  A better design would have been to attach a rope tot he leading edge of the cup on each side so the rope would pull the cup rather than push…I suspect we could have easily thrown a tennis ball 50+ yards.

Ballista Movie
Click to watch our first launch!

Isaac’s event went well and he got his credit.  I see more design enhancements in the coming weeks…this project isn’t over for me yet!  And that’s the best part in my mind…sure, the grade was the main driver for this project, but the best part was shared time and cussing with my son!