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!

9 thoughts on “Every boy needs a ballista

  1. Congrats to you both for a fine working weapon. However in your future endeavor to upgrade your weapon of mass fun, please remember to obey all of the laws of safety. Also, why leave the women out. A second one could assure a fine water balloon war.

  2. Well, it’s important to keep working on it—I’m certain that more power tools will be involved and maybe some overripe fruit. 🙂

  3. Girls are def not excluded. But every girl needs a trebuchet I think… We will be expanding and having a great battle this summer some time. The ballista already destroyed a watermelon!

  4. I built a trebuchet once upon a time but never got it to fire as far as I thought things should fly. I tweaked and tweaked on it until one day I just left it sitting in the garage and didn’t tweak on it anymore. Eventually I think I stripped the hardware off and burned the rest. Maybe someday I can find the hardware and try again.

    I’ve seen lots of people use leaf springs for their ballista!

  5. Leaf springs would be great…I’d be curious about what they used in place of the sinew to create the tension if they did it the old fashioned way

  6. The ones on Youtube use the leaf spring as the tension device and then use everything from rope to steel cable to fling the projectile. If I were building one with a leaf spring, I think I would opt for a steel cable!

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