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Baby Hatra Ballista
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Baby Hatra Ballista is a model of a ballista whose Archaeological evidence was found near Hatra Iraq.

The Hatra Ballista is described briefly in Greek and Roman Artillery 399 BC-AD 363. Minimal archaeological evidence resulted in artistic diagrams that I thought were compelling. I particularly like the way the arms have a lot of leverage during the bowline snap that leads me to think it may perform better than a classic bow or ballista.

I started this project to design the type of machine I'd like to use at the Zukapult contest, and also the 10 and under catapult category for the Punkin Chunk.

The Hatra Ballista model ready to fire a koosh!

This machine is of similar scale to Baby Ballista so that I can compete them against each other to see which design is best.

I built this ballista using the same basic 2x4s as with Baby Ballista, except this time I used hidden mortise and tenon joint pairs as described by Eric Marsden. The large outer part of the frame holds the crossbars apart. The smaller upright is a guess of mine based on where I'd like the arms to sit at idle, and the frame of the original Hatra Ballista.

The modiolus and epizygis are of similar dimension to the other ballista as well, but I've tried to advance the construction. Baby Ballista uses a mild steel bolt for an epizygis, and a wooden modiolus which rode over the wooden hole carrier. This had good friction, so pins were unneeded to prevent unwinding. Sadly, the bolts are slowly damaging the wooden modiolus. This time I have the wooden under washer, and a steel upper washer which should solve this problem. Unfortunately, the bar drifts over the steel too much.

A quick jig pinned to the workbench makes it easy to center the epizygis over the modiolus. I few quick zaps with a light-duty MIG welder keeps it that way.

To prevent slippage against the wooden under washer, I also welded lumps onto the bottom of the modiolus so they would dig into the wood of the under washer. Seemed like a good idea and they do their job well, but it ended up just being a little lumpy and required some extra work to solve.

A quick coat of paint on the modioluses and it's time to start winding the rope bundle. This image shows the first step where the rope is tied off and threaded through the various parts.

As each strand is laid in the torsion bundle, large locking pliers are used hold tension in the bundle. The fid tool is a bit of spring steel I cut out of a couch with electrical tape. To keep the tape from unsticking, I heat it up in a candle which causes it to shrink wrap over the rope and steel such that you need to cut the fid off when you are done. I used a braided nylon rope.

The arms are small lengths of ash which are actually cut-offs from arms we made for Mista Ballista. Old style arms had the bowstring loop over the end of the arm. I used a small bit of dowel pressed into the arm to provide the structure for holding the bowstring in place.

The stock for the ballista was measured and fitted based on the arm length, plus enough room to add a trigger and hook. With the arms at idle pointing forward, it is a strange thing to look at.

In the background, you can see Onager Jr in storage in the back of my shop.

The transverse regulea, plus some other supports complete the basic framework. A lame bowstring is just looped over the end of the arms.

Jeff and I then launched a small pillow around the basement for a while. Next up is to splice a new bowline for the little blighter, and build a launch stand.

Eventually, Amy helped by knitting a new pouch. I then spliced up a new bowline much as we did for Baby Ballista. This time I put some of the poly-p bowline as abrasion resistance where the mini seacatch will grab onto the bowline.

Ready to do some test launching in the back yard on a sunny day.

Once the bowline is pulled back, and the seacatch is hooked onto the hook, you can see the main frame beams are bending quite a bit. The original hatra ballista had metal sheething on the front which was likely used to prevent this. I'll need to add some of my own to solve this problem.

And here it is, ready to fire. This maiden launch sent the koosh 83 feet. As those who read here a lot know, the Koosh is my favorite comparison projectile. It is soft and vaguely safe to shoot indoors. Outside, it refuses to fly very far, making it easy to measure.

This hatra ballista was only mildly torqued up, but still outshot Baby Ballista's 70+ foot launch of last summer when it was tightened up pretty well. Baby Ballista has shot small rocks over 252ft in my field. I hope to see much greater measurements out of this little guy.

I think I have a new favorite torsion catapult design. Hoo ya!

Additional Pages for Baby Hatra Ballista
Baby Hatra Ballista Baby Hatra Ballista is a model of a ballista whose Archaeological evidence was found near Hatra Iraq.
Test Firing In July 2006 I brought the small Hatra Ballista out to my Dad's for some test firing. I recorded the results which you can compare against Baby Ballista's results. We stopped firing when it started to rain on us.

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