The Ballista

That truss arrangement is just about enough to give a fella ideas.

That’s cutting it as close as I dare on the acuteness of that string angle at full draw.

An inswinger’s bowstring is prone to interfering with the projectile fins when we try and stretch too much limb rotation out of them.

The 65 degrees of limb rotation, seen above, is about as much as can be managed with Phoenix, and still have her limbs be as long as reasonably possible.   This limitation is very deliberate.  The type of torsion spring I am making,  seems to yield it’s best performance with short limb rotations of 50 degrees or so.   Firefly convinced me of this to the point I decided to embrace the principle and make Phoenix as compact as possible.  The shorter limb rotation means I can get away with a stock length that is perhaps 18″ shorter than would otherwise be possible with some kind of 110 degree, over-rotating, specimen.

Short, sharp, and violent in it’s stroke, our ballista limb needs to be both light and immensely strong.  All this mock-up work is being done to help intuit how much weight we can trim away from the limb irons.

I premise this attempt by accepting the well known idea that late model Roman ballistas utilized iron strips and bowstring hooks on their limbs.  Therefore, at this stage, the existence of full length, limb irons, is a non-negotiable tenant of this project.  Puzzling the geometry at full scale, is my focus at the moment.

Phoenix’s full size spring mock-up has allowed me to make a start on the limb design.  There will be many cogitations on these parts before I can be confident that the limb is as light as possible, yet amply strong to handle heavy draw weights (3,000 lb.  at least, I’m hoping),  all the while exploring the historical utility of steel tension straps and steel end hooks for the bowstring.

That piece of steel, nestled up against my fake limb, is actually the prod for a small 200 lb. crossbow.  The prod is about 3/16″ thick, and seems about the right size to be a rudimentary model for the tension straps.

Gazing at it for hours on end helps bend my visualizations around what’s absolutely needed and what is not.  When it comes to limbs, the more “not” you can generate, the faster you can go.

The value of full scale mock-ups cannot be underestimated at this stage of spring development.

I’ll ponder on what I’m seeing here, for awhile.

No doubt all that’s needed is to deepen that scallop in the butt of the limb by a small amount.  That should provide enough clearance to prevent the spring chaffing on the straight stanchion as it is now.

In torsion catapults,  spring size really does matter.   ….So vital to keep our springs as girthful as possible!

So that’s 166 feet worth of 1/4″,  3 strand, nylon rope, going through a hole that is 2.630 inches in diameter.  This later is the same size as the hole in the field frame that the spring passes through.