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.

After a year or more of dormancy, the Phoenix project has started to stir again.

With mock-ups for the springs and limbs,  I am figuring the clearances needed inside the field frames for everything to operate smoothly.

The above photo gives some idea of how the wedge system will work.  The wedges shown here are just softwood.  Ipe seems like a good choice for the final version.  The wedges will need to resist some pretty massive crushing forces from the spring.

The spring being made in the above photo will probably not be used to power the final machine.   It is just another mock-up to help verify the limb design.  When the cord wrappings fill up the circle in the template, the spring will be ready to install in the field frame for a first real look at the clearances involved.  We want the spring to be as large as possible and still work within the field frame.  Also it is important to make the limb as thick as possible where it goes through the spring.  This will not only make the limb stronger at it’s weakest point, but will also force the two haves of the spring apart and so increase the leverage acting on the limb and make it’s action more powerful.

I have decided to put the Phoenix project on hold for the rest of the summer as there are more pressing concerns right now re: house building for my daughter.   Many thanks to those folks who have taken an interest in this project, and I’ll be back at it probably sometime in November.   Email dead for the duration.  Cheers,  Nick.

Two degree, counter-posed wedges, seem like a good starting point for our first attempt at an adaption of Mr. Philon’s souped-up torsion machine.

Those extra long wedges will be trimmed to length after the fine tuning is completed.   For experimental purposes the washers will also be capable of fine rotational adjustments.  There are 16 holes in the washers, each 22.5 degrees apart.   There is a 3 hole pattern in the vernier plate*,  with each hole set 15 degrees apart, so that the washers that control spring rotation can be locked down every 7 1/2 degrees.   The second set of three holes is so that there can be two locking pins for each station.   Next will be the radiused pusher bar that sits on top of the wedges.  And then on to springs and limbs.

*Counter plate, or counter washer, if you will.  I prefer “Vernier plate” as the Ampurias finds, whose geometry we are mimicking here, seem to contain the first deliberate use of a logarithmic scale similar to a modern day Vernier.

The notches and locking holes on Phoenix’s washers are complete.

Quite the labyrinth of mechanical possibility ain’t it?

Except for the locking holes and notches for the crossbars, the washers are about finished.

Why “modiolus of doom”?  Because right now I need some kind of hairy provocation to keep this project alive.

Time has a way of elongating, right before it runs out.  You think you have all the time in the world to finish something, and then …. Blam!… you’re done.  The meter runs out.  Which leaves the Catapult Gods smirking at you for the rest of eternity.   Damn their temporal omnipotence anyway!

Also, my laptop is getting hinky.  Email is down.  Correspondence not possible until I get it fixed.

…If I get it fixed.  A hermit-like submergence seems somehow linked to my ability to self-motivate.  Go figure.