The Ballista

Waiting for more elk backstraps from Granny’s, visions of Phoenix come to the boil.

Modeling the clearance available to high-collar type washers, seems like a good place to start.

Making a materials list.  Checkin’  it twice etc.

Initial processing of our sinew backstraps  has yielded this small bundle of coarse fibers.

These sinew fibers still need to be hand separated into smaller threads before twisting into cordage.  The bundle seen here contains the raw material from two elk backstraps.  There are seven more backstraps on order so that should give us at least five times the amount of fiber seen here.  I intend to turn it all into 3-ply cordage before making any further decisions on the scale of  the models. First things first, and all that.

Time is irrelevant when it is the journey that you seek.

Beyond making sure that obvious parameters like the draw weight and draw length are the same between our two model mangonels, there are some subtleties to this experiment that need to be considered.

If we utilize ball-bearing races between the washers and the counter-plates, that should remove enough of the erratic, frictional element to make putting a spring scale on the end of the washer tightening wrench a viable technique for getting torque readings on the pre-load tension of the torsion spring.  Thus we can garner more data on the sinew/nylon dynamic. (And then again, it may be just as valid to compare the pre-tension directly off of draw weight readings taken just as the arm starts to move off of it’s impact buffer.   This is only about comparison.   The KISS  principle is everything.)

It will be important to make suitable gauges to insure that the volume of  each of the two springs is as nearly identical to one other as possible.  Nylon and sinew have different densities,  and so weight is not a good comparative measure.   Besides, the limiting factor in fitting a spring into a field frame is clearly the volume, not the mass.

The larger we make these models the more accurate they will be as analogs for full sized machines.  The nylon powered version will be easy enough to prepare a spring for,  however, the sinew spring  leaves me feeling greedy for more elk backstraps to process. When it comes to scale: the more, the bigger, the better.

While we can make the models very accurate dimensionally to one another, and situate them securely and level for the testing, that does not necessarily prove that the initial launch angle of the projectile is the same between the two machines.   For this we will need the video camera and some kind of graduated backdrop to film against.

There is going to be a fair amount of work in making the models for this comparative analysis between sinew and nylon.  Because our approach is that of an actual bricks and mortar model shop, rather than just mathematical modelling based on a variety of assumptions about modulus of elasticity and permanent and temporary deformation etc, etc., it follows that we must be as scrupulous as possible in how these little wood and steel torsion engines are  designed and constructed. The same is true for how they are tuned and tested.

If we tune and perfect the shooting of the sinew based mangonel before finishing up the nylon spring,  this will allow us to know exactly what diameter that latter should be when it has had it’s pre-tension applied.  Proper sequencing, like this, will obviate any concerns about the  “tension/diameter shrinkage” ratio, and insure that the two different springs have equal volume.

We only aim to predict that if a competently rendered,  modern reconstruction that is powered by nylon, could shoot a given payload to a  range of X, then all other things being equal, it would likely shoot that same payload to a range of Y if powered by sinew.  Trying different values of arm rotation, and also different levels of pre-tension, should tell us a lot about the performance envelope of both materials.   That is all this about.  It is very important to define the focus at the outset.

I am waiting for ten backstraps of elk sinew to show up.  They are each a couple of feet long, and there should be enough sinew in them to make a torsion spring of, maybe, 1″ in diameter by 9″ long.  In any event,  the pair of small mangonels  that we are going to build will be sized by whatever we come up with for a small, prototype, sinew torsion spring.  The models will be considered simple test beds to discover the propulsion ratio between nylon and sinew.  They will have no historical authenticity attached to them and will be built to close enough tolerances (+/- .005″ all dims, should do it)  that mechanical consistency between the two machines will be assured.

There will be no need to complicate the experiment by fitting these one armed machines with a sling.  We’ll use a simple spoon type arm ala Monty Python,  and match the two machines in tuning as closely as possible.  Our only intent will be to find, as simply as possible, the comparative power of a sinew spring vs. a nylon one.  Naturally, both springs would need to be of the same size and have the same draw length and draw weight.  How far our two differently powered models will toss a particular sized steel ball, should give us a ratio with which we can predict how well a full-sized reconstruction equipped with nylon springs would perform if it was powered by good old-fashioned sinew.

This seems to be the only practical way to get a fix on what Firefly would be capable of if she was powered by authentic materials.  I did some rough calculations on the cost of enough raw sinew to make full sized springs for her.  The material alone would be over $10,000,   not forgetting the Herculean effort needed to process it into usable cordage. —  And with springs that precious it would be a dire moral challenge to take them near enough to their breaking point,  just to model the upper limit of  their performance. — So much better to explore this matter with smaller scale models where one can afford to challenge the Gods a mite, and have a failure or two to help peg the envelope.

Besides,  springs are just springs, and data describes them all.  Valid extrapolations are more useful than impractical fantasies of perfection that never come to fruition.

What it means to say, “a world made by hand”, is quickly made apparent when one first tries twisting cordage out of sinew in the ancient manner.   For right-handers, the simplest and most consistent technique for making high quality cordage goes like this: take two loose bundles of  lightly wetted sinew and pinch them between the thumb and forefinger of the left hand so they form a cross  like this:

While always holding on firm with the pinch of the left hand, take another pinch between the thumb and forefinger of the right hand and twist each of the two bundles, separately, and in a clockwise direction.  The left hand them creeps forward with an unrelenting pinch to lock in the two clockwise twists just performed with the right hand.  After that, the pinch of the right hand is reconfigured to grasp both of the recently twisted bundles in exactly the same manner as the left hand.  Then one makes sure all the twisted sinew in the pinch zone of the left hand is pulled as taut as possible by rolling the thumbs and forefingers of both hands into one another. Finally, while maintaining a strong pinch with both hands, make a concerted and violent twist with the wrist of the left hand so as to further tighten everything  in the same clockwise direction.  Be sure to lock in all the twisting with the pinch of that left hand, and then repeat the whole process.  If the pinch of the left hand remains constant, and never allows the twisted sinews to unwind, in a short time the wetted sinew will take a set and not untwist itself much at all.  With a few minutes of this simple exercise in hominid dexterity, you end up with something that looks like this.

As the work proceeds, new sinew fibers are laid into each of the two loose bundles to keep their volume consistent through out the process.    It is all just about as easy as can be.  No wonder our forebears spread to all corners of the planet.

Make your pesky cordage ye mortals!  Lift yourselves above the beasts!   Let your strong fingers pinch their way to the stars!

But…

…take care little hominids!   May the nets you weave not produce wonders more complex than you can maintain.  The more you need, the less you have.

My head’s spinning full of ideas on how to process dried sinew into a 2-ply (or maybe 3-ply) cordage of good strength.  The process will be dictated by the available supply of cheap backstraps.   (Small supply- hand twisting will do.   Substantial supply — better make a small spinning wheel.)

To start with we do some twisting.

If you use your hands to twist up a single ply strand in one direction,  and just keep on twisting until it wants to go kinky, then fold it in half, and with some gentle encouragement from your fingers,  the single ply will pop itself into a double ply.  Double ply is what happens when a  balanced and measured number of  kinks get smoothed into a spiral by a process known as reverse twist.  To the individual fibers it is a self-locking kind of arrangement, not unlike a Chinese finger puzzle that grips tighter the more linear strain is applied.  It just kinda happens naturally if you fool around with the proper twisting technique for awhile.

To our erstwhile hominid ancestors, this trick was enough of a boon to give them a huge survival advantage.

The propensity to form itself into a reverse twisted spiral (one ply clockwise, the other counter clockwise)  is fundamental to making cordage, and is a unique natural phenomenon that occurs with a wide range of fibrous materials.  Some folks consider it one of the major factors that allowed the development of our species.   Obviously for us humans, nothing happens without cordage.

Pretty kinky, huh?

There is no doubt that if I am to close the circle on this argument, “sinew” must figure prominently.   Let’s take a look at the stuff.

These two lovely hanks of processed backstrap sinew were obtained from Three Rivers Archery.   They are twelve inches long and are seen here folded in half.  My plan is to make a sinew bowstring for my 85 lb longbow, and then shoot the dickens out of it so as to season and confirm the integrity of the string.  Then the bowstring will be utilized to construct a torsion spring the size of a 4″ blunt cigar, and by my reckoning that will make a model of sufficient scale to explore the fundamental propulsive differences between sinew and nylon.

But first, that means a learning curve.   I must confess to never having worked with sinew before.  However, there are multiple articles on the interwebs that detail the procedure.  Volume two of The Traditional Bowyer’s Bible has a nice exposse on it all.