The Ballista

We see below, the new paddle limbs in all their glory.

The careful fit-up between the convex impact block and the bronze hardstop is a deliberate strategy aimed at  putting  the maximum level of control over the limb tips through out the entire firing cycle.  If I cannot adjust the limb tips to within  +/-  1/4″  from being  equidistant  from the center of the bolt groove,  the machine will never live up to all its accuracy potential.   By reducing the thickness of one of the bronze hardstops, it will be possible to make the limb that shows the least distance from the CBG (center of bolt groove) match the limb that shows the greater distance from CBG.   Performing this adjustment will insure that the bowstring is picked up by the catch at its relative exact center.  Also, it is important that the bowstring zips along the top of the bolt groove with out much downward pressure.  Friction is the enemy of speed.  This is best controlled by having a design that keeps the limb tips at the chosen altitude.  With the design chosen here, the interaction of the convex impact block and the concave bronze hardstop, creates a fulcrum that helps control and maintain the vertical disposition of the limb tips.   Our experience to date shows that the massive clamping grip from the torsion spring,  is  more than sufficient to prevent vertical slippage in the limb position.  Adding this fulcrum should make the whole dynamic of limb movement much more precise. Very important stuff, if your goal is accuracy.

I am getting that sensation again.  The one where I feel we are building to the moonshot, getting  this thing ready to launch.  Details, details……. everywhere I look, details.   A new backstop has been completed today.  Tomorrow I will make up half a dozen bolts.  After that, all we need is a new bowstring and it will be time to see how the new paddle limbs perform on the range.  If we can get a dozen or so shots showing a consistent velocity in the 800-1000 ft lbs. class,  we can start moving the target back towards the fifty yard line and see how well the machine can group its shots from there.

The Catapult God’s have returned to a better humour.

Above, our paddle limbs are almost finished.

Below, we see this odd looking creature from somewhere deep inside the Vatican.  The limbs sure look like paddles to me.  Anyway, it seems like I have heard thinner arguments than this staked out as scholarship.  (Note to self: locate immediately).

Photo from,  Greek and Roman Artillery 399 BC-AD 363,  Duncan B Campbell.

 The Catapult Gods have been in a bit of frump lately.  False leads, cracked hickory and wounded pride have all taken their toll.  We see in the photo below the whole sorry tale. 

Limb “A” was just a mock up to get the scallops properly located for limb “B”.  It was this limb “B”  design that went on to give us all our shooting results so far.  However, after 42 shots it had developed so many cracks in the scalloped area that I decided to upgrade to the limb “C” design.  It had a  full length steel brace and hook made from extremely tough PH15-5  material.   As good as this part of the “C” design was, it didn’t stop the hickory peavey handle from starting to split around the butt.   This proved disheartening to the point that yesterday I had a brief dalliance with design “D”.  It is basically a paddle shaped affair made from tw0 1″ planks of hickory backed with 1/4″ spring steel.   However, before I could finish it, I developed an irresistible urge to have another go at making the fancy  “C” design work, and so spent today in deep collaboration with mistress TIG to add on the extra tubing seen in design “E”. 

And it almost worked too.   After repeated testing of design “E” by taking the limbs up to 3000 lb. draw weight  (n0 actual shooting going on) it  became apparent  that the hickory was sustaining so many compression failures at the point that the wood exits the steel tube, it would not be a long lasting solution either.  And so, tomorrow I will delve into the ungainly looking  version “D” paddle limbs again.   To their credit, they are dead simple to build and should spread the load over the whole limb much more equally than any of the other designs.   On the negative, I cannot pretend that they seem as perfectly suited to the curved stanchions as I would like.  However, it is best to keep an open mind about these things.

The little anvil I had dangling from the bottom cross bar in the previous posting, was fine for organizing the strings and scoping out our new procedures.  However, nothing beats 16,000 lbs of Polish lathe to keep the bottom crossbar anchored while we pull on the top crossbar with a 2 ton chain hoist.  The come-along allows the field frame to be repositioned upwards after the spring has been stretched so that there is room to insert the bottom wedge.  The Romans could have used a purpose built stretching frame for these kind of chores.  In a pinch, the whole operation could be performed with nothing more than a stout tree trunk,  some rope, and somebody who knew how to tie timber hitches and make a Spanish windless.  Simple stuff really.

The upward pull from the chain hoist was around 1500 lbs.   The torsion spring could be seen to stretch 3/4″  in length before the wedges were inserted.  It is a good job the limb was installed first because there would be no squeezing it through the bundle after this much linear tension had been applied.  The convenience of being able to remove the field frames from the machine  cannot be underestimated when it comes to performing maintenance like this.   The Romans were really thinking when they made this design so mechanic friendly.  Working on a contraption like this surely gives one the sense that these were totally modern people in so many ways.   They must have had their own version of the KISS principle going on.  Smart, smart people.

 I have been experimenting with different techniques for making the torsion springs as straight as possible before applying any rotation to the washers.   The first thing was to rewind the springs on the jig and make them 1″ shorter.   Putting them in traction, as seen below,  made it much easier to straighten out the individual strings by hand.    Wedges will be installed underneath the crossbars to keep them spread as far apart as possible.   Hopefully this will reduce any of the excessive spiral that caused the new limbs to loose position.  With any luck we should be able to resume testing next week.

….Okay, okay, I’ll cop to it.  I’ve been having an aesthetic moment.   I will try not to let it happen again anytime soon.  Polishing is all very well, but the serious job of performance is what this project is supposed to be all about.

……….. deep into it.  Check back in about a week for a progress report on the current crop of improvements.  Me no bloggy for a bit.  Me just cut and weld and grind.

Negative on the projecting shield mentioned yesterday.  We will see if the crushed fibres in the hickory deepen or become untenable.  Right now, the depression in the wood serves to more fully lock the whole limb into the bundle. 

I sure wanted to fix it by welding on armour plate.  Occam’s razor can be such a bitch.

I had planned to perform various tests on the high spiral torsion springs to see if their performance is indeed as miserable as Philon claimed. As it turned out, the undesirable spiral proved so impossible to cope with, I had to abandon the notion of testing them at all.  Philon’s line that, “the strands are huddled up into a thick spiral” is an apt description of what we see in the following photo.

The photo was taken after one shot that registered a 3,000 lb draw weight. The limb repositioned itself forward by about 3/4”. The gap between the butt of the limb and the straight stanchion should only be 1/4”. All of this appears to be the result of the spring “huddling” up. That is to say, the two halves of the spring have moved closer together because of the excessive spiral. Their grip on the limb is compromised, and all the carefully worked out fore and aft clearances for the limb, are thrown out of whack.

We also note the brutal throttling of our beloved new limbs. 

In this photo we see the garroting effect of a tightly wound torsion spring on the unprotected throat of the limb. The hickory fibres have been crushed by the strings in the spring to a depth of around 1/8”. The fix will be to weld a projecting shield aft from the large steel collar. Probably that shield will need to terminate in some kind of a hook to engage the inboard half of the torsion spring.

The Catapult Gods do not yield up their secrets easily.  Our new limbs may have benefited from my original up-armoured mindset after all.  Not to worry.  The miracle from the Goddess TIG will fix these minor annoyances post haste.

…..So anyway:

From Marsden’s second volume on Ancient Greek and Roman Artillery, we read Philon’s advice for the more perfect maintenance of high powered torsion engines:

“There follows something else that is altogether inconvenient, unworkmanlike, and detrimental to the range. In the heat of shooting and pulling back, the string experiences a slackening and needs tightening again. The range of the shooting deteriorates because of the relaxation. But those who wish to tighten it cannot apply the stretching vertically and in a straight line, but do it by extra-twisting, imparting an extra twist unnaturally greater than is suitable. They think they are doing good, but they are actually doing the tension great harm and are reducing, I maintain, the velocity of the shooting and its force on impact. The engine looses its springiness because the strings are huddled up into a thick spiral and the spring, becoming askew, is robbed of its force and liveliness through the excessive, extra twisting.”

In the experience of this ancient master, it is far better to tension the spring bundle of a torsion engine, by pulling on its ends to cause it to stretch out longer, much like stretching a rubber band, than it is to rotate the washers and thereby twist the spring bundle into an ever more tightly wound spiral. This latter technique he roundly disclaims as being an unfortunate, lowbrow way of cranking up the power on a torsion engine. My personal experience with over 500 shots on the old Gallwey ballista (see earlier postings) indicates the fundamental importance of keeping the springs as straight as possible.

While I do not doubt these general principles that Philon lays out, I cannot quite shake the idea that maximum power will be achieved by twisting these torsion beasties up to the point we wring out every last drop of slackness.  Only then will they ever give up all their foot pounds of energy to us.  I believe that both tensioning techniques are appropriate, if they are sequenced properly and if the preload is carefully monitored. In terms of maintenance on these machines, it is very important that the linear expansion of the crossbars always be performed first, before applying any rotational tension.  In this sense, rotational tensioning of a ballista is not necessarily a bad thing.  It just has to be done in the right order and to the right degree.  In short, straight springs provide the best foundation for generating power in a torsion engine. However, in my experience,  you can always zip things up a bit by applying an appropriate level of twist at the end.   And come to think of it, Philon seems to be suggesting the same thing when he warns against “excessive” twisting.   Which, of course, is not the same as “no” twisting.

On another note:  Did I just talk myself into converting the crossbars on our machine into a wedge based system?………Old Philon would be so proud.

Me snooze now.  Blab later.