The Ballista

Decided to take the scenic route.

This next experiment will involve three, successively shorter, limb lengths. It will be a lot of fiddlin’ around. …..But, no use grumping. The Gods are never appeased by half measures.

What’s a minion supposed to do, anyway? I don’t want my catapult license revoked for failure to turn over all the stones in the garden. … Besides, every ponderous tome needs its fuel.

We start off with a limb length of 26″, and then move on to the 23″ and 20″ lengths. Limb length is measured from the center of the bundle to the center of the bowstring nock on the limb tip. (For our purposes, this is a new way of defining limb length. Previous limb lengths claimed in this blog were meant to reflect manufacturing, and are not comparable to these which speak more to leverage.) I will announce with a new posting when the limb length changes. Limb weight on the Mk IX’s is 8 lbs each.

…And so, off we go with those string-bean hooligans, the 26 inchers.

Shot #1
I decided to try the first shot with the bundles in a state of fair to middlin’ relaxitude. That is, the nylon in the springs is soft and plumpish when probed with a stout finger. About 1″ of lift for the limb away from its stanchion pocket, (when attacked by stern shoulder of fat old white guy). Velocity was, quite naturally, very slow. 150 feet per second with 377 gram bolt. I’m hoping this is actually very, very good. Those bundles are very, very slack. And my shoulder has turned touchingly weak.

Shot #2
Advanced all washers by 15 degrees (or 2 of the minimum rotational units in our vernier based locking system). The limb can still be manually lifted about 1/2″. The bundles appear firmer, but are obviously still mucho slack. The shot flew straight and true above the bolt groove. Nice balance. Velocity: 240 fps.

At this point in the testing I take careful measurement of the air-gap between the inside of the curved stanchion and the outside forward edge of the limb iron. Like so:

The port side limb shows a gap of .213, whie the starboard side is .330. All of the locking pegs occupy the same rotational position in the washer holes. That is, in all four stations, the peg positons match. It is decided to make the next advancement at the MRU (minimum rotational unit) of 7 1/2 degrees, for all locations except the lower washer on the starboard side. That we rotate 15 degrees and new measurements with the pin guages show us a gap of .222″ on the starboard side and .321 on the port side. How this assymetry in the gap moves back and forth as we tune the machine will signal the state of balance in the machine between the two power plants.

Shot # 3

No reading on chrono.

Shot # 4

With the washer rotation as mentioned last, shot # 4 shows a velocity of 285 250 fps. draw weight (3500 lbs -correction-) 3100 lbs, draw length 38″. Bundles are still mushy, 3/8″ of limb lift is still visible.

Shot # 5

The air gaps are measured at .260 starboard, .280 port. This only means that the resting point of the limbs relative to the surrounding steel framework (i.e. field frames & kamarion), has found this much clearance a happy and repeatable phenomenon. …… Yeah! team.

There is another thing going on, though. At 38″ of draw length, the starboard limb was observed to ride maybe 10 degrees above the plane of the flight deck. Ideally it should ride a couple of degrees below it. Damm! the dihedral! The fix is to induce more twist into the half of the bundle that rides above the plane of the limb. With an upward planing limb, the constriction that comes from an increase of torsion in the upper half of the bundle can be used to alter that dihedral into anhedral. Upward inclining into downward inclining.
Currently the starboard bottom washer, has an extra helping of MRU. Corrections as follows: SB zero degrees, ST +7 1/2 degrees, PB +7 1/2″, PT +7 1/2″. For ’ems wot ‘aint seen it, here’s the clobber we use to induce torsion into the bundles.

And we shoot….. Velocity- 268 fps. Draw weight 3500 lbs. Draw length 38″. Bolt weight 377 grams.

In a well tuned torsion spring, all the fibres bear a near equal share of the burden. I have found that as they age, rope torsion springs improve with use. That is to say, they become more internally balanced, as the tight ropes tend to loosen, and the loose ropes tend to tighten. This was true of the manila line we rigged our old Gallwey with, and also all the nylon 3-strand we have used over the years. Firefly is currently rigged with the Mk VI nylon springs, and they have been maturing nicely these last 150 shots. Movements in the stitching that hold the ropes in position indicate that there is a process of slipage over the cross-bars. These rotational movements of the ropes around the cross-bars tend to drive things towards a state of equal strain through out the spring.

A couple of days ago I removed the chaffing gear and could see the nylon spring in good detail. Happy to report, Zero damage from our grand nemesis, the dreaded Chaffing Trolls. Any bare metal rubbing on the spring is all the invitation they need. Eventually they can nibble into the ropes a fair way if the protective gear isn’t in order. Hungry little bastards, that they are.

…But, I digress.

Shot # 6

I took a check of the tautness of the bowstring by using my old subjective standard. One maxed-out Nick power. It’s a bit like a calibated torque wrench, except the data is measured by feel and compared by memory.

I usually record this measurement, not with a photo, but by remembering the apparent level of deflection that appears in the string right about the time a familiar stab of elbow pain occurs. The pain indicates we have arrived at maximum effort for this procedure.
…. Maybe a quick snapshot isn’t such a bad idea afterall. It is the amount of deflection in the string that we are trying to gauge. In any event, in past shooting I have noticed a perfect correlation between the tautness of the bowstring, and a marked improvement in the velocity of the bolt.
The greater leverage of the increased limb length of the Mk IX’s (3″ more than the MK VIII’s) produces less tension in the bowstring. (At least for any given level of strain in the spring.) It will be interesting to see if the aforementioned correlation continues to hold as the limb length, and presumedly string tautness, changes.

While tomes are usually tombs for all interred therein, there is some comfort in an experiment just begun. The illusion of animation it brings can propel us forward when all around sits becalmed in seas of bleak indifference. Cue the tiny violins.

….We proceed, with our tome.

All four washers are advanced 1 MRU, or as it is otherwise known, 7 1/2 degrees.

Velocity 263 fps. Draw weight 4000 lbs. Bolt, as before.

So why did the velocity go down to 263 fps after more tension was introduced into the bundles? I have seen this before. At this early stage of pre-loading the spring, the fibres collectively cycle between giving way, and standing strong. Twist ’em with enough preload — eventually they will all get the message and stand in more equal measure to one another as they bear the load.

That being said, I sense there is more velocity waiting for us as we shorten the limb. (There had better be, because 263 fps is appallingly slow for a 4,000 lb draw weight.) I will try a few more shots at this setting, and then prepare to shorten the limb by installing a new nock placement on the limb. Probably I should make some more bolts before going any further.

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As promised here are a few more shots with this long limb length, 4,000 lb draw weight, 38″ draw length, 377 gram bolt. The additional nock whippings have been added, and from here on in there will be no attempt to improve velocity by increasing washer rotation. This whole experiment is only about assessing the relative effects of limb length. We will get to hot-rodding after that has been figured.

Shot # 7 260 fps

Shot # 8 261 fps

Shot # 9 259 fps

Shot # 10 259 fps

Shot # 11 263 fps

Shot #12 261 fps

Okay, so with the last seven shots it looks like we have a nice stable baseline of around 260 feet per second. I would like to call attention to one obvious defecit with these long limbs, it will never be possible to go much beyond the 38″ draw length because the string angle becomes too acute to allow the use of finned projectiles. The interference issues inevitably leads to erratic bolt flight, what I have always called “waggle tails”. The following photo of the string angle gives some indication of how draw length becomes limited by these lanky 26″ limbs.

It is now time to wind up this stage of long limb testing and move on to the medium length.

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Well, okay. Maybe not just yet, time to wind it up. It is now a different day than the above. (And yes, I know: why didn’t I date those entries? ….. no excuse. Just gitin’ slow)

I was getting ready to switch the string down to the next station for the medium length foo-fer-rah, and discovered a distinct lack of tension in the at-rest bowstring. A shot was made. Velocity had dropped to a piddling 217 fps. Also the starboard limb had risen, dihedral fashion, a good ten degrees above the deck. I applied 7 1/2 degrees rotation to the top half of the starboard bundle. The constriction thus created tends to push upwardly incling limbs, downward towards the flight deck.

Another shot was made (# 13 by this count). Velocity was 277 fps. The limb rise was down to perhaps +2 degrees. I smacked downwards on the at-rest limb tip with a heavily weighted nylon mallet. Probably about 20 good whacks.

And then came, Shot # 14. Velocity was 280 fps. Starboard limb rise nominal @ -2 degrees. The mallet work had successfully reseated the limb in the bind of the spring.

We wipe our brow and shoot on.

Shot # 15. 277 fps.

Shot # 16. 276 fps.

Shot # 17. 277 fps.

Shot # 18. 277 fps.

That leaves a fella fair tuckered out. Final baseline for long limbs established @ 277 fps. Medium length testing imminent. Nap now.

The Mk IX limbs sit nestled in their nylon launch bays. Impatient racehorses, eager for the athletics in which they must soon engage.

We note with alarm that Mr. Owl has a big hole in his back. Ah! the treachery of gravity!

Athena is going to be pissed.

Ballast tanks one, two and three are filling with air.

All upgrades completed. Mk IX testing imminent.

What dread pattern lies beneath the scale of time? Molecules twisted and entwined for strength….

….yet the whole laid low beneath a confusion of rust and dirt and mangled empire.

What chilled vagary of the Gods, turns polished craft, into pitted relic?

Before you fell, how many true hearts did you pierce, sitting lonely in your tower?

Were you oiled and polished for parade? Shown off for courage?

Did the hands that spawned you, crowd eager at the kill?

Or were they making more, just like you?

Staunching the rot with promises of some more ardent glory?

What chance we, when all of what you were, is so much of what we are?

Ah!, the might of Rome! — largely a borrowed thing.

The game I’m playing may seem like a bit of a haphazard free-for-all, but there are rules you know. One of these has always been to respect the dimensionallity of the original artifacts, as expressed in the Baatz drawings. With today’s decision I am bound to break that smugness by venturing into the hypothetical, a bit.

By adding a spring steel plate to the straight stanchion, any worries about this sensitive area will be banished. (And good riddance, too.) The price for this happy confidence will be vowing to make a new set of field frames with stanchions made entirely from spring steel. That way we can easily stay true to the exact numbers in the Baatz.

The degree of confidence this upgrade will inspire when smacking at 5000 lbs again, is judged worthy of a minor dimensional transgression at this time. If the original machine had well forged components made from spring steel (I’m betting that steels with a max. tensile of over 100,000 psi would have been procureable by ancient artillery workshops), then the Battz dimensions suggest that this class of machine could easily have been capable of 5,000+ lb draw weights. The limits encountered by our current mild steel field frames (65,000 psi), shows that for sure.

While the Romans did not fully utilize the wootz method of steel production, its seems clear that the technique would likely have been available to them for the development of their super-weapons. For more on the underrated Roman steel industry, see the excellent paper by E.A. Ginzel, titled: Steel in Ancient Greece and Rome. (1995)

I quote him thus. (And thankyou, E.)

“Although the exact process was not understood, it was long known that juxtaposition of wrought iron to charcoal increased the hardness of the wrought iron. Two steel making processes were known and practiced in antiquity; the cementation process and the crucible process. The cementation process involved heating wrought iron in contact with a carbon source (usually charcoal) in such a way as to exclude exposure to air. In the crucible process wrought iron bars were melted in crucibles in which charcoal had been placed.

Steel tools made by the cementation process of Roman origin were found in Britain dating to the second century AD[17]. Carbon content varied irregularly throughout from 0% to 1.3%. It was this irregular distribution of carbon that made the cementation process, or “home-made” Roman steel less desirable.

It is suggested by Parr[18] that real production of steel began as early as 500 BC in India. This material was referred to as wootz. By Alexander’s time the production of wootz was a well established two step process using the crucible method. Two methods could be used, conversion from a cast iron form or conversion from a wrought iron form.

Although wootz steel and the Damascus steel manufacturing processes were probably introduced to the west around the time of Alexander, its significance was strangely under exploited, with one exception; the Damascus steel blade swords. To explain why Romans did not adopt or develop the wootz steel manufacturing process and Damascus forging methods must be speculative. Landels puts forward the suggestion that Roman furnace design made production of sufficient heat unattainable, yet he goes on to point out the 1150°C maximum could easily be extended to 1300°C using available technology[26]. Add to this the fact that the unreliable cementation process used by the Romans provided steels of medium carbon content and this alone could have reduced the temperature to melt the steel to a range reasonably achieved by existing Roman furnaces. The fact remains that opportunity to make improvements in the steel industry existed but was not used. Technical conditions existed during the Roman domination of Europe that have provided the Roman smiths the ability to produce the relatively high quality wootz type steel. The writer suggests two possibilities for the failure of good Roman smiths to make a wootz type steel. One possibility is that the serendipitous actions of forging a lump of the cast iron, that would have inevitably formed at some point during Roman smelting, did not occur. This was either because the Romans were always fastidious about keeping the unmalleable stuff out of their blooms, or they simply did not experiment with the dirty hard little buttons that would occasionally occur in the furnaces. A second possibility might be that the suppliers of the wootz steel kept the process a secret from their western customers.”

For our purposes, we need only remember that wootz steels would likely have been available to the Romans for high end purposes, even if this high grade material was not commonly manufactured by them and used in other industries. We might liken their use of wootz steels to the “strategic materials” so prized by modern states — valuable and rare, but certainly not unheard of. Their India trade was quite the thing, I hear.

For our doctrine of maximization in all things related to performance, that is enough of a track to follow to its endpoint.

Trying to explain to the Catapult Gods why their new toy is taking so long to complete makes a mere mortal feel pretty small.

Our expectations can lead us astray. The sage has warned us of this before, “… expect nothing, and live frugally on surprise.”

Easy to say, when you are a sage. An artisan must pump with blood and passion, before he can know the truth.

A couple of years ago one of Firefly’s field frames showed us a point of potential weakness in the Orsova design. While exploring some typically enthusiastic poundages, the straight stanchion on the port field frame took a slight and localized bend just beneath the loop that captures the shorter fork on the kamarion. In the photo below, the red arrows indicate the direction of this old injury.

It was this area of potential bending that forced us to limit draw weights to under 4000 pounds. At the time I had welded in a straight section of higher grade 4140 that was intended to beef up this area. It appears that some of my welding back then was not up to par as it is now possible to see a small crack forming on the outboard side of that stanchion. Easy to fix by grinding out the affected area and rewelding. I only mention it because it does highlight that same area as a place most likely to fail in the Orsova design. If I had it to do over again the field frames would be made entirely from 4130 or 4140 rather than the mild steel we had used at the start of this project.

I am pleased to report that now the kamarion has been disassembled from the machine, it is possible to see that its deliberate prebend (in a deflexed direction, to borrow from bowyer speak) is still there. In other words, if one were to look down on Firefly from an overhead position, the center of the kamarion should appear convex relative to the back of the machine. It is good to see that the kamarion has not taken a backward (reflexed) set when repeatedly stressed by a couple of hundred firing cycles. The kamarion is basically one big spring, and if it fails by having its center bend backwards too far at maximum draw, then the forks on the kamarion twist inwards towards the centerline of the machine. This in turn removes some of the support that prevents the top of the field frame from twisting inwards and down under the torsional loads. Not a good thing if one is playing with those higher draw weights.

The straight edge in the photo below gives some indication of this desirable, forward facing deflex in the kamarion. In short, the deflex acts to stiffen the kamarion, which in turn, protects that potential weakspot on the top of the straight stanchion. So glad it is still there.

Here is the setup that was used to tension the dacron back-cable.

And here is what happens when Mr. Man goes one clickity-click too many with this set up. (Note the depressions cut into the limb that were to be used to mechanically anchor the nock whippings.)

Fortunately I had a spare piece of ash, so all was not lost, and today a couple of usable limbs appeared out of this catasrophe.

A smallish whipping on the very end of the tip locked in the tension from the braided cable until additional epoxy soaked whippings could be added to form the nocks. Happily this cordage style nock forms a couple of high-tension compression rings that also protect the end grain on the ash. One day it would be intersting to try a similar binding rendered in sinew and hoof glue, or something.

…. Anyway, past experience has shown that inswingers can get away with minimal nocks so long as there is an additional retaining strap connecting the limb tip to the bowsrtring. More on that later.

These Mk IX limbs are all finished, save for a bit of rasp work and some sanding. They weigh exactly 8 pounds each, and are 3″ longer than the previous Mk VIII’s. Those previous limbs were a chunky 10 1/2 pounds each and are the ones responsible for all the data generated in the last year or so. We are hopeful that this new design of limb will develop much higher velocities.

Healing is almost complete. As a subject, torsion is more interesting when not investigated by one’s lumbar regions. …..Stupid, stupid, stupid.

One of Firefly’s old washer locking pins has been reassigned to locking the limb into its forward taper.

Here is a photo of that locking pin and some of the handy-dandy ferrule cement that holds it in place.

The 1″ deep hole for this 3/8″ pin should pick up enough ash to form an effective lock. The hole in the limb is only .002″ larger than the pin, and when installed that gap is filled with the marvelously sticky ferrule cement. Application of a hot iron to the flattened tip of the pin will allow for its removal.

While the sum of all the forces in the firing cycle always seems to propel the limb aggressively forward into the taper of the limb iron, it is well to guard against any backward forces that might be encountered when applying the old heave-ho to the tension strap. It is imperative that the tension strap remain unbudgeably anchored at either end of a non-compressible limb.