These beautiful CAD images I have brazenly copied from an excellent paper by Ildar Kayumov and Alexander Minchev.

arch strut 2

Ildar 1

 

Here is their paper and what it has to say about some interesting “new” ballista finds:    23_Kayumov_Minchev_ROMEC

By my reckoning, that short little winch handle would need to be connected to some fancy internal gearing to ever generate enough pull to be useful.  (“…to match the enormity of the task”, and all that.)  I like the idea of it though. Phoenix certainly has her ears perked in that direction when it comes to winches.

However, of greater interest to me here is the interpretation given the angled struts and their attachment points on the Kamarion.  Given the bending issues we have experienced in the past with the arch in Firefly’s Kamarion wanting to collapse backwards towards the winch,  Mr. Kayumov’s plan makes a lot of sense.

His use of simple rivets to attach the angled struts to the Kamarion may at first glance appear to be the simplest and most straight forward way to anchor these parts together. However, I believe the Romans went a little deeper on this and sought to make the machine something that could be easily disassembled.  A clue to the existence of this more advanced, take-apart design, lies in the loops and tangs on the field frames and Kamarion of the Orsova finds.   Those features make manifest the ancient designers train of thought:  a modular machine that could be quickly knocked apart or put back together again. (Think: packing the ballista on mules, shipboard transport,  yomping the whole thing up to the top of that ridge, assembling it up in a tower  etc.)  Presumably wedges were used to complete the mechanical lock up of all the joints.*  If this hunch about a take-apart machine is right, then it becomes highly desirable to use something other than rivets to connect the angled strut to the Kamarion.  Hence the following:

IMG_4813

The wedge has a shallow angle on it and, when driven into place, causes the yoke to grip the Kamarion with great force.  The locking pin is driven through the “sight” hole in the Kamarion and through a matching hole in the wedge.  This arrangement assures there is no lateral slippage of the yoke along the Kamarion, as well as locking the wedge in position so it can’t back out.

Firefly has successfully used this type of wedge and locking pin system to connect all the critical joints needed for her rapid disassembly. (That process takes me, on my own, about 20 minutes)  In all the shooting we have ever done, none of Firefly’s joints have ever loosened a jot.  Very stable she is.

Anyway,  that is my new, favorite explanation for those two holes in the Kamarion of the Orsova artifacts.

*Note, some scholars have suggested the wedges were made of hardwood.  My experience suggests that a metal wedge, driven into the joint and captured by some kind of retaining pin, is the ideal solution.  I fear wooden wedges would not hold up over time.  They would be smacked around by the vibration of a powerful machine and susceptible to all the usual vagaries of wood — splitting, shrinking etc.

Because Phoenix will be another “iron frame” ballista,  there is no reason not to favor all things ferrous if they offer an advantage.

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While I very much admire the artwork in those preceding Cad drawings of an Iron frame ballista, there are a few points in the design that I need to scrutinize before applying to Phoenix. I will list these areas below, and hope the authors of that fine paper, and anyone else for that matter, will understand where I’m coming from.  Just filling in all the minutiae before taking those first pecks at the Phoenix project.

(1) I have already noted that the winch handle that is shown, accurate representation of one of the Elenovo artifacts that it is, would not have had nearly enough leverage with a winch drum of the apparent diameter shown. Firefly’s levers for her hand winch are a whopping 5 1/2 feet long and her drums are 3 inches in diameter.   There is an extra set of pulleys to half the load,  and even with all that it takes close to 70 lbs. of down pressure on the end of the levers to drive that drum as it nears the full draw weight of 5,000 lbs. This is not to say the original crank artifact that was dug up is not long enough to draw back a powerful ballista, it is just that such a winch handle would need much more mechanical advantage than shown (i.e. a gear box, or as the authors of the paper suggest, a bloody big cheater bar for that vertical handle), otherwise it would be little more than a toy in terms of power.

Medieval crossbows and their block and tackle cocking devices, with multiple pulleys and several falls of rope, had very small diameter drums of about 1″ to give them lots of power, and yet their winch handles appear to be about the same length as the one shown in the artwork. What were the winches of those medieval crossbows rated for in terms of pulling power? not much more than a puny 1500 lbs or so, even for the bigger ones.   (……… Which is only relevant because that ain’t the kind of ballistic territory I’m figuring on for the Phoenix.)

(2) The end caps of the field frames on the original artifacts show 4 through holes that hold the locking pins for the bronze washers. It is very important to have enough holes in the washers to allow small adjustments in their rotation. Firefly’s rotational discrimination in her washers can be advanced in 7 1/2 degree increments. These incremental adjustments are the only ones available for tuning the thrust of the two bundles after the ballista is assembled and in use.  Also of note are the catapult finds from Ampurias that demonstrate this 7 1/2 degrees of adjustment.  Some folks have mentioned that the tuning of the bundles is done with linear stretching, and it is true that when the spring is fabricated by winding it around the crossbars it is best to tension the bundles as equally as possible. But in the real world, things get out of wack no matter how carefully the springs are balanced by the linear stretching that is applied when they are made.   (Yes, I know,  “wedge” machines can offer some adjustments after assembly,  but in my experience it is too rough and imprecise a way of performing delicate negotiations between a pair of ornery springs.  In fairness, though, more experiments with differently angled wedges are needed to make that statement absolutely.)  What is clear is that during the lifetime of the springs the need for fine adjustment of the individual spring power is essential to keep twin armed ballistas performing properly. Again, I respectfully remind anyone who might actually be reading any of this, these things I speak of are not just armchair theories. Firefly has been there and gone through all of this.   For a scheme based on rotational adjustments,  7 1/2 degrees or finer seems about right to achieve balance in the springs.  Very important if you ever want to tame those waggle tails!

(3)   This next one is a bit tricky.  I say that because Firefly has only ever tested one particular length of spring.  With longer springs perhaps she would do a better job of using the 120 degrees (or whatever it is) of extra limb rotation depicted in the artwork.  The longest limb rotation I ever tried with Firefly was 110 degrees and the performance was fairly abysmal.  The formula that I hit upon for getting power out of Firefly was pretty simple and it favored much less limb rotation.  The formula goes like this: if the tension on the bowstring is really taut in the at rest position, power is excellent.  To have this extra tautness in the at rest position it is necessary to have the bundles cranked up very high with rotational pre-tension.   With Firefly’s length of spring, this tautness can only be achieved with limb rotations of about 45 degrees.  That is because when the rotaional pre-tension is up this high, the draw weight starts to stack heavily at around 45 degrees. To put it plainly, she ain’t a going no further.  Big limb rotations and long draw lengths are kind of  a non-starter for Firefly, fortunately however,  that does not necessarily matter when it comes to generating velocity with inswingers.  Our experiments have  shown that it is that final snap of the bowstring as it closes the power stroke that is responsible for a lot of the projectile’s final velocity.  To put it another way: loose, at-rest string with Firefly using 110 degrees of limb rotation  and her shots were only puddling along at a meager 225 fps.  Very taut, at-rest bowstring, with a short 45 degrees of limb rotation, and she was hammering the clouds at 360 fps and more.

………So anyway, our current prejudice on this matter suggests that really fast inswingers  do better with much less limb rotation than is shown in the paper’s artwork.  Deep down though, I intend to remain agnostic on the matter because the case for greater limb rotation needs more exploration than I’ve given it so far.  Go figure….

Back in 2009, April 6, I posted this bit of frippery.

compxxx

Of course it ain’t exactly frivolous.  Those are actual measurements taken from Firefly as she cranks her arms back and forth.  It’s pretty easy to intuit the speed increase if you ponder the relationship between the draw lengths and the limb angles a bit.  The string and limb basically move in unison, therefore the string has to cover the same amount of distance it did in the last half of the draw as it did in the first half, only it has to do it in the briefer interval of time afforded by the smaller 31 degrees of limb rotation.  With an inswinger, that is where the short rotation zip comes from.  Just that simple.

And now for that moment of snapola:

Click for vid   20121107141039(2)       Bolt is a Dura Europos style, 340 grams,  359 feet per second, 1500 foot pounds of muzzle energy. Maximum range is 900 yards.

Here is a video of our farthest shot yet, 998 yards.  Click for Vid:   20121107135533(1)   Projectile is a 276 gram Dura bolt moving at 375 fps.

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(5)  If this laundry list were to turn itself into an apology for shorter draw lengths, I would remind the reader of the monumental advantages of high power in a small package.  Very Tacticool, as they say these days.   Phoenix will be at least 2 feet shorter than Firefly, and have greater relative power due to her improved structural strength and ability to handle higher draw weights.   The advantages of shorter machines in towers or on shipboard or wherever, would be a welcome advantage derived from the inswinger’s natural form of compound leverage. (See above envelope.)

(6)  I really don’t know where this goofy list is going anymore, so let me just say,  many thanks to Mr. Kayumov and Mr. Minchev for being such unwittingly good sports and appearing in this posting.   You are helping Phoenix to get her gaps filled in. Cheers!

 

2 Responses to “Hence the following”


  1. Charles W. Fink says:

    In blacksmithing holes are punched and tapered, and cone shaped pins are easy to forge and with gravity, self center and tighten. Between the archimedes screw and the antikathera mechanism it seems like a worm gear winch would have been a simple , strong solution the ancients would have invented. It just needs to disengage after cocking to cycle quickly. A diagonally folded and sewn strap of woven cloth might have the strength to fit in a finite space. They probably built a lot of these machines and some were better than others. That enough of one was found by people that cared to figure it out is practically a miracle. Its such an elegant looking machine, such thin limbs, to do so much. The strength of the materials must be balanced so there is no weakest link. You must know more about it than anyone. It seems like you have to balance what you know you could build, with what you think they could have built. Stainless steel cable would be cheating, but don’t underestimate what our ancestors were capable of. Their concrete was more durable than ours and can withstand salt water over time. This planet is littered with huge stone structures we can’t explain. I don’t know how we lost so much history that our origins are so clouded but there seems to be a missing link between ‘cavemen’ and Rome being laid out like a wheel before it was built. Pictures are pictures. I think you have earned creative license to do your best.


  2. nick says:

    I have considered using high tensile, carbon steel cable in lieu of the Dacron tension strap used on the back of the limbs. I suspect there would have been some very high end metal workers somewhere within trading distance of the Roman empire back in ancient times. High end applications require high end sourcing for strategic materials. For the tension straps, either high quality steel cables or rods would seem to be natural choices if they were available. Some folks grumble at this notion because the Romans are not generally noted for being excellent metal smiths themselves. They are missing the point a bit. These machines were the high tech beasties of their day. And, as we all know, high tech devices of doom are generally well fed entities inside a military state. They had steel. It follows that somewhere there was some steel that was better than others. Now the hunt is on. “Find me what I want Igor, I have an idea!” The imaginative historian suspects the resourcefulness of the people he studies is greater than the record strictly allows.

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