Full Version: Torsion Artillery Compared to Tension Artillery
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Tension artillery holds several advantages over torsion artillery. It was easier to construct and maintain, and composite bows could be better protected against dampness as springs. Nonetheless, it were torsion engines which came to provide the mainstay of ancient artillery arsenals throughout most of antiquity, whereas tension bow machines practically disappeared from the records (and archaeological evidence) after the 3rd-2nd century BC.

According to the standard view, this was due to torsion machines being much more powerful than tension crossbows. Marsden, going into some detail about the respective qualities of the two systems, ascribes to tension bow machine "a relatively modest performance" (Marsden I, p. 63). This opinion is also corrobated by the ancient engineer Heron himself: "... [Greek engineers] wished to increase both the size of the missile and the force of projection. They sought to make the arms of the bow more powerful, but they could not realize their intention by the use of composite bows." (Marsden I, p. 16) So, looking for better ways elsewhere, the Greeks came to realize that torsion shooters were more powerful and relied on them thereafter, so the accepted wisdom goes.

However, to the best of my knowledge, the view of torsion machines holding much more power has never been subjected to close scrutiny or been proven by experiments or calculations. Modern experts universally assume a superior performance but fail just as regularly to provide evidence for it. So, could it be merely a myth which tries to rationalize the rise of torsion engines in retrospect? Where is the evidence?
Ancient artillery is not my field, but it seems to me that Torsion casting machines could be more easily taken apart, packed in a compact mass, and transported. Also, parts of one could more easily be cannibalized to keep others working.
Quote:Modern experts universally assume a superior performance but fail just as regularly to provide evidence for it. ... Where is the evidence?
I cannot give an answer, but mistakes are often made like this. My list of common errors is, essentially, a list of questions that should have been asked, but weren't. I would not be surprised if torsion is, indeed, not better than tension.
Hello, there were a number of papers written regarding torsion artillery, tension e counterweight.

The one most usefull to us was "Mechanics of the Onager" V.J. Hart and M.J.T. Lewis (Journal of Engineering Mathematics - 1986) because of a simple stated assumption, that the Elastic Force of a Rope Bundle could be roughly used as that of a cilinder of the same material, meaning that friction and fiber displacement forces could be considered negligible.

What we found out in real field testing is that Torsion Artillery requires small adjustments since the fibers will keep streching (somewhat overriden by using a strong pretensioning apparatus).

We also found that simple "Tension Artillery" depends exclusivly on the fibers (or construction) of the tensioning element leading to a varied degree of fiability (all trees are by nature anisotropic).

So while the Torsion Artillery is harder to build, it also deals with the simple nature of a single variable present in Tension Artillery by introducing a bucket load of variables, that when put together in roughly the same way (Vitruvian Proportion for example) will make very similar weapons.

If you go on our website, we have a simple explanation for the involved equations in Torsion vs Counterweight (we feel that Tension in classical era could be, well a dice game).
Quote:"Mechanics of the Onager" V.J. Hart and M.J.T. Lewis (Journal of Engineering Mathematics - 1986)

I flipped through it, a lot of formulas, but nothing about a comparison between torsion and tension shooters.
No, they just rebuilt an onager and built the phisycal model afetr it.

It was most usefull to us because of their finding regarding the "cilinder of material"

What we need to think about when build torsion and tension artillery is on Force.

Everything will reduce itself to Force, what can the material cope with, what do we need.

Torsion artillery has the huge advantage of "calibrating" the for├že you want to use. You do that like the Greeks and Romans found out empiricly (and that Hart and Lewis proved cientificly) you increase the D of the washers, which by their reasoning also increased the distance between washers. Since a cilinder volume is base x height this will wield an exponential increase in Force.

But although they could increase the Engine Force by simply making larger washers, then you have other limiting factors such as the break strain of the arms, and the break strain of the hole carriers.

What we've found, not only by running Computer simulations but also by firing on the field, is that the arms will break first than anything when they are made of wood.

From here we can extrapolate that being the arm wood fibers the first to give, due to elastic forces demanded of them, a tension engine without some serious metal reinforcements (or laminated metal arms as a whole)would be not as strong simply because of material limitations.

When da Vinci planed his "giant crossbow" he planned composite arms, if in the classical era you could build such wood/wood composites or wood/metal composites, it is likelly that Tension would give Torsion a run for its money.

Something we wouldn't mind doing if we could get a grant (or just acess to some extensometers, tension meters and ultimate stress/strain machines - but Eng. School is long gone.)

With the known technology level I'd say that from a builder POW, torsion would be far easier, an ultimately stronger.
I am still asking myself how the literature assumes that torsion artillery is more powerful than tension. I mean this is the generally acknowledged rationale for why the Greeks soon switched from bow machines to torsion catapults. Yet, the experts never set out to prove this. And what is the common denominator for comparisons? Can you compare the volume of torsion springs with the diameter of a bow that simple? Can one say for a given projectile weight, a torsion catapult shoots farther and faster?