The purpose:

Although a number of very excellent works have been published that contain sections about late model,  iron framed,  Roman ballistas,  it seems that there is relatively little written about full scale reconstructions that delve into the probable performance characteristics of these machines.   This is hardly surprising given the amount of work involved making a reconstruction that is both plausibly authentic,  yet powerful and accurate enough to be considered a viable weapon of war.   While it is far beyond the scope of this study to investigate all the forms and sizes of iron framed ballistas in use during the late Roman empire, it is possible for us to explore one of them.

The field frame and kamarion that were discovered in Orsova,  contain most of the  important pieces of information needed to make a working replica.  These  can be listed quite simply as:  (1) the diameter of the holes in the end caps on the field frames,  (2) the distance apart that the rope bundles occupied when installed in those field frames, (3)  the geometric relationship between the stanchions and loops on the field frames,  relative to the tangs on the forks of the kamarion.  The artifacts also provide the dimension of the arch in the kamarion  and the internal size of the loops on the field frames.   The overall height of the field frame provides a suggestion of how long the torsion springs would have been, but without the washers and crossbars there is no way to know for sure.

With the salient dimensions mentioned above, it has been possible to reconstruct a machine where the geometry of the artifacts makes simple and practical sense.  There is an internal logic contained in the relationship between the original field frame and the kamarion , that once fully understood, makes any attempt to reassemble them using interpretations from the ancient texts, a secondary and less compelling matter.

The purpose of this project is to suggest not only how the original Orsova artifacts fitted together relative to one another, but also to perform  a complete range of ballistic tests using a full scale reconstruction.  Our task is to reproduce the highest levels of performance  possible with a machine based on the Orsova artifacts.   Because the Romans had generations to perfect the performance of their late model ballistas, it seems logical that if we are to accurately model the capabilities of the original machine,  then we  must make every effort to have our reconstruction shoot as powerfully, accurately and reliably,  as it possibly can.  The safe bet is that the Romans certainly did.

The premise:

Our starting point for making a full scale reconstruction of  the Orsova ballista were these drawings taken from,  Recent Finds in Ancient Artillery, by Prof. Dietwulf  Baatz.

Later in the project it became necessary to make a new and stronger kamarion,  at that point we became aware of the following drawing by Aitor Iriarte from his paper in  JRMES  11  2000   47-75 .   Only the cross sectional dimensions from this print were used to make the second kamarion.

The above drawings are the only dimensioned sources that were used in making our reconstruction.  In those instances where actual dimensions were not called out, every effort was made to scale these drawings accurately   The premise we are operating on is that these drawings are accurate representations of the original artifacts.    If there is any debate about their validity,  it is not the concern of this paper, or our reconstruction efforts, to resolve those issues.  These drawings represent the given parameters for the following body of work.  Everything rests on them.

Organizing principles

There are three basic principles governing this project:

(1)  Honor the artifacts by not changing any critical, performance related dimensions. (ie.  the hole diameter in the field frames, length of the kamarion etc.)

(2)  When in doubt on how to proceed, always opt for the design alternative that will yield maximum performance and reliability,  consistent with rule one.   This will insure that our intent,  at least, will always stay on the same path as the original creators of the Orsova artifacts.

(3)  Use the minimum amount of modern materials;  and when we do, have a very good reason for doing so.

As with any enterprise of this scope there are always provisos.   The following caveats address the major concerns posed by this last principle.

Proviso 1 —  Cordage.

The Romans probably used sinew to power their torsion engines.  Sinew is made from tendons.  These tissues, extracted from animals during rendering,  have high elasticity and strength not found in other natural fibers.   The historical record shows that horsehair or human hair could be used to make torsion springs, but it is doubtful these two alternatives would have bested sinew in high end power.   Because it is far beyond the scope and expertise of this artificer to attempt to duplicate a full size torsion spring in sinew, we will need to turn to modern materials to power our machine.

Of the modern cordage that is available to make torsion springs for catapults, nylon is commonly held to be the most suitable. ( Perhaps at a later date we can extrapolate the probable performance of the Roman springs by making small scale models where working with sinew would be a more practical undertaking.  By doing a comparative study of the propulsive qualities of nylon vs. sinew on a small scale, a simple efficiency ratio could be developed and applied to the  shooting data from our nylon powered, reconstruction.  This, in turn,  should give us a fair idea of how well the original Orsova machine would have performed with its sinew based torsion springs.)

For the same reasons mentioned above, the bowstrings used on our machine are made from dacron rather than sinew.  Comparative small scale tests may also be run at some time to ascertain the likely differences between this modern material and sinew.

Proviso 2 —  Fasteners

Because the Romans used rivets in place of any fastening system based on screw threads,  we need to indicate our reasons for using modern nuts and bolts to secure various parts of our machine.  Largely it is a matter of convenience when it comes to swapping out parts on the machine as the design progresses.  In every instance that a screw thread has been used, the job could have been done by expert riveting.  Of course with rivets, we would lose the ease and flexibility that comes with modern nuts and bolts;  and because the higher purpose of our effort is to test for ballistic performance, not the intricacies of Roman riveting technique,  this deviation from authentic fastening technique was judged allowable.

Proviso 3 —  Machining and welding.

While the Romans had all the benefits of generations of skilled artisans and virtually unlimited manpower to make their super weapons, this project has only one individual, albeit with a well equipped machine shop, to attempt a modern day reconstruction.  In those instances where CNC machining or Tig welding have been used,  careful consideration has been given to make sure the design being replicated could have been made with the forging techniques available to the Romans.

Proviso 4 —  Safety

The limbs of our machine are the weak link likely to suffer catastrophic failure.  Here it was deemed prudent to utilize kevlar webbing, laminated to the front and back of the limb, to limit the amount of flying debris if some kind of breakage should occur.

Given these limitations, it is still expected that some useful insights might be gained about the performance of the original machine by attempting to build and shoot a full size reconstruction.

To be continued……

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