I take the liberty of quoting some recent correspondence with Dr. Tracey Rihll, after I sent her some videos of Firefly’s latest exploits.  Tracey is the senior lecturer in the Department of Classics and Ancient History at the University Swansea and also author of that excellent book,  The Catapult.   Her extensive treatment of the subject is pretty much required reading for all catapult geeks.

—– Original Message —– From: Rihll T.E. To: Watts Unique Sent: October 19, 2011 12:58 AM Subject: RE: Firefly in action.

These are really useful Nick; thanks. I assume in the second one when you seem to touch a small device in front of the slider with a wooden pole, shortly before pulling the trigger, you are checking a velocity meter or something similar; please correct me if that’s not what’s going on.

Re: accuracy, the passage that springs to mind is Caesar, siege of Avaricum (Gallic War 7.25; see also 8.41, discussed in my book p. 97).

Have you tested range yet? It would be nice to know whether the figure of 400 yards/metres has any substance, and if so, what exactly!

All the best


Dear Tracey,

Yes, that procedure with the wooden pole is where I initiate the chronograph.  I have 30 seconds to make the shot before it stops transmitting the microwave radiation for the Doppler reading.

I have not tested for range yet.  That should be coming right up. However, my previous ballista  (the one I call the Gallwey and appears in the beginning of my blog),  had the same power of 1200 + foot pounds and 300 feet per second, and it would toss a 5, 000 grain bolt almost exactly 400 yards .  Lighter bolts made it out to 600 + yards. That old outswinger machine has springs more than twice the size of Firefly,  so it looks like the inswinger design really does favor high power in a compact package.

I realize now that utilizing the full 90 degree rotation of the arms and longer draw, are not necessarily advantageous in the inswinger design.  The power seems to come from inducing high levels of twist into the bundles when they are at rest, while, of course, keeping them as straight as possible.  Ideally the sweep of the limbs is short and violent, rather than long and sustained.  The draw lengths on Firefly have dropped to 33 1/2″  from the 50″ inches I had been using at the start.  This (and the fact they wouldn’t have needed space for the dynanometer) means that the Orsova engine could have been a good two feet shorter than Firefly, giving much more room to operate it in a cramped  guard tower.  It may also mean I need to make Firefly, Mk. II.

A quick word on the subjects of power and accuracy:  I feel that demonstrating how accurate these machines would have been is one area where experimental archaeology can make some valid contributions.  When we talk about “power” as it relates to these full scale reconstructions of torsion engines, we are invariably talking about the power derived from nylon springs instead of sinew springs.  Many assumptions about the equivalence of these two materials have to be made.  On the other hand,   the accuracy that the machine can produce is dependent more on quality construction, good mechanical design, and careful tuning, than it is anything else.  When it comes to the shooting accuracy of these machines, the modern experimenter can have a high degree of confidence that he is not straying beyond the bounds of what the Romans could have achieved.

Thank you for helping to keep me inspired these last few years as I worked on all this.  It made all the difference.  Nick.

2 Responses to “Thank you, Dr. Rihll.”

  1. Randi Richert says:

    Your posts and research are always interesting to the rest of the twisted-geek community. You are posting real numbers and thinking/building in real time with no slavish devotion to any pet theory, kudos! Your observation that Firefly has similar output with significantly smaller modioli echoes Aitor’s observation that his Cheiroballistra had @ 30-40% more power as an inswinger that in its original outswinger configuration. This may turn out to be a very important distinction.
    If you do make a Mark II version, possibly based on your quickly modfied sketch from the other day, I suggest that you may want to reduce the rail/forend that extends beyond the frame/ladder to a simple I-beam affair as evidenced on Trajan’s column. It would help offset the reduced mass of the case. Although a forehand portion is included in Heron’s design, it is totally superfluous on a sliderless stand mounted weapon. If you do, I’d love to photoshop it and my base/mulecart back into the Trajanic images.

  2. Nick Watts says:

    Thank you Randi. I am carving pumkins for the twisted-geek community at this very moment. More later…

    If I do commit to Firefly Mk. II, there are many details of historical authenticity I want to revist, (e.g. the holes in the forks on the kamerion). The case and forend are also right at the top of this list because they are so fundamental to other aspects of the design. I am open to anything, especially if it makes some sense of what that ancient rock carver was trying to communicate to us.

    On the photoshop question,… naturally, shop away old chap.

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