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Onager - First Components Delivered
#16
Quote:Right both Tom Feeley and Len Morgan are providing advice right now so hopefully we won't embarrass them too badly :oops:
Glad to hear you're taking advice, Martin. Of course, what many RAT readers won't know is that we know virtually nothing about the design of the onager!

The design you're following was suggested by Sir Ralph Payne-Gallwey way back when. I prefer the design suggested for Napoleon III -- a serious Roman enthusiast -- by Lieutenant-Colonel Verchere de Reffye, chiefly because Michael Lewis has demonstrated (theoretically, of course) that De Reffye's forward-sloping buffer is more efficient than Payne-Gallwey's vertical one.

Unfortunately, we have no onager manual that would settle the debate! Smile
posted by Duncan B Campbell
https://ninth-legion.blogspot.com/
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#17
Hmm. I'm using the technical treatise by Eric Marsden. Here' he suggests the vertical stop is better as the inclined stop resulted in frequent breaking of the arm.

BUT, here's what I'll do. If you can point me in the direction of the design documentation, I'll build an inclined stop as well. We're building two Onagers at the same time, so why not build one of each?
MARCVS VLPIVS NERVA (aka Martin McAree)

www.romanarmy.ie

Legion Ireland - Roman Military Society of Ireland
Legionis XX Valeria Victrix Cohors VIII

[email protected]

[email protected]
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#18
Nerva, the very best of luck on your kicking ass! A full sized one (whatever that is) would be tremendously huge and heavy, and yours will be heavy enough. How you ever adjusted aim for one of those suckers once you had them in place I can't guess. It wasn't easy, however they did it.

Maybe I didnt see it on the site link you provided, but can you give us some details on your project? Kind of wood used, hand tools and power tools you plan to deploy, what you plan to make the rope out of, etc. Artillary freaks need details, after all.

Thanks.
Dane Donato
Legio III Cyrenaica
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#19
Wow Dane, I'll do my best :wink:
MARCVS VLPIVS NERVA (aka Martin McAree)

www.romanarmy.ie

Legion Ireland - Roman Military Society of Ireland
Legionis XX Valeria Victrix Cohors VIII

[email protected]

[email protected]
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#20
Quote:Hmm. I'm using the technical treatise by Eric Marsden. Here' he suggests the vertical stop is better as the inclined stop resulted in frequent breaking of the arm.
As far as I recall, Marsden never mentions the forward-sloping buffer suggested by De Reffye. He basically compared Schramm's backward-sloping buffer with Payne-Gallwey's vertical buffer, and preferred the latter.

I don't remember anything about Marsden experiencing arm breakages. What's the page ref?

It's true that Schramm initially had problems because he tried to stop the arm too soon -- after it had swung through only about 37º (the same as his ballista's arms!) -- and had to increase the angle of his buffer. But it still sloped backwards.

Wasn't it Payne-Gallwey who had all the arm breakages? (Also, iirc, he couldn't secure a backward-sloping buffer firmly enough to stop the arm thrusting it away from the frame -- surely a fault in his joinery skills rather than in Schramm's backward-sloping buffer design.)

The De Reffye buffer, on the other hand, slopes forwards. That's the beauty of his underrated design. The longer the swing, the better. (Prefiguring the trebuchet, where the arm has maximum swing!)

Of course, you then have the problem of pulling the arm back again -- I guess you'd need to employ a combination of pushing and pulling. I think that's why the Payne-Gallwey design has proved so popular -- it's dead easy to operate. But is it accurate? (btw I don't remember Payne-Gallwey ever arguing the case -- he simply presented it as a fait accompli. Didn't he?)
posted by Duncan B Campbell
https://ninth-legion.blogspot.com/
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#21
Hi Duncan

Forward sloping/backward sloping? :lol: My copy of Marsden is loaned at the moment to the man making the frame (Sean Shields) but I'll get it back tomorrow and take a look again and give you the references.

Your original comment interests me though. You say the 'forward' sloping stop proved more efficient. More efficient than what? Presumably the vertical stop? But, more efficient in what sense? Longer range, higher accuracy, better load throwing capacity, Less arm breakages?

I will say this though, the Onager in your illustration has great advantages in terms of Mobility. The stop could easily be detached from the main body for transport. Do you have any other design details? I'd be very grateful :wink:
MARCVS VLPIVS NERVA (aka Martin McAree)

www.romanarmy.ie

Legion Ireland - Roman Military Society of Ireland
Legionis XX Valeria Victrix Cohors VIII

[email protected]

[email protected]
Reply
#22
Quote:Your original comment interests me though. You say the 'forward' sloping stop proved more efficient. More efficient than what? Presumably the vertical stop? But, more efficient in what sense? Longer range, higher accuracy, better load throwing capacity, Less arm breakages?
Yes / not necessarily / not sure what you mean / probably (but that's down to the size of the buffer). Big Grin Theoretically, of course.

The problem is that De Reffye's design has never been taken up. Marsden seems to have been unaware of it -- from memory, he doesn't mention it anywhere. And everyone has tended to follow the design tried and tested over 25 years by the Ermine Street Guard. So there has been no history of practical testing, and hence no opportunity to tweak the design.

Michael Lewis's theory is published in an obscure mathematical journal. (Hart, V. G. & M. J. T. Lewis, "Mechanics of the Onager", Journal of Engineering Mathematics 20, 1986, pp. 345-65 -- good luck finding that one!)
He (and his mathematical collaborator) found that, unlike the ballista, the onager cannot be defined by a nice simple mathematical formula. As you have probably realised, there are a number of variables: arm length, arm thickness, sling length, finger angle, and buffer angle. We have no idea about any of these!

Lewis tested a De Reffye-style model versus a Payne-Gallwey-style model (note: not full-size machines), and found that, all variables being equal, the De Reffye generally gave a longer range. Might be worth investigating -- but it would be expensive!! Confusedhock:
posted by Duncan B Campbell
https://ninth-legion.blogspot.com/
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#23
Hmmm...let me chew that one over for a while, but thanks for the info :wink:
MARCVS VLPIVS NERVA (aka Martin McAree)

www.romanarmy.ie

Legion Ireland - Roman Military Society of Ireland
Legionis XX Valeria Victrix Cohors VIII

[email protected]

[email protected]
Reply
#24
Guys, I have my copy of Marsden, Technical Treatises in front of me. Payne-Gallwey set up his buffer to halt the arm at 75 degrees to the horizontal, but as Duncan pointed out, the jointry kept coming loose. He then, according to Marsden, put his buffer to stop the arm at full upright position of the arm.

Schramm was religious about keeping his arm stopping in it's forwad momentum at 67.5 percent, and set the bufffer at that angle, and the arm kept breaking, so he later placed his buffer at 75 degrees, and that resolved the breakage issues. Marsden reports that his small machine worked prefectly following Payne-Gallwey, and he never needed the spare arm he had built, just incase.

None of this should be taken religioiusly, though. Payne-Gallwey, in his book of the crossbow, illustrates the thumb ring for archery as being worn backwards, and if one were too try it that way, it would be an ugly injury he would suffer.

Dane
Dane Donato
Legio III Cyrenaica
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