Onager questions.

[Maximus Minnenus]

Greetings everyone,
I have been kicking around building an onager for about two years.  Bought some books but the information on the onager is so painfully thin, I don't think I have ever seen even a source say they were x number of feet long in the base.

Now in digging around I found someone that roughly figured out the thickness of the band by measuring the weight of the projectile and distance it could through.  It makes a few assumptions from there. 
His statement is that a "full sized" Onager would roughly be 20 US feet.  Now that would make the bottom two rails about
4 tons if they were made out of decent oak.   Now I can get trees at little to no charge and rough em in with a chain saw.
but with a total weight of about 5 tons I am not going to be moving it with out special equipment.  So I was thinking half size.
at one ton per rail I could move this with a truck, abet in several trips. 
Relevant paper here
  
     Now I am of the anti-arrest post crowd.  I read on here a discussion from several people about different ideas about what would work better.  Things like the design with a pad low and angled to allow a full swing. 
And someone (Robert perhaps) put up a design with the tension arm in the very front.
But my first question is assuming that the design of the Onager dose not allow the arm to smack into a cross beam why bother arresting the arm at all?  I know the texts tell us it will splinter stone work but I won't fire it on cement.
The local police are going to have a heart attack when they get a call about a 10' version for the first time.  So outside of town in a open field will be the firing place.  Unless it is going to break the swing arm I could care less if it hits the ground.

Now I don't have the ability to steam bend something this big.  So in order to get it off the ground and provide more swing for the bottom of the arm I am going to put "feet" if you will under the four corners.  And notch them into place.  I am thinking that it will provide enough height and if that won't do it I will just dig under the arm a bit.  The feet will be the same size as the rails of the Onager and 2 or three feet long for stability.   Does this sound stable enough?  Not that I am going to be anywhere in the area when I set it off for the first time but I don't want to destroy it on the first shot either.

Lastly the ever present elephant in the room. 
Sinew and horse hair are out as cord.  Human hair would cost as much as paying someone to make one out of sinew.
So has anyone found anything better or have ideas for something that would work better then triple strand pre-stretchered nylon?  I have considered used bungee jumping rope even but the rebound may be a bit dangerous.  
Obviously I haven't made any progress on that one. 

I have constructed a small model when this idea first came up and it worked well enough but I used surgical tubing (think wrist rocket tube) for the model and not totally surprisingly it flipped around when fired but a half sized one should be heavy enough to mostly stay put when fired.  So since correct rope is out I am looking for something else that will work.

Sorry for the length.

[Gunthamund Hasding]

welcome to the forums

Just one comment on the size

I really think you plan too big, the local police will be of course unhappy but so the more the local hospitals. I would not dare to try to span that monster, or if you do use an engine to span it and stay 300 ft away, if it breaks people around could be hurt. and if you fire such a monster be sure not to have anything in is front for half a mile enven a mile at least.

I think that one about 5ft big would be more than enough

Just my comments from what I saw that the onagers of different groups throw.

Anyhow I wish you good luck with all and keep us posted

BTW as per forum rules you should have your real name in your Signature

[Flavivs Aetivs]

The Payne-Galleway Onager design is a commonly perpetuated misconception, you are wasting so much energy by stopping the arm short like that. This is what a real Onager looked like:



The reason for the angled stop (in actuality they used a mound of dirt), is that way when the rock is released from the arm at the top of the swing it has the maximum amount of energy transferred into the stone.

And yes, they were known to splinter stonework but could not actually tear down fortifications. The Traction Trebuchet is what it took to do that, which didn't come about until the late 6th century and wasn't very accurate.

D.B. Campbell could tell you more.

[Maximus Minnenus]

The Payne-Galleway Onager design is a commonly perpetuated misconception, you are wasting so much energy by stopping the arm short like that. This is what a real Onager looked like:

I am not going to build in a stop at all.   I can not see any reason for the presence of it at all.  I guess I was more worried about the thing breaking the arm by hitting the dirt.  Short of that I won't bother with one.

---

Just one comment on the size

I really think you plan too big, the local police will be of course unhappy but so the more the local hospitals. I would not dare to try to span that monster, or if you do use an engine to span it and stay 300 ft away, if it breaks people around could be hurt. and if you fire such a monster be sure not to have anything in is front for half a mile enven a mile at least.

Ok thanks for the feed back.  
I know the city would have kittens if I ever fired it with in shooting range of the city let alone inside city limits.
I have a farm I can shoot it on, and no worries about the dangers.  I fully understand that this thing might be as dangerous to the person on the trigger as someone down range.   The idea is to keep upping the strength of the tension on the ropes a little at a time.   More importantly to see if it is tearing its self apart as the tension increases.
This would be the first serious build of a catapult I have attempted so even if everything else is correct I could make a mistake that causes a catastrophic failure.
And I don't plan to end up on the news in a "hold my beer and watch this moment".  Nor be that guy and cause city's to start banning the owning of ancient weapons.
That is why I was asking about the size.   We are all guessing, and my theory (ok guess) was that they built them on site with the metal and rope being the only thing they carted around.   But in the description it says that the legs were bent.
It is even harder for me to see the ancients going through that trouble on a battle field.  if needs be they would have dug out under the arm.  (My fall back plan if the feet are unworkable.) But no where do we a guess about size.
The paper I linked states that from that gentlemans math they would have used a 20' onager to get the ranges in question.
That was the main thrust of that part of the question.  I am aware that he took a educated guess on the size.
If you assume that he is correct and account for using inferior rope and only half the size of onager it should only fire 200 yards or best case 300.  Still dangerous enough to command respect indeed.  And the trigger and ratchets will not even look appropriate to the time.   Safety indeed will be more important.  I like my limbs connected to my body and intend to keep them there.  Using motor to draw the arm back is a good idea.  Although Roberts idea of a slanted drum to pull it back with is elegant and simple.  

Ok got it.  Will fix that post haste.
Thanks again for the comments.

Who spends the less interested portions of his life as a unassuming grocer
Daniel Watters.

[Robert Vermaat]

Ok got it.  Will fix that post haste.
Thanks again for the comments.

Who spends the less interested portions of his life as a unassuming grocer
Daniel Watters.

Hi Daniel,

Please add your real (first) name to your username or your signature (in your profile).
It's a forum rule.
Regards,
Robert
Moderator

[Flavivs Aetivs]

If you assume that he is correct and account for using inferior rope and only half the size of onager it should only fire 200 yards or best case 300. Still dangerous enough to command respect indeed. And the trigger and ratchets will not even look appropriate to the time. Safety indeed will be more important. I like my limbs connected to my body and intend to keep them there. Using motor to draw the arm back is a good idea. Although Roberts idea of a slanted drum to pull it back with is elegant and simple.

200-300 is about right. The smaller late Roman inswinging-style Manuballista, which fired bolts instead of stone projectiles, can do over 1000.

[Robert]

Salve Daniel,

The design I posted was based on the only period description of the onager. If you can get your hands on trees easy enough, it would stand to reason you select a curved trunk, hew it in half and the square it. This would give the humped shape and would lift the cordbundle and base of the arm well off the ground.
From your replies I gather you are not really all that concerned with authenticity, which is a bit of a shame, as why build the thing, then? Flax or hemp rope will give you good torque, as you pre-tension your bundle using the rachet. As the onager uses a sling, this greatly increases the range. Note also the downward bend in the end of the arm. This is based on the description as well. Pre-tensioning your rope will also Ensure you will break the arm if you allow it to crash into the ground unchecked. Also, it will put great strain on your construction. The ancient source clearly describes an arresting mattress. The rock, by the way, is long gone before the arm is arrested by the padded mound.

Presently, I am engaged in researching and building a traction catapult. I hope to publish the results in Medival Warefare Magazine later this year. Less Romans these days, more Franks!

[Flavivs Aetivs]

Traction Trebuchet? Cool, although the Romans adopted it before the Franks.