Catapult – Onager

Onager with SlingWhen was the onager (catapult) invented? Who invented it? How big was it? Can I make one?

An onager is a type of catapult that has one arm and uses torsion for hurling the projectile. Large onagers were used as siege engines and could launch huge projectiles to be used to smash through walls and fortifications. It could also be loaded with many smaller stones or fiery pitch to use against enemy troops or to bombard the inside of a fortress.

When was the Onager Invented

The onager was invented around 250 B.C.  I had a hard time finding out this information amongst all of the garbage listed out there on the internet. This is a really hard question to pin down. But 250 B.C. is the answer I found and I will give an explanation as to why I believe it is accurate. I will also try to explain away some of the other, wrong answers you might find out there.

The first mention of a one-arm torsion catapult is by Philon of Byzantium in his book, Mechanike Syntaxis (Compendium of Mechanics) and he lived from 280 B.C. to 220 B.C. It is referred to in one section, called Belopoeica (Artillery). The reference can be found in Greek and Roman Artillery: Technical Treatises by author E.W. Marsden. I sometimes see a reference number like this (Pol 91.36) which I am pretty sure refers to the Belopoeica by Philon, but I haven’t found it yet.

Our best, early description of an onager came from a Roman writer, named Ammianus Marcellius. He roughly gave the following explanation:

The onager’s framework is made out of two beams from oak, which curve into humps. In the middle they have quite large holes in them, in which strong sinew ropes are stretched and twisted. A long arm is then inserted between the bundle of rope, at it’s end it has a pin and a pouch. It strikes on a huge buffer with a sack stuffed with fine chaff and secured by tight binding. When it comes to combat, a round stone (often clay balls with Greek fire in them, which explode on impact and burst into flames) is put in the pouch and the arm is winched down. Then, the master artilleryman strikes the pin with a hammer, and with a big blow, the stone is launched towards it’s target.

Ammianus Marcellius
his account of Roman History from 354 AD to 378 AD

Why is it So Hard to Figure Out When this Device was Invented?

Here is my speculative answer to this question based on hearsay from the internet.

The onager, a one-armed torsion catapult was built as an improvement to the ballista, which is a two-armed torsion catapult. It was basic military research and design for siege engines. The ballista was a great weapon, but it was complicated to use and had lots of moving parts that could break. The Roman engineers got together and created a much simpler design. (They took off one arm.) Since it was a military invention, they didn’t want the enemies to find out about it, so they didn’t write it down in detail. What they did write was meant to be confusing or in code so that the enemy couldn’t re-create one. Some internet historians believe that the diagrams and descriptions given were purposefully created with mistakes.

Also, the names for the different types of catapults were different in different times and different places. The scorpion was a title given to both the onager and the ballista at different times. That type of thing just adds to the confused misinformation. But in truth, we just don’t have very much documentation about onagers.

Sir Ralph Payne-Gallwey, in his book, The History, Construction and Effects in Warfare of the Projectile Throwing Engines of the Ancients states:

It is astonishing what a large number of catapults and balistas were sometimes used in a siege. For instance, at the conquest of Carthage, B.C. 146, 120 great catapults and 200 small ones were taken from the defenders, besides 33 great balistas and 52 small ones (Livy).

Sir Ralph Payne-Gallwey
The History, Construction and Effects in Warfare of the Projectile Throwing Engines of the Ancients

But we now know that he made stuff up to support his claims. The book “The Art of the Catapult” states:

146 B.C. – The Roman historian Livy writes that over 400 onagers and ballistae were used at the Roman siege of Carthage.

The Art of the Catapult

Were they getting their information from Payne-Gallwey? Maybe they didn’t realize that Philon had already talked about onagers. Or perhaps they didn’t realize that a one-armed, torsion catapult is an onager. Who knows?

Sometimes people write that the onager was mentioned by Philon in 200 B.C., but this seems odd to me since he died around 220 B.C.

Why the Onager was Invented

The onager was invented to replace the complicated ballista. The onager is much easier to use and required fewer men to operate. It had only one arm, which meant it needed only one torsion spring, so it didn’t have as many moving parts to worry about. This means that more soldiers could learn to use it. It did not have the range that the ballista had, though.

The Romans would include an onager with a Centuria, which is a group of about one hundred soldiers. It took twelve soldiers to operate the onager.

Power Source

It is a torsion engine that derives it’s power from a large skein of twisted ropes. The ropes need to be strong and elastic, so often were made from hair or sinew.

Skein of Twisted Rope

Sling or Cup

The proper onager probably used only a sling. The sling provides added power to the projectile when it is hurled. The length of the sling adds to the length of the arm with almost no additional weight, so it would seem silly not to use such an advantage.Catapult - Onager

Sometimes we will see a picture of an onager with a cup on the end, though. Most times, actually. This is likely due to a book authored by Sir Ralph Payne-Gallwey called The Crossbow. When the author built his onager, he incorrectly added a carved out cup at the end of the swinging arm. This required the arm to be thick enough to support this cup which in turn slowed down the speed of the projectile since some of the hurling power was being used to lift the heavy arm. The book had a large influence on the way people thought about catapults and created the misconception of the cup. In an Appendix he wrote to the book, Sir Payne-Gallwey rebuilt the onager with a thinner arm and a sling. This second onager produced much better results and was much closer to the actual shape of the onager.

Did Onagers Have Wheels

A proper onager wouldn’t have had wheels. This is because when the arm was released, the power put through to the projectile was so much that the onager would buck and jump. If there had been wheels on the onager, the whole catapult would roll backwards violently and leave the projectile sitting not very far away. Imagine a fool standing on a skateboard and throwing a bowling ball forwards. What’s going to happen? The skateboard will roll backwards and the fool will likely fall off. The bowling ball will not go very far and may even end up landing on the person who threw it. Since the goal was to get as heavy a projectile as possible to travel as far as possible, it would have been foolish to put wheels on the onager, even on a small one.

Also, an onager used as a siege-engine for destroying walls would have been so large that wheels would have been impractical. It would have been too heavy to move and the wheels would have gotten stuck in the ground. The half-sized catapult that Sir Payne-Gallwey made was two tons so a full sized one would have been twice that. Such large onagers would have been built or at least assembled on site. When large onager’s were made, special areas designed to soak up the compression would have to be created or it would buck and destroy the ground under it, causing it to lose it’s aim.

So, why do all the pictures seem to show the onager with wheels? That’s because the drawings were created by artists who would make up the parts of the machine that were left out of the sketches. Also, since the onager was a military machine, the drawings would often be left purposefully incomplete or inaccurate to prevent the enemy from getting hold of them and building one. Mostly, I suspect it is due to the enormous influence the book, The Crossbow had on the world since the pictures in the book showed the onager with wheels.

How Big is an Onager

The onager can be different sizes ranging from large enough to hurl stones through castle walls (siege engine) to small enough to be a desktop catapult. The size is determined by the size of the projectile you want to hurl. There aren’t any good, accurate, historical examples that I could find. We do know the size of some of the stones the catapults hurled and the distance the stone traveled. With this information, we can deduce the size of the onager that hurled it.


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