Onagers in the Roman Legions

[D B Campbell]

Mr. Campbell is the expert here on artillery, he'll be able to help you when he sees this topic.

After an introduction like that, it would be churlish not to respond. :-)

As the OP mentions ballistas and onagers, I'm guessing we're in the second or third century AD. (Flippancy alert.) The ratios mentioned come from Vegetius, so we don't know if they're true or if he made them up. (End of flippancy.)

It's certainly difficult to imagine a legion dragging cart-loads of onagers on the march, but everything we know about Roman artillery (which, frankly, isn't that much) suggests that each legion was, indeed, responsible for its own catapults. Remember that artillery probably wasn't stored in a tensioned condition, so "rot and humidity damage" needn't have been a big problem, as there were specialists in charge of it, in any case.

I can't imagine anybody quibbling if your fictional legion has to start manufacturing onagers for a forthcoming siege. If you have any follow-up questions, I'm sure someone will pitch in to help.