Help needed building an onager

[D B Campbell]

BUT this is a takedown model, not a strung ballista. So that would put the pre-tensioning in a whole different perspective IMHO. Unless the rope would have been wound after the onager had been assembled. This would then however be conflicting with the idea of rapid deployment, would it not?

I don't think anyone has ever suggested that these were rapid-deployment machines, Robert. I certainly wouldn't subscribe to the notion of "quick assembly", except in the sense that Roman engineers settling down for a siege would be able to knock some up in fairly short order, especially if they had brought the major components with them. (Sadly, the sources do not describe how this worked.) The word "rapid" doesn't really suit the onager: Ammianus seems to suggest (during the siege of Amida) that, once deployed, they were a major pain to manoeuvre about. I tried to give a flavour of this in the cover painting of Siege Warfare in the Roman World, which shows onagers being deployed in defence of a walled town:
[attachment=5847]Siege_Warfare_in_the_Roman_World.jpg[/attachment]

I have been looking at the picture of that huge machine shown by D B Campbell, also if an ornager of this size could be dismanteled and transported is there any evidence of the Romans also transporting their amunition as well.

In my opinion, ammunition was prepared on the spot, as well as the machine. The Romans show no particular diligence in collecting "ballista balls" after use, which is why we find so many of them.

Also, please remember that Brian Delf's painting is of the large onager described by Ammianus. I tried to calibrate it as a one-talent machine (i.e. the spring is powerful enough to launch a 26kg boulder) requiring eight men to winch back the arm. I have no doubt that there were smaller versions!