>>543801523
>it looked like you could do coal liquefaction (with nuke steam, and barreled heavy oil to bootstrap it), or even basic coal liquefaction with only asteroid products.
My bad, it is impossible to achieve without advanced asteroid processing to provide sulfur for free, the amount of coal and petroleum you would need for both sulfur and plastic make it a negative loop

Even then making plastic in space is far too resource hungry to be a viable alternative
A cannon magazine is equivalent to 10 cannon shell + 1 plastic bar and 2 iron plate, it would unironically require almost 10 time the infrastructure/power and 15 time the amount of carbonic asteroid to achieve a steady 1 shell/second, most of that is taken by the carbon>coal>>petroleum>plastic chain
Compare that to what you need for 1 rocket/second and you can understand why i say the cannon turret doesn't invalidate the rocket turret

Also rocket capacity for cannon mag is 4 per rocket, 2 for uranium, shipping massive amount of plastic from planetside is the only sensible option and even that require a massive cargo hold