The Expanse and its compatibility with OA

[The Astronomer]

On Starlark: I thought the radiators have to be on almost immediately after turning on the engine....

Yes, but they won't work when the ship is stationary, and aren't deployed in the first few minutes of flight. The first few gigawatts are used to heat some of the water propellant into steam, adding a small amount of added delta vee.

From my experience (with Children of a Dead Earth), oxygen, and by extension, water, is a hilariously bad propellant for thermal rockets. If you need additional thrust, use hydrogen or methane instead of water. If you're an interstellar spacecraft, where delta-v is extremely important and thrust is of little concern, remove water propellant and turn on the radiator right from the beginning.