- "to just heat the water directly" Recombination of hydrogen/oxygen gives you flame temperatures far north of >3,000 K. This is probably the simplest way to "

- "to just heat the water directly" Recombination of hydrogen/oxygen gives you flame temperatures far north of >3,000 K. This is probably the simplest way to "just heat the water", in this temperature range. I imagine a "conventional" 3,000 K heat exchanger (for example, electric-resistive) would be very difficult to engineer. (Those also exist, and you can look up "resistojet" to find alternative approaches to compare against. As best as I can tell, those tend to operate at much lower temperatures; and the high-performing ones use hydrazines as their (mono)propellant, boosting the final temperature by exothermic chemical decomposition. I might have misunderstood the literature).