Ben W
Chess Grandmaster (Supervised)
True, supplying non-LEO depots really does call for a Spaceship-only design, perhaps with ion thrusters for propulsion instead of methalox. (But with methalox as the payload.) How long would it take for an ion-propulsion kick stage to boost a full methalox depot from LEO to GEO? Napkin math: experimental ion thrusters can provide 5N thrust with 100kW power. Let's attach 100 of these to a kick stage, with nuclear to supply 10MW. That would give 500N of thrust, or about 1/25,000 the thrust of Starship with six Raptors. (Ion engines are ~70% efficient, so let's imagine we can dissipate 3MW of heat somehow; this may require a truly massive radiator.) If it takes 4 minutes of thrust for an ordinary Starship to reach GEO from LEO, it would take this ion thruster about two months. Once the depot fuel is used by an outbound Starship, the kick stage can boost the empty depot back to LEO for another cycle.Unfortunately, the GEO tank farm would be untenable. You're going to spend propellant to get your tanker there, and then back again for another trip. That means you're using propellant to push the tanker and its return propellant around. The amount of propellant coming from the Earth explodes in mass. You'd be killing yourself just trying to get one load of propellant to the GEO tank farm.
Then again, if such a kick stage is feasible, it would probably end up being used instead of methalox for pretty much all space-to-space missions, with the possible exception of moving crew quickly between LEO and low lunar orbit?
I've been trying to wrap my head around an optimal way of doing round trips, but it's just so ugly without an LMO tank farm supported by a Moon base. A fully-tanked stack requires 4,600 tons of propellants. Optimistically assuming that a Starship tanker could get 200 tons to LEO, that would mean twenty-three launches to get a LEO stack loaded.
LMO = low-medium (eccentric) Earth orbit? What value would the Moon base provide? (I don't think liquid-methane ISPP is viable on the Moon?)
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