In article ,
says...

Hmm, presumably though, they need to use power fromsomewhere if going out
past Mars. The supply of spares will obviously need sorting out as well.
somehow one never sees this sort of problem depicted.

True, but it does happen. Mir had spares and ISS has spares for just
such an occasion. One of the big reasons the Russians were so happy for
Shuttle-Mir was that the shuttle could carry far more to Mir than a tiny
Progress. It was desperately in need of spares when the shuttle started
visiting.

Jeff
--
"the perennial claim that hypersonic airbreathing propulsion would
magically make space launch cheaper is nonsense -- LOX is much cheaper
than advanced airbreathing engines, and so are the tanks to put it in
and the extra thrust to carry it." - Henry Spencer