I am interested in orbital refueling, not for satellites (though that’s a good idea) but for manned exploration and infrastructure. For instance, to support a lunar base we would want regular flights with reusable craft. The best mission profile would depend on an inter-orbital transfer vehicle that never lands. Instead it shuttles between LEO and lunar rendezvous with landers that stay at the moon and never return to earth. There is a lot of refueling in these schemes. Space-X’s Mars plan for instance, depends heavily on refueling in orbit.
A quick search shows there have been some refueling studies as well as tests and at least some space station refueling. They discuss the problem of boil-off of cryogenic fuels, and list various solutions.
But one thing I have not seen much about is liquid surging and settling. When under thrust it’s not a problem, but in micro gravity, how can we keep the liquid at the drain? Bladders wouldn’t work with most fuels. Centrifugal propellant settling would certainly work, but how? A rotating baffle could spin the liquid in the tank, but it would create turbulence against the walls. Also the way oxygen tanks like to explode, it would be nice to keep machinery out of the tanks. That’s what happened to Apollo 13. One could rotate the whole depot and either have a counter-rotating connection, or spin the ship when it docks, a la 2001. But that’s an awful lot of work just to feed a drain.
Does anyone know how it is envisioned to handle weightless liquids in tanks?