Before Lunar Orbit rendezvous made it possible for a single Saturn V to launch a complete lunar mission, the expected mission profile included launching a rather large lunar lander to Earth orbit, mating it with n upper stage, and then fueling the whole thing using specialized tanker spacecraft. NASA lucked out with the LOR concept; while some considerable work had been done on the tankers, the fact is that NASA really didn’t know *how* to do zero-gravity propellant transfer. When the word came down to stop working on the tankers, there were undoubtedly quite a number of quiet expressions of relief.

Still: the ability to do major propellant tanking in space will be vital for a real interplanetary economy. SpaceX will need to be able to do that for many of the lunar and Mars missions planned for Starship. So, it seems that on-orbit tanking is back on the menu.

NASA agrees to work with SpaceX on orbital refueling technology