Fun idea to think about. FedEx (and many others) refuels at Ted Stevens Anchorage Airport, and it looks like they have 8 refueling pads. Obviously thats a primary function of Ted Stevens and the reason why it has 3 long runways. I dont know how long the refueling stopover is, but the delay cost cant be that bad compared to an efficient, ground-based refueling operation. Then there are other challenges, like a KC-46 (the latest 767 based tanker), can only carry enough fuel to for 3 747-Fs, (or 6 half-fills). Speculating, but these jumbo jets also create so much downdraft causing a trailing plane to fall into an aerodynamic "hole," so that they need a lot of space between planes. The KC-46 costs about $150mm (flyaway cost), plus you have pilot crews, ground crews, etc to maintain them. A robotic system isn't going to look too terribly different, getting a gigantic auxiliary fuel tank up into the air. But hey, solve those problems, you might have something!