[Blood of Tyrants]

Since the tanker would necessarily fly at a much lower speed and lower altitude, I can’t see the advantage of developing the complex technology involved in mid air LOX and liquid hydrogen transfer.

[Jeff F]

A fair amount of fuel would need to be burned in a fully loaded craft just to reach 35,000 feet. Even worse, the stubby little wings would not be able to lift a lot of weight at the much lower takeoff speeds. If you could defer adding 100,000 pounds or so of LOX until you are at cruising speed, then you gain a proportional amount of payload capacity. Plus you are already traveling near mach 1, seven miles up, and in much thinner air.

As a comparison, the Space Shuttle burned nearly a fifth of its LH2 and LOX in order to reach 35,000 feet, and that is with the assist of two solid rock boosters.