True, but the fuel to get up to that altitude is a significant cost that can't be ignored, and the mass must be carried at takeoff (unless you want to solve the

True, but the fuel to get up to that altitude is a significant cost that can't be ignored, and the mass must be carried at takeoff (unless you want to solve the problem the way the USAF does, but I don't even want to think about passenger-rating in-flight refueling). The design flaw that finally killed the Concorde was precisely related to that --- the need for large fuel tanks and light structures left the vehicle very vulnerable to hull damage. At the other end, you have to make sure you're taking sufficient fuel for diversion to alternate airports (and there might not be a lot of airports willing/capable of taking the vehicle---that also affected the Concorde) as well as time to orbit waiting for a landing slot (if LAX is your destination, that's non-trivial). This already affects standard passenger routes --- short hops are more expensive because the aircraft doesn't spend much (or any) time at cruise altitude, where they're much more fuel-efficient. Yes, obviously these vehicles would be more economical for long over-water flights, but the problem is only partially mitigated by a longer time in an efficient regime. Those takeoff/landing costs don't go away.