Henry Spencer (who I expect will be at Space Access this year, after missing last year for the first time ever) explains (once again) why the future of space launch continues to lie with rockets, despite the superficial appeal of not having to carry oxidizer.
This is an important point that I’d never thought about explicitly:
The pure-rocket design was more than twice as heavy as X-30 at takeoff, because of all that LOX. On the other hand, its empty weight – the part you have to build and maintain – was 40% less than X-30’s. It was about half the size. Its fuel and oxidiser together cost less than half as much per flight as X-30’s fuel. And finally, because it quickly climbed out of the atmosphere and did its accelerating in vacuum, it had to endure rather lower stresses and less than 1% of X-30’s friction heating. Which approach would be easier and cheaper to operate was pretty obvious.
This implies that a rocket powered vehicle will have much better off-design (higher delta V, such as more altitude or higher inclination) performance than the air breather, because its dry mass that has to be given the additional velocity is much less. It also means that it will be cheaper to deorbit, and the thermal load will be less, for a given wing area (assuming that it has wings, which an air breather certainly would). I suspect that no matter what the technology level, air-breathing launchers are doomed to remain the equivalent of flying cars — interesting in theory, but never achieved in practice.