In your hypothetical, the initial flight is doing almost none of the real work and you might as well just launch that booster from the ground. Look at the rocket equation. You have to achieve a given delta-v to get into orbit. That drives the required mass ratio of the rocket. Getting up to 500 mph or whatever a B-52 can do basically has negligible impact on the required delta-v, so the require rocket size is no smaller.<p>SCRAMJETS are interesting because they can get up to a significant fraction of the required delta-v while in air-breathing mode. They can do that because unlike a traditional jet engine, they don&#x27;t have to slow air down to subsonic speeds for combustion. That&#x27;s what drives the maximum speed of traditional jet engines.<p>Edit: saw your other response. Delta-v is still an issue for suborbital trajectories. Hitting 100 km altitude requires a speed of about 3,200 mph. If you&#x27;ve got a subsonic air breathing stage, you&#x27;re still not getting much benefit. The big benefit of SCRAMJETS for suborbital flight is that they can hit the required speeds in air-breathing mode the whole way. No need to carry around heavy oxidizer.