Re: Do the math
What has to be done for faster speeds is that the inflow has to be actually "captured" and slowed down to where you can do something useful with it. One aerospace guy, whose name I forget, pointed out that an airburner moving at these speeds is using a lot of energy "accelerating" the air it uses to its own velocity, which of course any regular rocket has to do with its fuel. It negates even more of the advantages of using ambient air for combustion and reaction mass.