Speaking of DARPA, the agency now wants to get serious about a two-stage (turbojet+high speed system) propulsion option, dubbed Vulcan, which would allow a plan to leave a runways and accelerate to Mach 4, approximately one Mach number above what the storied SR-71 could do.
COMMENT: The USAF has wanted this kind of ability for a long time, and has seriously pursued it off and on (as budgets and changing priorities dictated). It's not as easy as it sounds to jump from a Mach 3 to a Mach 4 engine: every gain in those speed regimes takes hard work. And a propulsion system which can go from runway to Mach 4 is more of challenge, since, as DARPA points out, it's not going to be a single engine - it has to be some sort of two-stage device like the turbo/PDE option mentioned in the announcement. For a single engine, the high-speed end is outside what turbojets and their derivatives can do and the low-speed requirement means it can't be a relatively simple ramjet or a cutting-edge scramjet (which has a lot more development ahead of it and really only makes since for a much higher speed regime). I've known quite experienced USAF people to express the opinion that transatmospheric (long-range, zero to Mach 12 - 18) aircraft should not be too hard to develop, which made me want to quote Engineer Scott: "I can't change the laws of physics!"
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