While some of us have long wondered about sightings of the mystical “Aurora” delta winged super jet since the 1990s which was allegedly powered by some sort of pulse jet (along with a string of donuts vapour trail) one high speed jet concept more likely to be or become real is the Lockheed Martin Skunk Works’ proposal for a hypersonic “Son of Blackbird” reconnaissance/strike aircraft called SR-72. The proposed SR-72 craft reported by Flightglobal’s Steve Trimble apparently uses an combined-cycle engine using turbofan jet for speeds up to Mach 2.5 and a ramjet/scramjet path for speeds Mach 4-6. Skunk Works remains silent about how the middle bit of the speed regime is powered but may include using a pre-cooler technology like Reaction Engines Limited’s rocket based cycle SABRE engine to up the mass flow rate.
Likewise, cruising at Mach 6 will create a lot of heat for the craft to get rid of. Generally, once above Mach 4, it is best to get out of the atmosphere. We shall have to wait and see whether the Skunk Works manages to solve these problems. One note of caution here. Not all of the legendary Skunk Works stuff works out that well.
The Skunk Works’ X-33 prototype for a planned eventual Venturestar single-stage-to-orbit vehicle to replace the Space Shuttle never flew. Officially, its overly ambitious composite tankage let it down and caused its cancellation, but, in truth, it was never likely to solve the problem of achieving single-stage-to-orbit performance using conventional non-air-breathing rocket engines anyway.