Boeing is planning a two-step process beginning with flight tests of an F-16-sized, single-engine proof-of-concept hypersonic precursor vehicle leading to a twin-engine, full-scale operational vehicle with about the same dimensions as the 107-ft.-long SR-71.

Boeing will expand on research from its past X-43 and X-51 Waverider experimental aircraft, which were tests of unmanned hypersonic planes, as the company refines a new aircraft design.

Boeing and Lockheed Martin are studying turbine-based combined cycle (TBCC) engines along with Orbital ATK and Aerojet Rocketdyne.

A TBCC engine would use a conventional turbojet to achieve speeds up to about Mach 3, the limit for a turbojet, and then transition to a dual ramjet/scramjet, which must be traveling at speeds over Mach 3 to work properly, compressing air from the intake to achieve combustion without an axial compressor. The ramjet/scramjet would then carry an aircraft to speeds over Mach 5. The plane would need to transition back to to the turbojet to slow down and land.