Lockheed Martin and P&W’s Rocketdyne division are working on a hybrid propulsion system for hypersonic flight, (paid subscription required) which uses a turbojet to take off and accelerate to Mach 2+, with a dual mode ramjet (DMRJ), it does supersonic combustion at higher speeds, taking over beyond that, with the turbojet being cocooned.

It’s kind of a “Son of Blackswift.”

As it currently stands, wind tunnel tests are proceeding on the DMRJ (phase 1), with a system integrating both the turbojet and ramjet being testing together in phase 2, and eventually testing of the system in flight.