Variable cycle engine

To reduce aircraft drag at supercruise, SST engines require a high specific thrust (net thrust/airflow) to minimize the powerplant's cross-sectional area.

This implies a high jet velocity supersonic cruise and at take-off, which makes the aircraft noisy.

In the 1970s, Boeing modified a Pratt & Whitney JT8D to use a tandem fan configuration and successfully demonstrated the switch from series to parallel operation (and vice versa) with the engine running, albeit at partial power.

During take-off and approach the engine behaves much like a conventional turbofan, with an acceptable jet noise level (i.e., low specific thrust).

However, for supersonic cruise, the fan variable inlet guide vanes and auxiliary intake close to minimize bypass flow and increase specific thrust.

[3] Examples include the General Electric XA100 and the Pratt & Whitney XA101, as well as the propulsion system for the Next Generation Air Dominance (NGAD) fighter.

The Rolls Royce Ultrafan is the largest and most efficient engine to allow multiple turbine speeds.

"[5] Startup Astro Mechanica is developing what it calls a turboelectric-adaptive jet engine that shifts from turbofan to turbojet to ramjet mode as it accelerates from a standing start to a projected Mach 6.

[6][7][8] Instead of a fixed gearbox, it uses an electric motor to turn the turbine(s) behind the fan at an ideal speed for each phase of flight.

The electric motor adaptively controls the propulsion unit, allowing it to behave like a turbofan, turbojet, or ramjet depending on airspeed.

[9][7][10] It is in some respects similar to turbo-electric marine engines that allow propellers to turn at a different speed than the steam turbines that power them.