Variable cycle engine

A variable cycle engine (VCE) is an aircraft jet engine that is designed to operate efficiently under mixed flight conditions, such as subsonic, transonic and supersonic.

The next generation of Supersonic transport (SST) may require some form of VCE. SST engines require a high specific thrust (net thrust/airflow) at supersonic cruise to keep the cross-sectional area of the powerplant to a minimum, so as to reduce aircraft drag. Unfortunately, this implies a high jet velocity not only at supersonic cruise, but at take-off, which makes the aircraft noisy.

A high specific thrust engine has a high jet velocity by definition, as the following approximate equation for net thrust implies:

${\displaystyle F_{n}={\dot {m}}\cdot (V_{jfe}-V_{a})}$

where:

${\displaystyle {\dot {m}}={\frac {d}{dt}}m=\,}$ intake mass flow rate

...
Wikipedia