Variable cycle engine | Wikipedia audio article
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SUMMARY
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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 supercruise 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:
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{\displaystyle F_{n}={\dot {m}}\cdot (V_{jfe}-V_{a})}
where:
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{\displaystyle {\dot {m}}={\frac {d}{dt}}m=\,}
intake mass flow rate
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{\displaystyle V_{jfe}=\,}
fully expanded jet velocity (in the exhaust plume)
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{\displaystyle V_{a}=\,}
aircraft flight velocity
Rearranging the above equation, specific thrust is given by:
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{\displaystyle {\frac {F_{n}}{\dot {m}}}=(V_{jfe}-V_{a})}
So for zero flight velocity, specific thrust is directly proportional to jet velocity.
The Rolls-Royce/Snecma Olympus 593 in Concorde had a high specific thrust in supersonic cruise and at dry take-off power. This alone would have made the engines noisy, but the problem was compounded by the need for a modest amount of afterburning (reheat) at take-off (and Transonic Acceleration). An SST VCE would have to increase the engine airflow substantially at take-off, to reduce the jet velocity at a given thrust (i.e. a lower specific thrust)
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