A variable cycle engine (VCE), also referred to as adaptive cycle engine (ACE), 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. To keep aircraft drag at supercruise down, SST engines require a high specific thrust (net thrust/airflow) to keep the cross-sectional area of the powerplant to a minimum. 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: where: intake mass flow rate fully expanded jet velocity (in the exhaust plume) aircraft flight velocity Rearranging the above equation, specific thrust is given by: 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) Examples One SST VCE concept is the tandem fan engine. The engine has two fans, both mounted on the low-pressure shaft, with a significant axial gap between the units. In normal flight, the engine is in the series mode, with the flow leaving the front fan passing directly into the second fan, the engine behaving much like a normal turbofan. However, for take-off, climb-out, final-descent and approach, the front fan is allowed to discharge directly through an auxiliary nozzle on the underside of the powerplant nacelle. Auxiliary intakes are opened on each side of the powerplant, allowing air to enter the rear fan and progress through the rest of the engine. Operating the fans in this parallel mode substantially increases the total airflow of the engine at a thrust, resulting in a lower jet velocity and a quieter engine. Back in the 1970s, Boeing modified a Pratt & Whitney JT8D to a tandem fan configuration and successfully demonstrated the switch from series to parallel operation (and vice versa) with the engine running, albeit at