Variable cycle engine

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.

An advanced technology engine is a turbine engine that allows different turbines to spin at different, individually optimum speeds, instead of at one speed for all. It emerged on larger airplanes, before finding other applications.

The next generation of supersonic transport (SST) may require some form of VCE. 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 during supersonic cruise and at take-off, which makes the aircraft noisy.

Specific thrust

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

F

n

=

m

˙

(

V

jfe

−

V

a

)

{\displaystyle F_{\text{n}}={\dot {m}}\left(V_{\text{jfe}}-V_{\text{a}}\right)}

where:

m

˙

=

d

d

t

m

,

{\textstyle {\dot {m}}={\frac {d}{dt}}m,\,}

intake mass flow rate

V

jfe

,

{\displaystyle V_{\text{jfe}},\,}

fully expanded jet velocity (in the exhaust plume)

V

a

,

{\displaystyle V_{\text{a}},\,}

aircraft flight velocity

Rearranging the equation, specific thrust is given by:

F

n

m

˙

=

V

jfe

−

V

a

{\displaystyle {\frac {F_{\text{n}}}{\dot {m}}}=V_{\text{jfe}}-V_{\text{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. This made the engines noisy. The problem was compounded by the need for a modest amount of afterburning (reheat) at take-off (and transonic acceleration).

Concepts

Tandem fan

One SST VCE concept is the tandem fan engine.