Wikipedia
Jet engines and other gas turbine engines are often uprated by adding a zero-stage, sometimes written ' '0' stage', to the front of a compressor. At a given core size, adding a stage to the front of the compressor not only increases the cycle overall pressure ratio, but increases the core mass flow. A further uprating may be done by adding another stage in front of the previously-added zero stage, in which case the new one may be known as a zero-zero stage.
A comparison with other ways of uprating an existing engine without drastically redesigning the engine shows for a particular case, eg the Rolls-Royce/SNECMA M45H, the thrust could have been increased by 25% with a zero-staged l-p compressor or 10% with either an improved HP turbine or with water injection.
Zero-staging is also combined with other modifications to provide increased thrust or lower turbine temperature. It may be required for an existing aircraft weight increase, or for a new application, as shown by the following examples.
A 15-stage Rolls-Royce Avon powered the Lightning F.1. A zero-stage, together with a new turbine, was added (total 16 stages) for the Caravelle III. A zero-zero stage was added (total 17 stages) for the Caravelle VI.
The 7-stage Snecma Atar D was used in the Mystere II. A zero-stage was added (total 8 stages) for the E and G used in the Vautour and Super Mystere B.2. A zero-zero stage (total 9 stages), together with a 2-stage turbine was added for the Atar 8 and 9 used in the Mirage III.
The Rolls-Royce/Snecma Olympus 593 started with a 6-stage LP compressor. As the Concorde increased in weight during the design phase the take-off thrust requirement increased. The engine was given a zero-stage to the compressor, a redesigned turbine and partial reheat.
Examples of zero-staging for land-based gas turbines are the aeroderivative GE LM2500+ and the heavy-duty GE MS5002B. An alternative to zero-staging used by some OEMs is supercharging the compressor with a fan driven by an electric motor.
Zero-staging is demonstrated by the following relationship:
$w_2 = (w_2 \sqrt{T_3}/P_3) * (P_3/P_2) * (\sqrt{T_2/T_3}) * (P_2/\sqrt{T_2}) \,$
where:
core mass flow = w
core size =$(w_2 \sqrt{T_3}/P_3) \,$
core total head pressure ratio = (P/P)
inverse of core total head temperature ratio = T/T i.e. (P/P )
core entry total pressure = P
core entry total temperature = T
So basically, increasing (P/P) increases w .
On the other hand, adding a stage to the rear of the compressor increases overall pressure ratio, decreases core size, but has no effect on core flow. This option also needs a Turbine with a significantly smaller flow capacity to drive the compressor.