Zero-stage

Jet engines and other gas turbine engines are often uprated by adding a zero-stage, sometimes written '0' stage,^[1] to the front of a compressor.^[2] 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.^[3]

Zero-staging is also combined with other modifications to provide increased thrust or lower turbine temperature.^[4] 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.^[4]

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.^[5]

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.^[2]

Examples of zero-staging for land-based gas turbines are the aeroderivative GE LM2500+^[6] and the heavy-duty GE MS5002B.^[7] An alternative to zero-staging used by some OEMs is supercharging the compressor with a fan driven by an electric motor.^[8]

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 = $w_2 \,$

core size =$(w_2 \sqrt{T_3}/P_3) \,$

core total head pressure ratio = $(P_3/P_2) \,$

inverse of core total head temperature ratio = $T_2/T_3 \,$ i.e. ($P_3/P_2 \,$)

core entry total pressure = $P_2 \,$

core entry total temperature = $T_2 \,$

So basically, increasing $(P_3/P_2) \,$ increases $w_2 \,$.

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.

References

1. "Optimization Of Military Compressors For Weight and Volume" Keith Garwood, AGARD-CP-421, Propulsion and Energetics Panel 69th Symposium, Paris, 4-8 May 1987
2. Hooker, Sir Stanley (1984). Not much of an Engineer, P. 153. Airlife Publishing Ltd, Shewsbury, England ISBN 0906393353.
3. https://www.flightglobal.com/pdfarchive/view/1974/1974%20-%200622.html
4. ""Rolls-Royce Aero Engines" Bill Gunston, Patrick Stephens Limited 1989, ISBN 1-85260-037-3, p.141-142
5. https://www.flightglobal.com/pdfarchive/view/1959/1959%20-%200803.html
6. http://www.ewp.rpi.edu/hartford/~roberk/IS_Climate/Papers/T36-TUT02.pdf
7. https://powergen.gepower.com/content/dam/gepower-pgdp/global/en_US/documents/technical/ger/ger-4171-perf-reliability-improvements-ms5002-gas-turbines.pdf
8. http://www.fernengineering.com/pdf/gt_upgrade_options.pdf