T stage

T-stages, sometimes called booster stages, are mounted on the low pressure (LP) shaft of some turbofan engines directly behind the fan.

T-stages are used to increase overall pressure ratio and, for a given core size, the core mass flow. This is demonstrated by the following relationship:

w 2 = ( w 2 T 3 / P 3 ) ) ∗ ( P 3 / P 2 ) ∗ ( P 2 / P 1 ) ∗ ( P 1 / T 1 ) / ( T 3 / T 2 ∗ T 2 / T 1 ) , where: hp compressor entry mass flow = w 2 core size = ( w 2 T 3 / P 3 ) hp compressor total head pressure ratio = P 3 / P 2 lp compressor total head pressure ratio = P 2 / P 1 lp compressor entry total pressure = P 1 lp compressor entry total temperature = T 1 hp compressor total head temperature ratio = T 3 / T 2 lp compressor total head temperature ratio = T 2 / T 1 which varies more slowly than P 2 / P 1

So as P 2 / P 1 increases with the addition of T-stages, w 2 also increases.

T-stages are a popular method for uprating the thrust of an engine (see, for example the Pratt & Whitney Canada PW500).

The alternative is to place a zero-stage, mounted on the HP shaft, at the front of the HP compressor. This approach requires a significant change in the HP turbine, whereas a T-stage can, if necessary, be accommodated by simply adding another stage to the rear of the LP turbine.

Although T-stages usually only supercharge the core stream, some engines do feature a deliberately oversized intermediate pressure (IP) compressor, which compresses both the core flow and a proportion of the bypass flow. This enhances the stability of the T-stages during throttling. Where necessary, the alternative is to employ blow-off valves.