What is "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:

$w_2 = (w_2 \sqrt{T_3}/P_3) )* (P_3/P_2)* (P_2/P_1)*( P_1/ \sqrt{T_1})/ (\sqrt{T_3/T_2} * \sqrt{T_2/T_1} ),$ where: hp compressor entry mass flow = w  core size = $(w_2 \sqrt{T_3}/P_3) \,$ hp compressor total head pressure ratio = P/P  lp compressor total head pressure ratio = P/P  lp compressor entry total pressure = P  lp compressor entry total temperature = T  hp compressor total head temperature ratio = T/T  lp compressor total head temperature ratio = T/T  which varies more slowly than P/P 

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