Lewy's example

T-stages, sometimes called boost 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:

$w_2=(w_2\sqrt{T_3}/P_3)*(P_2/\sqrt{T_2})*(P_3/P_2)/\sqrt{T_3/T_2}$

where:

core mass flow = $w_2$

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

core entry total pressure = $P_2$

core entry total temperature = $T_2$

core total head pressure ratio = $P_3/P_2$

core total head temperature ratio = $T_3/T_2$ which varies more slowly than $P_3/P_2$

Note:

in this case, the core refers to the whole of the gas generator compression, not just that of the high pressure (HP) compressor.

The first two terms - before the second multiply - are constant. The value of the remainder of the expression increases as the core pressure ratio 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 an additional 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.