# T-stage

T-stages, sometimes called booster stages, are mounted on the low pressure (LP) shaft of some turbofan engines directly behind the fan.[1]

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:

${\displaystyle w_{2}=(w_{2}{\sqrt {T_{3}}}/P_{3}))*(P_{3}/P_{2})*(P_{2}/P_{1})*(P_{1}/{\sqrt {T_{1}}})/({\sqrt {T_{3}/T_{2}}}*{\sqrt {T_{2}/T_{1}}}),}$
where:
hp compressor entry mass flow = ${\displaystyle w_{2}\,}$
core size = ${\displaystyle (w_{2}{\sqrt {T_{3}}}/P_{3})\,}$
hp compressor total head pressure ratio = ${\displaystyle P_{3}/P_{2}\,}$
lp compressor total head pressure ratio = ${\displaystyle P_{2}/P_{1}\,}$
lp compressor entry total pressure = ${\displaystyle P_{1}\,}$
lp compressor entry total temperature = ${\displaystyle T_{1}\,}$
hp compressor total head temperature ratio = ${\displaystyle T_{3}/T_{2}\,}$
lp compressor total head temperature ratio = ${\displaystyle T_{2}/T_{1}\,}$ which varies more slowly than ${\displaystyle P_{2}/P_{1}\,}$

So as ${\displaystyle P_{2}/P_{1}\,}$ increases with the addition of T-stages, ${\displaystyle w_{2}\,}$ 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.