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: where: hp compressor entry mass flow = core size = hp compressor total head pressure ratio = lp compressor total head pressure ratio = lp compressor entry total pressure = lp compressor entry total temperature = hp compressor total head temperature ratio = lp compressor total head temperature ratio = which varies more slowly than So as increases with the addition of T-stages, 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. References Jet engines