A conventional turbofan is divided into high and low pressure spools, each consisting of a compressor and turbine. The low-pressure spool drives the fan and provides most of the propulsive power. The fan works best at slower speeds, while the rest of the spool - the compressors and turbines - are more efficient running at high speeds. Rolls-Royce tackled this by introducing a third, intermediate spool, which allows the fan, compressors and turbines to run at speeds closer to optimum. P&W's option, never attempted before at this high power level, is to introduce a gear between the low spool and the fan. The reduction gear allows the shaft to run three times faster than the fan and reduces significantly the number of stages and parts.
Source: Flight International
