The introduction of by-pass engines was made possible by the two-spool compressor.

As demand for higher by-pass ratios grew a large diameter, front stage fan was introduced in the LP spool.

This became necessary to provide the higher air mass flows required to produce the by-pass air, and to satisfy the core engine needs.

The fan passes much more air than is required for combustion.

A fan blade is specifically designed to produce a large mass flow at its outer span whilst providing the correct mass flow to the core engine from its inner span.

This is a tall order when using two-spool compressors, as the fan speed has to be regulated to suit the operating speed of the remaining LP or ‘booster’ stages.

Problems are encountered during deceleration when the LP compressor will deliver too much air to the decelerating HP compressor.

The temptation to produce a three-spool engine has resulted in some interesting concepts.

Using three spools allows the fan to rotate at its optimum speed leaving the remaining intermediate and high-pressure spools to independently adjust to their required operating speeds.

Rolls Royce successfully introduced three spools with their RB211, the smaller RB199, and the Trent engine families.

Other designers resorted to a fan which was to run through a reduction gear from the LP spool – the geared fan.

This still ties the fan to the related speed requirement of the LP booster stages.

Many engines are two-spool and require variable inlet guide vanes (VIGVs) and variable stator vanes (VSVs) to accommodate air velocity changes from the fan into the core engine.