Threespool

Three-spool refers to a multi-spool configuration in which three concentric shafts drive separate compressor and turbine stages within a single engine or system. The term is most commonly applied to turbofan aircraft engines, where low-, intermediate-, and high-pressure spools operate at different speeds to optimize performance across a wide flight envelope.

In aviation, three-spool engines provide independent speed control of the different compressor stages, allowing better matching of the compressor and turbine across varying pressures and temperatures. This can improve fuel efficiency, throttle response, and overall propulsion efficiency, especially at light takeoff power and at cruise. The arrangement also enables higher overall pressure ratios and more flexible optimization of the engine cycle.

Historically, the three-spool concept is closely associated with Rolls-Royce, which developed and popularized three-spool turbofan designs beginning with the RB.211 in the 1970s and continuing with the Trent family. Notable examples include the RB.211 and its derivatives, the Trent 700 (A330), Trent 900 (A380), Trent 1000 (Boeing 787), and the Trent XWB (A350). These engines underpin several wide-body airframes and have contributed to the adoption of three-spool architectures in modern commercial aviation.

Compared with two-spool designs, three-spool engines are more mechanically complex, heavier, and typically more expensive to manufacture and maintain. They also require more sophisticated control systems to coordinate the three independent spools. Despite these drawbacks, the efficiency and performance benefits have sustained their use in certain programs.

See also: turbofan, multi-spool engine, Rolls-Royce Trent family, RB.211.