What does the term "spool speed" refer to in gas turbine engines?

Spool speed in gas turbine engines specifically refers to the rotational speed of the engine's rotating components. This term encompasses the speeds of various rotating parts such as the turbine, compressor, and other associated components within the engine.

Understanding spool speed is crucial as it directly affects the engine's performance, efficiency, and operational characteristics. Each component must operate within a specific speed range to optimize airflow, combustion, and thrust output while preventing mechanical failure due to excessive speeds. Therefore, monitoring and controlling spool speed is essential for ensuring safe and efficient engine operation.

In this context, the other options do not accurately capture the full definition of spool speed; they refer to specific aspects of the engine rather than the overall concept of the rotational speed of all rotating components.