Definition
A turbine engine with two independent rotating assemblies (spools), each consisting of a compressor connected by a shaft to its own turbine. The low-pressure spool and high-pressure spool turn at different speeds, with the high-pressure spool's shaft passing through the hollow center of the low-pressure shaft. Each spool is driven only by its matching turbine and is free to find its most efficient rotational speed.
Plain English
A jet engine built with two separate spinning sections instead of one. Each section spins at its own best speed, which makes the engine more efficient and easier to start than an engine with a single spinning assembly.
Context Anchor
Seen in turbine engine descriptions, maintenance training, and cockpit engine indications such as separate low-speed and high-speed rotor readings.
Derivation
Spool' here refers to the rotating compressor-shaft-turbine assembly, named because it resembles a spool (a cylinder with a shaft through it). 'Two-spool' simply means two such assemblies, one nested inside the other.
Why Pilots Care
Provides better efficiency and smoother performance across a wide range of speeds compared with single-spool designs.
Intuition Check
Do not picture two thread reels inside the engine. Here, spool means a rotating shaft group made of compressor and turbine parts that turn together.
Example Sentence 1
Most modern turbofan engines use a two-spool design, with the fan and low-pressure compressor on one spool and the high-pressure compressor on the other.
Example Sentence 2
The two-spool engine maintained steady thrust while the low-pressure spool adjusted to changing flight conditions.