I am a student and I'm trying to determine how a spool works in an engine. What effect does it have depending on the number of spools on the performance of the engine?

I tried to find it on Google but did not got any solid answers.


3 Answers 3


A spool in a gas turbine consists of a compressor (or fan), a connecting shaft and a turbine, see this answer. The purpose of the spool is to compress air driven by combustion gasses through the turbine. If not all energy is used in the turbine, it is either used in the next turbine or expanded in an exhaust nozzle.

The image below depicts in dark/light blue the high pressure spool (typically called the N2 spool), on the left the individual compressor stages that form the High Pressure Compressor, HPC, in the middle a connecting shaft piece (or drum) which connects to the turbine. The light blue shaft parts are also known as hubs and form the start and the end of the spool: enter image description here

HP rotors of CFM56-7B. Adapted from CFM56-7B Familiarization Manual

The rationale behind using multiple spools is given by the type of gas turbine and the maximum compression ratio. To increase the propulsive efficiency, large amounts of flow need to be accelerated a little, this leads to large blades (typically called the fan) which may not be rotated too fast to prevent the tip speed to reach the speed of sound (for the sake of losses and structural integrity). Furthermore, given the fact that gas turbines operate at different power settings than the design value, where the axial flow through the machine is constant, a lower power setting will cause less fuel to be added to the cycle and as such less power generated in the turbine to drive the compressor resulting in a lower spool speed and thus lower compression ratio. This (less) compressed air is in volume larger than the design (high power) situation and will therefore increase the axial speed. This increase of axial speed causes blades to stall unless measures are taken (variable stator vanes, or air bleed). A compression ratio of 8:1 is typically the maximum for a single spool. To obtain higher compression ratios, to increase the cycle efficiency, multiple spools can be added to mitigate the problems at part power.


I like Oscar's answer about what is a spool. I'd like to add more information on the number of spools

Going from 1 spool to 2 spools takes you from a turbojet (1 spool) to a turbofan (2 spools). There are many good existing questions on this site regarding turbojet vs turbofan so I won't go into that.

2 spools vs 3 spools is different. Both configurations would be turbofans, with one spool being the fan, aka a low pressure compressor, driven by a low pressure turbine. The fan generates the majority of the thrust of the engine. The remaining 1 or 2 spools form the core of the engine, which generates the hot gas flow necessary to power the low pressure turbine. GE generally chooses 2 spool designs, whereas Rolls Royce generally chooses 3. The difference is a tradeoff between mechanical complexity and aerodynamic complexity.

With a two spool design, there is only one spool for the high pressure compressor. Each individual compressor stage only gets a small compression ratio, so to get the overall compression ratio that you want, you need to stack multiple compressor stages together, say 9 - 11 of them. Now, at one particular design condition, say cruise, every single stage can be completely matched to work together perfectly. But at any other design condition, say takeoff, the stages aren't perfectly matched. The front stages might be pumping more air than the back stages really want to handle. You'd really like it if you could slow down the front stages a little bit and speed up the back stages. But you can't, they are all one one shaft. So make it work (with good efficiency, good stall margin, etc) you have to play all kinds of games with the aerodynamics: variable geometry like VSVs, bleed valves, very detailed CFD models, etc.

Now with the three spool design, you have two spools for the core. So instead of single high pressure compressor, you can split that into an intermediate pressure compressor and a high pressure compressor. So now you have more flexibility. On those off design conditions, you can slow down the front stages a little bit and speed up the back stages, because they are on different spools, so you don't need fancy aerodynamic design. But the price you pay for that is mechanical complexity. Every spool is a rotating shaft that has to be supported by bearings. So now you need more bearings. Bearings have to be supported by something, so now you need frames and internal supports to hold them, and they need lubrication, etc. All these parts need to be bolted together somehow. Overall you have a lot more parts to the structure.

  • $\begingroup$ Are you sure turbojets are always 1 spool? AFAIK, the Olympus 593 that powered Concorde was a 2 spool design. $\endgroup$
    – Bianfable
    Feb 18 at 10:59
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    $\begingroup$ @Bianfable Also the P&W J57, a 2 spool turbojet. But, I don't think there are many. $\endgroup$
    – 0scar
    Feb 18 at 11:06

Start with the background: in a jet engine the air flows through the inlet into the compressor (to increase pressure), then through the burner (combustion chamber) to increase energy, then through the turbine (to extract power to turn the compressor) and finally out through the nozzle to provide thrust. In a single spool engine, the compressor and turbine stages are connected by a single shaft - so that is a one spool engine.

High compression ratio is needed for efficiency and it is difficult to increase on a single shaft so the dual spool engine was created. The air goes from inlet to the "low pressure compressor" then the "high pressure compressor" (LPC and HPC). The turbine has stages attached to each compressor (the HPT and LPT).

  • $\begingroup$ Does the lpc and hpc rotates in different directions to each in 2 and 3 spool engines, and Does the shaft drives spool ? $\endgroup$ Feb 17 at 17:21
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    $\begingroup$ Spool is a term to link the compressor, turbine and the shaft that connects them. Generally, the rotation is in the same direction. For example, at full thrust the F100 in the F-15 or F-16 has the fan at N1= 10 000 rpm and the HPC at about 12500 rpm so the difference is only about 2500 rpm. If they were counterrotating, some one would have to design and build real high speed bearings. They have, the F119 in the F-20 is similar in size but is counterrotating. The gyroscopic effect should be better in the latter. $\endgroup$
    – W H G
    Feb 17 at 20:51

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