# What are the limitations of using a single shaft between the compressor and turbine in a jet engine?

Question Background

I'm studying the internal structure and components of a turbojet. I can see there is a shaft between the compressor and the turbine, so that energy is mechanically extracted from the exiting gas and used to help the compressor rotate. You can appreciate it in the next image:

Question

I understand the shaft is there so that the turbine helps the compressor move. But this introduces a serious constraint: both the compressor and the turbine need to rotate at the same speed. Is this true in general? How is this managed when there are several shafts connecting several compressors/turbines? Are reduction boxes needed here or do they, in fact, move at the same speed? Is this a big deal, does it introduce any serious limitation/drawbacks regarding the exiting air power?

Just started studying jets, hope the answers are not very trivial.

• This answer explains multiple shafts. I'm sure there are others lurking around here. Commented Mar 7, 2016 at 19:21
• "used to help the compressor rotate" - No, no, no - not 'help' - the shaft is essential to forcing the compressor to rotate. Commented Dec 15, 2023 at 13:34

How is this managed when there are several shafts connecting several compressors/turbines?

The different shaft will rotate at different speed from each other, but the relative compressor/turbine pairs will rotate at the same speed.

Expanding a little, in the most complex case that I heard of, there are 3 shafts: High Pressure (HP), Intermediate-Pressure (IP) and Low Pressure (LP)

Image source

HP turbine and HP compressor will rotate at the same speed, but this is not necessarily equal (in fact is highly likely that it will be different, otherwise there would not be the need for different shafts) to the speed of the IP compressor/turbine couple. And this will be different also from the speed of the LP turbine/fan couple.

Are reduction boxes needed here or do they, in fact, move at the same speed?

They generally rotate at the same speed. And generally you don't want gear boxes in the central section of your engine. Apart from the maintenance nightmare that they would become, given the rotational speeds and energies involved, they would be extremely heavy, inefficient and would constitute a critical system quite prone to failure.

An exception are the fans, they have enough space and are in front of all the rest of the engine, allowing the housing of a gearbox within their enclosure. You can find a better discussion about this case in this question; keep in mind that here you have 3 compressor stages with only 2 turbine stages, the gearbox was necessary to have a speed differential between fan and IP compressor without requiring 3 turbine stages and an additional shaft. (thanks to @fooot for pointing the PW1000G out)

Is this a big deal, does it introduce any serious limitation/drawbacks regarding the exiting air power?

As everything in engineering, there are trade-offs. The ideal performance would be obtained via an infinite set of infinitesimally thin turbines/compressors, each rotating at a slightly different speed, but this is not technologically feasible. The engine designers will then settle for a slightly sub-optimal performance, but it will be actually achievable with 1 or 2 (3 in extreme cases) discrete shafts.

• I wasn't expecting such a detailed answer. Thanks. Commented Mar 7, 2016 at 19:26
• @fooot I was wrongly focusing on the intermediate section. I will add that to my answer. Commented Mar 7, 2016 at 20:28
• @fooot If you mean PW1000G are only GTF on any airliner so far, that not correct. There are a few examples from past. Most notables are Garrette TFE731 (total >11000 built) and Lycoming ALF 502. But they are smaller engines and PW1000G series is the first in medium thrust class with High BPR. Looks like RR UltraFan will be the first one in the large thrust class.
– jayS
Commented Sep 7, 2017 at 10:00