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I was wondering why some turbofan engine have multi-stage fans.

e.g. F-119 engine:

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F100

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Saturn AL-31

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All three engines you show are for fighter jets flying at supersonic speeds. These engines need a low bypass ratio: Why do military turbofan engines use a low bypass ratio?

To achieve more thrust at lower bypass ratios, the bypass air needs to be accelerated more, which requires more than one fan stage because axial compressors can only add a limited pressure difference per stage:

Low bypass engines often have a multistage fan which generates a low volume but relatively high speed air stream whereas high bypass engines usually have a single stage fan which generates a high volume but relatively low speed air stream.

(SKYbrary - Turbofan Engine)

In principle, high bypass ratio engines where more air is accelerated less are more efficient and therefore used on modern airliners, but they are not suited for supersonic flight.

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  • $\begingroup$ Supersonic is not the driver, but the basic idea in the answer is correct. Fighter planes do not fly supersonic most of the times, and for supersonic speeds most planes require the afterburner (excluding supercruising fighters). Note that the engine in afterburner mode is basically an engine operating in max dry power with enabled afterburner and opened exhaust nozzle. Upstream of the afterburner, up to the front of the fan, the engine doesn't even know it's flying supersonic. $\endgroup$
    – 0scar
    Commented Mar 5, 2021 at 23:38

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