From what I understand, the fan(s) collect(s) and compress(es) air into the compression chambers, where fuel is added to the air & combusted to create an even bigger pressure/thrust, right before the fuel-air mix hits the final chamber where even more fuel can be added & combusted (if you go +afterburner).

My question is this: If it's very wet out, and the engine is sucking up multiple gallons of water per second, then exactly how is combustion achieved with all that water contaminating the air?

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    $\begingroup$ A turboFAN engine with an afterburner seems like an unusual combination. Do you have an example of an engine that is built that way? $\endgroup$
    – Ralph J
    Jul 10, 2017 at 14:28
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    $\begingroup$ @RalphJ aviation.stackexchange.com/q/36903/4108 $\endgroup$
    – Sanchises
    Jul 10, 2017 at 15:04

1 Answer 1


The air is compressed in the compressor section of the engine and heats up. Hot air is able to hold disproportionally more water in gaseous form than cold air, so all that water will vaporize and enter the combustion process together with the air. Since it is already vaporized, it will not absorb that evaporation enthalpy again that makes water such a good extinguishing agent. On the other hand, since the evaporation enthalpy was sucked from the air in the compression process, the air will be cooler when it enters the combustion chamber, so more fuel can be burnt (up to the stoichiometric ratio of course) and power can be increased.

This trick was used on high performance piston engines and early jets. The Boeing 727 would inject water into the compressor on purpose on hot and high take-offs to increase engine thrust.

If the amount of water entering the engine overwhelms the absorbing capacity of the hot air, the engine runs the risk of flaming out. Regular precipitation does not pose this threat, however.

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    $\begingroup$ "Regular precipitation does not pose this threat, however." Relevant video $\endgroup$ Jul 10, 2017 at 14:01
  • $\begingroup$ How about turbofans with a larger bypass ratio? Do they rely on premixing, or centrifugal water removal, or? $\endgroup$
    – Sanchises
    Jul 10, 2017 at 15:05
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    $\begingroup$ Fun fact, water injection does roughly the same thing as nitrous (NOS) in a high-performance car $\endgroup$
    – gillonba
    Jul 10, 2017 at 16:18
  • $\begingroup$ @Sanchises: The centrifugal water removal comes standard on turbofans. That is one reason why on the 727 the water was injected after the first compressor stages. The other is that freezing of the water inside the intake could be avoided this way. $\endgroup$ Jul 10, 2017 at 16:58
  • $\begingroup$ Is the water vaporization increasing the volume and thus the pressure a factor here? $\endgroup$
    – yo'
    Jul 10, 2017 at 18:15

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