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Are any jet engines used in combat aircraft water cooled?

If they are not, and instead are air cooled, how can the engine ensure consistent cooling? Wouldn't the airflow passing through whatever radiator implementation it has be greatly affected by the airspeed? Consider low subsonic numbers with heavy maneuvering in a dogfight, vs. mach 2 sustained flight?

While I can imagine that variable air inlet for the radiator would change the airflow, I really have no idea how this would work.

I'm also aware that certain rocket engines, e.g. the F-1 allow the fuel itself to circle around the thrust chamber to cool it. Perhaps a similar system is used in jet engines.

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Jet engines are cooled and temperature managed by the mass air flow itself which is used to surround and limit the boundaries of the flame, keeping it away from the burner can walls, and dilute the heat of the flame as it passes to the turbine (only a minority of the mass airflow's oxygen is actually burned - which is why afterburners work; there's lots of leftover oxygen to work with).

If you're talking about using a closed liquid cooling system with heat exchangers taking heat from the burner can and turbine somehow, well, I've never heard of a water jacketed burner can, and how you could use liquid in a closed system to cool a turbine, who knows.

Liquid cooling was used in the past as supplemental cooling to increase available power by spraying water directly into the mass flow as mentioned here, but in that case the water is a consumable.

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  • $\begingroup$ Don't forget that the Harrier also uses water injection... $\endgroup$ – UnrecognizedFallingObject Aug 10 at 12:59
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    $\begingroup$ Actually, the most important part of temperature management is fuel supply. $\endgroup$ – Peter Kämpf Aug 22 at 15:10
  • $\begingroup$ From the standpoint of temperature as a limit on power yes, and when your ITT margin is gone, your engine is worn out. Here we're really talking about a "temperature control system" used as coolant as it were, and in that case it's really just clever management of the air flow surplus to oxidation requirements. $\endgroup$ – John K Aug 22 at 19:15
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The thing most in need of cooling inside a jet engine is usually the high pressure turbine blades. The air entering the first stage of the turbine is in the neighborhood of 2500F (https://en.wikipedia.org/wiki/Turbine_blade). This is often higher than the melting temperature of the blades themselves. These blades are cooled using air bled off from the high pressure compressor, which is typically at a "cool" temperature of 1000F or so. Water would not be much help in cooling here, as it would instantly vaporize.

EDIT: @hobbes I was about to say the same thing. Beyond that, where does all of the water come from? You'll have to bring it with you on the plane. Based on data from here (https://leehamnews.com/2017/01/27/bjorns-corner-aircraft-engines-operation-part-2/) a CFM56 core has a mass flow rate at cruise of about 50 pounds/s. 1-3% of that is used for turbine cooling, so let's just call it 1 pounds/s. So during a two hour flight, you are using about 7200 pounds of air for cooling. Let's say that water is 10x efficient at cooling, so if you replace air cooling with water cooling, you need to carry 720 pounds of water along with you for each engine, plus a tank to store it in, a pump, and tubing. Overall at least 1500 pounds increase to overall aircraft weight. That means you can either carry less passengers (less revenue) or have to carry more fuel (more expense). That's a big hit versus air cooling which is basically free in terms of weight.

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    $\begingroup$ On the contrary, water would be (from a thermal perspective) much better at cooling, since it starts out at a much lower temperature, it consumes a metric forktonne of heat in the vapourisation process, and all that comes way before it gets merely as hot as the cooling air usually used starts out as. $\endgroup$ – Sean Aug 10 at 2:27
  • $\begingroup$ Water would need to be at a very high pressure to keep it from boiling inside the turbine blade. That makes the construction (which is rotating at 10,000 rpm, remember) heavier. Pumping water from the stationary part of the engine to the rotating shaft would require a seal that has to withstand hundreds of bar, which is difficult to achieve. Even filling the cooling passages with water instead of air would add considerable weight. $\endgroup$ – Hobbes Aug 10 at 9:54
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The closest I could find for closed-cyle fluid cooling was a 1975 NASA study for supersonic aircraft, but this used no radiators, and wasn't for the engine. Instead, circulating refrigerant moved heat from the wing and fuselage to the fuel tank containing liquid hydrogen, in the hope that titanium skins could be replaced with cheaper aluminum. The conclusion was that this would help only at airspeeds much greater than Mach 3.

Even for piston-powered aircraft (and NASCAR automobiles for that matter), the drag due to airflow through a radiator can be enormous. Some 1930's racing aircraft used radiators inset flush to the wings for this reason.

It may just be that the jet engine's mass airflow suffices, so there's no justification for the extra complexity and weight of a dedicated heat exchanger.

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    $\begingroup$ A radiator doesn't have to produce a large drag penalty, or even any drag penalty at all - in fact, a properly-designed radiator can actually produce net thrust, by using the air it heats as reaction mass. $\endgroup$ – Sean Aug 9 at 20:39
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    $\begingroup$ Neato! ...it looks like the Meredith Effect has been tried in Formula 1 cars, but not in jet aircraft. Answered another way, because a turbine's combustion isn't blanketed in an enclosed cylinder head, maybe the nonmoving metal parts don't heat up enough to warrant liquid cooling - the airflow is enough. (Sending coolant through the turbine blades seems ghastly complicated.) $\endgroup$ – Camille Goudeseune Aug 9 at 20:55
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    $\begingroup$ @CamilleGoudeseune Most jet aircraft do send coolant through the turbine blades, but the coolant is gas not liquid. See aerospaceengineeringblog.com/turbine-cooling for pictures. $\endgroup$ – alephzero Aug 10 at 10:20
  • $\begingroup$ Good grief, I had no idea that modern turbine blades look like ant farms. $\endgroup$ – Camille Goudeseune Aug 10 at 14:10
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The KC-135 used to have water cooling during takeoff to increase thrust during a critical phase of flight (decreasing runway required and increasing max weight). When the KC-135 transitioned to the R-model, the new high bypass engines no longer required water cooling.

The MQ-9 is a combat aircraft, and has a turboprop, and therefor is technically a jet. The extended range variant uses an alcohol-water injection system for takeoffs at high gross weights in certain environmental conditions in order to provide cooling for max thrust at takeoff.

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If you mean liquid cooling like the piston engine in your car then no, as stated above boundary layer air cooling is used to keep vanes cool, shape the flame in the combustion chamber to stop it touching anything. There is a large amount of air available and means to control it. The Pegasus engine in the Harrier uses water methanol as a thrust augmentation system, cooling the compressor air, making it more dense, making more power.

See here :- How does jet engine water injection work?

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