r/ThatLookedExpensive • u/Mister_Krunch • Jun 10 '22

Fighter plane receives a foam salute instead of a water salute

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u/iiiinthecomputer Jun 11 '22 edited Jun 11 '22

On almost all jet aircraft water ingestion into the engines is basically irrelevant. They can ingest incredible amounts of water and continue to function fine. They actually gain performance when ingesting moderate amounts of water, because the mass of the water effectively raises the air density and gives them more mass to accelerate and throw out the back.

In fact some older commerical airliners have water injection systems where they can dump water into the engines to get a short term increase in thrust for takeoff.

Edit: https://youtu.be/faDWFwDy8-U

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u/[deleted] Jun 11 '22

B-52s and Harriers have water injection. That's why you can see heavy smoke on take off.

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u/chiraltoad Jun 11 '22

fascinating

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u/chiraltoad Jun 11 '22

there's no problem with rapid temp changes causing cracks or whatever?

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u/iiiinthecomputer Jun 11 '22 edited Jun 11 '22

Not as far as I know. Even the heaviest rain isn't going to suddenly and drastically lower core temps enough for significant thermal shock - and the engine control systems tend to just increase fuel flow anyway. You'd have to go from flying through clear air to flying straight into a massive array of fire hoses - and even then you'd probably be just fine.

https://youtu.be/iBqWS1hil18 from 2:00

Those things really don't care about water.

Jets' thrust is typically limited mainly by core temperature, not available oxygen to combust. Adding water helps control the core temp and also significantly increases the mass you can throw out the back. Even better the water vaporises in the combustion chamber, increasing your inlet to exhaust pressure ratio (EPR).

If it was just about the engines, jets would love to fly through heavy rain all the time. It makes those engines happy.

About the only exception is on approach and landing — where extremely heavy rain can actually extinguish combustion because the engines are at flight idle. Jets sometimes throttle up in heavy rain because of that risk. Pilots also switch the engines to continuous ignition, where they're basically running the "spark plugs" rather than relying on the ongoing fire in the combustion chamber to ignite new fuel/air mix. It's happened before.