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Is it possibly intentional? Eg maybe they started burning at idle thrust after landing, to empty out the tanks instead of just venting them?

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There's no "idle thrust" on Falcon 9 since even the lowest thrust is enough to lift the almost empty booster. You also don't want to completely run out of fuel because your engine will probably explode. This was just a residual fire.

> You also don't want to completely run out of fuel because your engine will probably explode.

Can anyone suggest keywords/links about this? Sounds interesting.

One problem here would probably be turbopumps [1] running dry. These things run very close to their limits when fully stressed and would presumably run higher still if they no longer needed to pressurize anything.

The fuel is also used as a coolant [2] so you would not want to run out of fuel in the plumbing before you run out of propellants in the combustion chamber.

[1] https://en.wikipedia.org/wiki/Turbopump

[2] https://en.wikipedia.org/wiki/Merlin_(rocket_engine_family) (search "regeneratively cooled")

I am not a rocket scientist ( IANARS ? )

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