Watched the Apollo 11 famous footage of the launch and the camera perspective was from the ground focused on the ascending rocket. When it came time for staging and dropping the first stage, we can clearly see cutoff of rocket motors, separation and ignition of the second stage.
One thing that puzzled me always was the first stage as it falls away is seen trailing vapor, residue from its shut off engines. This reasonably has to be residual unburned oxidizer/propellant. its enough of a plume to be visible even 30+ seconds later it is still leaking away as the second stage carries the rocket further away and away.
Im asking bc every ounce, every bit of weight is calculated for and certainly fuel is no exception. Why lug the fuel up there just to shut the engines off presumably early and not burn it. Any reason for this inefficiency?
EDIT: Including link to video. [https://www.youtube.com/watch?v=dhTvadtW2dc](https://www.youtube.com/watch?v=dhTvadtW2dc) Begin watching at time 36:38
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It’s dangerous to run liquid-fueled rocket engines dry. That’s because of the pumps that need to run at insane speeds to keep the fuel flowing to the engine at a specific pressure. If those pumps are interrupted by a lack of fuel, you get cavitation and/or pumps moving at even faster velocities, which causes them to sieze up, break apart, or even explode. None of those three options are something you want in a giant drum full of rocket fuel vapors.
So in order to keep that from happening, the engines (and therefore the pumps) are set to cutoff when the fuel tank gets *really* low, but not empty.
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