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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Great answers here, but don’t forget as well that these rockets also have to be overfueled because they leak on the launchpad as well.
As the liquid oxygen and liquid hydrogen sit on the pad, they warm up, which causes them to boil, and the overpressure has to be bled off to prevent damage. This is why they seem to steam and smoke on the pad. But that is fuel being released, so they have to overfueled to account for that loss.
Since the temperature may vary from launch to launch, they have to be designed to carry enough fuel for the entire flight no matter how much is bled off before launch.
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