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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The engines are not designed to be run out of fuel. If the fuel pumps run out of fuel for its gas generator power source the loss of fuel pressure will cause a backfire into the fuel system and the engine explodes. If you get an air pocket in the fuel into the cooling jacket of the rocket engine you get a hot spot without cooling where the metal melts within a fraction of a second and the engine explodes. If the spark gap igniter runs out of fuel through its separate fuel supply before the main engine cuts out you get a backfire through the igniter causing the engine to explode. If you get gassious oxygen in the oxygen supply lines you get a very rich combustion and when liquid oxygen is reintroduced to the engine it can cause a huge explosion destroying the engine. And since they were running multiple engines if they lost one or two of the side engines you would get asymmetric thrust and the rocket would start spinning uncontrollably.
So basically the rockets are not made to run out of fuel. That means that as soon as the fuel and oxygen levels in the tanks gets dangerously low they have to controllably shut down the engine. But there would still be quite a lot of fuel in the very long and thick lines going around in the engine, including the cooling jacket. And that is what some of the plume is. I know they did have purge systems which used compressed helium to clean out the fuel lines but that was mainly a concern for ground testing and ground aborts so I am not sure if they were activated in flight. But you would still get fuel and oxygen boiling pushing unburned fuel out the lines.
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