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
In: 113
Because it is better to have a fully controlled separation than letting the engines completely run dry.
If you just let the engines run fully until they run out of propellant, well, do you really think all of the engines are going to run out at the exact same instant? No, chances are at least one of the five would have a little extra fuel in the line for a half second longer than the other ones. And even that little difference with unbalanced thrust can start to throw the rocket off balance and maybe make it lose control in the air.
Plus, once those engines turn off. There’s no real way to steer the rocket, all steering was done by gimbaling (turning/tilting) the engines. So if the engines are off, the rocket is just flying with no controlled inputs and could again lose balance and fail.
So, it’s better to just manually cut the engines off at a specific controlled time and start the second stage up at a specific time than to try and use up the last couple pounds of fuel.
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