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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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.
For a variety of reasons, if you achieve your desired velocity by a desired altitude EARLY, you’re gonna shut down EARLY. Who knows- wind, humidity, design error, this batch of oxidizer is extra oxidizery who knows. But you still shut off when you need to because your payload has no means of “coming back” if you overshoot its target orbit/trajectory. Oh and to that end, what if there’s a problem with one of your engines on that stage. Like what we saw with several of the Raptor engines on the Starship booster (it lost 3? 4? – literally exploded) – so the remaining engines may have to burn longer to achieve the same delta-v, and <something something rocket equation> fewer engines burning for longer may NOT use the same qty fuel as more engines burning for less time. So, when the stage drops, it may have extra fuel. And a bit extra in general for contingencies. But not much.
– not an expert or anything just armchair rocket fan.
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.
The first stage is pushing you up right up until it’s not. Once it’s out it’s slowing you down.
If you wait until you’re sure it’s 100% completely empty to ditch it, you’re liable to lose delta V.
Much of the fuel systems for the early stages also run via pressure, so you have to keep a certain level of positive pressure in the fuel tanks in order to keep the stage going. Once that pressure drops below a certain point it’s time to separate.
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.
Wow guys what amazing answers. This makes sense. Sacrifice some unburned propellant/oxidizer and shut down the engines at a predetermined time vs running them dry has multiple safety, stability and performance reasons.
Why would you wait for one stage to run dry before you jettison it? You’d lose thrust/momentum in between stages.
To add to the great answers that have already been given: you want the rocket engine to shut down in a controlled manner. [SpaceX’s third ever launch is an example](https://en.wikipedia.org/wiki/Falcon_1#Third_flight) of this going wrong: the first stage shut down, and the first and second stages separated normally. But before the second stage’s engine ignited, some remaining fuel in the first stage engine evaporated, this was enough to push the first stage slightly forward into the second stage and damage it.
it’s for safety concerns.
you could let the engines run completely dry, but then you would have ot make sure all engines run dry at the exact same time(as to not impact the rocket’s route itself), hence for better control, the engines are cut off manually as to ensure they all shut off together.
you also gotta consider that early designs for rocket engines rely on very powerful pumps that need ot mantain a certain level of pressure which is unsafe if the engine loop is allowed ot run completely dry.
if this means having a waste a lil fuel…for the safety of the astrounauts so be it.
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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