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In rocket launch videos, we often observe liquid propellant rocket engines running at full thrust for several seconds before actual liftoff. We understand that liquid propellant rockets consume a significant amount of propellant before liftoff, (between 0.5% and 0.75% for the Space Shuttle).

We also observe that rocket tanks remain connected till the last seconds with quick disconnect lines. See for instance this slow motion images of the Space Shuttle liftoff when the quick disconnect quit contact at minute 9 and minute 29.

Hence my question, is/can the tank be filled up through the quick disconnect line until the last seconds before liftoff, in order to maximize the rocket efficiency?

the world is not flat
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    I don't think it totally answers your question but you might be interested in this answer: https://space.stackexchange.com/a/34977/6944 – Organic Marble May 10 '20 at 20:37
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    I always thought that connection were hold so that in case of an abort, they can empty the tanks before sending human investigating. – Manu H May 11 '20 at 12:57

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Neither Ariane nor the Space Shuttle are refueled while the engines are running. I don't know of other launch vehicles that have extended periods where the engines are running before liftoff.

The price for this is relatively low. The main tank has to be a little bit bigger to fit the extra fuel, but on the plus side it eliminates a lot of error sources.

The purpose of the exercise is to check if the engines are ready for flight. Topping off the tank would require some system to regulate the pressure of the incoming fuel precisely, to emulate the behavior of a tank that is pressurized as in flight. This system may obscure a problem with the engine, such as the in-flight pressurization not working or an inducer being defective.

So, having an extra system like that in the loop partly defeats the purpose of starting the engines early.

Rikki-Tikki-Tavi
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  • Is there a source for that? It makes sense, but a link to documentation is preferred. – Anton Hengst May 11 '20 at 20:06
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    For shuttle there is a detailed countdown here: https://science.ksc.nasa.gov/shuttle/countdown/count.html You can see TERMINATE LOX REPLENISH at -4:55 and TERMINATE LH2 REPLENISH at -1:57. Both well before the engine start command. – Organic Marble May 12 '20 at 00:18
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    @AntonHengst I don't have a quotable source, but I have discussed this question with people familiar with the engine in the past. – Rikki-Tikki-Tavi May 12 '20 at 11:50