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It seems that the norm is to (fast-) fill fuel and oxidiser tanks from the bottom. For example, on this drawing of the Saturn S-IC stage you can see the LOX and RP-1 fill valves at the bottom of the respective tanks (hard to read - click through to see higher-resolution images; the fill valves are near the AB and CD plane markers):

S-IC drawing

(source: Mike Jetzer/heroicrelics.org - higher resolution images available)

The SII stage LOX tank had the fill valve at the bottom, but LH2 fill valve seems to be at the top (see figures 50 and 60 respectively in this document, from here).

This answer related to STS also shows the STS External Tank to be filled from the bottom.

Naively, I would think that filling from the top is much easier, as you don't have to fight the pressure of the propellant already in the tank, either with a pump or by putting the supply tank higher than the target tank (communicating vessels).

What is the motivation to fill the tanks from the bottom?

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  • $\begingroup$ I don't know how to find a source for this, but it's simply because that's where the tank outlet is, and to add a large, duplicate inlet valve/pipe at the top would add weight. $\endgroup$
    – Organic Marble
    Commented Apr 19, 2021 at 15:13
  • $\begingroup$ @OrganicMarble "outlet" as in "goes to the engines" or "outlet" as in "drain"? I guess it makes sense to combine drain and fill if those flows are comparable. $\endgroup$
    – Ludo
    Commented Apr 19, 2021 at 15:17
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    $\begingroup$ Goes to the engines. See also space.stackexchange.com/q/39342/6944 $\endgroup$
    – Organic Marble
    Commented Apr 19, 2021 at 15:17
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    $\begingroup$ "I would think that filling from the top is much easier, as you don't have to fight the pressure of the propellant already in the tank": the opposite is true. Until the tank is completely full, the pressure of the propellant in a line going to the top of the tank will be greater than the pressure of the propellant at the bottom of the tank. $\endgroup$
    – Christopher James Huff
    Commented Apr 19, 2021 at 15:19
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    $\begingroup$ Dynamically you also have the fact that you're lifting propellant to the top of the tank and dropping it in. Depending on the propellant, this could damage internal structures, cause excessive vibration, form froth, heat the propellant, etc. And filling from the bottom lets you use the same plumbing for draining. $\endgroup$
    – Christopher James Huff
    Commented Apr 19, 2021 at 17:10

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It actually takes less pumping power to fill a tank from the bottom. Just like the pressure experienced by a diver depends on their depth, the pressure at the bottom of a vessel (whether a tank or a vertical pipeline) depends only on the column depth of the liquid within that vessel. The pressure in a line going to the top of the tank will be equal to the pressure at the bottom of a completely full tank, the bottom of the tank will have lower pressure up until it is completely filled.

Also, if you fill from the top, you're lifting propellant to the top of the tank and dropping it in. That extra energy has to go somewhere, and could potentially damage internal structures, cause excessive vibration, form froth, excessively heat the propellant, etc.

Finally, you not only need to fill the tanks, you need to be able to empty them as well. Filling from the bottom lets you use the same plumbing for draining, and possibly share some of it with the plumbing to the engines.

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