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Each F-1 engine had two fuel feedlines connecting it to the fuel tank as shown in these contemporary photographs and drawings.

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The corresponding LOX system only had one feedline.

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The engine had a single turbopump which pumped both propellants.

Why was the engine and stage designed to utilize dual feedlines for the fuel?

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This was done for load balancing purposes.

The turbopump arrangement was as follows:

  • the LOX pump at the top
  • the fuel pump in the middle
  • the turbine at the bottom

The fuel was fed from two sides of the pump, and there were also two outlets. In this way, any fluctuation in input or output pressure was balanced, avoiding sideways forces acting on the pump shaft.

LOX was fed from the top directly above the shaft, so input pressure fluctuations did not induce sideways forces. The LOX pump had two outlets for the same load-balancing reason.

As an aside: the LOX inlet flow was pushing down on the pump shaft. This force was balanced by the fuel flow by having the fuel inlet below the outlet, thus generating a force in upward direction cancelling the downward force from the LOX.

This is explained nicely in the "NASA Saturn V Owners' Workshop Manual" by David Woods (ISBN-13: 9780857338280; ISBN-10: 0857338285). In the edition I have the relevant section starts on p. 36.

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    $\begingroup$ This is a good start - I saw that the fuel turbopump had two inlets - but why not split the feedline right before the engine instead of running dual ones all the way from the tank with the associated weight cost? Double the pre-valves, double the gimbal sections, etc. Looking for the rationale for a complete dual feedline. $\endgroup$
    – Organic Marble
    May 31, 2021 at 15:27
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    $\begingroup$ @OrganicMarble fair enough, I was too quick to conflate "inlet" and "feed line"... $\endgroup$
    – Ludo
    May 31, 2021 at 15:45
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    $\begingroup$ @OrganicMarble given the two inlets, this diagram suggests that having two feed lines was just the least-complex way to get fuel from the tank to the pump inlets. Not a lot of space there, and putting T- or Y-splits in those lines may not have been feasible with such high flow rates, since splits would introduce rather sharp corners... Still looking for something better... $\endgroup$
    – Ludo
    May 31, 2021 at 19:11
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    $\begingroup$ I looked in the obvious places like Stages to Saturn but didn't see anything. I feel like there must have been some compelling reason to duplicate all the valves and gimbal lines instead of running just one and splitting it at the engine, but I could not find it. $\endgroup$
    – Organic Marble
    May 31, 2021 at 19:15
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    $\begingroup$ I agree with Ludo here. As the fuel tank was directly adjacent to the engines, two separate fuel lines are the obvious solution and a Y-junction requires justification. The oxygen tank was on top of the fuel tank and had much longer feed lines, but only one per engine. (@OrganicMarble) $\endgroup$
    – Rainer P.
    Jun 1, 2021 at 9:25

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