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I understand from many sites that in the first stage of the Saturn V, the RP1 tank was pressurized by high pressure Helium tanks, but the LOX tanks were pressurized using gaseous oxygen. Is this correct? If yes, why so?

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Helium was used to pressurize the RP-1 tank to save weight. Nitrogen is much cheaper but its density is 1.250 kg/m$^3$, helium is 0.1785 kg/m$^3$, so 1.071 kg is saved for every cubic meter of emptied tank volume.

The first stage RP-1 tank volume of the Saturn V was 770 m$^3$, so 824 kg were saved by using helium instead of nitrogen. During the flight the tank pressure was 3 bar instead of 1 bar, so three times the weight, 2,474 kg was saved.

The tanks for compressed helium were mounted inside the LOX tank. The low temperature increased the density of helium thus a smaller and lighter tank could be used to store more helium at the same pressure. To avoid exceeding the rated tank pressure, helium loading could be finished only when tanks were cold.

The propellant tanks included special fill and drain points to handle heavy-duty lines used to fill the big vessels at high rates; up to 7300 liters (2000 gallons) of RP-1 per minute. If left to its own devices inside the tank, the RP-1 would have settled into strata of varying temperatures, a highly undesirable situation, so the S-IC incorporated a fuel conditioning system to "stir" over 730 000 liters (200 000 gallons) of RP-1 gently by continuously bubbling gaseous nitrogen through the feed lines and the fuel tank prior to launch. To ensure proper engine start and operation, a fuel pressurization system contributed to good pressure at the fuel turbopump inlets where 10 fuel lines (two per engine) funneled RP-1 to the engines at 4900 liters (1350 gallons) per second. During the countdown, pressurization was supplied by a ground source, but during flight, a helium pressurant was supplied from elongated bottles stored, not on the fuel tank, but submerged in the liquid oxygen (LOX) tank. In this medium, the liquid helium in the bottles was in a much more compatible environment, because the cold temperature of the liquid helium containers could have frozen the RP-1 fuel. There were additional advantages to their location in the colder LOX tank. Immersed in liquid oxygen, the cryogenic effect on the aluminum bottles allowed them to be charged to higher pressures. They were also lighter, because the cryogenic environment permitted manufacture of the helium bottles with one-half the wall thickness of a noncryogenic bottle. Produced by the Martin Company, the four helium bottles, 6 meters long and 56 centimeters in diameter, were aluminum extensions of unique length. Ducts carried the cooling helium down through heat exchangers on the F-1 engines, then carried heated, expanded gaseous helium back to the top of the fuel tank for ullage pressure.

From this NASA page.

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Because all you have to do to get gaseous oxygen is take liquid oxygen and warm it up a bit. That means just tapping off a little gas from the engine and routing it back to the tank to keep the ullage pressure where it needs to be.

Contrast that with RP-1, which doesn't really vaporize enough to get the pressure where it needs to be. For that, they had to supply ullage gas.

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    $\begingroup$ Good graphics here if you'd like to use them: ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090016301.pdf $\endgroup$ – Organic Marble Apr 24 '20 at 17:08
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    $\begingroup$ I guess technically, you could use the oxygen to pressure the fuel tank as well. Maybe adding a small spark might lead to even more pressure, at least for a while... $\endgroup$ – mlk Apr 25 '20 at 14:07

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