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What is the pressure levels inside rocket liquid oxygen tanks, at launch time?

The question is for the minimum and maximum (known) levels of pressure inside such tanks.

I am asking for any rocket since such data is not really easy to find.

If the pressure inside Atlas V is X, then the pressure for Delta V should be similar, or at least in the same order of magnitude.

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Russell Borogove
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2 Answers2

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According to the Flight Manual, the Saturn V upper stage LOX tanks were maintained at 38-41 psi (2.6-2.8 bar).

The first-stage LOX tanks were kept at lower pressure, 20-24 psi (1.4-1.7 bar).

It appears those values may have varied slightly on different flights.

Russell Borogove
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  • Why the pressure is higher în the second stage? – Joe Jobs Jan 14 '21 at 02:54
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    Different engine designs, J-2 on the upper stages, F-1 on the lower. I don't know why the engines were designed for different inlet pressures. – Russell Borogove Jan 14 '21 at 03:23
  • Interesting. First stage ~ same as shuttle. – Organic Marble Jan 14 '21 at 05:08
  • Why is the pressure not higher in all stages; isn't more propellant better? Or is the pressure largely a function of the volume/geometry of the storage vessel due to liquid oxygen being incompressible? – aroth Jan 14 '21 at 11:45
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    @aroth The pressure is maintained by supplied pressurant in the ullage above the liquid; it has little to do with the amount of propellant. Higher pressure = heavier structure = less payload (although some pressure stablizes the tank); the lowest pressure you can get away with is used. – Organic Marble Jan 14 '21 at 13:21
  • I guess the hydrogen tanks have similar pressure levels. But how about kerosene? I guess it is very close to one bar? – Joe Jobs Jan 14 '21 at 15:00
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    I’ve provided links in my answer which can yield additional information. – Russell Borogove Jan 14 '21 at 16:17
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The ullage in the space shuttle external tank liquid oxygen tank was pressurized by ground-supplied helium to its flight pressure level of 20-22 psig (1.4 - 1.5 bar) at 2 minutes and 55 seconds before liftoff. Before that its vent/relief valve was open and the tank was approximately at ambient pressure. After liftoff ullage pressure was maintained at the same level by tapping gaseous oxygen from the space shuttle main engines.

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Organic Marble
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