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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2 Answers 2


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.

  • $\begingroup$ Why the pressure is higher în the second stage? $\endgroup$
    – Joe Jobs
    Jan 14, 2021 at 2:54
  • 1
    $\begingroup$ 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. $\endgroup$ Jan 14, 2021 at 3:23
  • $\begingroup$ Interesting. First stage ~ same as shuttle. $\endgroup$ Jan 14, 2021 at 5:08
  • 2
    $\begingroup$ @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. $\endgroup$ Jan 14, 2021 at 13:21
  • 1
    $\begingroup$ I’ve provided links in my answer which can yield additional information. $\endgroup$ Jan 14, 2021 at 16:17

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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