Organic Marble just answered a question about Apollo 13 in terms of the storage of Oxygen, and posted some fascinating stuff, including the fact that Oxygen was stored as a super-critical fluid.

I was just wondering what the benefits of storing Oxygen as a super-critical fluid were and why this was done for Apollo 13? As a follow-up question I was also wondering if this is still the standard for storage of Oxygen?

Note: I know very little of fluid-dynamics.


3 Answers 3


The same system was used on Shuttle - allow me to discuss that, the design philosophy applies to Apollo as well (Shuttle deleted the fans though, and had a special Avoid-Apollo-13-circuit in the O2 tanks).

A supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist.

(wikipedia link in question)

The lack of distinct phases is important for systems like the Apollo and Shuttle cryo systems. The heat transfer properties of gaseous O2 and liquid O2 are quite different - if the fluid was allowed to have gas bubbles in it, hot spots could occur on the heater surfaces adjacent to bubbles, which could be disastrous in the pure O2 environment.

Keeping the O2 and H2 cryogens for the fuel cells at supercritical conditions is a smart design for several reasons.

  • There is no concern about keeping the fluids at the tank outlet. The supercritical fluids occupy the entire tank volume.
  • It's simple to manage the properties of the fluids - it can be done with a relatively straightforward heater / pressure sensor control system.
  • No pumps or other devices are needed to expel the fluids, the high pressure in the tanks does that for you.
  • No slosh dynamics because no liquid/vapor boundary (h/t to Tristan for the comment, also mentioned in the reference here)

Here are tank quantity / pressure / temperature graphs for the Shuttle tanks.

enter image description here

enter image description here

Downsides include having to use power to run the heaters, relatively heavy and expensive tanks - they have to withstand high pressures, and are vacuum-jacketed, and of course, the danger of running heaters in a pure O2 environment.

Shuttle had a special circuit in its O2 tanks to prevent an Apollo 13 type disaster. Sensors measured the current going into and out of the heater panels. If the in- and out- currents weren't very similar, a short on the heaters was suspected, and the heaters were tripped off.

enter image description here

Source: Orbiter Systems Instructor Console Handbook (not online)

There's a nice description of the Orbiter cryo system in the Press Manual. Here's an O2 tank system schematic from there.

enter image description here

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    $\begingroup$ @MagicOctopusUrn Let me check the rules, they were pretty paranoid about it, may not have used the whole heater system. $\endgroup$ Commented Oct 7, 2019 at 19:57
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    $\begingroup$ Well, you've got a fluid in a tank at 850 psi, if you open a valve connected to that tank, it's gonna come squirting out! Then the pressure starts to drop, and when it drops enough, the heaters come on and bring the pressure back up. I'll add some words about supercritical fluids at the top of the answer. $\endgroup$ Commented Oct 7, 2019 at 20:03
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    $\begingroup$ @MagicOctopusUrn there were redundant A and B heaters in each tank as shown on the drawing. The flight rule A9-255 says if a heater trips off while it's powered (so presumably really a short) the redundant heater will be used only under certain special circumstances too long to explain in a comment - but they really didn't want to use that tank at all if they didn't have to archive.org/details/flight_rules_generic/page/n1485 $\endgroup$ Commented Oct 7, 2019 at 20:10
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    $\begingroup$ There's also the benefit of no slosh dynamics affecting CG or loads. $\endgroup$
    – Tristan
    Commented Oct 8, 2019 at 13:55
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    $\begingroup$ @MagicOctopusUrn: I don't think "not needing any pumps" is directly related to supercriticality in any way — it's just a useful side effect of keeping the tanks at high pressure, which keeping the contents above the critical pressure requires anyway. But supercriticality does mean that the entire tank is filled with a single homogeneous phase, rather than having liquid in some parts of the tank and gas in others, which simplifies some other things (e.g. no sloshing, no need for ullage motors to allow safely starting the engines in microgravity). $\endgroup$ Commented Oct 8, 2019 at 14:00

I'm not a chemist but I'll go out on a limb and suggest a way to resolve some issues in comments.

It looks to me as though as long as you are above both the critical pressure and critical temperature at the same time, it's a supercritical fluid; thus the name.

So as long as the pressure is above 50.4 bar and the temperature is above 154.5 K (-118.6 C) it's supercritical. And in a tank it's going to be either all supercritical or none, unless you have a transient gradient in temperature or pressure.

This excellent answer explains that the supercritical phase of oxygen and many other gases can often behaive similarly to a "normal" ideal gas and not be "wonky with all sorts of amazing, bizarre properties." I strongly recommend giving a read!

enter image description here

above: https://www.engineeringtoolbox.com/oxygen-d_1422.html

below: https://en.wikipedia.org/wiki/File:Phase-diag2.svg

enter image description here

Here's a video of what a liquid + gas phase transitioning to a supercritical state looks like. The line where the surface of the liquid meets the gas just fades away and the color (this happens to be chlorine) becomes half way between the darker liquid and the lighter gas. Pretty cool, especially if you watch how it converts back to liquid + gas at the end!

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    $\begingroup$ That's a neat video. I assume what we're seeing at the end is transient critical opalescence as the gas and liquid phases become distinct and then gradually separate under gravity? I.e. basically the phases start out all mixed up as the fluid hits the critical point, and then the liquid phase gradually rains down while the gas bubbles up? $\endgroup$ Commented Oct 8, 2019 at 16:39
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    $\begingroup$ @uhoh Wow! You have the best videos, that was immensely cool. It helps to actually see what everyone was talking about with having "all parts of the tank in the same phase". It's weird to wrap your mind around. $\endgroup$ Commented Oct 8, 2019 at 18:47
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    $\begingroup$ That was indeed cool, I've never witnessed the transition visually. $\endgroup$ Commented Oct 8, 2019 at 19:32
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    $\begingroup$ @Uwe I've just asked Is oxygen above the critical point always supercritical fluid? Would it still appear to roughly follow the ideal gas law? Can you have a look and make sure you're comfortable with what I've written? $\endgroup$
    – uhoh
    Commented Oct 9, 2019 at 13:10
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    $\begingroup$ @Uwe You're talking about "liquid-like supercritical fluid", in your bottle it's gas-like, which is pretty much just a gas and it's being technically "supercritical" is usually ignored. $\endgroup$
    – Mithoron
    Commented Oct 9, 2019 at 16:48

If you want to store as much oxygen in a given volume as possible, you have to increase the density significantly. There's two ways to accomplish that.

  • Low temperature (cyrogenic liquid)
  • High pressure (supercritical fluid)

If you don't need to store it for very long (say during a launch), then cyrogenic liquid oxygen has lots of benefits. You get maximum density and the tanks don't have to withstand high pressures.

But for an extended mission, cyrogenic storage is problematic. You either need to have capacity to handle significant boiloff, or you need active cooling systems (which require power, mass, and complexity). The alternative is to let it come up to ambient temperature and put up with the high pressures that requires.

So on a medium-to-long duration mission like Apollo, cryogenic storage costs a lot. That makes the lower density of super critical fluids become an acceptable trade off.

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    $\begingroup$ But the supercritical fluid exists at high pressure AND low temperature. Look at the diagrams in the answer by Organic Marble. The critical point of oxygen is at 154.581 K, 5.043 MPa. The boiling point at 90.188 K is somewhat lower. The tanks for the Apollo fuel cells were designed for high pressure and thermal insulation for low temperature. $\endgroup$
    – Uwe
    Commented Oct 8, 2019 at 9:36
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    $\begingroup$ The Shuttle and Apollo tanks were both cryogenic and supercritical. Your answer is incorrect. $\endgroup$ Commented Oct 8, 2019 at 13:55
  • $\begingroup$ The comment is right, but the answer still makes sense because cryogenic fluids require lower temperature and supercritical fluids require high pressure. $\endgroup$
    – Pere
    Commented Oct 9, 2019 at 12:15
  • $\begingroup$ this answer about supercritical oxygen is a good read chemistry.stackexchange.com/a/122246/16035 $\endgroup$
    – uhoh
    Commented Oct 9, 2019 at 16:23

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