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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.

Organic Marble
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Magic Octopus Urn
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3 Answers3

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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

Organic Marble
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  • Wow the tid-bit about the failsafe put into place after Apollo 13 was interesting, there's two because they're redundant right? If one heater fails or is shut off due to a malfunction, the other can continue to run? – Magic Octopus Urn Oct 07 '19 at 19:56
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    @MagicOctopusUrn Let me check the rules, they were pretty paranoid about it, may not have used the whole heater system. – Organic Marble Oct 07 '19 at 19:57
  • If I want to know a bit more about how super-critical fluids remove the need for pumps should I ask that in a separate question? Is it simply because the turbidity of swapping states automatically distributes the fluid homogeneously? – Magic Octopus Urn Oct 07 '19 at 20:02
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    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. – Organic Marble Oct 07 '19 at 20:03
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    @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 https://archive.org/details/flight_rules_generic/page/n1485 – Organic Marble Oct 07 '19 at 20:10
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    There's also the benefit of no slosh dynamics affecting CG or loads. – Tristan Oct 08 '19 at 13:55
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    @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). – Ilmari Karonen Oct 08 '19 at 14:00
  • @IlmariKaronen that was an eloquent way of explaining it to someone who has no prior knowledge of fluid dynamics. Thank you for the comment :)! – Magic Octopus Urn Oct 08 '19 at 14:20
  • this answer about supercritical oxygen is a good read https://chemistry.stackexchange.com/a/122246/16035 – uhoh Oct 09 '19 at 16:22
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    @uhoh nice, and appears to address Uwe's diving-bottle concerns well. We used the Z form of the ideal gas law in the Shuttle Mission Simulator for high pressure helium tanks. – Organic Marble Oct 09 '19 at 16:25
  • How do you deal with the curve in the graph as the fuel levels are depleted? It seems as if the rest of the system has to rapidly adjust the pressure and temparature as fuel is expended. Do you think that is a good follow up question? – Magic Octopus Urn Oct 09 '19 at 17:48
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    @MagicOctopusUrn the heaters were set to keep the pressure up around 850 psi (O2), 220 psi (H2), way above the "vapor dome". – Organic Marble Oct 09 '19 at 18:09
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    @OrganicMarble ahhh... I was misreading the graph, I got it now. Dumb mistake. – Magic Octopus Urn Oct 09 '19 at 18:11
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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!

uhoh
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    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? – Ilmari Karonen Oct 08 '19 at 16:39
  • @IlmariKaronen it could be, I don't know, but here's another interesting video; https://youtu.be/GEr3NxsPTOA – uhoh Oct 08 '19 at 16:56
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    @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. – Magic Octopus Urn Oct 08 '19 at 18:47
  • But oxygen sold in steel bottles under a pressure of 200 bar at room temperature is gaseous, not supercritical. The ideal gas law is valid for oxygen between 1 and 200 bar. – Uwe Oct 08 '19 at 19:15
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    That was indeed cool, I've never witnessed the transition visually. – Organic Marble Oct 08 '19 at 19:32
  • @Uwe all I have are the sources shown and the conclusions they explain. What you've said contradicts them, "it's not supercritical at room temperature and 200 bar" do you have a supporting source for that? Let's get to the bottom of this the SE way, looking for the best supporting sources for facts cited. I'll be at the library tomorrow so I can do a proper literature search. – uhoh Oct 08 '19 at 20:37
  • Scuba divers use the ideak gas law for air and oxygen bottles to calculate the amount of remaining gas for 200 down to 10 bar. This would not be possible if there is supercritical oxygen in the tank. – Uwe Oct 09 '19 at 08:30
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    @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? – uhoh Oct 09 '19 at 13:10
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    @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. – Mithoron Oct 09 '19 at 16:48
  • https://space.stackexchange.com/a/37049/18825 .. I am confused, no sloshing then what this anomally talks about? – zephyr0110 Oct 09 '19 at 17:38
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    @Prakhar "no sloshing" discussed on this page only applies to supercritical oxygen. Those two apollo spacecraft had liquid propellants, and some cryogenic liquid oxygen as well. – uhoh Oct 09 '19 at 17:46
  • @uhoh So, only apollo 13 had such storage? – zephyr0110 Oct 09 '19 at 17:48
  • @Prakhar "no sloshing" only applies to gases and supercritical fluids (which are a lot like gases in this context). Liquid propellants are extremely common and they slosh like crazy! Spacecraft have all kinds of anti-slosh precautions. It's just that in the case of Apollo 11 (and a few others) those precautions may not have been good enough in some situations. – uhoh Oct 09 '19 at 17:52
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    @Prakhar your link is referring to sloshing of propellants used in the Apollo propulsion system. This whole page is about reactants used in the Apollo and Shuttle electrical system. – Organic Marble Oct 09 '19 at 18:18
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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.

BowlOfRed
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    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. – Uwe Oct 08 '19 at 09:36
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    The Shuttle and Apollo tanks were both cryogenic and supercritical. Your answer is incorrect. – Organic Marble Oct 08 '19 at 13:55
  • The comment is right, but the answer still makes sense because cryogenic fluids require lower temperature and supercritical fluids require high pressure. – Pere Oct 09 '19 at 12:15
  • this answer about supercritical oxygen is a good read https://chemistry.stackexchange.com/a/122246/16035 – uhoh Oct 09 '19 at 16:23