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In a question on another Stack Exchange site a proposal was forwarded to premix oxygen and hydrogen in the stoichiometric molar ratio of 1:2 to be used as rocket fuel, as opposed to the standard method of storing each liquified gas (LOX, LH2) separately.

According to Wikipedia (here and here) the freezing and boiling points of oxygen (presumably at standard pressure) are around 54 and 90 Kelvin, while those of hydrogen are around 14 and 20 Kelvin, respectively, suggesting that oxygen would freeze well before gaseous hydrogen would condense into a liquid.

However, one might (naively) wonder if oxygen might still exist as a solution in liquid hydrogen at some useful fraction, and at a pressure of roughly one atmosphere or less for practicality of storage on a weight-conscious rocket.

Even the wording of this abstract makes this sound fairly unlikely, but without some confirmation from someone more knowledgable it's hard to address the other linked question.

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  • $\begingroup$ It sounds like new famous last words. Interest in liquefaction of such mixture isn't particularly reasonable. $\endgroup$ – Mithoron Feb 21 '18 at 0:35
  • $\begingroup$ @Lex I've edited the question, trying to word it a bit more carefully in hopes that it could be reopened. Still waiting for the edit to be accepted... $\endgroup$ – uhoh Feb 21 '18 at 6:10
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    $\begingroup$ @uhoh You took quite some liberty here, but I see no harm in doing that. However, If you, Lex, don't like these edits, please edit it yourself. $\endgroup$ – Martin - マーチン Feb 21 '18 at 7:07
  • $\begingroup$ @Martin-マーチン indeed, but since we're both looking at the same original question elsewhere I'm pretty sure this is the same basic question, or at least close enough so that the answer will be helpful there. I'd already left a comment for Lex there as well. Thanks! $\endgroup$ – uhoh Feb 21 '18 at 7:10
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    $\begingroup$ Looks good to me. $\endgroup$ – Lex Feb 21 '18 at 20:17
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It would be the other way round. Since the oxygen is the first to liquify, it would be hydrogen dissolved into liquid oxygen.

In this article they show the following calculated vapour liquid equilibrium diagram.

enter image description here

The 2:1 mixture, $x_{\ce{H2}}=0.67$, would exist at around 85 K and 50 bar.

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  • $\begingroup$ This is great and extremely helpful! Any conclusions or available info that might address "...and at a pressure of roughly one atmosphere or less for practicality of storage on a weight-conscious rocket." $\endgroup$ – uhoh Feb 21 '18 at 10:12
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    $\begingroup$ From what I could get from the paper, I think the x-axis in your graph is a mass ratio not molar ratio. This leads to x_H2 = 0.11 which is great since it gives a less extreme temp. $\endgroup$ – Lex Feb 22 '18 at 1:35

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