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I remember reading somewhere that a catalyst was used to change the ratio of ortho- vs para-hydrogen before it is loaded as LH2 propellant on a rocket.

What is this exactly, and why is it necessary? I've read this paragraph from Hydrogen Fundamentals on this hydrogen safety website, but I still don't completely understand the implications for rocket fueling.

Liquid hydrogen (LH2) has the advantage of extreme cleanliness and the more economic type of storage, however, on the expense of a significant energy consumption of about one third of its heat of combustion. Another drawback is the unavoidable loss by boil off which is typical to maintain the cold temperature in the tank. The evaporation rate is even enhanced when ortho hydrogen is stored. The heat liberated during the ortho-para conversion at 20 K is huge with 670 kJ/kg compared to a figure of 446 kJ/kg for the latent heat of vaporization at the same temperature. This represents a safety issue requiring a design of the hydrogen loop which is able to remove the heat of conversion in a safe manner.

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The problem is that the transition produces enough energy to boil the LH2. As explained on the old sci.space.history group:

Skipping the gory quantum-mechanical details... there are two energy states of the hydrogen molecule, ortho and para. At room temperature, hydrogen is about 3/4 ortho. At liquid-hydrogen temperatures, the stable state is almost all para. But the ortho-para conversion is slow, so if you just liquefy hydrogen, what you get is still 3/4 ortho, and it slowly converts itself to para. This is trouble because the ortho-para transition releases a modest amount of energy, and it doesn't take much energy input to boil off liquid hydrogen. In fact, the transition will boil off all the liquid.

The fix is to find a catalyst which will speed up the transition, and put some of that in your hydrogen liquefier, so the transition will happen while the liquefier is sucking heat out of the hydrogen anyway. This is now quite routine.

The difference in properties between the two are quite small; some thermal properties are slightly different, as I recall.

I suppose ortho-hydrogen ought to have a very slightly higher Isp than para-hydrogen, but the difference is slight and the difficulties of handling and storage loom large.

This particular problem is pretty much unique to hydrogen, fortunately. Hydrogen and helium, and to some extent neon, show oddities in their behavior which are visible effects of quantum mechanics -- they are sometimes spoken of as the "quantum gases". (This contributes to some of the peculiarities of liquid hydrogen, like its very low boiling point and its very low density.) Hydrogen is the only one of the three which has molecules containing more than one atom, and hence the only one which shows an ortho-para distinction.

In practical terms this means

As a result of this slow change, a thermally isolated tank of liquid hydrogen prepared without conversion to the para form will lose about 1 percent of its volume during the first day of storage.

(this is from the reference linked below)

A number of catalysts have been used: chromic oxide, paramagnetic ferric oxide, hydrous manganese oxide. More info here.

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  • $\begingroup$ That's a great description, thanks! It's an internal degree of freedom that can hold heat and release it later. From this it sounds like you do do the conversion as part of the cool down process and then it's done, but I thought I remembered something related to doing it again just before -or as part of - the loading of the LH2 into a tank before launch. Am I misremebering? $\endgroup$ – uhoh Jul 28 '16 at 12:52
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    $\begingroup$ From my shuttle knothole, I believe it was delivered to the pad as para- and stayed that way without further treatment. It sounds like keeping quantities of the stuff around in a mixed ortho/para state would be, at the least, uneconomical. But other vehicles may have worked differently. $\endgroup$ – Organic Marble Jul 28 '16 at 13:05
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    $\begingroup$ Note: there is ortho/para-helium, but the spin alignments are those of the 2 electrons instead of like H2's spatially separated protons. $\endgroup$ – amI Sep 20 '18 at 21:31

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