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From: (Henry Spencer)
Subject: Re: Aerial Propellant Transfer Revisited
Date: Mon, 3 Jul 2000 22:43:58 GMT

In article <8jqhmq$qn2$>,  <> wrote:
>  Just as a quibble, you don't *have* to insulate LH2 plumbing in all
>cases.  The issue is that LH2 is cold enough you get liquid air
>formation on the lines, and that for most ground operations you don't
>want a 20% LOX mixture dripping off the plumbing into other things.

I'll quibble right back. :-)

First, a relatively minor item:  one reason why people worry about liquid
air dripping off LH2 plumbing is that it's *not* just 20% LOX; it's more
like 50%.  Yes, air is only 21% oxygen, but oxygen also has a higher
boiling point than nitrogen, so it's more willing to condense.  Working
out the exact composition of the condensate requires a chart, but from
memory, 50% LOX is about the right number.  And that's just the initial
composition; the nitrogen will boil off faster too...

>usual solution is to spray foam the LH2 lines (and usually the LOX lines
>too, to avoid too much frost formation -- which is heavy in the initial
>stages of launch).

True, it's usually considered a better bargain to insulate LOX lines too,
in the ground hardware.  Flight LOX plumbing usually isn't insulated
unless there is something underneath which could be damaged by falling
ice.  Witness the ice falling off an Atlas or Saturn during launch.  The
shuttle ET LOX tank was originally supposed to be mostly uninsulated,
until they noticed the possibility of tile damage on the orbiter.  (That's
also why the "beanie cap" umbilical was improvised, late in development --
to avoid icing on the LOX vent.)

>From a design standpoint, for automated ground
>operations you don't *need* to insulate LH2 lines any more than LOX
>lines with the right design of ground ops hardware -- but most folks do
>to avoid potential hazards...

Here I will proceed to a more serious quibble:  insulation on LH2 lines is
usually mandatory even without the liquid-air hazards, because LH2 is so
much more sensitive to heat leaks.  With LOX, people can accept a bit of
boiloff; with LH2, the density and heat capacity of the fluid are so low
that you'd lose huge amounts of it.  Forming one liter of liquid air boils
over 12 liters of liquid hydrogen, and the liquid air runs off instead of
sticking around and forming an insulating crust the way frost does.

>   For something like Pioneer's orbital refueling concept, I think you
>can make a case that you don't have to insulate the LH2 lines, if any.

There aren't any -- Pioneer isn't using LH2.  I brought LH2 in only as a
comparison, because so many people assume that the extreme measures needed
for handling LH2 are required for any cryogenic fluid.

>...The LH2 line insulation would also avoid this, and
>also prevent the possibility of 20% LOX enriched oxidzier being added
>into the jet intakes -- and having engine surges would not help...

As I recall, Mitch has said that they worried a lot about LOX ingestion
(of 100% LOX, from leakage when disconnecting the LOX probe), but on
closer examination, found it to be completely a non-problem.
Microsoft shouldn't be broken up.       |  Henry Spencer
It should be shut down.  -- Phil Agre   |      (aka

From: (Henry Spencer)
Subject: Re: Liquid Hydrogen
Date: Sat, 8 Jul 2000 22:49:04 GMT

In article <>,
K129000 <> wrote:
>I came across the mention of parahydrogen in the report I was reading.  I know
>that there are two type of hydrogen and I think that there is something about
>electron spin or orbit direction that plays a part.
>What are these types?  What are the major differences?  Is there a sizable
>impact on Isp for one type of LH2 over another?

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.
Microsoft shouldn't be broken up.       |  Henry Spencer
It should be shut down.  -- Phil Agre   |      (aka

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