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From: (Henry Spencer)
Subject: Re: Propellant leakage (was: A nice waste of money.)
Date: Sun, 6 Sep 1998 03:35:26 GMT

In article <>, Bruce P. Dunn <> wrote:
>> >Okay, the internal pressure would go up, but if the tank is sufficiently
>> >strong to resist the internal pressure of the warming hydrogen, wouldn't
>> >it stay liquid?
>> Yes, more or less... but that is a big "if"...
>1) "Critical temperature is that temperature above which a gas cannot be
>liquified by pressure alone."

There's a reason why I said "more or less". :-)  I didn't want to get into
an explanation of supercritical fluids...

Normally, a gas is loose molecules bouncing around with a lot of open
space in between, and a liquid is tightly-packed molecules that slide over
each other without ever opening up much space in between.  But when you
compress a gas, keeping it warm enough that it doesn't liquefy, the
molecules start spending more and more time in contact, and the gaps start
to close up.  And when you heat a liquid, keeping it pressurized enough
that it doesn't boil, the molecular motion gets vigorous enough that the
molecules can't hang onto each other as well, and gaps start to open up.
Carry both of these trends far enough, and gas and liquid blur into each
other to the point where there is no longer a sharp boundary.

>In other words, once the hypothetical container of liquid hydrogen warms
>above 33.25 K, the liquid *will* evaporate...

Not really.  What you get above the critical point is not a gas, but
rather something that is sort of a vague hybrid between liquid and gas.
Its properties are not a clear-cut match with either one.  At quite low
densities it will be rather gas-like.  At liquid-like densities -- which
can be achieved without any fundamental difficulty, just a small matter of
engineering :-) -- its behavior will be rather liquid-like.  And there is
no sudden transition between the two at a well-defined point, just a
gradual change of properties.

Liquid hydrogen does not evaporate or otherwise suddenly change state at
33.25K.  It just gradually gets more gas-like as you warm it past that
point, and if you then lower the pressure while holding temperature
constant, you will see a gradual change to what everyone will agree is a
gas, with no actual boiling at any point.

(With hydrogen, you actually might not be able to tell the difference :-),
because LH2 is already somewhat gas-like -- in particular, it is quite
compressible by liquid standards.  For fundamental reasons, the "quantum
fluids" -- LH2, LHe, and to some extent LNe -- behave, roughly speaking,
as if they were warmer than they actually are.  Solid hydrogen has
semi-liquid behavior, and liquid hydrogen has semi-gas behavior.)

If you have a tank of LH2 with a bit of GH2 above it, and you warm it past
the critical point (and the tank is strong enough to contain the internal
pressure at all times, and it's well-stirred), what you will see is not
sudden boiling, but a blurring and disappearance of the liquid surface,
leaving you with a tank full of a homogeneous, rather compressible,
substance with near-liquid density.

"Cannot be liquefied" should be read as "cannot be caused to undergo an
abrupt phase transition between gas and liquid", not as "cannot be caused
to have liquid-like properties".

Sorry, Bruce, no point for this one. :-)
Being the last man on the Moon is a |  Henry Spencer
very dubious honor. -- Gene Cernan  |      (aka

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