Re: Liquid Hydrogen Refrigeration

In article <IDvGxv.91F@xxxxxxxxxxxxx>,
henry@xxxxxxxxxxxxx (Henry Spencer) writes:
> [Spitzer] solves it only in a loose sense. With elaborate insulation wrapped
> around a modest-sized tank, it gets boiloff rate down to the point where
> it won't boil dry for at least a couple of years.

The passive boiloff rate for Spitzer is equivalent to 1 mW of heat
input to a 360 liter tank.

> Liquid helium is harder than liquid hydrogen, yes. But on the other hand,
> liquid-hydrogen tanks for propulsion will be far bigger -- they will
> dominate the vehicle in a way that Spitzer's tank doesn't -- and they
> can't tolerate a lot of boiloff, and mass issues limit the insulation.

On the other hand, not only is the boiling point higher for hydrogen
than for helium, the boiloff gas has a lot more enthalpy. This
enthalpy can counteract heat conduction through structural
components.

Many years ago, I posted the calculation of the equilibrium
temperature of a body at 1 AU. Now I can no longer find it, but as I
recall the answer is about 5 K. Of course this assumes complete
shielding from the Sun and no nearby warm objects; the heating comes
from light from zodiacal dust particles. Because of the T^4
dependence of radiation power, the equilibrium temperature is
negligible compared to the 20 K liquid hydrogen temperature. The
problem is that radiation at 20 K is awfully inefficient, but the
"reservoir" temperature is plenty low enough.

Quite a lot has been learned about space cryogenics in the past
several years. I don't think long-term hydrogen storage in deep
space can _by itself_ be considered a difficult problem any more.
However, storage near a planet is much more challenging, as Henry
mentioned. Also, for a real mission, other requirements may conflict
with the insulation and shielding requirements.

--
Steve Willner Phone 617-495-7123 swillner@xxxxxxxxxxxxxxx
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
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