From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: Chemical Logistics In Earth/Lunar Space
Date: Fri, 9 Jul 1999 15:40:52 GMT

In article <7lrobg$1i5$1@nnrp1.deja.com>, gbaikie  <gbaikie@hotmail.com> wrote:
>>[superconductors]
>
>I wonder what the average Lunar temperature is a couple meters to a
>kilometer below the surface.

A couple of meters down, it is circa -20degC over most of the Moon.  Maybe
colder near the poles -- we have no data from there -- but note that the
lunar regolith is an *excellent* insulator, and the interior is warm.

A kilometer down is harder to say, but at the Apollo sites, near the
surface, temperature increases 1-2deg/m as you go deeper.  This may
taper off farther down.

Except *perhaps* near the poles at quite shallow depths, it is unlikely
that there is any point within the Moon at temperatures suitable for any
known superconductor.

>Say, you had a good mirror and put in various orbits. Have the mirror
>point towards the sun and behind the mirror in it's shadow have a
>sphere filled helium. What temperatures would the helium be at various
>distance from the Sun?

There are limits to this, because the mirror itself becomes warm and
radiates heat.  The coldest yet achieved by passive cooling has been about
50K (which took more than just a mirror!).  With good design, at about
Earth's distance from the Sun but in deep space well away from any warm
planetary surfaces, it should not be hard to reach liquid-nitrogen
temperature (circa 70K), it just perhaps maybe might be possible to get
liquid-hydrogen temperature (20K) although it's never been done and you
can find people who will argue about it, and it is almost certainly not
possible to get liquid-helium temperatures (circa 4K).

>Since this should block most of the solar radiation- other factors such
>as cosmic radation and solar wind might affect temperature...

Those are relatively minor effects; the only thing that can really compete
with the Sun is a warm planet nearby.  The dominant problem is the heat of
the Sun, via warm intermediaries like planets, sunshades, mirrors, etc.
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: LH2, LOX in orbit (was: Tanker-C Question)
Date: Wed, 13 Sep 2000 23:12:15 GMT

In article <dcits02cte.fsf@storm.nws.noaa.gov>,
Mark Oberfield  <oberfiel@haze.nws.noaa.gov> wrote:
>     > I'd say an orbiting zero-g liquifaction plant would be much
>     > more difficult than simply devising ways to store and transfer
>     > cryogenic fluids on orbit.
>
>What about passive radiators?  Since the blackbody temperature
>of empty space is around ~4K that sufficient to liquify hydrogen.  But
>I suppose being in LEO this would be difficult to keep the radiators
>shielded and pointed away from earth, moon and sun all the time.

Exactly.  The temperature of deep space is not just the black-body
temperature, by the way, because things like stars contribute some extra.
But the dominant problem in the inner solar system is that the Sun, and
any warm planets which happen to be nearby, contribute a lot more.  The
best passive cooling systems in LEO to date deliver 40K or thereabouts
(COBE after its helium ran out), and 70-80K is a better estimate of what
you can get by straightforward methods.  There is talk that you can do
better, *maybe* enough for passive hydrogen storage, but it's unproven...

An important secondary problem, also, is that because thermal radiation
emission scales with the *fourth power* of temperature, cryogenic
radiators emit very little heat (i.e., they supply very little
refrigeration) per square meter of radiator area.  So reaching very
low temperatures with passive cooling and sane radiator areas requires
truly fanatical minimization of heat leaks, which is quite difficult
for big tanks.
--
Microsoft shouldn't be broken up.       |  Henry Spencer   henry@spsystems.net
It should be shut down.  -- Phil Agre   |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: LH2, LOX in orbit (was: Tanker-C Question)
Date: Thu, 14 Sep 2000 20:49:11 GMT

In article <39C07DD7.4C65B6BE@gnelson.demon.co.uk>,
Graham Nelson  <graham@gnelson.demon.co.uk> wrote:
>> Exactly.  The temperature of deep space is not just the black-body
>> temperature, by the way, because things like stars contribute some extra.
>
>Is the heating effect of distant stars (well, stars other than the
>Sun) actually measurable? Good heavens.

There's enough starlight to be noticeable compared to the cosmic
black-body background, because the stars are so much hotter.  Not huge but
not entirely negligible.  Likewise, in the solar system there is quite a
bit of infrared emission from dust in the asteroid belt... which is why
the Terrestrial Planet Finder, which wants to work in IR, is planned to be
located out around Jupiter.
--
Microsoft shouldn't be broken up.       |  Henry Spencer   henry@spsystems.net
It should be shut down.  -- Phil Agre   |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.policy
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Alternatives to LH/LOX
Date: Tue, 10 Oct 2000 00:05:19 GMT

In article <8rtfvu$o74$1@nnrp1.deja.com>,  <wmook@my-deja.com> wrote:
>Its clear to see that since the background heat level of the universe
>is about 4K, and the boiling point of LH is higher than that, there's
>really no problem with long term storage of LH in a properly designed
>tank.

That would be true, more or less, in interstellar space.  Within the inner
solar system, the picture is rather different.  There, the lowest feasible
equilibrium temperature, without really drastic measures, is 70-100K.
There are people who will tell you that passively-cooled storage of LH2 is
possible, *but* this is unverified speculation involving severely pushing
the technology.  If you want known technology, long-term storage of LH2
requires active refrigeration.

>...again, with a properly designed tank, LH can be stored
>efficiently at Earth-Sol distances or greater.

References?

>What is important for chemical rockets is SPECIFIC IMPULSE.

That's one of the things that are important for chemical rockets.  Not the
only one.

>If we assume a structural fraction of 1/10th for both vehicles...

Don't forget that achieving a given structural fraction is considerably
more difficult for LOX/LH2 than for denser propellants.  The difference
is not small.
--
Microsoft shouldn't be broken up.       |  Henry Spencer   henry@spsystems.net
It should be shut down.  -- Phil Agre   |      (aka henry@zoo.toronto.edu)