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Subject: SPACE Digest V10 #237

SPACE Digest                                     Volume 10 : Issue 237

Today's Topics:
               Re: Solar power vs RTGs
            Re: Moon Colonies / Ant Tanks?
               Re: Solar power vs RTGs
               Re: Solar power
----------------------------------------------------------------------

Date: 12 Nov 89 04:55:46 GMT
From: mailrus!jarvis.csri.toronto.edu!utgpu!utzoo![e-mail]  (Henry Spencer)
Subject: Re: Solar power vs RTGs

In article <[e-mail]> [e-mail] (Tom Neff) writes:
>... If the questioner wants to know why Christic et alia didn't
>protest Viking and Voyager... they weren't around at the time.

Similar groups were, however.  They just weren't paying attention.

>One thing that even Henry can agree on is that the "cost effectiveness"
>of RTGs is largely a chimera.  DOE sells NASA plutonium fuel at heavily
>subsidized prices...

I don't recall NASA ever being too worried about how much their power
systems cost, actually.  I think the relevant cost measures here are
weight, reliability, and design tradeoffs, not money.  RTGs are lighter
than just making the arrays bigger, especially when you have to make them
extra-big to allow for radiation damage in the planetary Van Allen belts,
but the weight comparison against concentrator designs is less obvious.
The major reliability issue is the unproven nature of concentrator solar
systems, although it doesn't strike me as a big problem.  The tradeoffs,
as usual, point both ways.  RTGs emit radiation, which is a nuisance;
it's not an accident that Voyager's scan-platform boom is 180 degrees
away from the RTG boom.  Solar panels are big, especially if they have
even bigger concentrating mirrors, and structural weight and flexibility
are an issue when it's time to maneuver.  Solar arrays stop producing
power when in shadow, i.e. behind a planet, and this means batteries,
which are a weight and reliability headache.  (Voyager has no batteries
at all; it doesn't need them much, and a capacitor bank was judged to
be a better way of dealing with surges when very long life was a major
design goal.)  Solar arrays have to be pointed at the Sun, which means
attitude restrictions and mechanical complications.  Solar arrays are
a pain to deploy from a folded launch configuration, as witness the TVSat 1
failure, but on the other hand RTGs need cooling before and during launch
and are a safety worry.  Both run hot, but RTGs are worse because their
power-conversion system is very inefficient.  And so on.
--
A bit of tolerance is worth a  |     Henry Spencer at U of Toronto Zoology
megabyte of flaming.           | uunet!attcan!utzoo!henry [e-mail]

------------------------------

Date: 11 Nov 89 23:58:28 GMT
From: cs.utexas.edu!mailrus!jarvis.csri.toronto.edu!utgpu!utzoo![e-mail]  (Henry Spencer)
Subject: Re: Moon Colonies / Ant Tanks?

In article <[e-mail]> [e-mail] (Tom Neff) writes:
>>Somebody once asked Arthur C. Clarke, who lives in Sri Lanka, what
>>it would be like to live in a space colony.  He replied, ``You'll
>>have to ask my friend Isaac Asimov.  He lives in New York.''...
>A city like New York may look like a steel cave to someone like Clarke,
>but it assuredly is not.  It's a breathing organic entity literally
>filled with life and ecology.  It's quite diverse and TOTALLY
>uncontrolled...

This is unquestionably true, but entirely irrelevant.  The hinterlands
of New York exist not because they are necessary to the psychological
well-being of the Manhattanites, but because they are necessary for
economic and logistic reasons... reasons which do not apply on the Moon.

There is no reason why a well-established lunar colony can't have quite
a bit of life and diversity, although better control would definitely be
needed.  (By the way, the biology of in New York is *not* "TOTALLY"
uncontrolled, as attempts are made to control undesirable life forms.)

>In truth, the only places where people truly live indoors in
>closed environments are things like SAC bases and prisons...

This is closer to the point.  It is unquestionably true that many people
in the city spend a good bit of their time outdoors.  The point is, *some
don't*.  Asimov is an example.  I'm another, actually.  I would miss not
being able to stroll outside or open the window... but in fact I don't
do either very often, and I am fairly sure I could get along without them
if there were compensating factors (which do not exist in prisons and
SAC bases).  I do spend about an hour a day outside on a typical weekday,
but that's simply because I live half an hour's walk from work.  I am not
emotionally attached to that half-hour walk, and indeed it's a damn
nuisance much of the time.  The window in my office does not open, and
since I like it warm -- my friends claim I'm a hothouse plant -- there
aren't that many days when I'm comfortable opening my apartment windows.
In winter in particular, there are months when I'd happily give up all
need to go outside.  (That was even more true in Saskatchewan; at -20C,
the popularity of outdoor sports is rather limited.)  Some people simply
aren't bothered by such an environment.
--
A bit of tolerance is worth a  |     Henry Spencer at U of Toronto Zoology
megabyte of flaming.           | uunet!attcan!utzoo!henry [e-mail]

------------------------------

Date: 12 Nov 89 01:39:12 GMT
From: bfmny0![e-mail]  (Tom Neff)
Subject: Re: Solar power vs RTGs

In article <[e-mail]> [e-mail] (Henry Spencer) writes:
>>Oh, as a side note, has anyone asked the various groups that opposed
>>Galileo why they didn't speak up about any of the other probes or
>>satellites that have used RTGs?
>
>The obvious answer is that they just hadn't noticed them.  Don't expect
>rationality from those people.

Also don't expect better answers than the "obvious" from some around
here.  :-)  If the questioner wants to know why Christic et alia didn't
protest Viking and Voyager... they weren't around at the time.  If the
questioner want to know why Christic et alia aren't making any noise
about Ulysses, Cassini, and CRAF, along with the various proposed SDI
test vehicles that would use RTGs and reactors, the answer is that they
are!  Although you probably won't hear about it until the next lawsuit.
The coalition's position, from what I can figure, is that NASA
deliberately ignores alternative energy options like concentrated solar
arrays because it wants to secure a toehold for nuclear powered
spacecraft of all kinds, both for its own convenience and for the SDI
office.  I think this is reading too much into NASA's motives, as it
presumes them competent to manage a decent conspiracy. :-)  But it is
quite possible that Not-Invented-Here Syndrome (just say no to NIHS!)
reared its head along the way.

One thing that even Henry can agree on is that the "cost effectiveness"
of RTGs is largely a chimera.  DOE sells NASA plutonium fuel at heavily
subsidized prices.  If the full cost of producing the fuel were factored
in, RTG power would be considerably more expensive.  Of course this is
nothing new for NASA, whose every launch is "costed" out using similar
subsidy-driven fantasy figures.
--
"My God, Thiokol, when do you      \\    Tom Neff
want me to launch?  Next April?"   \\    [e-mail]

------------------------------

Date: 11 Nov 89 23:18:22 GMT
From: jarvis.csri.toronto.edu!utgpu!utzoo![e-mail]  (Henry Spencer)
Subject: Re: Solar power

In article <[e-mail]> [e-mail] (setenv WALRUS green) writes:
>I have heard several people claim that there is indeed enough
>light out near Jupiter for it to be practical to use solar panels.
>Now, I'm curious: is it true?

It undoubtedly can be done, with sufficiently advanced designs.  Gerry
O'Neill once calculated that solar power is practical out to a good
fraction of a light year, if you're willing to use a big enough mirror
as a concentrator.

>Or is it that the panels would just have to be too huge...

The collecting area has to be big.  This means either big panels or a
concentrating mirror.  Big panels are heavy, and concentrating technology
of that sort (as opposed to small concentrator lenses to run souped-up cells
at high temperature) is not well developed yet.

>... What kind of
>power output can you expect at varying distances from the sun?

It's basically just a function of collecting area.  To get the same power
output twice as far out, you need four times the area, courtesy of the
inverse-square law.  If using a concentrating mirror, deduct a little bit
for absorption and inaccuracies in it.  If not using a mirror, or if
operating at a wide range of distances without changing mirror geometry,
some correction for varying cell temperatures will be necessary.

>Oh, as a side note, has anyone asked the various groups that opposed
>Galileo why they didn't speak up about any of the other probes or
>satellites that have used RTGs?

The obvious answer is that they just hadn't noticed them.  Don't expect
rationality from those people.
--
A bit of tolerance is worth a  |     Henry Spencer at U of Toronto Zoology
megabyte of flaming.           | uunet!attcan!utzoo!henry [e-mail]

------------------------------

End of SPACE Digest V10 #237
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