The Moon, space, and Capt. Smith. ( Digest Number 328)

Sorry I didn't address your specific question and mirror angles earlier.

Along the way, I'll comment on a few other topics you raise on the SSI list.

>There was a reference in one of the links to solar chemistry.
>They're writing about reducing CaCO2 to CaO + CO. Using the CaO for
>cement. That'll be useful to me in my studies in a week or so, when
>I start looking at building materials.

Back when the space station program included the 'solar dynamic' power
system, Boeing (or rather, the part that was then McDonnell Douglas), built
a test article. This was also around the time of the space exploration
initiative (SEI) and they studied it's potential use on the Moon. Jim
Garvey (sp?) did an internal corporate study on the scintering of lunar
dust (simulate) into bricks - using only the heat. I think he reported on
the results at an SSI conference. I seem to recall his bricks were fused
together very fast - quicker than he expected. I don't know what
assumptions he made about the different conditions of Earth atmosphere vs
lunar near-vacuum. (Also, I'm not sure what became of that big 'solar
cooker' they made, but a couple years ago (it's gone now), someone set up a
demonstration solar power system next to the Pentagon. Washington area
drivers could see the pretty mirror structure as they crossed the bridge
across the Potomac.)

>So, back to the solar question. Is there any geometry for the
>heliostats that can make use of ALL of the mirrors during the lunar
>day? How would a field be laid out so that the mirrors between the
>tower and the sun can be used?

When the sun is just rising above the horizon, a mirror in front of the
tower could reflect some light onto the tower - but the amount would be
small because not much of the mirror would be perpendicular to the
direction of the Sun.

Suppose the focus on the tower is 30 degrees above the horizon from the
point of view of the mirror. Another way of saying that is the focus is 60
degrees from the vertical. If the Sun is coming up in the opposite
direction (90 degrees from the vertical) then the Sun to focus angle is 150
degrees. Halfway between would be 75 degrees. This means the mirror would
point 15 degrees from the vertical toward the rising sun. But the
cross-section area of the mirror that actually intersects sunlight would
only be sin(15) or about 25% of the mirror area. Another way of see it is
to realize that to bounce the horizontal sunlight up 30 degrees toward the
tower, the edge of the flat plane of the mirror is lifted up half that
angle, or 15 degrees.

In the limiting case, where the tower is far away (picture a tiny tower,
only 2 degrees above the horizon, that you can cover with your thumb held
at arms length), then the mirror stays flat (or maybe tilts up by 1
degree). But the rising sun does not hit much of this mirror flat on the
ground (sin(1deg) ~1.7%). It just contributes a tiny bit - but who could
expect much more? It was dark just before that! :-) Wait a little while,
and the sun is high enough to start focusing it toward the tower.

With those tiny 'glancing incidence' angles, you can appreciate the problem
of the design of an X-ray telescope, that can only focus light using small
angle deflections. Both the Chandra AXAF telescope and the European XMM use
designs of tubes of polished metal that slightly angle x-rays back to a
focus. They don't get much, with those sine factors, but enough to collect.

> Date: Mon, 18 Feb 2002 08:13:18 -0000

I'm presuming a habitat could last as long as the pyramids, but as
>I'm researching this topic I'm finding out about the destrective
>effects of radiation and thermal stresses on materials.
>
>The literature on SPS presume a limited life before either
>refurbishment or replacement, so what makes habitats immune?
>
>How long could the whole space manufacturing infrastructure (lunar
>and asteroid mining, lunar power sats, mass drivers, orbital
>manufacturing, etc) last before it has to be rebuilt?
>
>I'm starting to question the economics if a major part of the
>infrastructure and habitat have to be rebuilt in order to mitigate
>the space environment damage.
>
>OTH, if they can last 100s-1000s of years, then full steam ahead,
>say I.

Thermal stresses would be strongest for those items going through many
light/dark cycles. Low Earth and low lunar orbits (LEO, LLO) would be
highest on this scale, followed by medium, high, and geosynchronous orbits
(MEO, HEO, GEO), then lunar surface. Object that stay on the surface of the
Moon would get deep hot and cold cycles each month. Objects in space far
from the Earth and Moon, like L3 L4 L5, might have thermal cycles based on
their own design but not much from the natural environment. The high energy
radiation effects would be a steady, long term factor to consider. In
general, these kinds of design considerations might require a bump up in
the safety factors, to extend the life - but not a drastic change. If you
can get large scale space manufacturing to work economically, then adding a
bit more mass would not change things.

> Date: Mon, 18 Feb 2002 08:17:01 -0000

>I'm assuming that the mass drivers on the lunar surface will operate
>with some form of superconductor, and that their efficiency will be
>diminished if they have to operate at the temperatures of the lunar
>noon.

Superconductors would probably help, although mass drivers could be deigned
without them. The design trade would involve the issues of
creating/providing those materials on the Moon, and the reliability of
those components in the whole operation. My guess is they would still win
out, but eventually those issues would need to be addressed.

>Are their any studies on how efficiency would be effected by day
>time termperatures and whether it would be cost effective to place a
>flimsy roof over the mass driver field to block the sun, thereby
>lowering the temperature.

Definitely cost effective. A bit of foil can create a shadow, and if you
can get a good view of the dark sky, you can radiate heat into that 3K
(-270C, -455F) sky. Thermal control is always an important issue in space -
and you should always use that dark cold sky when you can. I suppose having
a view of the dark lunar sky will become a lunar real-estate issue in the
future. It does not help much to have your radiators pointed at some other
bright surface on the Moon that is also getting overheated in that intense
sunlight. :-)

A final note.

We have to operate on several levels in thinking about these topics. We
have to make the case to do this at all, and assume economic construction
techniques - while starting with an industry that has lived for 40 years on
finding the most expensive way to do anything. We have to turn around
and convince the industry that their future lies in lower cost operations
for large scale customers. We have to find an initial pathway to this
future, but still encourage the visions of the follow-on projects that
motivate others. We have to convince the government that this future is
good enough to encourage, but not allow them to try to do it all
themselves, in the traditional expensive 'cost-plus' paradigm.

As the years go by, we have to play the role of Captain Smith, who told the
British investors of the Virginia Company they should *keep* supporting the
Jamestown colony, even after 20 years, although they had never found gold.
He said it was *still* worthwhile, because someday, maybe - all this new
land he had mapped (the Chesapeake, Potomac, etc.) might have a value of a
few thousand dollars worth of fish. :-)

- Eric

PS. Did you know at google.com, in the 'groups' section, you can search
usenet newsgroup postings for the past 20 years? There were 35000 hits for
keywords 'space studies institute' for example. Scary that all my flames
are archived for eternity.

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