"When Zubrin factors in lack of cloudy atmosphere, and the ability to track
the sun, together with conversion efficiencies, he calculates an orbital
solar array would outperform a land-based one by a factor of 3. If the
surface array is designed with sun-tracking panels, however, he estimates
this factor would shrink to 1.5, and concludes this is too slim a margin to
justify building the arrays in orbit."

As I recall, Zubrin was including transmission losses in this number (i.e.
only about 50% of the power generated by an SPS would be usable electricity
on earth in the same sense as an earth based solar TD plant.)

"Also, Zubrin seems to assume that billion-ton space habitats will be needed
for building SPS from space resources (a Stanford Torus would mass 10
million tons, a Bernal Sphere would come in at under 4 million tons, and a
simple Space Manufacturing Facility would mass much less still). Past a
certain point, even O'Neill conceded that SPS would precede large,
Earth-like habitats, and not the other way around."

First off, Zubrin directly attacks O'Neill's economic notions of space
colonies. I agree with O'Neill's evaluation that eventually a planetary
society growing at a steady exponential rate (which, unfortunately, does not
describe humanity at this time) would eventually need to rely on space based
colonies due to the volumetric efficiencies of planets. That, however, is
for the very distant future; we have an awful lot of problems to fix before
we get there, not the least of which is humanity's overall moral framework,
which is moving away from the biblical baseline which ensures this continued
growth (i.e.: "Be fruitful and increase in number; fill the earth and subdue
it." - Genesis 1:28) Also, I recall Zubrin saying that pre-extant space
colonies would be needed to motivate SPS not build them. A large space
infrastructure is needed to ensure the 5000fold reduction in the cost of
producing and delivering SPS materials into space. A good SPS is harder to
engineer than a similar size space colony, the first several of which would
probably have large amounts of lunar regolith-based concrete in their
structures. An SPS needs to support a large area of either PVA wings or TDG
reflectors, which in turn needs to resist much greater maneuvering and solar
pressure induced torques for how much they weigh, as compared to a space
colony with only the power systems needed for its own use. The space colony
will probably be built first merely because it is so much easier to build
(Mir and ISS could be used as vestigial examples.)

"Zubrin says even assuming a lunar mass-driver could deliver ores to GEO at
1/10,000th of current launch prices,"

That would lead to about $40/kg, which is probably about the minimum that
could possibly be accomplished for GEO delivery.

"But is there in fact any basis for comparison between rocketry and launch
via electromagnetic forces? A M.I.T. study concluded that a lunar
mass-driver could launch ore into orbit for a cost of around 10
cents/kilogram."

Here Mike Combs has not defined a comparison baseline. Zubrin was talking
about GEO delivery, and has left the M.I.T. study at "into orbit", which is
probably Lunar LEO, or at best, Lunar-Earth L1 intercept. Both of these
destinations are a considerable ways from GEO delivery. I'd have to look at
this M.I.T. study, I'm guessing that it would have optimistic estimates of
the cost of developing, qualifying, building, calibrating, and operating a
lunar mass driver (comparing a tabletop mass driver to a lunar launching
facility would be a bit like comparing a pair of scissors to a scrapyard
shear.) On a more positive note, such a mass driver would form a justifying
argument for either a Earth-Sun L1 based SPS (where SOHO is located), or a
Moon-Earth L2 based SPS. The reason is that local solar power would not be
available to the lunar launch facility during the two week lunar night; an
SPS would ensure the launch facility is operational on a continuous basis.
The SPS would probably be launched by the lunar launcher, as even a half
operational lunar mass driver would be far more economical than launching
materials from Earth, except perhaps via beanstalk elevator.

"Zubrin says that, '...the size and complexity of the O'Neill
operation...boggles the mind'. Certainly all can agree on this point. But it
seems inescapable that building self-sufficient settlements on the surface
of Mars of comparable capacity would require at least an equivalent amount
of infrastructure not only be launched into Earth orbit, but propelled the
additional distance to Mars, and then soft-landed on the surface. Zubrin is
well known as an advocate of the position that this is within our
capabilities."

Oops...I think Combs missed that where ever you are building colonies, the
materials have to come from somewhere. In the case of colonies on Mars,
only infrastructure items needed to get a Mars colony started on its own
industries are required, after which a Mars colony can refine its own
materials from Martian material. An O'Neill-type colony can't pull its own
structural materials out of thin air...that is not even available at space
colony sites! Nope, an O'Neill colony will be, at best, thousands of miles
away from the nearest quarry. Because Earth has such an onerous gravity
well, even with Bluestar like technology combined with space tethers...or
even beanstalk elevators, it is obvious that planetary colonies will precede
space colonies.

The bottom line is that it is not worth making SPS to generate power for
Earth: we have plenty of our own effective energy resources. Making SPS to
power space infrastructure where generating power locally is much more
onerous...that is where the SPS will shine if it does at all (i.e. for an
O'Neill type colony, it may be easier for the colony to have a dedicated
free floating co-located SPS than to generate power with structurally
connected arrays. For a colony on a heliocentric orbit, an SPS would be in
a lower orbit than the colony, and use solar pressure for lift, thus
maintaining the same orbital period as the colony. In this manner the solar
pressure loads on the power plant would not be borne by the colony
structure, and the two separate spacecraft would be much lighter and more
managable than a single self-sufficient colony.)

On 7/5/06, Arthur Smith <apsmith@...> wrote:
>
> Zubrin got it wrong. His focus on Mars perhaps distorted his views on
> the subject. Mike Combs has a nice "debunking" here:
>
> http://members.aol.com/oscarcombs/spacsetl.htm#zubrin
>
> On the comparison of terrestrial and space solar power, the effects of
> launch and other costs, etc., there is my article on the subject
> available here:
>
> http://www.aps.org/units/fps/newsletters/2004/april/article2.cfm
> PDF: http://www.nsschapters.org/ny/longisland/articles/fetter_jan_05.pdf
>
> (the html version lost some subscripts and mathematical notation).
>
> Arthur
>
>
>
>
> Yahoo! Groups Links
>
>
>
>
>
>
>


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