From: solarpowersatelliteplace@yahoogroups.com
[mailto:solarpowersatelliteplace@yahoogroups.com] On Behalf Of Terry
Wilson


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.)


At the time I wrote the article, I was able to find a reference to the
MIT study, but not the actual study itself. Sorry I can't provide more
info. As a guess, assuming the MIT study was following what we might
call the O'Neill set of assumptions, this would be delivery to the
Moon-Earth L-2 point. Delivering payloads from L-2 to HEO (assumed SPS
construction site) ought to not cost terribly much, given that we're
talking about two locations fairly high up in the Earth's gravity well.


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.


I know that the High Frontier studies were perfectly happy with a lunar
mass driver with a 50% duty cycle, pointing out that some downtime for
maintenance would be necessary in any case. I remember at an early
point there was a small nuclear reactor at the moon base. Later on, the
reactor was gone and the base was powered solely by PV arrays. I've
always suspected that they concluded that extending the duty cycle of
the mass driver was not worth the political hassle of launching a nuke
to the moon.


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.


No, what I was thinking about when writing that is the industrial
"seeds" needed to get started: mining equipment, ore refineries, and the
first simple manufacturing facilities. The only operational complexity
present with High Frontier which is absent from Mars is the mass-driver
firing of construction materials to L-1, and their subsequent transport
to HEO. So of course Zubrin chooses to make a big deal of this. But to
me, those two parts of the operation are not necessarily the most
complex or expensive parts of the overall operation.


Because Earth has such an onerous gravity
well, even with Bluster like technology combined with space
tethers...or
even beanstalk elevators, it is obvious that planetary colonies
will precede
space colonies.


To me it's not so obvious. I think it will turn on which is more
expensive; getting needed construction materials off of the moon or an
asteroid, or operating at a much greater distance from Earth.


(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.)


Hmmm. I'd always considered attached arrays to be more advantageous
than a separate microwave-linked platform, but I hadn't considered this
angle before. Interesting.





Regards,

Mike Combs




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