[ExI] Space based solar (was: global warming again.)

Keith Henson hkeithhenson at gmail.com
Fri Mar 20 17:47:48 UTC 2009

On Fri, Mar 20, 2009 at 9:43 AM, John K Clark <jonkc at bellsouth.net> wrote:
> "Keith Henson" <hkeithhenson at gmail.com>
>> Space based solar provisionally looks like the best idea.
> But you think using lunar material for most of the mass that makes up those
> power satellites is a bad idea; it's never been entirely clear to me why you
> think all of the material should be launched from the very bottom of Earth's
> deep gravity well and not just use that option for the hard to make stuff
> like computer chips. Solar cells are easier to make than computer chips but
> you need to make vastly more of them. If we can make them on the Earth
> why can't we make them on the moon?

I was one of the *founders* of the L5 Society, which was based on
using lunar materials.  The problem now is that the need for building
power sats is acute.  There just isn't the time to build up lunar
mining and industry to build power sats that way.  I wish there was.

Deep as the earth's gravity well is it's not expensive in theory to
get out of it.  A 100% efficient moving cable space elevator will take
a kg to GEO for 15 cents of energy.  Used to build power sats, that
much energy can be paid back in a day or two.  Rockets are about 3
percent efficient.  They pay back the lift energy in about 40 days,
and the fuel cost is about $5/kg.  The problem is the rocket equation
which says for chemical exhaust velocities you only get about 1 part
in 60 of liftoff mass to GEO and the high cost of highly stressed
rocket hardware.

> It seems to me that if power satellites are ever going to be economical,
> and I'm not sure they ever will be, then you'll have to use lunar material
> for at least 95% of their mass. But I could be wrong.

A lift cost to GEO of under $100/kg results in power for a penny a kWh
and liquid fuels from that power for a dollar a gallon.

Incidentally, solar cells may not be the best approach.  At 60%
efficient, thermal engines may be a better approach.

Space resources may still play a big role.  Invar may be material of
choice to build power sats.  If so, the pace of building them exhausts
the nickel mines on earth in short order.  Asteroid 1986 DA is a 2.3
km chunk of nickel iron.  It is 140 m/sec from GEO.  You still need
really low cost to GEO since an extraction plant could easily mass
50,000 tons.


> John K Clark
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