From: eder@hsvaic.boeing.com (Dani Eder)
Newsgroups: sci.space
Subject: Re: SPS feasibility (WAS: SPS fouling astronomy)
Date: 17 Aug 92 15:48:05 GMT
Distribution: na
Organization: Boeing AI Center, Huntsville, AL

gary@ke4zv.uucp (Gary Coffman) writes:

In reference to Earth heat balance calculations, I disagree with
Mr. Coffman.  If terrestrial solar power is used, whether PV or
thermal, you have to locally increase absorbtion of sunlight, since
both PV cells and solar collectors are effectively darker than the
ground over which they are built.  A darker earth is a warmer earth,
despite the shifting of the heat load from the collector to the end
using machines by way of electrical transmission lines.

In the case of microwave (not laser) SPS, the receiving antenna
(rectenna) is metal, which tends to reflect more sunlight than
the gound over which it is built.  If needed, the rectenna can
be painted white to reflect even more.  The reduced absorbtion of
sunlight is compensated by the increased absorbtion of microwaves.
If you choose your parameters carefully, you can make the total
impact on the Earth's heat balance zero.  Another free variable
you can play with is that if SPS's are in orbit around the Earth,
the ones on the sunward side cast shadows, and the ones on the
opposite side reflect sunlight back to the Earth.  By engineering
and how you aim them, you can adjust the effect on the Earth's
heat balance by some undetermined amount (it will have to wait
until you get a design).

You are correct in the case of a laser SPS that there will be
large amounts of waste heat to deal with, since laser-->electric
conversion efficiencies are no better than 50%, while microwave
--> electric conversion efficiencies are closer to 85%.


>Earth's heat balance. Normal heat input from the Sun for the Earth is
>about 140 TW so the additional 1 TW is less than a 1% change. But due to

The radius of the Earth is 6378000m.  Thus the cross sectional area
is 1.278x10^14 square meters.  The solar constant is 1390 Watts/sqaure
meter, so the total flux impinging on the Earth is 1.776x10^17 Watts.
The albedo of the Earth is 0.36, so 64% of the flux is absorbed, i.e.
1.137x10^17 Watts, or 113,700 TW.

>Now what's happening on the Moon? Gas lasers are very inefficient devices. 
>I don't have CO2 numbers at hand, but He-Ne efficiencies are around 0.1%. 
>Giving an advanced CO2 laser a 10% efficiency is likely generous. Now to 
>convert the IR back to electricity the Earth based receiver is either going 
>to be photovoltaic cells, or some thermal system. In either case, efficiency 
>is under 30%. That means that 34 TW of electricity has to be input to the laser 
Gas lasers are not proposed by power beaming researchers (by the way,
I currently AM a power beaming researcher for NASA.).  At the Laser
Power Beaming Photovoltaics/FEL Interactions workshop, 25 March 1992

The Jet Propulsion Laboratory reported measured actual laser-->PV
conversion efficiencies of 29% for Gallium Arsenide cells.  The
theoretical limit is 57% for GaAs at 800 nm, which is slightly into
the infra-red.

It is proposed to use a free-electron laser for making the beam.  The
FEL in theory is an efficient device.  In practice, large ones are
expected to work in the 5-10% efficiency range.  Personally, I prefer
the microwave SPS to the laser SPS, since the conversion efficiencies
at both ends are much higher.


>>> No we say that we must finally open up the last frontier to development. No 
>>> single technology or service from space will justify the expense BUT, taken
>>> together, the development of the resources of the solar system, whether it 
>>> be solar energy from space, materials from the Moon, asteroids, and other 
>>> planets will raise the planetary standard of living to a height that will 
>>> make today look like the abode of dirt dwellers.

>So we lose money on every sale, but we make it up in volume?

I take issue with both statements above.  The comsat business alone
is sufficient market to stimulate the initial opening of space as a
frontier, if you are smart about serving that market.  Launching
comsats on chemical rockets is not being smart about it, because they
are terribly expensive one-use throw-aways.  Rocket performance has
stagnated in the mid-300 second specific impulse range for earth-to-
orbit for 30 years.  It's no wonder the market has not opened up.

The second comment misses the point - if you have more markets to
spread the development cost of your transportation system over, you
can justify it where a single market would not.

>>>(the point of this thread is to find out), but why should we want to
>>>import bulk material from space to Earth? Few extraterrestrial ores
>>>match the quality of those found on Earth, save for iron-nickel
>>>asteroids, and there's already plenty of steel in landfills. Even if
>>>you could import diamonds from the Moon, gold from the asteroids, or
>>>star sapphires from Mars, could you beat the transportation costs?

Iron-nickel asteroids, being a type of steel (9% Ni, 1% Co), have
a ready market on Earth on the order of $200 billion per year.  The
quality of steel would be higher than that used on Earth today for
most applications.  What is needed is a way of retrieving it for less
than $400/ton.

Diamonds currently go for about $1000 per carat, or $5 M per kg.
Even today the transport cost to and from the Lunar surface is
more like $100,000 per kg, so diamonds would make sense.  Gold
is not known to be present in quantity in asteroids, but platinum
group metals are (in association with the Iron-nickel.)  On Earth
Pt-group metals are rare precisely because they were soluble in the
iron, and sank to the core of the planet when the Fe-Ni core formed.
In asteroidal Fe-Ni, the cores have been exposed by impact, and the
material is accessible.  Now, Pt goes for about $400/oz, or $13,000/kg.
Currently this is about what it costs to launch stuff to Earth orbit,
so a mining return ratio of more than 1 would be required to make
money (return more than one kg of platinum per kg of mining equipment
brought to Earth orbit).  If launch costs were reduced greatly,
then it would be easier to make money this way.

>I am realistic enough to know it won't make a material difference here on
>Earth. Like Columbus' "discovery" of the New World, the riches ultimately 
>fall to the inhabitants of the New World, not those left behind in the Old 
>World. So it is with space. People living in space will be the primary 
>beneficiaries of space activity. Earth can attempt to play Hudson Bay
>Company, but eventually Earth has to manage by itself. I don't have a 
>problem with that as a justification for supporting space. "To boldly go..." 
>is sufficient reason to support space activities. The human race, wherever
>they may be, will be the beneficiaries of that process, not Earthlings.

As a final comment: the SPS discussion has sidestepped any question
of fusion becoming a power source for the Earth.  By this I mean
plain ordinary fusion rather than He-3 based hype.  I am aware of two
lines of inquiry that may lead to easily produced fusion power
(i.e. Mr. Fusion).  The first is the Plasmak type machine being worked
on by Paul Koloc, wherein the plasma confinenment fields are run
in the plasma itself (rather than in a surrounding machine, such as
the Tokamak design).  The second appraoch is the Electric Fusion
machine being designed by Robert Bussard.  This uses an electric
potential well to cause the protons to collide in the center.  Both
have great potential.  If cheap and easy fusion is available, a lot
of current problems could go away.

Dani

-- 
Dani Eder/Boeing/Advanced Civil Space/(205)464-2697(w)/232-7467(h)/
Rt.1, Box 188-2, Athens AL 35611/Member: Space Studies Institute
Physical Location: 34deg 37' N 86deg 43' W +100m alt.
***THE ABOVE IS NOT THE OPINION OF THE BOEING COMPANY OR ITS MANAGEMENT.***
Newsgroups: sci.space,sci.physics