🚀 go-pugleaf

>From OTA@S1-A Sat Dec 12 03:51:56 1981

SPACE Digest                                      Volume 2 : Issue 57

Today's Topics:
			      Space Elevator
			     space elevators
			  Launching Laser Costs
	       Atmospheric distortion and high power lasers
		      cost of power for laser launch
			     Penthouse please
		 Laser launches from the Moon or in Orbit
			   Penthouse, Yes Sir!
			     Penthouse, P.S.
			      Strong cables
----------------------------------------------------------------------

Date: 11 Dec 1981 05:55:28-PST
From: CSVAX.wildbill at Berkeley
To: space-enthusiasts@MIT-MC
Subject: Space Elevator

The concept has been around for quite some time. I first remember encountering
it about 10 years ago, at which time a friend and I did a calculation
about how big it would have to be to support its own weight. As I recall,
using steel as the material (as we did in our test case) requires that the
cable be several times as big around as the earth at its widest point in
order to be 1 inch around at sea level. There are obviously some problems here.

Arthur C. Clarke did an excellent treatment of this idea a few years back
in his novel \\The Fountains of Paradise//. He avoided the problem by inventing
a super-strong material with just the right properties for building such a
device to keep the story going.

------------------------------

Date: 11 Dec 1981 1106-CST
From: Clyde Hoover <CC.CLYDE at UTEXAS-20>
Subject: space elevators
To: space at MIT-MC

	The idea of elevators into space is not new, I recall seeing it
mentioned in a Sunday-suppliment magazine in the mid-60s. For a more
proper treatment (rather than OMNI), read Arthur C. Clarke's 
"The Fountains of Paradise".
-------

------------------------------

Date: 11 Dec 1981 0954-PST
From: Paul Dietz <DIETZ at USC-ECL>
Subject: Launching Laser Costs
To: space at MIT-MC

That figure of $2500 per ton for th energy to launch a payload with 
a laser is probably low, because a gigawatt laser will consume a lot more
than 1 gigawatt of power.  Don't forget R&D costs, either.

To solve the turbulence problem we can put the laser on top of a
mountain, just like a telescope.  In this respect the launcher has an
advantage over a laser weapon, because the weapon has to be able to work
anywhere.  A good choice for a launch site would be one of the Hawaiian
shield volcanoes - there is very little turbulence there.  
-------

------------------------------

From: BRUC@MIT-ML
Date: 12/11/81 14:48:54
Subject: Atmospheric distortion and high power lasers

BRUC@MIT-ML 12/11/81 14:48:54 Re: Atmospheric distortion and high power lasers
To: space at MIT-MC
	If a laser pulse is sufficiently short, the air molecules
won't have time to move in order to affect the index of refraction. 
Atmospheric distortion really wouldn't be a problem. Absorption is a problem
with visble wavelength light under certain weather conditions. Building
it in the desert would be a good solution.

------------------------------

Date:  11 December 1981 15:08 cst
From:  VaughanW at HI-Multics (Bill Vaughan)
Subject:  cost of power for laser launch
Sender:  VaughanW.REFLECS at HI-Multics
To:  Space-Enthusiasts at MIT-MC

You can't assume that big lasers are plugged into somebody's power line,
and therefore you can't use $/KWH that are based on such an assumption.

Here's one way to do it, if you *insist* on using electrical rather than
chemical energy:

Compute the size of a 1GWatt power plant.  Figure out what it costs to
build it.  Assume some realistic interest rate.  (No fair using 3 or 4
percent.)  Add the costs to the cost of your laser.  I hope you also
used a realistic interest rate for the laser.  I think you will find
that your fuel costs are negligible by comparison with your interest
costs.

Maybe you can get some of your costs back by agreeing to sell power to
people who don't mind losing it for some time either side of a launch.

Seriously, peaking power is much more costly than baseline load, and you
are talking about a gigawatt of peaking power.

------------------------------

Date:  11 December 1981 18:02 est
From:  Tavares.Multics at MIT-Multics
Subject:  Penthouse please
To:  Space-Enthusiasts at MIT-MC
In-Reply-To:  Message of 11 December 1981 06:02 est from Ted Anderson

The idea isn't exactly new.  Arthur Clarke wrote a medium-sized novel
around this idea several years ago.  More technology from the man who
invented the value of the geosynchronous orbit-- perhaps the NEXT big
value of the geosynchronous orbit?!

------------------------------

Date: 11 Dec 1981 1452-PST
From: Bob Amsler <AMSLER at SRI-AI>
Subject: Laser launches from the Moon or in Orbit
To: space at MIT-AI

If  atmospheric dissipation of laser's  is the major problem
to their  successful  use then  wouldn't  it be  logical  to
propose them as a means  of propulsion in space and from the
moon,  with the shuttle  as the Earth-bound  vehicle. Is the
shuttle's design economical for OTHER than trips through the
atmosphere?, i.e. would we  be likely to use the shuttle for
orbiting the moon or landing on it. Does No Air = No Shuttle
for space travel?
-------

------------------------------

Date: 11 December 1981 2234-EST (Friday)
From: Hans Moravec at CMU-10A (R110HM60)
To: space at MIT-MC, Wedekind.ES at PARC-MAXC
Subject:  Penthouse, Yes Sir!
CC: Hans Moravec at CMU-10A

	Cables for travel to earth orbit are hardly more far out than
chemical rockets or cannon.  They become practical when bulk materials
exhibiting one tenth the laboratory strength/weight ratios of single
crystals become available.  The best materials these days are Graphite
composites and the synthetic Kevlar, both of which have about six
times the strength/weight of steel.  Another factor of six improvement
would do it, well within the measured, not to mention theoretical,
limits for normal matter.  Chemical rockets have the same problem -
only the most energetic fuels will do.  The shuttle's main engines
operate within a factor of two of theoretical limits.

	The anchored cable you mention is the simplest, but also most
expensive, of a whole class of cable based schemes.  In principal you
can build a cable to synch orbit (22,000 mi above surface) and beyond
out of any material.  The secret is to taper it so that the cross
section at each level is just large enough to support the weight of
the cable below.  The cable starts thin on the ground, gets fatter as
you go up, is thickest at synchronous altitude, and thins down again
farther out.  The problem is that the ultimate taper (ratio of cross
section at synch height to that at ground) is exponential in the
strength to weight ratio of the material.  Using steel you would need
a taper of about 10^40.  Kevlar or graphite brings it down to about 10^7
(almost reasonable), and with another factor of six it becomes 10 and
merely a large engineering project.  (The mass ratio of a rocket that can
leave earth is similarly sensitive to the specific energy of the fuel.)
There are at least two novels centering on this concept - Arthur Clarke's
"The Fountains of Paradise" and Charles Sheffield's "The Web Between
the Worlds".

	A very simple variant of the idea which is practical today
is to have long cables spinning in orbit.  A long tapered
Kevlar cable orbiting the sun at the same distance as earth
and spinning about its own axis could, on each half rotation,
boost a payload massing about one hundredth of the cable's mass from a
Venus/Earth minimum energy trajectory to one that takes it to Mars.
The cable would lose orbital velocity in this maneuver, but would
get it back when an equivalent mass used it to get back from Mars
to Venus.  Such inertial energy storage could make the energy
cost of solar system commuting practically zero!  The maneuver
involves docking with the end of the cable, hanging on for about a
half turn and letting go, with your velocity vector much changed.
The docking with the end of the cable could be made as slow and
low g as desired, because a cable with a given delta v capability
can be built either short and fast spinning, for high turnover
but tricky docking, or arbitrarily long and thin and slow.  The
mass remains unchanged as you excercise this tradeoff.

	So in conclusion, another factor of five to ten improvement
in high strength materials will make the space elevator concept in
all its variants not only practical but practically humdrum.
Ten to twenty years at the outside in my estimate, but I suppose
it would not be conservative to count on it.  (And we're all
conservative, right?)

------------------------------

Date: 11 December 1981 2237-EST (Friday)
From: Hans Moravec at CMU-10A (R110HM60)
To: space at MIT-MC, Wedekind.ES at PARC-MAXC
Subject:  Penthouse, P.S.
CC: Hans Moravec at CMU-10A

Further reading available through this net address.

------------------------------

From: FONER@MIT-AI
Date: 12/12/81 01:16:34
Subject: Strong cables

FONER@MIT-AI 12/12/81 01:16:34 Re: Strong cables
To: Space-Enthusiasts at MIT-MC
I got a report about a year ago from DuPont on their interesting
material called Kevlar, which is some five times stronger than steel
for its weight and has some other interesting properties to boot.

They mentioned the "skyhook" idea, but commented that even though
Kevlar was strong, it wasn't \that/ strong.  They did, however,
include some useful engineernig data on Kevlar (of course!) and on the
skyhook idea (which was nice of them).

I'll dig the report out tomorrow and send any useful parts to the
list.

						<LNF>

------------------------------

End of SPACE Digest
*******************