From: ederd@bcstec.ca.boeing.com (Dani Eder)
Subject: Re: Space Elevator II. The story continues.
Date: Jun 13 1995
Newsgroups: sci.space.tech

Andrew Nowicki <anowicki@delphi.com> writes:

>Skyhook has to be made of a material having very high specific
>strength, i.e., Spectra or Kevlar. Both plastics decay rapidly due to
>thermal fatigue and space radiation. Therefore, skyhook is not
>durable.

Sigh, WAKE UP!  Kevlar and Spectra have not been the highest strength
to weight materials for almost a decade.  Currently 1 million psi carbon
fiber (as in Amoco T1000) is the highest strength.  According to
their representatives, they can probably get 1.1 to 1.2 million psi
in a production fiber if anyone needs enough of it to get them to
go through the job of setting up a production line for it.  Most
carbon fiber buyers want either higher stiffness with lower strength,
or will accept lower strength at lower cost.

Carbon does excellently against thermal, but it is brittle and is
attacked by atomic oxygen, requiring a coating in low orbits.

Dani Eder

From: ederd@bcstec.ca.boeing.com (Dani Eder)
Subject: Re: Space Elevator II. The story continues.
Date: Jun 19 1995
Newsgroups: sci.space.tech

Andrew Nowicki <anowicki@delphi.com> writes:

>I wonder if it is possible to make a tether from fibers alone. If the
>fibers are long and their surface is jagged, a braided fiber rope may
>be feasible. On the other hand, uncoated carbon fibers are brittle
>and may be weakened by rubbing against each other.

Solid rocket motor makers have been winding motor cases with essentially
bare carbon fiber for a number of years now.  The typical form it comes
from the fiber maker in is a 'tow' consisting of 12,000 fibers each about
5 microns across.  For handling and protection from atomic oxygen, you
can fabricate a 'tape' consisting of two layers of aluminum foil
sandwiching as many tows laid side to side as you need for strength.
The foil is sealed to the carbon using heat and pressure, both of which
will be far, far lower than the carbon can deal with.  So you might have
a tape 5 mm wide by 0.5 mm thick, with an ultimate strength of 19,000 N
(about 4,200 lb).  Wind this in reels, taking care not to kink it.  
Put aluminum end pieces on the tape so you have a way to connect
lengths of tape together end-to-end or side-to-side.

Dani Eder

From: Dani Eder <ederd@worldnet.att.net>
Subject: Re: space elevator / cable
Date: May 08 1997
Newsgroups: sci.space.tech

Peter Hanely wrote:

> Spectra 1000 has the characteristic length of 315 km!?!
> That might be enough for me to reconsider a tapered tether concept.
> thats 7 times the strongest fiber I had specs for(c. 1960's tech)
> -

And Toray Industries (http://www.toray.com/html/carbon_fibers.html)
T-1000G carbon fiber has a strength of 924,000 psi and a density of
0.0654 lb/cu inch.  Therefore it has a characteristic length of 
14.1 million inches, or 359 km.

There is supposed to be a slightly better version called T-1000 that
has a tensile strength of 1 million psi available.  The last time
I talked to a carbon fiber representative (about 5 years ago), I was
told that the PAN fiber process could probably be pushed to 1.1 to
1.2 million psi if someone wanted to badly enough.  At 1.15 million
psi tensile strength, the characteristic length is 446 km.

The PAN fiber process starts with polyacrylonitrile fibers (a plastic)
and 'chars' them to get pure carbon fiber.  Note that Toray just
announced they are building a carbon fiber plant here in Decaur, AL
(next to Huntsville, AL) with a capacity of 1800 tons of carbon fiber
a year.  I'm not sure if the main reason is that Monsanto has a plant
that makes the PAN fibers next door, or that Huntsville AL is full of
aerospace companies that use carbon fiber a lot, or both.  But it 
sure is nice for a space tether enthusiast to get a factory right
nearby.

Note that much of the carbon fiber produced by Toray is not so much
'high strength', as it is 'high stiffness'.  The high stiffness fibers
are still strong (on the order of 500,000 psi), but they have
extraordinary stiffness, which is important for making things like
jet fighter wings that won't bend or twist under extreme loads.

And remember, a minimum weight space elevator is not a hanging
cable from GEO to the ground, it is a tower up from the ground
meeting a hanging cable from GEO, with the same taper ratio on
both.

Since compression strengths of carbon composites are about 28% of
the tensile strength of the pure fiber, we can calculate theoretical
taper factors as follows:

Equivalent depth of gravity well from GEO to ground in 1 g terms =
6,223 km neglecting orbital motion.  With orbital motion, reduced to
about 6,000 km.  Divide by 1.28 to account for tower, then hanging
portion covers equivalent of 4,687 km at 1 g.  with 446 km scale
length, we have 10.5 scale lengths.  Then the taper ratio is 
e^(10.5) = 36,683:1, or 191.5 in diameter.  If the 'payload' of the
elevator is 1 million lbs, then the cable is one square inch
at the narrow end, and 16 ft square at GEO.

So, with no allowance for factors of safety and parasitic structure
(stuff on the cable besides fiber strands), we arrive at a 
maximum cable diameter that is somewhat bigger than the combined
cables on a large suspension bridge.  So we are already at the 
'theoretical feasibility' point in materials strength.  We have
a ways to go before we reach the 'practical economics' point.

Most of the way from GEO, the cable will be close to the maximum
diameter.  Therefore the cable mass will be something on the
order of 1 billion tons, or about one thousand centuries of
world carbon fiber production at present rates.

Dani Eder