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From: eder@ssc-vax.UUCP (Dani Eder)
Newsgroups: net.space
Subject: Re: Realistic space elevators
Message-ID: <604@ssc-vax.UUCP>
Date: Sun, 13-Nov-83 00:10:07 EST
Article-I.D.: ssc-vax.604
Posted: Sun Nov 13 00:10:07 1983
Date-Received: Wed, 9-Nov-83 04:31:03 EST
References: <13208@sri-arpa.UUCP>
Organization: Boeing Aerospace, Seattle
Lines: 36

x                                              6 November 1983

     Unfortunately, current materials technology does not allow the
construction of a reasonable geo-synchronous tower.  But you can do
some interesting things with current structural materials.

     The key concept to understand in dealing with tall structures
is 'scale height'.  In a tower made of a given material, it is
the maximum height a constant section column can be built, and can
be found by dividing the compressive strength (lb/in**2) by the
density (lb/in**3).  The result is in inches.  In a cable hanging
from the sky, it is the maximum length a constant section cable
can be before breaking, and is found the same way.

     Some examples: Steel 240,000 psi , .3 lb/in**3 = 800000 in = 12.6 mi
         Kevlar 3650 MPa , 1500 kg/m**3 , 9.8 m/s**2 (note, 1 gravity
 is assumed in english units, must be explicit in metric) = 248 km

     If you want to build a minimum weight structure taller than one
scale height, you taper the column or cable by a factor of e (2.718...)
per scale height.  One interesting material is graphite reinforced epoxy,
which we use here at Boeing in commercial airplanes.  The scale height,
allowing a factor of saftey for real world design, is 50 km.  It is
quite possible to build a tower that reaches into low earth orbit.

     Fortunately, as you get away from the earth's surface, gravity
is less, so the scale length increases.  Unfortunately, the number
of scale lengths to GEO, even for kevlar, is 26.  This means the tower
has to weigh e**26 = 1.45x10**11 times the 'payload'.  If we could
grow saphhire fibers, with a theoretical strength of 2.8 million psi,
then the tower would only weigh 5000 times the payload, which would
make it a feasible transportation system.

                                                 Dani Eder
                                                 Boeing Aerospace