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Date: Sun, 28 Apr 91 01:45:09 -0400 (EDT)
Subject: SPACE Digest V13 #474
SPACE Digest Volume 13 : Issue 474
Today's Topics:
Re: ALS vs. Saturn V
Writer needs help getting his facts straight.
Re: NASP
Re: Saturn V and the ALS
Orbital Elements of Saturnian system
Re: Shuttle Reliability (was: Re: Saturn V and the ALS)
Re: Incentives
Re: Saturn V and the ALS
Re: Transportation Tethers (Beanstalks)
Alexander Abian wants to blow up the moon?
NASP
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>>* Given a medium-sized comet or carbonaceous chondrite asteroid captured
>> into GEO and a mature space manufacturing industry, a Kevlar "beanstalk"
>> might be built from GEO all the way to the Earth's surface. The
>
>>The main research problems are creating high-tensile, low mass fibers
>>beyond Kevlar for such structures, and learning how to manufacture them
>>from space materials.
>>
>With all the speculation I read about beanstalks, I've yet to hear
>someone say we currently have developed the materials technology to
>build one, yet you seem to be saying that Kevlar has the requisite tensile
>strength. Is this right? I thought beanstalks were waiting for a
>quantum leap in science, like the monomolecular filaments Arthur
>Clarke posited in "Gardens in the Sky" (or a title much like that).
Well, the other day I was thinking about beanstalks (btw, what are
the differences between beanstalks, skyhooks, tethers, etc.? Are
beanstalks and skyhooks just different names, or are skyhooks not
anchored to the earth? ... just curious) and worked out the math for
what I think is called a log taper going from the Earth's surface to
somewhere past geosynch. The idea is that you vary the diameter of
the cable as needed to support the tension so you don't spend too
much cable just holding up other cable. I simplified assuming that
the only forces on any segment of the cable are the tensions at either
end, the Earth's gravity, and "centrifugal force" (I know it doesn't
exist, but it holds satellites up...). I got this:
dT
---- = T S^-1 ( G Me / r^2 - phi^2 r ) dr
dr
where T is the tension on the cable at radius r, G and Me are
the gravitational constant and the mass of the Earth, and phi is
the rotational velocity of the Earth in radians/sec. S is
the cable's tensile strength divided by its density (which I
think is called the specific tensile strength). These numbers
come out to:
G*Me = 3.99 * 10^14 N*m*m/kg
phi = 7.27 * 10^-5 rad/sec
Tensile strength of Kevlar ~ 3 * 10^9 Pascals
Density of Kevlar ~ 1.5*10^3 kg/m^3
S = 2*10^6 m^2/s^2
(actually, my source here is unclear, this is probably a Kevlar-
epoxy composite; it will do for ballpark computation...)
Anyway, the equation above solves to this mess:
G Me phi^2 r^2
------- + ---------
T^-1 = k e^ S r 2 S
Setting the tension at sea level to, oh, 10^9 N (this being a
margin for safety and for payloads), k works out to 2.5 * 10^-23 1/N.
Then the tension at geosynchronous orbit (r=4.23*10^7 meters) is
3.4 * 10^19 N, for a cable diameter of about 60 kilometers. Yikes.
(The cable tension and diameter are largest at geosynch, for reasonably
obvious reasons.)
Shedding a few orders of magnitude, if the sea level tension
is 10^6 N (100 tons), k=2.5 * 10^-20 1/N, and the tension at geosynch
is 3.4 * 10^16 N, diameter maybe 2 km. Slightly more reasonable
but still pretty farfetched.
Note that the dependence on S is (like everything else) exponential.
A single order of magnitude improvement in S would reduce the cable
diameter at GEO to a bit over four feet! Given, an order of magnitude
*is* a "quantum leap in technology", but less than an order of
magnitude will still have a very good effect on cable size...
I'm also leaving out such things as structural redundancy for what
amounts to a thirty-thousand-mile-high skyscraper, shielding it from
weather (and terrorists), maintenance, insurance (what if it breaks
and falls on your insurer?) ... not to mention the incredibly
difficult job of putting the beanstalk *up* in the first place.
--
wiml@milton.acs.washington.edu Seattle, Washington
(William Lewis) | 47 41' 15" N 122 42' 58" W
"Just remember, wherever you go ... you're stuck there."
------------------------------
Date: 26 Apr 91 21:31:35 GMT
From: agate!bionet!uwm.edu!zaphod.mps.ohio-state.edu!pacific.mps.ohio-state.edu!linac!midway!ellis.uchicago.edu!esti@ucbvax.Berkeley.EDU (Paul A. Estin)
Subject: Alexander Abian wants to blow up the moon?
Very late last night (around 4 or 5 AM) I tuned in to CBS's
"Nightwatch" news program in the middle of a "debate" between Bevan
French of NASA and some crazy loon at Iowa State University named
Alexander Abian.
My question is basically... who IS this Abian guy and what
exactly is he trying to say? Maybe it was made clearer in the
segment's beginning (which I missed), but while I watched, Abian was
ranting maniacally about changing the system of the Universe and
moving satellites to Earth orbit and blowing up the moon to change the
Earth's axis... or something like that. He wasn't terribly clear.
Abian also used some weird analogies, such as {refusing to accept the
possibility of working to a different possible configuration for our
solar system} is like {refusing to accept the possibility of
nonrealistic painting like Picasso's} (to which French replied,
"Having a nose on the side of the face may be nice artistically, but
on a real person it's rather hard to breathe that way."). More
generally, he seemed more of an "artiste" gone wild than an
astrophysicist.
Now, has anyone heard of this guy Abian, and does anyone
understand what he's talking about? He's one of the more interesting
nuts I've come across.
-----
"Think of terraforming as a very large example of performance art."
