From: pgf@srl01.cacs.usl.edu ("Phil G. Fraering")
Newsgroups: sci.space
Subject: golden oldie: launch idea
Date: 23 Nov 92 22:17:37 GMT
Article-I.D.: cs.By6wtt.I6u.1
Distribution: sci
Organization: [via International Space University]
X-Added: Forwarded by Space Digest
Original-Sender: isu@VACATION.VENARI.CS.CMU.EDU

Here's yet another: - pgf :

The orbiting linear accelerator article (I thought both the article
and the idea were extremely good) was

Roger D. Arnold and Donald Kingsbury,  The Spaceport,
Part 1: Analog v99 #11  November 1979  pp 48:67  and
Part 2: Analog v99 #12  December 1979  pp 61:77

They propose an accelerator length of 600 km subjecting payloads to
5g, with an active stiffening system on the structure.  Neither the
mass nor the complexity is obviously lower than a cable performing
the same task:  Imagine a cable in low earth orbit that spins in
the plane of the orbit so that the spin just cancels the orbital
velocity at the points where the cable tips come closest to the ground.
The cable is like two spokes of a giant wheel that is rolling on the
earth's surface at orbital speed.  A flying machine can now jump
up and grab the cable end at its lowest and slowest point (for a few
seconds the tip is actually stationary with respect to the ground,
just like the portion of the rim of a rolling wheel in contact with the
ground is momentarily stopped).  The cable can actually enter the
atmosphere (and with terminal guidance and high precision, it could
even kiss the ground), so the job of docking with it is simpler than for
the linear accelerator spaceport.  The payload then hangs on to the end,
and lets the cable flip it around to be flung off at high velocity
later.  At the top of the swing the cable tip is moving at twice
the (orbital) velocity of the cable's center of mass, and if the
payload lets go then, it is sent off with a factor of more that sqrt(2)
beyond escape velocity.  The cable loses some orbital momentum in the
process, wich it can regain from incoming payloads, or high specific
impulse engines at its middle, just like the orbiting linac.

Such a non-anchored skyhook can be build low and spinning fast,
or long and orbiting high and turning slow.  If you build one
to orbit at synchronous height, it has most of the properties of
the synchronous beanstalk.  It turns out that there is a lower
orbit which is optimum in the sense that it minimizes the taper
required by the cable.  The length of such an optimum cable is
one third the diameter of the earth (this is a general principle;
cute, huh?).  So we have the cable about 4000 km long, with its
center orbiting 2000 km above the surface.  With a material
that can make a beanstalk with a taper of 100, we can make an
optimum rolling cable like this with a taper of only 10, using
100 times less material for the same payload capacity.  The
rolling cable can hoist 1/50 of its own mass on each touchdown.
Such touchdowns happen every 20 minutes, in succession at six
equally spaced points around the orbit.  The cable is very long
relative to the depth of the atmosphere, and because of the scale
and the cycloidal shape of the tip trajectory, the cable ends
appear to descend from the sky vertically on each touchdown, with
a continuous upward acceleration of 1.4 g.  They stab downwards into the
atmosphere at a tame 2 km/sec, slow to a dead stop for an instant at
their lowest point, and accelerate gently upwards to leave in the same
way. The tip stays in the atmosphere five minutes each touchdown.

The material of the cable (if graphite) has a tensile strength of
at least 3 million pounds per square inch, so one or two square inches
at the cable ends is certainly sufficient for most tasks. The average
cross section would then be about five square inches.  This gives
the whole rolling skyhook somewhat the scale and geometry of a
typical transatlantic telephone cable, except that the graphite
is five times less massive than the copper and steel of the phone cable.

It seems at least possibly cheaper to me than the accelerator, but cost
analyses would have to decide.  The big advantage of the accelerator
is that it can be engineered entirely with known materials and techniques,
while the cable awaits the next increment in high strength materials.

Re: collisions with aircraft, I agree that most of the time a taut 
3 million psi, inch diameter, cable would be to a slow moving aluminum
plane much like a cheese cutter is to a piece of cheese.  Almost all
of the cable is above the atmosphere, however, and a collision at orbital
velocity would be another matter.  The hit probability is no greater
than for a big satellite. The rolling cable is 4000 km long and about
5 cm in diameter.  This gives it the same "frontal" surface area as
a 500 meter diameter sphere.  A collision would not be much of a disaster
on the ground, because the small cable diameter insures that the
cables burns up on reentry (though the sheet of flame across the sky as
several thousand kms burn simultaneously should be interesting).  Still
the cost to the owner (or insurance) and to the payload on the cable
at the time certainly make this event undesirable.  Some kind of
Norad (or coast guard) traffic control or monitoring would seem
worthwhile.  Given a few hours or days warning a skyhook can dodge
a few kilometers, but it will probably be the least maneuverable
object in earth orbit.  It will probably have to be given right of way
most of the time, just as law of the sea gives oil tankers right of way.

Here are a few more skyhook references:

Arthur C. Clarke, The Fountains of Paradise,
    Harcourt, Brace and Jovanovich, 1978.

Charles Sheffield, The Web Between the Worlds, Ace SF, 1979.

Charles Sheffield, How to Build a Beanstalk,
    Destinies Vol 1 #4, Aug-Sep 79, pp 41:68,  Ace books.

Charles Sheffield, Skystalk, Destinies Vol 1 #4, Aug-Sep 79, pp 7:39

Charles Sheffield, Summertide, Destinies Vol 3 #2, Aug 81, pp 16:84


And yet another person doesn't sign his name!

-- 
Phil Fraering
"...drag them, kicking and screaming, into the Century of the Fruitbat."
<<- Terry Pratchett, _Reaper Man_
Newsgroups: sci.space