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From: hogg@utcsri.UUCP (John Hogg)
Newsgroups: net.space
Subject: Re: Another Use for Tethers
Message-ID: <1976@utcsri.UUCP>
Date: Tue, 21-Jan-86 17:02:18 EST
Article-I.D.: utcsri.1976
Posted: Tue Jan 21 17:02:18 1986
Date-Received: Tue, 21-Jan-86 17:09:39 EST
References: <8601161312.AA15050@s1-b.arpa> <98@dg_rtp.UUCP> <6307@utzoo.UUCP>
Reply-To: hogg@utcsri.UUCP (John Hogg)
Organization: CSRI, University of Toronto
Lines: 39
Summary: 

In article <6307@utzoo.UUCP> henry@utzoo.UUCP (Henry Spencer) writes:
>> The notion is to extend two tether ends (with some small mass or other
>> on each) away from the station, one primaryward, the other
>> anti-primaryward.  Tidal forces will produce a tension on the string...
>> Then merely use a solar powered electric motor (with the tension on the
>> tether as an "anchor") ...
>
>Not the space telescope, but a number of satellites are stabilized this
>way.  It's called "gravity gradient" stabilization, because the reason
>for the tidal forces is that the primaryward and antiprimaryward masses
>are at different distances from the primary and hence feel slightly
>different gravitational accelerations from it.  Tethers probably would
>not do; you need rigid or semi-rigid structures to transmit torque...

Actually, tethers might provide an advantage here.  You need a solid lever
arm to change a force into a torque.  This should ideally be as large as
possible.  However, if it must be small, a tradeoff can be made made by
increasing the force, which can be done by creating a large gravity
gradient via a long tether.  In other words, a case can be made for a
small (=> light) satellite, a small (=> also light) plumb bob and a
LONG string.  Kevlar cable will weigh less than a rigid boom providing
the same stabilizing torque.

I have no idea what vibrational properties a design of this type would
have, and that could be the most serious problem.

Note, by the way, that two balancing tethers are not required.
Furthermore, for a given amount of cable, it will be far more efficient to
have one long tail (up or down, dealer's choice) than two small ones; in
fact, for half the plumb bob mass (one instead of two) you'll get twice the
gradient force.  There's an r^2 in there.
-- 

John Hogg
Computer Systems Research Institute, UofT
...utzoo!utcsri!hogg

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