skyhooks again

From: spike66 (spike66@attbi.com)
Date: Tue Feb 11 2003 - 22:47:25 MST

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    Damien Broderick has written story which has a
    skyhook being grown by a nanoreplicator. As I
    read the story, one of the many difficulties with
    making an orbit-synchronous cable occurred to
    me: that the cable would need to get an enormous
    amount of angular momentum from somewhere in order
    to keep one end pointed at the planet.

    Imagine a planet with a high-carbon satellite
    conveniently located in synchronous orbit. This
    satellite is tidelocked to the planet, that is
    the same face always points toward the planet.
    So the sidereal rotation rate of the satellite
    is one rev per "day".

    Imagine nanobes on the satellite decide to build a
    diamondoid cable down to the surface. As they
    do so, they manufacture and pay out cable into
    space to maintain the CG at synchronous altitude.

    By the law of conservation of angular momentum,
    the sidereal rotation rate of the satellite will
    slow down as the moment of inertia of the satellite
    goes up. One can imagine that a completed cable/
    satellite system would have a jillion times the
    moment of inertia of the satellite without the
    cable, so the sidereal rotation would have slowed
    a jillionfold, so the cable is no longer synchronous.
    The satellite still hangs in one spot over the
    planet, but appears to be rotating about its
    axis at about one revolution per day.

    One way to deal with this is to spin the satellite
    at a jillion revs per day before the construction
    begins, which presents its own practical difficulties.

    I wrote a paper once before about the problems
    with making a geosynchronous cable, but I just
    thought of this new one yesterday.

    Has anyone seen a study which deals with this
    angular momentum problem? Anders, Robert, have any
    of you space cable fans recognized this problem?

    spike



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