ST401385@BROWNVM.BITNET (01/17/86)
Angular momentum is conserved in space. This means that if rockets
are fired on the ends of long tethers connected to a space station,
it will increase the angular momentum of the system, whether the
tether is a rigid rod, a cable, silly putty, whatever. (Assuming,
however, that a pointing device exists to keep the rocket pointed
in the direction of the desired spin, perpendicular to the station-rocket
line.)
The important question is how the angular momentum gets transfered
to the station if the tether is a flexible line. What will happen in
the simplest case is that the line starts to wrap around the station.
If you visualize the situation for a moment, you will see that while
it does so the tension on the cable exerts a torque on the station
(proportional to the radius of the station divided by the distance
to the rocket.) This torque transfers angular momentum to the station.
When the mass whacks into the station, the sudden jerk on the line
transfers the remainder of the angular momentum.
If the mass sticks, this is the end. Otherwise, the mass will
begin to unwind. Strangely enough, initially this process also
torques up the station. Very shortly, though, the cable tension
goes slack. The mass moves away from the station (by 'centrifugal
force'), but the rotating station unwinds cable faster than the
mass moves out. If you cut the cable at this point, you have a
rotating station. If not, when the mass reaches the end of the string,
WHACK! There's another big jerk, and the station loses a bunch of
angular momentum (This process is known as "Yo-Yo despin." Really.
It's occasionally used on sounding rockets which are spun during
boost, but need to be nonrotating at apogee.)
Of course, that's in the simplest case. In real life strings
are subject to all sorts of vibrations and such when you jerk them.
This whole process would work much better if, instead of letting
the cable wind around the station, you reel it in on some sort of
roller.
---Geoffrey Landis <GL>
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