REM@IMSSS (Robert Elton Maas, this host known locally only) (01/17/86)
dietz@SLB-DOLL.CSNET (Paul Dietz) writes:
D>Even if they aren't used for launching things, tethers may be very
D>useful for generating angular momentum in spinning space structures.
D>For example, a space station could be spun up by extending two very
D>long cables with small reaction engines on the ends. The cables would
D>be spun up and, because of the long moment arms, would acquire large
D>amounts of angular momentum.
Andrew Folkins replies:
AF> Date: 13 Jan 86 19:00:40 GMT
AF> From: ulysses!burl!clyde!watmath!utzoo!utcsri!ubc-vision!alberta!cadomin!and
rew@ucbvax.berkeley.edu (Andrew Folkins)
(That's an awfully long path!! Are you really 9 hops away from the Arpanet??)
AF> Umm, what stops the cables from just wrapping around the station? This
AF> scheme would work, but you would need rigid 'towers' instead of cables,
Indeed, I missed that point in original reading of Dietz's idea, the
idea is nonsense because flexible objects don't transmit torque which
is how levers work in the first place. If you attached rockets that
were set to pull in a certain geographical direction, the tether would
just deform around to where the rocket was pulling straight along the
line of the tether. If you attached rockets that were set to apply
angular momentum, the tether would wind up around the space station
until the rockets were resting against the side of the station. If you
just attached the rockets to the end of the flexible tether without
any control mechanism, they'd gyrate in random directions depending on
momentrary torques due to the twisting/winding/pulling tethers. You
need a lightweight but rigid tower, and during the preceding STS
mission such were put together and taken apart several times to test
in-space construction of such objects. Now that has one more practical
application it seems, supplying rigid moment arms (levers) for
efficiently spinning up a station.
AF> ... the mass needed for these towers might be large enough to
AF> offset any fuel savings.
Perhaps. This will take an engineering decision. With a very bulky
station, very lightweight long beams, and many weeks time to spin up
the station, beam-levering may be a viable technique.
AF> One last point, the cables will be stationary only with respect
AF> to the station, and they will still have a considerable amount of angular
AF> momentum. Retracting them would spin up the station even more, just like
AF> a spinning figure skater.
You can also keep the arms partly extended and use slight extension or
retraction to servo the angular speed of the station to compensate for
motion of people and materials around within the station (walking
toward the center of the station causes the station to spin a little
faster, which may upset some pre-programmed astronomical observations
or ship dockings etc.), as an alterative to having an attitude-control
rocket consuming lots of fuel to do that. Other methods are spinning
weights on end of tethers which are reeled in or out (you don't gain
mechanical advantage, but you can still transfer angular momentum that
way if you reel slowly enough to avoid whiplashing the tethers), or
having a spinning wheel with an electrical motor for moving the
station with respect to the wheel to effectively transfer angular
momentum between the wheel and the station, or having two spinning
wheels at different RPM with gearing between them and the station to
transfer angular momentum with virtually no electrical energy needed
except during startup.
AF> All ideas in this message are fictional. Any resemblance, to any idea,
AF> living or dead, is purely coincidental.
I can make my disclaimer sillier than yours (first header wars, now
disclamer wars, what's next, pornographic multimedia-mail wars??):