Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.3 4.3bsd-beta 6/6/85; site ucbvax.BERKELEY.EDU Path: utzoo!watmath!clyde!burl!ulysses!ucbvax!space From: REM@IMSSS (Robert Elton Maas, this host known locally only) Newsgroups: net.space Subject: can't use flexible tether as if it were a rigid lever Message-ID: <8601170609.AA18916@s1-b.arpa> Date: Fri, 17-Jan-86 01:10:10 EST Article-I.D.: s1-b.8601170609.AA18916 Posted: Fri Jan 17 01:10:10 1986 Date-Received: Sat, 18-Jan-86 00:53:16 EST Sender: daemon@ucbvax.BERKELEY.EDU Reply-To: REM%IMSSS@SU-SCORE.ARPA Organization: The ARPA Internet Lines: 67 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??):