Date: Mon, 10 Aug 92 05:05:45 From: Space Digest maintainer Reply-To: Space-request@isu.isunet.edu Subject: Space Digest V15 #092 To: Space Digest Readers Precedence: bulk Space Digest Mon, 10 Aug 92 Volume 15 : Issue 092 Today's Topics: Info on Challenger accident Modified Tether physics (was Re: Physics of shuttle & tether) Welcome to the Space Digest!! Please send your messages to "space@isu.isunet.edu", and (un)subscription requests of the form "Subscribe Space " to one of these addresses: listserv@uga (BITNET), rice::boyle (SPAN/NSInet), utadnx::utspan::rice::boyle (THENET), or space-REQUEST@isu.isunet.edu (Internet). ---------------------------------------------------------------------- Date: Mon, 10 Aug 1992 05:41:54 GMT From: "J. Zufi" Subject: Info on Challenger accident Newsgroups: sci.space I was introduced to this news group only last year. When Challenger was destoryed in 1987 (?), was this newsgroup around, and if so were there official responses/analysises of the accident posted? If so, where could I find them? Thanks. -- [ Jonathan Zufi | rda834x@nellads.cc.monash.edu.au [ Robotics & Digital Technology | zuf@yoyo.cc.monash.edu.au [ Monash Univeristy, Caulfield | "A winner never quits; a quitter never wins" [ Melbourne, Australia | traditional. ------------------------------ Date: 9 Aug 92 12:07:08 From: Craig Powderkeg DeForest Subject: Modified Tether physics (was Re: Physics of shuttle & tether) Newsgroups: sci.space,sci.space.shuttle In a well-informed but not-well-thought-out response to my article, seds%cspara.decnet@Fedex.Msfc.Nasa.Gov writes: Craig I don't mean this to flame, just explain tether dynamics from the folks who did the experiments. ... [Well-referenced Description of VxB electrodynamics of the tether deleted] > [my description of large plasma conduction paths deleted] This explanation is wrong for the reasons stated in the equations above. The max emf is generated when you are traveling at right angles to the magnetic field in question not along it. The best orbit is one that is equatorial and very low to increase both v and B to their max values. This statement (no, silly, the one just above! :-) is wrong for *exactly* *the* *same* reasons it cites! The problem to avoid here is that *any* conductive path in the neighborhood of the shuttle, will incur the exact same electromotive force -- so if you ran two wires to make a circuit between the shuttle and the ball, for example: ^ wires |\ <-- shuttle | | | v | |========================================O <-- ball |__\ then the emf would exactly cancel between the two wires, and you'd get no useful voltage out! Clearly this is true for wires in any small configuration about the shuttle in a constant B-field (and the Earth's B-field *is* pretty constant over shuttle-sized distances). The argument also applies to free electrons (ie conduction through the plasma) with paths in the vicinity of the shuttle, as well (since any free electron path could be replaced by a wire of the exact same shape). The whole point of having a long plasma path is to do the cross-field return transit far away, so that on the return trip from the shuttle to the ball, the charges don't lose the energy they gained while coming from the ball. The relevant point in the cited reference is: ... Plasma contactors a the tether ends proved this contact, establishing a current loop (a so called "phantom loop") through through the tether, external plasma, and ionosphere. Although processes in the plasma and ionosphere are not clearly understood at this time, it is believed that the current path is like that shown in figure 3.19. (with no fig. 3.19 attached...). While I don't claim to know everything that's going on in the ionosphere (That comes *after* the PhD :-) my whole argument from before is that the unknown current path mentioned, has to go far from the environment of the shuttle, to avoid the problem I've just described. Meanwhile, Dennis hits home about tether gravity-gradients: This gravity gradient force differential is on the order of magnitude of 1 X 10-7 g wheras the gradient due to the center of mass shift and momentum transfer is on the order of 1 X 10-5 g, a much greater force. Ouch. Serves me right for not thinking about the numbers before posting -- I `knew' that the two effects were there, and simplified the wrong way... But the effect is still (in the Shuttle's reference frame) one of a constant-gradient gravity field. [Engineering details omitted for brevity.] Thanks for the correction and additional information. Here are Dennis's references again: Reference here is the Tethers in Space Handbook -Second Edition- National Aeronautics and Space Administration Office of Advanced Program Development, NASA HQ Code MD (now code DD) Washington, DC 20546 [Book produced under NASA contract # NASW-4341. Edward J Brazill, NASA HQ contract monitor. Also see NASA Conference Publication 2422 Applications of Tether in Space Workshop Proceedings Vols 1 and 2. [Proceedings of a workshop held in Venice, Italy, Octover 15-17, 1985] Hope this helps to clear the confusion up on tether power generation So do I... -- Craig DeForest: zowie@banneker.stanford.edu *or* craig@reed.bitnet "So, if you guys make a living looking at the SUN, why do you spend so much time at the SYNCHROTRON, working UNDERGROUND at NIGHT?" ------------------------------ End of Space Digest Volume 15 : Issue 092 ------------------------------