Date: Mon, 10 Aug 92 05:05:45    
From: Space Digest maintainer <digests@isu.isunet.edu>
Reply-To: Space-request@isu.isunet.edu
Subject: Space Digest V15 #092
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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)

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Date: Mon, 10 Aug 1992 05:41:54 GMT
From: "J. Zufi" <rda834x@nella4.cc.monash.edu.au>
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 <zowie@daedalus.stanford.edu>
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?"

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End of Space Digest Volume 15 : Issue 092
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