Space & Astronomy/Space and Astronomy (October 1993).iso/pc/text/spacedig/v13_6/v13_645.txt

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Subject: SPACE Digest V13 #645

SPACE Digest                                     Volume 13 : Issue 645

Today's Topics:
           Re: Galileo Antenna (was Re: Amputation)
              Re:  SPACE Digest V13 #556
         Re: Self-sustaining infrastructures
           Re: Galileo Antenna (was Re: Amputation)
          Re: Extra Terrestrial Intelligence
              Re: SPACE Digest V13 #541
      Re: Fred vs. Exploration: head-to-head competition
       Beginner's guide to space plasma physics wanted
         Re: Self-sustaining infrastructures
   What Kennedy forgot to say... (was: Re:  SPACE Digest V13 #556)
    Refueling satellites (was Re: Self-sustaining infrastructures)
               Re: satellite refuelling

Administrivia:

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----------------------------------------------------------------------

Date: 27 May 91 02:37:09 GMT
From: world!ksr![e-mail]  (Chris Jones)
Subject: Re: Galileo Antenna (was Re: Amputation)

In article <[e-mail]>, akerman@qucis (Richard Akerman) writes:
>In article <[e-mail]> [e-mail] (Doug Mohney) writes:
>>
>>You are implying that Galileo's antenna deployment problems are a direct
>>result of the trajectory. or that flying it on Shuttle damaged it. Could you
>>please produce some direct evidence of this? I wasn't aware that JPL had
>>figured out why the antenna was gorked.
>>
>>Could be just a part which failed. Could be a lot of things. I'm sure both sci.
>>space and JPL would appreciate the documentation of failure and why.
>
>Well, certainly the high-gain antenna cover was necessary because of the Venus
>flyby, but I guess the question is whether the "umbrella" design of the
>HG antenna was used because of the flyby or if it had been intended for the
>spacecraft even if it had been launched by a Centaur upper stage from the
>shuttle.  Anyone know?

I believe any high gain antenna with Galileo's data rate and operating from
Jovian distances is going to be larger than will fit into any standard payload
fairing.  It's pretty much a given that it will require some sort of compact
stowage and subsequent deployment.  This particular design is the same as is
used for TDRS satellites, I recall.  It wasn't changed between the
Shuttle-Centaur direct flight plan and the Shuttle-IUS VEEGA flight plan, save
for the addition of the sun shield and the delay in deploying it.  The unusual
aspect of Galileo's mission is the length of time the antenna is kept
undeployed, and I'm sure there is concern that this caused or contributed to
the deployment problem.  As has been pointed out, though, there are many
possibilities, and JPL seems to be taking a very cautious approach to resolving
the problem (as well they might, since they do have time, and don't want to
make things worse by hasty action).

--
Chris Jones    [e-mail]    {uunet,harvard,world}!ksr!clj

------------------------------

Date:    Sun, 26 May 91 03:50 EDT
From: Mike Bishop                          <[e-mail]>
Subject: Re:  SPACE Digest V13 #556



.....special session of Congress

     May 25, 1961


  "I believe that this nation should commit itself to achieving
the goal, before this decade is out, of landing a man in the
moon and returning him safely to earth."

      - John F. Kennedy




-------------------------------------------------------------
Mike Bishop              "If your on the right track, you'll
<[e-mail]>      get run over if you just sit there"
SAS Institute Inc.
Cary, NC                       - Will Rogers
--------------------------------------------------------------

------------------------------

Date: 26 May 91 17:51:58 GMT
From: swrinde!cs.utexas.edu!sun-barr!newstop!exodus!concertina.Eng.Sun.COM![e-mail]  (Steve Hix)
Subject: Re: Self-sustaining infrastructures

In article <[e-mail]> [e-mail] (Henry Spencer) writes:
>In article <[e-mail]> [e-mail] (Frank Crary) writes:
>>On this note, I would like to mention that the lifetime of a satellite in
>>Low Earth orbit is limited by its fuel supply...
>
>Another issue here, by the way, is that satellites fuelled by hydrazine also
>suffer from slow degradation of the catalysts they use to break it down in
>their thrusters.  So just refuelling them has limits.
>
>>... Can anyone comment on the viability of a
>>for-profit Low Earth orbit infrastructure for the purpose of re-fueling
>>satellites?
>
>constellation.  Refuelling geostationary comsats would be much more promising.
>Getting from one side of Clarke orbit to the other is not quick, but it *is*
>relatively cheap, and there usually is plenty of advance warning that a bird
>is running low.  Of course, you'd have to convince the comsat builders to
>start providing for refuelling.

Don't just refuel them: build the comsats with removable attitude-control packs.
Pop out the old one, pop in the new one, and take the old one back for refurbishing
and refueling (so as to deal with the catalyst degradation problem, etc. at the
same time).

Btw, would it make sense to build Clarke-orbit comsats with gravity-gradient
stabilization?  Or do they *really* have to have station-holding capability?




--
------------
  The only drawback with morning is that it comes
    at such an inconvenient time of day.
------------

------------------------------

Date: 27 May 91 01:22:11 GMT
From: agate!lightning.Berkeley.EDU![e-mail]  (Frank Crary)
Subject: Re: Galileo Antenna (was Re: Amputation)

In article <[e-mail]> [e-mail] (Richard Akerman) writes:
> ...but I guess the question is whether the "umbrella" design of the
>HG antenna was used because of the flyby or if it had been intended for the
>spacecraft even if it had been launched by a Centaur upper stage from the
>shuttle.  Anyone know?

The high gain antenna was designed and built long before the Challenger
accident. No one at the time had ANY idea that a Venus fly-by would be
involved. The addition of the fly-by had NOTHING to do with the antenna
design.

                       Frank Crary

------------------------------

Date: 26 May 91 18:27:39 GMT
From: agate!earthquake.Berkeley.EDU![e-mail]  (Frank Crary)
Subject: Re: Extra Terrestrial Intelligence

In article <[e-mail]> [e-mail] (Steinn Sigurdsson) writes:
>Open clusters would be fairly bad for a civilization with a long term
>view, too violent and too short lived. If the halo population has
>planets it would be the best bet, although the stars may be too red
>and dim. In either case there are plenty of second generation G
>dwarfs, and we _know_ they can suffice.

Clusters are only violent and short lived on an astrononical time scale.
On the scale of a civilization, they are quite constant. Remember, ALL
human civilization is less than 10,000 years old. As long as a cluster
doesn't change much on a scale of 100,000 to 1,000,000 years (which it
doesn't, unless it is VERY young and still has alot of O and B stars...)
I don't think colonists would mind. (Note this is based on a extrapolation
of social behavior resulting from a single data point, e.g. mankind.)
Along a similar line, ALL teraforming concepts are "short lived:" they
create a Earth-like environment on Mars or Venus or whatever that will last
at most 10 million years. People still talk about them as if they were
perminant.

Halo population stars are, in general, metal-poor and would not, in all
probability, have any planets.

                           Frank Crary

------------------------------

Date: 26 May 91 18:22:53 GMT
From: usc!jarthur!nntp-server.caltech.edu!hamlet.caltech.edu![e-mail]  (Lydick, Carl)
Subject: Re: SPACE Digest V13 #541

In article <[e-mail]>, [e-mail] (Walter Dnes    ) writes...
>     So we no longer burn a hydrogen-oxygen mixture in the shuttle's
>rockets... instead we react hydrogen & oxygen in a fuel cell to
>generate electricity to power the motor that reels/unreels the
>tether. I still fail to see the difference. Now, how do you get the
>hydrogen/oxygen up to the station ?
>
>> >shuttle will tend to remain in the same orbit as the station, *UNLESS
>> >IT FIRES ITS ROCKET ENGINES TO DE-ORBIT*. That's what the shuttle has
>> >to do right now.
>>
>> It doesn't have to fire it's engines.  It has to expel enough mass at
>> high enough speed to match the momentum change it wants.  Rockets are
>> traditional, but in this case, you could use the station itself as
>> 'reaction mass'.
>
>     Assuming that the station can maintain its structural
>integrity through several years of heavy-duty jolting to kick
>the shuttles out, *WHERE DOES THE ENERGY FOR THE "KICK" COME
>FROM* ? A magnetic-repulsion launcher would require electricity,
>which would take us back to fuel cells.

The energy comes from lowering the shuttle back to the lower orbit prior to
reentry.  This, of course, requires that you have some way to store the energy.
There will, of course, be losses.  But using maneuvering rockets, the losses
are 100%, so you're bound to save SOMETHING using a tether.  You don't have to
kick the shuttle hard enough to drop it all the way to the lower orbit; just
hard enough that tidal forces can maintain a tension on the tether.
--------------------------------------------------------------------------------
Carl J Lydick    HEPnet/NSI: SOL1::CARL    Internet: [e-mail]

------------------------------

Date: 26 May 91 17:53:57 GMT
From: swrinde!sdd.hp.com!spool.mu.edu!news.nd.edu!mentor.cc.purdue.edu!pop.stat.purdue.edu![e-mail]  (Herman Rubin)
Subject: Re: Fred vs. Exploration: head-to-head competition

In article <[e-mail]>, [e-mail] (Charles Radley) writes:

            ................

>     The makeup of comets......I would rather have skin makeup for my
> wife.    Really Mr. Szabo, your attempts at justifying space science
> as being somehow more worthwile than manned spaceflight are
> absolutely parochial to the point of silliness.

Radley and Szabo have illustrated my point that the government should
not be in the business at all.  Different people want different things,
and have different priorities, and it is important that governments not
decide what should and should not be done.  Let those who wish to support
exploration support exploration, those who wish to support man is space
support it, and those who wish to support welfare project support them.

We need diversity, and not direction by bureaucrats or ignorant Congress-
people.  The funding should not come from taxes, but from those who wish
to invest or donate to the cause.  There will be cooperation between the
various groups, as there is now, and there will not be the problem of
trying to get majority support, or even convince politicians.
--
Herman Rubin, Dept. of Statistics, Purdue Univ., West Lafayette IN47907-1399
Phone: [phone]
[e-mail] (Internet, bitnet)   {purdue,pur-ee}!l.cc!hrubin(UUCP)

------------------------------

Date: 26 May 91 22:49:52 GMT
From: agate!spool.mu.edu!uwm.edu!zaphod.mps.ohio-state.edu!wuarchive!rex!rouge!pc.usl.edu![e-mail]  (Phil Fraering)
Subject: Beginner's guide to space plasma physics wanted

I'm looking for a good book on plasma physics for a senior level
Physics student that can at least partially explain magnetic fields
entrailed in plasmas and their behavior.


I'd also like a good introduction to the work of Hannes Alfven (please
excuse the plain ascii...).

Thanks a lot!!! Post or e-mail, depending on your preference.

Phil Fraering || Usenet (?):[e-mail] || YellNet: 318/365-5418

Standard disclaimer, whatever a disclaimer is, applies.

''It hardly mattered now; it was, in fact, a fine and enviable
madness, this delusion that all questions have answers, and nothing is
beyond the reach of a strong left arm.`` - Larry Niven and Jerry
Pournelle, _The Mote in God's Eye_

------------------------------

Date: 27 May 91 01:33:44 GMT
From: agate!lightning.Berkeley.EDU![e-mail]  (Frank Crary)
Subject: Re: Self-sustaining infrastructures

In article <[e-mail]> [e-mail] (Steve Hix) writes:
>
>Don't just refuel them: build the comsats with removable attitude-control packs.
>Pop out the old one, pop in the new one, and take the old one back for refurbishing
>and refueling (so as to deal with the catalyst degradation problem, etc. at the
>same time).
>
I think refueling would be easier to do, even if it might be a less effecient
way to extend the lifetime of satellites. Replacing the station keeping
system would invlove, at least, screwing in bolts. Refueling just requires
connecting a hose. This would be easier to do teleroboticly, and may even
be possible with a satellite not designed for refueling (or failing that,
it would require very little re-design.)

>Btw, would it make sense to build Clarke-orbit comsats with gravity-gradient
>stabilization?  Or do they *really* have to have station-holding capability?
>
They REALLY need station keeping. This is to keep the same position in orbit.
Without this, the satellite would wind up in a geostationary orbit over the
Indian Ocean (the gravitational minimum potential point in the geostationary
orbit.) Gravity-gradient stabilization would only effect the attitude of the
spacecraft, not its orbit. In any case, this effect, as well as other "tidal"
forces, decreases with the CUBE of distance. As a result, it is VERY weak
at geostationary heights.

                      Frank Crary

------------------------------

Date: 27 May 91 00:41:05 GMT
From: usc!zaphod.mps.ohio-state.edu!uwm.edu!csd4.csd.uwm.edu![e-mail]  (Mark William Hopkins)
Subject: What Kennedy forgot to say... (was: Re:  SPACE Digest V13 #556)

In article <[e-mail]> [e-mail] (Mike Bishop) writes:
>     May 25, 1961
>
>  "I believe that this nation should commit itself to achieving
>the goal, before this decade is out, of landing a man in the
>moon and returning him safely to earth."
>
>      - John F. Kennedy

"... and we should follow it up with sustained exploration of the moon, leading
eventually to the establishment of lunar bases.  With this experience, I
believe that we shall have the capacity to forge ahead to land a man and woman
on Mars BEFORE THE END OF THIS CENTURY, and settle the first colonies shortly
thereafter."

This is what he would have ultimately said if he had lived...

America lost its sense of direction in the 1960's.  As a people, we no longer
knew why we were in Vietnam, we no longer knew why we were going into space,
Congress no longer knew how to maintain finances properly :), pluralism becamse
a curse on the future instead of a cause for future celebration.

All this could be traced, in one significant stroke of insight, to the loss of
vision ... ultimately to the loss of the man in '63.

Hey, that's what you get for overreliance on someone else's vision!  It might
be called leadership addiction. :)

------------------------------

Date: 27 May 91 03:47:05 GMT
From: news-server.csri.toronto.edu!helios.physics.utoronto.ca!aurora.physics.utoronto.ca![e-mail]  (Christopher Neufeld)
Subject: Refueling satellites (was Re: Self-sustaining infrastructures)


   On the subject of refueling satellites, can anybody tell us whether
the fuel tank is usually kept in the spun or despun section of a
satellite with both? Some tanks are equipped with a forcing membrane to
feed the fuel. Are any fed centrifugally, possibly with a gas injector
to maintain a constant pressure? In short, would there be any
difficulties with putting the fuel system on an inertially fixed
platform? If some of the thrusters have to go on the spun section this
must result in some fancy plumbing at the bearing.


--
 Christopher Neufeld....Just a graduate student  | There no place like $FC58
 [e-mail]    Ad astra! | They're $FF69-ing my
 cneufeld@{pnet91,pro-cco}.cts.com               | every word! Send for a
 "Don't edit reality for the sake of simplicity" | free $A56E.

------------------------------

Date: 27 May 91 22:20:54 GMT
From: swrinde!zaphod.mps.ohio-state.edu!rpi!news-server.csri.toronto.edu!utzoo![e-mail]  (Henry Spencer)
Subject: Re: satellite refuelling

In article <[e-mail]> [e-mail] (Fraering Philip) writes:
>\new development (pun unintentional) in comsat thrusters is using resistance
>/grids or electric arcs to superheat the exhaust from a hydrazine thruster.
>
>Couldn't an electric heating element be used as a catalyst?

I'd expect so -- the stuff does decompose if you heat it enough -- but there
may be some problem with that.  The resistojet-boosted hydrazine designs
I've seen do seem to make catalysis and heating separate steps, if memory
serves.  I don't know why offhand.

Actually, I can think of one possibility -- the conditions of decomposition
influence the chemical composition of the exhaust, and hence the exhaust
velocity -- but I don't know whether that applies.
--
"We're thinking about upgrading from    | Henry Spencer @ U of Toronto Zoology
SunOS 4.1.1 to SunOS 3.5."              |  [e-mail]  utzoo!henry

------------------------------

End of SPACE Digest V13 #645
*******************