Space & Astronomy/Space and Astronomy (October 1993).iso/pc/text/spacedig/v13_6/v13_622.txt

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Subject: SPACE Digest V13 #622

SPACE Digest                                     Volume 13 : Issue 622

Today's Topics:
          Re: Tethers (was Re: Gas Guns and Tethers
          Re: What comes after Fred's death?
               Laser numbers sent to me
             Re: Asteroid mining
             Re: Building Infrastructure
             Re: 2001 and "The Endeavor"
                Re: lifeboats
          Perspective: SS Freedom vs. Space Science
      Re: Fred vs. Exploration: head-to-head competition

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

Date: 21 May 91 13:18:06 GMT
From: usc!zaphod.mps.ohio-state.edu!sol.ctr.columbia.edu!emory!ox.com!fmsrl7![e-mail]  (Ron Carter)
Subject: Re: Tethers (was Re: Gas Guns and Tethers

In article <[e-mail]>, ke4zv!gary (Gary Coffman) writes:
>In article <[e-mail]> [e-mail] (Ron Carter) writes:
[tidal/centrifugal acceleration is 4e-6 m/sec^2/m vertical sep]
>Unless I totally screwed up the integration, it would take 11.5 days
>for the shuttle to drop 50 km in this system. Is that reasonable?

I think not.

Simplifying assumption:  tether tension is set so that there is a
net 10 N downward acceleration on the Shuttle.  Initial separation
is 50 meters, initial tension is 10 N.  Shuttle mass is 1e5 kg.

d = .5 * a * t^2  implies  t = sqrt( 2 * d / a )

d = 5e4 m (minus a bit)
a = 1e-4 m/sec^2

t = sqrt(2 * 5e4m/1e-4 m/sec^2) = sqrt(1e9 sec^2) = 3.16e4 sec = 8.78 hours.

If tether tension was not increased to maintain a constant acceleration,
but was kept to a constant proportion of the tidal/centrifugal force,
then the Shuttle's speed would increase exponentially and this time
would be MUCH shorter.  I haven't worked the diffeq yet, but the
acceleration rises roughly linearly with increasing distance, so...

Would you mind posting your analysis if it's so different?
Perhaps you caught some glitch that I ignored.  The requirement
of rotating the relative velocity vector at omega might make a
big difference, and my above simple analysis would have missed it.
I also didn't take into account the requirement for tidal force
to spin up the tether; as the two masses separate, their angular
inertia increases, and angular momentum must be input to keep its
rate of spin at omega (1/orbit).  This must come from tidal torque;
the lower mass moves forward as spin slows, and the upper moves
backward, yielding a tidal torque increasing the spin.

>I'm confused by the energy balances here. It seems you could get
>electrical energy *or* you could change the station's orbit. How
>are you dividing the energy gain from converting gravitational
>potential energy and orbital kinetic energy (I assume) between the
>reel and the orbit changing?

No, you get both.  The energy input to the reel is the integral
of the tension over the length of extension; the tension is > 0
whenever the two centers-of-mass are not co-incident.  The orbit
change is a result of the tether holding *both* end masses in
disequilibrium; the upper one is moving faster than circular
orbit speed, but the downward pull gives it a greater omega
than it would maintain in free fall.  The lower one is moving
slower than circular orbit speed, but the upward tension keeps
it from falling into a lower orbit and maintains its lower omega.
So long as the two halves are held together by the tether, their
center of mass, somewhere between, is what is really "in orbit".
Oh, yes, they've also picked up some energy of rotation about
their center of mass, due to constant omega but increased angular
inertia about the CG.  I haven't analyzed the system to find
out exactly where this comes from.

When the tether is released, both masses go into elliptical orbits.
The upper one follows an ellipse with the perigee at the point of
release, and the lower follows an ellipse with the apogee at the
point of release.  The upper one goes even higher, the lower one
goes even lower.

>Another thing I can't quite picture is the fact that the shuttle,
>in dropping to a lower orbit, has got to go faster to stay in that
>orbit. It will lead the station by more and more as the cable is
>unreeled. Doesn't the shuttle need more energy in order to go faster?

If the Shuttle were in an inertial frame, you would be correct.
However, the tether tension pulling up accelerates it, offsets
the force of gravity, and allows it to maintain a circular path
while moving slower than circular-orbit speed.  So, as long as
the tether remains vertical, as the Shuttle goes down, it also
slows down, and stays slower until it lets go of the tether.

>Isn't this the energy gained by the shuttle in dropping to the lower
>orbit? Doesn't that mean there is none left over to raise the station's
>orbit or generate electricity from turning the reel? Have I completely
>misunderstood something?

To that last:  yup.

If you go back to your orbital mechanics, you'll find that the
kinetic energy >gained< by going to a lower circular orbit is
exactly 1/2 the gravitational energy >lost< in dropping lower.
If the Shuttle drops lower, but doesn't pick up the speed
commensurate with the drop (it's part of a larger assemblage
with a center-of-mass somewhere above), guess where the "lost"
kinetic energy goes?  It goes into the tether reel and upper mass.

>I just reread what I wrote in the last paragraph. I must be missing
>something, *my* questions sound screwy to me. The shuttle can't gain
>enough energy from a drop (delta h) to increase it's orbital speed
>(delta v) enough to establish a stable orbit. If it did, it wouldn't
>drop anymore. What's the real geometry of the situation?

Well, if you let the tether *go*, you'd establish a stable elliptical
orbit (assuming it doesn't intersect something, like air or mountains).
The reason your questions sound screwy is that you're trying to apply
orbital mechanics to a part of the system which does not contain the
center of mass (it is not in free fall), and conservation laws to an
open system (half of a system connected by a tether, which transfers
momentum and energy).

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

Date: 21 May 91 07:52:50 GMT
From: agate!maelstrom.Berkeley.EDU![e-mail]  (Frank Crary)
Subject: Re: What comes after Fred's death?

In article <[e-mail]> [e-mail] (Steve Willner) writes:
>Hasn't it occurred to anyone that life sciences research (keeping a
>couple of astronauts in orbit for a year or so) can be done without a
>space station?  All that should be needed are another couple of
>shuttle orbiters, extended duration modifications (up to 30 days is
>already planned), and rendezvous capability.
>

Not really. First of all, it would be nice to have a large sample of
human subjects, e.g. 4 or more per year. To do this by shuffeling
people back and forth between orbiters would take up over half the
crew of the shuttle. This would mean loosing the bulk of the research
talent on each shuttle mission. Second, it would be a real mental
hardship (something I generally like to ignore.) for someone to
spend a year in such a confined space (or series of small, identical
confined spaces.) I doubt that it would be considered a good idea by
the NASA phych people. Third, and most important, it would require
12 flights a year of the Space Shuttle system. The greatest number
of launches to date is, I believe 9. Even with additional orbiters,
the rate could not go above 14 (without building a new Vehicle Assembly
Building, Shuttle Processing Facility, etc...) And 14 flights assumes
everything goes perfectly. I doubt the Shuttle could do it.

Anyway, I would rather see, say, 50 rats as the subject. The sample size
is larger, and you can disect the rats after the flight.

                      Frank Crary
                      UC Berkeley

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

Date: 21 May 91 17:59:12 GMT
From: agate!spool.mu.edu!sdd.hp.com!caen!ox.com!fmsrl7![e-mail]  (Ron Carter)
Subject: Laser numbers sent to me

I'm sure Doug wouldn't mind me posting this information rather
than having to sum it up in my own words.
=====================================================================
Date: Tue, 21 May 91 11:45:46 -0500
Message-Id: <[e-mail]>
From: sharkey!citi!gatech!aries.scs.uiuc.edu!mcdonald (Doug McDonald)
To: fmsrl7!wreck (Ron Carter)
Subject: Re: Laser launchers

In article <[e-mail]> [e-mail] (Ron Carter) writes:
>Incidentally, the characteristic impedance of free space is about
>377 ohms per square, if memory serves.  1 KW/cm^2 implies an RMS
>electric field strength of 614 volts/cm, which is nowhere near
>the dielectric breakdown point of air.  100 GW/m^2 is 10 MW/cm^2,
>implying an RMS FS of about 61 KV/cm.   That's where I'd start
>to worry about things turning to plasma.

Examples: a 10 joule 1 microsecond CO2 laser produces 10 megawatts. This
produces breakdown in air at a cross section of roughly 1 square millimeter,
or 1 gigawatt per square centimeter, 10^15 watts per square meter.

At 20 gigawatts per square centimeterat 1.06 microns I have not observed
breakdown, for 10^-10 second pulses.

HOWEVER, this is really an unimportant point for laser launchers. Those
figures are  for air at ONE ATMOSPHERE. At lower pressures the powers get less -
much less indeed. I have no hard numbers, though. You probably
could count on not having breakdown problems up to maybe 5 or 10 miles.
Then, of course, at say 50 or 100 miles up again lasers will for sure
propagate nicely.

Doug McDonald
=====================================================================

[Ron again.]

It appears that a system running a peak power of 1 GW/m^2 would have
a rather large safety factor before breakdown of air becomes a problem.
If necessary, it could run higher power levels early and late, and
coast through troublesomely conductive parts of the atmosphere.

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

Date: 21 May 91 19:33:31 GMT
From: agate!spool.mu.edu!think.com!zaphod.mps.ohio-state.edu!rpi!news-server.csri.toronto.edu!utzoo![e-mail]  (Henry Spencer)
Subject: Re: Asteroid mining

In article <[e-mail]> [e-mail] (James Davis Nicoll) writes:
>    The Sudbury asteroid has two advantages that allow it to produce
>nickel at a lower cost than nickel produced by exploiting other asteroids.

Just to nit-pick:  it is not in fact clear that the Sudbury mines are mining
an asteroid.  The ore body is clearly the result of an impact, but whether
it is the remains of the impacting body, or the remains of metal-rich magma
released from deep in the crust by the impact, was not settled last I heard.
--
And the bean-counter replied,           | Henry Spencer @ U of Toronto Zoology
"beans are more important".             |  [e-mail]  utzoo!henry

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

Date: 22 May 91 03:57:48 GMT
From: cis.ohio-state.edu!zaphod.mps.ohio-state.edu!uakari.primate.wisc.edu!dali.cs.montana.edu!ogicse!sequent!muncher.sequent.com![e-mail]
Subject: Re: Building Infrastructure

In article <[e-mail]> [e-mail] (Henry Spencer) writes:

>My understanding has always been that there was
>no particular market for *air* mail before the Kelly Act.  Customers just
>didn't think the faster delivery was worth anything to them.

This is a nit.  Mail was a very large current market, and it was well
known and much desired that mail be delivered as quickly as possible.
Airplanes were proven as mail carriers before the Act.  Furthermore,
mail was already a government business, like defense.  Finally,
"bean counters" -- people who actually do the arithmetic instead
of daydreaming -- did market surveys and worked out the dollar figures.

When was the last time somebody did a marketing survey and revenue/cost
statement for a space station?


--
Nick Szabo                [e-mail]
"If you understand something the first time you see it, you probably
knew it already.  The more bewildered you are, the more successful
the mission was." -- Ed Stone, Voyager space explorer

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

Date: 22 May 91 04:31:29 GMT
From: cis.ohio-state.edu!zaphod.mps.ohio-state.edu!uakari.primate.wisc.edu!caen!uwm.edu!csd4.csd.uwm.edu![e-mail]  (Mark William Hopkins)
Subject: Re: 2001 and "The Endeavor"

In article <[e-mail]> [e-mail] (Rick Ottolini) writes:
(2001)
>When I first saw the movie, a year before the moon landing, I thought many of
>the things in the movie would occur by that time.  It looks like they won't.

It looks like they won't in the public sector.  When Kennedy died, America
lost its vision, the last vestige (the moon landing) of his vision spent itself
in short time, and everything went after that.

But don't rule out the private sector by any means.  A few enterprising souls,
not constrained by excessive politicking or ingrown bureaucracies, will make
all the difference probably by even the date mentioned above.

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

Date: 23 May 91 21:51:15 GMT
From: agate!linus!linus!cyclone![e-mail]  (S. J. Okay)
Subject: Re: lifeboats

In article <[e-mail]> [e-mail] (Henry Spencer) writes:
>If all you want is a bit of maneuverability for rendezvous, given an
>accurate launch I think you can get by with my suggestion -- mount the
>SM thruster quads on whatever adapter you're using to hold retros.

This was my original point about lifeboats and my  query about the CM.
A lifeboat is exactly that, its a small shell/can to hold a crew and
recovery device and MAYBE thrusters if picking up your astronauts
wherever they land is a problem.  A small kick-thruster package to
get you pointed in the right attitude for re-entry is about all you
need for a lifeboat. Anything else isn't a lifeboat per se and is
more like a Hermes or something. If you want your Hermes or ECRV
to double as a lifeboat, thats another thing entirely.

---Steve

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

Date: 20 May 91 18:14:31 GMT
From: agate!spool.mu.edu!sdd.hp.com!elroy.jpl.nasa.gov!usc!nic.csu.net!csun.edu!corona![e-mail]  (Stephen Walton)
Subject: Perspective: SS Freedom vs. Space Science

I've scanned (not read thoroughly, don't have the time) the various
back and forth over the recent House action.  As someone who faxed a
letter to the committee asking them to continue funding space science,
I suppose I bear some responsibility for the current situation.
However:  the letter I wrote simply asked the committee to preserve
space science funding.  As I understood the situation of two weeks ago,
the House wanted a $700M cut from NASA, and Truly was taking the
position that this cut should come entirely out of space science
because of the recent de-scoping of Freedom.  A cut of this magnitude,
according to my NASA sources, would have meant the cancellation of
*every* space science mission not already flying, as well as
significant cuts in NASA support of ground-based astronomy.

My letter, which I'd be glad to send the text of to anyone who cares to
read it, simply pointed out that space science has real benefits.  I also
quoted the Augustine Commission report to the effect that they recommended
that space science funding be kept at its current level of 20% (that's
right, 1/5th) of the NASA budget and that the strategic plan from the
Office of Space Science and Applications be maintained.  I did not mention
SS Freedom or any other government program as being less worthy of funding
than space science;  I simply asked the committee to do what they could to
preserve an active space science program despite current budget problems.
Similar letters were no doubt sent by other astronomers and members of
the space science community.  The decision to zero out SS Freedom came as
a complete surprise.
--
Stephen Walton, Dept. of Physics & Astronomy, Cal State Univ. Northridge
    "Lately it occurs to me/What a long, strange trip it's been"

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

Date: 20 May 91 22:40:10 GMT
From: agate!spool.mu.edu!mips!zaphod.mps.ohio-state.edu!swrinde!elroy.jpl.nasa.gov!jpl-devvax![e-mail]  (Douglas Creel)
Subject: Re: Fred vs. Exploration: head-to-head competition

I find it humorous how people cite the "wisdom" of Congres when it suits their
own personal beliefs, and then attack Congress when its decisions don't support
their views. How someone can trust the judgement of a Congressional
subcommittee, which is nothing but a bunch of lawyers whose knowledge of science
and engineering is woefully pitiful, over that of NASAs' leadership is beyond
my comprehension. And by the way, NASA's leadership is compromised of more
than just astronauts. I don't recall William Lenoir ever riding on a shuttle,
and last time I checked he was in favor of the station.

As far as manned space drawing funds away from "space science" projects, there
certainly was no shortage of unmanned missions to the planets during the
sixties and early seventies, and that was during the height of the Apollo
Program. The shuttle only seemed to draw away funds from space science
because space scientists were getting a slice of a smaller pie. If you look
at the percentage of NASA funds devoted to space science during the Apollo
program, and the percentage during the shuttle program, I'd wager that there
wouldn't be much of a difference at all. In fact, one area which has suffered
since the Apollo era has been NASA's technology base. Much of what has been
accomplished in the last decade has been born on the investment in research
done well in the past. If we're going to cut Fred, the we should plow some of
that money in to R&D in such projects as NASP, Aerobrake experiment, nuclear
propulsion and advanced life support systems.

How are we ever going to build a permanent presence in space if we don't
invest the funds to find out how to do it. As far as I see it, the main reason
for building Fred isn't microgravity research, or life sciences research but
simply stated this:

We should build a space station, so we can learn how to build a space station.

If this sounds like a circular argument, it is not. Only by operating a space
station will we learn the lessons necessary to teach us how to operate
efficiently and affordably in space in the future.  An investment in Fred is
an investment in the future. Those who attack the space station the most, are
of course those who feel they will personally lose the most. No shame here.
But don't use catch phrases like "pork barrel" to justify attacks on Fred,
Because AXAF and SIRTF and CRAF/Cassini are every much pork barrel projects
as Space Station Freedom is.

Douglas D. Creel
Navigation Systems Section (Mars Observer Project)
Jet Propulsion Laboratory

P.S. Oh, and as you can see, my livelihood also depends on space science funding
     and not manned space funding, so my views are not influenced by personal
     gains or losses.

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

End of SPACE Digest V13 #622
*******************