Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/SPACEDIG/V13_5/V13_573.TXT

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Subject: SPACE Digest V13 #573

SPACE Digest                                     Volume 13 : Issue 573

Today's Topics:
             Re: Laser launchers
              Re: Why the space station?
          Re: SPACE STATION FREEDOM WOUNDED
              Re: Why the space station?
        Re: Tethers (was Re: Laser launchers)
  Re: Desaturation of the Reaction wheels on the Magellan Spacecraft
               Re: Saturn V and the ALS
       POTENTIAL MAJOR SOLAR FLARE WARNING - REINSTATED
             Re: Space Station Cancelled

Administrivia:

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

Date: 16 May 91 11:24:08 GMT
From: att!emory!wa4mei!ke4zv![e-mail]  (Gary Coffman)
Subject: Re: Laser launchers

In article <[e-mail]> [e-mail] (Ron Carter) writes:
>
>In article <[e-mail]> [e-mail] (Gary Coffman) writes:
>>Well we have to decide first how much power is needed at the target.
>>Let's assume a payload mass of 1000 kg.
>
>You are thinking way too big.  The proposal of which I'm aware
>was for a 13.6 kg payload.  (30 pounds.)  So cut all your numbers
>by 2 orders of magnitude.
>
>(No, the purpose was NOT to orbit satellites or people.  The
>purpose was to orbit bulk commodities for a space station or
>other program, such as fuel, oxygen, or water.)

So laser launchers *aren't* HLV replacements. That's where this all
started. I thought I was thinking *small* when I postulated 1000 kg.

>>.... We must attain the highest specific impulse that we can to keep
>>the amount of dead weight reaction mass to a minimum. This in turn means
>>that we need to deliver the same amounts of power to the payload as
>>would be supplied by a chemical rocket, again, gigawatts.
>
>There's another faulty assumption.  Chemical rockets have far more
>than the amount of reaction mass required for the minimum-energy
>push to orbit, and must expend more energy to lift it.  The minimum
>energy mass-ratio for a rocket using a constant exhaust velocity
>is about 4.  Typical rockets run 20 and up.

Less than one order of magnitude difference.

>> To avoid
>>turning the atmosphere to plasma, we need to keep beam intensity below
>>about 100 Kw/m^2.
>
>That's only 10 W/cm^2, a trifling amount of power.  You are talking out
>of your hat.  Another poster mailed me information about a laser he used.
>Its beam intensity was around 300 W/cm^2.  It did not ionize air, it did
>not even cause thermal blooming.  Once again, you are off by at least 2
>orders of magnitude.  It is not surprising that your conclusions are faulty.

You're right there. I went back and checked the articles where the figure
came from. 1 kw/cm^2 .... big oops, I multiplied by 100 instead of 10,000
to convert from cm^2 to m^2.

>>I have read studies, and I have done back of the envelope calculations
>>myself. No chemical bond known to science has the strength to support
>>a geosync to earth tether.
>
>Knowing what you've posted in this thread, you probably ignored
>the decrease in acceleration with altitude when calculating
>stresses.  This throws your calculations way out of whack.
>
>I'll see if I can find my analysis of the iron-whisker tether.
>When I can post my calculations, I'll expect you to point out
>exactly where they are faulty.

No, I made allowance for decrease in acceleration with altitude,
and for change in tension with length. It's been pointed out to me
in email however that I was considering a constant diameter tether
when I should have been looking at a *tapered* tether.
That makes a major difference in the mass of the tether. That makes a
beanstalk barely concievable with a maximum diameter of some 1100 km
at the maximum tension point and tapering to a few meters at the ends.
It still seems well beyond practicality to me. I'd be willing to look
at your figures though, I might have multiplied by the wrong constant
again. (blush)

Gary

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

Date: 16 May 91 10:31:33 GMT
From: att!emory!wa4mei!ke4zv![e-mail]  (Gary Coffman)
Subject: Re: Why the space station?

In article <[e-mail]> [e-mail] (Ken Sheppardson) writes:
>[e-mail] writes:
>>[e-mail] (Gary Coffman) writes:
>>>
>>>If you assume that you are never going to do large scale in space assembly
>>
>>that would give us knowledge about solar system resources with which
>>to solve the problem.  Right _now_, Congress is deciding on Fred
>>vs. SIRTF, a powerful infrared telescope which would discover thousands
>>of new asteroids and comets.  Fred is incredibly destructivce of the
>>long-term potential for large scale space assembly.
>
>
>  [...hmmm...maybe Queen Isabella should've given the money to those
>   folks who wanted to build a REALLY BIG telescope to look in the
>   other direction to see if they could see around the world the other
>   way....] :)

This reminds me of the story about the drunk looking for his car keys
under a street lamp even though he had dropped them across the street.
"Oh, but it's so much easier to see over here."

We're plodding along groping step by step toward the goal of starting
the car while Nick is off chasing rainbows under the street lamp.
If we don't have on orbit facilities to use those postulated asteroids
and comets with their postulated valuable resources, it won't matter
today whether we know their orbital parameters or not. One day in the
distant future when our aspirations finally outstrip our ability to supply
them from Earth, when we're ready to attempt O'Neil colonies or other things
of similar magnitude, then we'll be ready to start thinking about exotic
mining and refining. Until then such things as SIRTF merely serve to
scratch our curiosity itch while diverting limited budget away from
building the infrastructure needed as a first step in ultimately
exploiting such resources.

I think Nick wants to scuttle our present halting first steps, we're
barely beyond the dugout canoe stage, because he sees the possibility
of 747s on the horizon. But without those halting first steps, Columbus
wouldn't have had his advanced caravels and Boeing would never have
existed to build those 747s.

Gary

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

Date: 16 May 91 08:27:51 GMT
From: rochester!sol![e-mail]  (Brian Yamauchi)
Subject: Re: SPACE STATION FREEDOM WOUNDED

In article <[e-mail]> [e-mail] (Allen W. Sherzer) writes:

>The House HUD/VA/IA Appropriations Subcommittee marked up the NASA
>appropriation this morning. They zeroed out ALL station funding.
>Unless the Senate restores the money and it survives conference,
>the space station will be cancled.

>looks like the space science people who lobbied had an effect.

Another question: is the money zeroed or just the program?

In other words, suppose NASA asked for $14 billion of which $3 billion
was going to the station.  Does this mean NASA just gets $11 billion
or does it mean they get $14 billion but can't spend any money on the
station?  If it's the former, I can't see any reason for space
scientists to celebrate.  On the other hand, if it's the latter, I can
think of a lot of useful things to do with a spare $3 billion...
--
_______________________________________________________________________________

Brian Yamauchi                University of Rochester
[e-mail]        Department of Computer Science
_______________________________________________________________________________

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

Date: 16 May 91 15:46:57 GMT
From: usc!cs.utexas.edu!news-server.csri.toronto.edu!utzoo![e-mail]  (Henry Spencer)
Subject: Re: Why the space station?

In article <[e-mail]> [e-mail] writes:
>Nor should we.  I don't know any explorer worth his or her salt
>that advocates spending $tens of billions for one mission at the
>expensive of several dozen other missions, such as orbital telescopes,
>flyby probes, lunar and planetary orbiters, comet rendesvous, etc.

Hmm, odd that you think the Galileo people are not "explorers worth their
salt".  (That project very definitely ate everyone's lunch for a while.)
Clearly we have a very selective definition of worthiness here.
--
And the bean-counter replied,           | Henry Spencer @ U of Toronto Zoology
"beans are more important".             |  [e-mail]  utzoo!henry

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

Date: 16 May 91 07:55:47 GMT
From: att!emory!wa4mei!ke4zv![e-mail]  (Gary Coffman)
Subject: Re: Tethers (was Re: Laser launchers)

In article <[e-mail]> [e-mail] (Christopher Neufeld) writes:
>In article <[e-mail]> [e-mail] (Gary Coffman) writes:
>>
>>Let's look at a typical strawman tether launch. Let's propose a 1000 km
>>tether whose center of mass is in a 700 km orbit. To hook on to the
>>lower end of the tether, you need to be going faster than the orbital
>>velocity of a 700 km orbit and be at an altitude of 200 km. If you've
>>already gone that far, you've paid most of the cost to orbit.

Not a good strawman tether it turns out. The tether isn't rotating which
turns out to be the real advantage of tethers as opposed to beanstalks.

[Good explanation of tether mechanics deleted]

>>So the
>>tether doesn't buy you that much. And you've added considerable Rube
>>Goldberg complication to the scheduling of your launch. Above an airless
>>body like the Moon, a tether begins to make a lot of sense. But for
>>Earth based launches, you've got to keep the end of the tether above
>>the majority of air resistance or it'll end up in your lap.
>>
>   Clearly it can buy you quite a bit. You have to watch out for air
>effects, but the tether is moving quite slowly relative to the ground
>when it's in air. You might be able to get away with a tether which
>drops down to a couple of dozen kilometres altitude without too much
>trouble with air, and a cargo aircraft to that altitude costs
>considerably less than a rocket which carries the same payload to Earth
>escape, even when you figure in the fuel which the tether will need to
>consume in order to maintain its orbit. If the air is thick enough to
>bother a tether, it's probably thick enough to hold an airplane up, so
>they should be able to mate nicely. The airplane doesn't have to be
>going anywhere near orbital velocity.

From the point of view of air drag, the tether presents a nearly end
on view of itself rather than the broadside I had assumed when it
approaches the lower atmosphere.

>>It always comes back to fighting that pesky air. That's why I keep
>>harping on turning that liability into an asset by using wings and
>>air breathing engines to get above most of it.
>>
>   That's exactly what you'd do with such a tether system. The tether
>would represent a significant advantage in getting the payloads out,
>though.

Thanks to Christopher and a few others who sent me helpful email, I've
begun to appreciate the usefulness of rotating tethers. Note to Christopher,
I ran some numbers with the help of your equations. Earth to geosync
beanstalks seem to be one engineering project likely never to be built
even if materials science gets lots better. These things are huge. A
maximum diameter of 1100 km, wow.

Gary

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

Date: 17 May 91 01:56:09 GMT
From: mintaka!olivea!samsung!cs.utexas.edu!news-server.csri.toronto.edu!utzoo![e-mail]  (Henry Spencer)
Subject: Re: Desaturation of the Reaction wheels on the Magellan Spacecraft

In article <[e-mail]> [e-mail] writes:
>    I've got a question for everyone out their. Why do the reaction
>wheels have to be desaturated on the magellan spacecraft? ...
>If the spacecraft was in a reference frame with no forces acting on it,
>desaturation would not be necessary. Does anyone know what kind of forces
>would be acting on the spacecraft that would put it in a non equilibrium
>situation? ...

There are a number of sources of "disturbance torques" that eventually
need to be dealt with.  Any spacecraft orbiting near a planet runs into
tidal torques.  If the planet has an atmosphere, aerodynamic torques are
noticeable out far beyond the usual limits of atmospheric effects.  If
it has a magnetic field, magnetic torques can be an issue.  Any asymmetric
spacecraft experiences torques from light pressure.  Micrometeorites can
cause small random torques.

>1) a frictional force coming from the upper atmosphere--how far does the
>venutian atmosphere extend anyways?

Well beyond Magellan's orbit, for this purpose.

>2) momentum from the solar wind

Insignificant, usually, compared to light pressure.  (The solar wind is
ionized plasma; it is not a generic term for "everything the Sun emits".)
--
And the bean-counter replied,           | Henry Spencer @ U of Toronto Zoology
"beans are more important".             |  [e-mail]  utzoo!henry

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

Date: 16 May 91 14:01:33 GMT
From: mojo![e-mail]  (Doug Mohney)
Subject: Re: Saturn V and the ALS

\In article <[e-mail]>, [e-mail] (Allen W. Sherzer) writes:

>>  There is no
>>commitment to any near-term development of heavylift launch capability
>>or manned expendables.
>
>Put a Soyuz on a Titan. It will work just fine. Use the money saved
>to build a space station. Send the spacelabs up to LEO and keep them
>there. Service with Titan IV as needed.

HOW MUCH TIME AND MONEY IS IT GOING TO COST TO CERTIFY SOYUZ ON A TITAN?
Do you have any clue? Let me give you a few hints here: A) Translating the
documentation and technical specs into English B) Certifying Titan IV as
man-rateable C) Integrating Soyuz and Titan into a package & "dry-firing" sans
crew at least once or twice D) Writing, testing and working out new launch
procedures for your little hybred at the Cape.

Come up with some numbers, Mr. Sherzer. You manage to produce them for Titan IV
and the Heavy Lift Delta.

You can't snap your fingers and do this. Nor will you get ANY support (non,
zip) for it. It's bad enough to get people to accept Soviet UNMANNED launchers.
Not to mention the political lobbying from the unions to "Buy American" if
your idea was half-heartedly considered.

>Heavy lift would be nice but we can do without it for a while.
>
>>Orbiter production should be terminated only when
>>such efforts are approved, funded, and well under way.  They aren't.
>
>And they never will be the way things are now. All the money which
>could go to buying HLV's is spent operating the Shuttle.

You still ignore what will happen to Shuttle-dependent payloads, including
scientific experiments designed to fit into the bay, SpaceLabs, and cooperative
efforts with the European Community.

Perhaps you really don't care about sacrificing ESA/NASA joint ventures, but
I think ESA has been screwed enough already.

     Signature envy: quality of some people to put 24+ lines in their .sigs
  -- >                  [e-mail]                        < --

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

Date: Thu, 16 May 91 13:01:34 MDT
From: oler <@BITNET.CC.CMU.EDU:[e-mail]> (CARY OLER)
Subject: POTENTIAL MAJOR SOLAR FLARE WARNING - REINSTATED
X-St-Vmsmail-To: st%"[e-mail]"

                /\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\

                     POTENTIAL MAJOR FLARE WARNING

                /\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\

                             May 16, 1991


ATTENTION:

     As was noted in the last Major Solar Flare Alert, the Potential Major
Solar Flare Warning has been reinstated until Region 6619 departs from the
western solar hemisphere.  The last major flare spawned by this region
earlier today was a complete surprise (for the reasons noted in the alert)
and was not expected to produce anything major.  This obviously turned out to
be an incorrect assumption, and proves just how unpredictable and elusive
major flares can be.

     This warning will remain in effect until further notice.


                /\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\

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

Date: 16 May 91 17:04:32 GMT
From: usc!samsung!rex!rouge![e-mail]  (Fraering Philip)
Subject: Re: Space Station Cancelled

In article <[e-mail]> [e-mail] (Reg Clemens) writes:

>Lets hope they have the courage to keep it cancled.. Then we can get
>on with doing some REAL Space Science.


I don't think Freedom's a good idea, but let's face facts:

1. It will probably have its budget restored by the Senate.

2. Killing Freedom will not transfer its money to space science.


What is happening is that money is going from the less pork-barrel
projects (yes, even Freedom) and into the pork-barrel construction
business.

Of course, it may be that a lot of the cheap office blocks that
go up will house space scientists and astronomers and aerospace
engineers, and therefore help space exploration and exploitation,

But I doubt it.


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

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

End of SPACE Digest V13 #573
*******************