Message-ID: <McABDDu00WBw82Mk5n@andrew.cmu.edu>
Precedence: junk
Reply-To: space+@Andrew.CMU.EDU
From: space-request+@Andrew.CMU.EDU
To: space+@Andrew.CMU.EDU
Date: Wed, 15 May 91 02:23:12 -0400 (EDT)
Subject: SPACE Digest V13 #553

SPACE Digest                                     Volume 13 : Issue 553

Today's Topics:
			 Re: Laser launchers
		      Re: Honking at cyclists...
       Re: Ethics of Terraforming (was Re: Terraforming Venus)
		      Re: Honking at cyclists...

Administrivia:

    Submissions to the SPACE Digest/sci.space should be mailed to
  space+@andrew.cmu.edu.  Other mail, esp. [un]subscription requests,
  should be sent to space-request+@andrew.cmu.edu, or, if urgent, to
			 tm2b+@andrew.cmu.edu

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

Date: 13 May 91 17:10:56 GMT
From: cis.ohio-state.edu!zaphod.mps.ohio-state.edu!sol.ctr.columbia.edu!emory!wa4mei!ke4zv!gary@tut.cis.ohio-state.edu  (Gary Coffman)
Subject: Re: Laser launchers

In article <41687@fmsrl7.UUCP> wreck@fmsrl7.UUCP (Ron Carter) writes:
>In article <2777@ke4zv.UUCP>, gary@ke4zv.UUCP (Gary Coffman) writes:
>>Sorry, but a laser launch system must carry "reaction mass" to be heated
>>by the laser beam. This *dead weight* produces no useful energy input.
>>It's mass and tankage mass must still be carried aloft by energy input
>           ^^^^^^^^^^^^^^^^
>>from the laser beam alone.
>
>Current concepts have eliminated the tankage by carrying the propellant
>as a solid (ice).  This eliminates materials costs for the tankage and
>gives a huge savings in manufacturing costs; freezers are very very
>cheap to run and reproduce simple shapes (like slugs) quite well!

This implies an extremely precise application of energy to the ice.
An extremely small pointing error would cause off center heating of
the block and off center thrust. I've heard this mentioned as a way
of steering the vehicle, but such precise control seems well beyond
state of the art as I understand it. Therefore I was considering a
machined metal chamber to contain and direct the heated exhaust. This
then implies tankage and pumps. If such precision is possible in the
neccessarily high power beam, then all bets are off. Because of the
previously mentioned problems with atmospheric blooming, I don't 
think such precise control is possible.

>>Granted that at very low altitudes the 
>>atmosphere itself may be used as a working fluid, but above roughly 
>>5 km the atmosphere is not dense enough to carry the energy load and
>>internal reaction mass must be carried. It makes little sense to carry
>>*dead weight* when that same mass could release useful chemical energy.
>
>Only in the naive case.  There are two points to consider:
>
>1.)	Exhaust velocity.  Laser propulsion can impart far
>	more energy to a gram of propellant than can chemical
>	reactions.  This lowers the mass-ratio and the total
>	energy required.

This implies that the beam power density must exceed the chemical energy 
released by a conventional rocket. Again I fall back on the contention
that such high power density isn't practical through the atmosphere.

>2.)	Cost of carrying chemical fuels.  They require tankage,
>	nozzles, pumps for liquids, heavy walls for solids, and
>	material and manufacturing for all of these.  Laser
>	propulsion has the potential to eliminate them entirely.

An ideal chemical rocket has a mass ratio of about 4, so eliminating
the tanks, engines, etc will reduce the mass of the vehicle by less
than 20% if we want any payload. This is significant, but not awe inspiring.

>Are you at all capable of thinking beyond the naive case?

I guess I could ask you the same question. You are making some mighty
big assumptions that, in my opinion, are rather shakey.

>>Using a space based laser is even worse in some respects than using a
>>ground based laser. At least on the ground there is a large sink for
>>the waste heat produced in the process of generating the beam. At least
>>on the ground the massive tankage required for the laser reactants doesn't
>>have to be heaved into space.
>>... the mass required to be delivered to orbit to operate the
>>laser far exceeds the mass of fuel needed to deliver the payload directly
>>to orbit.
>
>Now you are assuming chemical lasers.  Why would anyone carry a
>chemical laser, which needs reactants which are used once and
>exhausted, into space to launch things with?  Why not excimer
>lasers, or free-electron lasers, or solar-UV-pumped gas lasers?

As I mentioned in a previous posting, cooling of any but gas dynamic
lasers is a difficult problem on earth where a large heatsink is
available. In the vaccum of space, I believe that getting rid of
the waste heat will be much more difficult. Therefore I assumed that
a space based laser would have to be of the gas dynamic type to avoid
melting from it's own waste heat. If you have a plausible cooling 
method for such lasers, I'd love to hear it.

>When you were knocking efficiency, you were talking about
>electrically driven CO2 lasers on the ground (worst case, except
>perhaps He-Ne lasers).   Now that you're bashing space-based
>lasers, you are talking about chemical reaction lasers (worst
>possible choice).  You can't have it both ways!

Well yes you can. Different enviornments require different assumptions.
On earth, electrically powered lasers make sense because there is
no waste exhaust that needs to be dealt with. A large laser launch
facility that launches often would have considerable enviornmental
impact from the exhaust of a gas dynamic laser alone, not to mention
effects caused by the beam. A large gas dynamic launch laser would
produce as much exhaust, probably more, than a comparable power chemical
rocket engine. Meanwhile a space based laser has the problem of
disposing of waste heat efficiently. Really huge radiators capable
of disposing of gigawatts would be required for all but a pass through 
gas dynamic laser.

>Take your straw-man engineering proposals and compost them, please.

Take your naive dream laser launchers and look at the dirty little
engineering details.

>>The higher exhaust velocity for a given heat input that light molecules
>>like H2 give you is extremely valuable in increasing performance.
>
>You can't get any significant exhaust velocity out of H2 without
>supplying energy to it externally; you can't react H2 with H2.
>If you burn it with O2, you've got H2O.  Laser-launch systems
>will probably use (surprise!) H2O!

Ah, but if they use straight H2, and because they supply the energy
externally they can, they would get better efficiency.

>>The blooming problem is more than just a defocusing of the beam that
>>can be solved by adaptive optics. At the beam power levels required,
>>the atmosphere is turned to a superheated plasma....
>
>Pray tell what those power levels are, and why we have to have
>these high power levels anywhere except in the immediate vicinity
>of the back end of the target.  Tell me why we can't just plug
>in this power density limit as a design criterion for the optical
>system.

We know several things about the beam power levels.

1. The power level will have to be at least as high as the power level
   produced by a chemical rocket lifting the same mass. Gigawatts.
2. The power level must be high enough to turn enough ice to explosive
   plasma to give the same thrust level as a chemical rocket. Gigawatts.
3. A beam power level high enough to turn ice to plasma will also be
   high enough to turn air to plasma.
4. For a highly dispersed beam source (multiple lasers in a widely
   spaced ground array) the geometry of the situation shows that 
   somewhere below the apex of the converging beams the power density
   reaches the plasma point of air.
5. As air pressure decreases, the power needed to ionize what air remains
   decreases. 

From 4 we can determine that beam guidance will be sufficiently disrupted
that your block of ice won't receive the precision heating necessary to
avoid off center thrust. From 5 we can say that problems will occur at
lower beam power levels as the vehicle gets higher (until it gets *really*
high anyway). From 1 and 2 we can say that we're talking about incredible
beam power densities, in the gigawatts per square meter range. From 3
we can feel sure that the beam *will* bloom.  If we restrict beam power
levels to less than 1 megawatt per square meter, we are talking at least
a thousand beams for a 1 gigawatt launcher. And as they converge, some point, 
at least a meter from the target, beam power density approaches the full power 
of a single beam system. During the early phases of launch, the beams will
be converging at nearly right angles to the vehicle. This seems to preclude
precise steering of your ice vehicles.

>>A one square inch kelvar tether to geosync orbit would weigh 45.2 million 
>>pounds. I don't have the handbook here at the terminal, but kelvar doesn't
>>approach 45 million pounds per square inch tensile strength by orders of
>>magnitude. It couldn't support itself, much less a usable payload.
>
>More oversimplifying.  This ignores reduced weight as altitude
>increases, tapering and improvement in materials.  To quote
>one of the experts (lightly editted):
>
>-From: andrew.cmu.edu!hm02+ (Hans P. Moravec)
>-Newsgroups: sci.physics
>-Subject: Orbital elevators (was Ringworld)
>-Message-Id: <MbGToH600Uh7MAjn0Y@andrew.cmu.edu>
>-Date: 21 Nov 90 04:05:39 GMT
>-
>[material deleted]
>-    For Earth the taper with Kevlar would be 10^11, and the cable could 
>-hoist only about one trillionth of itys own mass -- which I admit makes
>-it impractical!   But the required taper is exponential in the weight to
>-strength ratio of the material, and a material only five times better than
>                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>-Kevlar (within the measured strength/weights of single crystals of various
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>-substances) would bring the taper ratio to 160 and the mass fraction to
> ^^^^^^^^^^^
>-1/2000. 
>
>So much for "physical impossibility".  It is just something that
>materials science can't quite do today.  It is quite possible.

Ok, I concede this one. I hadn't considered a tapered cable. Still, growing
a single crystaline strand, WITH A TAPER, that would reach to geosync, sounds 
rather unlikely for the simple reason that crystals like to determine their
own shape. I still think this is a fundamental rather than developmental
problem. Note that we are talking about beanstalks here and not dynamic
tethers. They have another set of practical problems as launcher replacements.

Gary

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

Date: 14 May 91 00:16:36 GMT
From: cis.ohio-state.edu!zaphod.mps.ohio-state.edu!samsung!noose.ecn.purdue.edu!en.ecn.purdue.edu!irvine@tut.cis.ohio-state.edu  (/dev/null)
Subject: Re: Honking at cyclists...

In article <1991May13.180909.16448@watdragon.waterloo.edu>, jdnicoll@watyew.uwaterloo.ca (James Davis Nicoll) writes:
> >
> 	So, what happens at the core of human settlement when the core
> regions have been stripped of useable resources, and new resources from the 
> outlying regions can't be shipped back fast enough to suit demand? There
> is that nasty speed of light restriction on travel, not to mention how 
> expensive shipping objects is likely to be over interstellar distances.
> I suppose it'd be like fungus; colonise a region, breed like hell until
> the local resources are gone, and send out spores to repeat the process.
>
Exactly!  Its not impossible, just expensive to ship goods.  Rome in 
the days of the Empire relied on Egyptian grain ships that took months
to reach the hungry populace of non-self sufficient Rome.  But in our
case months are turned into DECADES here.  These shipments would last longer
than many governments and companies.  And I doubt that colonists would be
really interested in shipping off their produce and things to be consumed
by 'the core' in exchange for 50 year old consumer goods!

I think resource management will be the key to solar exploration and
colonizatiopn, rather than the 19th century planeary 'rape' that is
implied.  There will be very little trade, probably limited to
unique local products.  Well, unless really cheap cargo ships are made...
Even in the core, technology will develop that will renew resources
that are now considered 'non-renewable.'  Population will be limited by
the cost of resources in a bizarre supply/demand like curve (very 
Malthusian.....).
 
> 	Ignoring C, what do you do when you've 'eaten' the universe?
> It only takes millenia of growth at our current rate to convert the
> universe into human flesh,

Despite my personal disagreement with how long it will take to 'eat'
the universe.  We can renew resources.  Higher science and technology
that expansion will require will (hopefully) tech us a lot about
efficient use of resources as well as how to renew the so-called
'non-renewable' resources.  Also, I don't think that the WHOLE
universe will ever be turned into 'human flesh' - I don't like
the gloom and doom of Malthus, but his theories have some validity.
When resources become scarcer, people will not have as many kids because
the expense of resources, population drops until it becomes cheaper, essentially
a damped harmonic of growth when resources are scarce.

> birth rate will drop, if our industrial growth continues at the current
> rate, we'll 'eat' the Universe industrially in a few millenia. As far
> as I know, the Universe is finite, so there are limits (which can be very
> large, depending on your assumptions about how efficiently we can use the
> Universe) to growth.

All things considered - no one knows how big the universe is, or how in
5000 years we can 'eat' it?  Only 5000 years?  (few thousand = 5000 :) )
100 billion stars in a galaxy, with billions of galaxies.  A googol-plex
of stars for STARTERS, and only 5000 years.  Our galaxy is 100,000 light
years across.  If we could turn material into human flesh as fast as the
speed of light in all directions, it would take 100,000 years for our
galaxy!  200 + million years for the nearest next galaxy, etc!

I think you might be off by a factor of 1000 or even 10000 for our galaxy
alone. :)
> 
> 	I suspect that the current conditions are akin to a phase
> change, and population after the current industrial revolution will
> be somewhat steady, but much higher than prior to the IR. 
> 
I think that poulation will expand in the exponential fashion that it has
until everyone has TV and 1000 channels ( TV-the most effective birth
control! :) ), or resources dry up.  
Then, as costs of resources (food, etc) rise, people will not increase in the
numbers they have and population will steady off.
There is also a trend that favors resource renewal rather than industrial age
plundering.  I think we are growing out of our 'throw-away society' phase.
(At least I hope so!)

-- 
+-----------------------------------------------------------------------+
| Society of Philosophers, Luminaries,       	| Brent L. Irvine       | 
| and Other Professional Thinking People.....   | Only my own ramblings |
+-----------------------------------------------------------------------+

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

Date: 13 May 91 18:16:45 GMT
From: voder!pyramid!lstowell@ucbvax.Berkeley.EDU  (Lon Stowell)
Subject: Re: Ethics of Terraforming (was Re: Terraforming Venus)

In article <DLBRES10.91May11150108@pc.usl.edu> dlbres10@pc.usl.edu (Fraering Philip) writes:
>
>Uh, perhaps the face shouldn't be discussed on this newsgroup.
   
   You are implying that "space" enthusiasts have absolutely no
   sense of humor?   

   I resent the implication.

>I don't mean to imply that the face is or is not an artifact, it's
>just that at this stage of the game, discussion of the face is
>pointless, and blanket statements about the face such as "Does anyone

    Interesting statement, given that it is unlikely that anyone
    has taken a fairly good look at it.   Who knows what the
    doggone thing is.....and that is PRECISELY the point!  

    Frankly, I would much rather take a good look at some core
    bores from Mt. Olympus.  

>elxe think it sorta looks like Maggie Thatcher?" could end up
>putting you in a situation where you're stepping on a lot of
>people's blue suede shoes oops, I mean toes.... :-)

   Thank you, I'll take that as a compliment.  Sometimes toes
   NEED stepping on.

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

Date: 14 May 91 03:39:24 GMT
From: cis.ohio-state.edu!zaphod.mps.ohio-state.edu!wuarchive!rex!rouge!dlbres10@tut.cis.ohio-state.edu  (Fraering Philip)
Subject: Re: Honking at cyclists...

Actually, everyone else out there knows we have
potential to use up the entire universe. That's why
very shortly the Earth will be destroyed to make way
for an interstellar bypass.

--
Phil Fraering || Usenet (?):dlbres10@pc.usl.edu || 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 #553
*******************