Aucbvax.5499
fa.space
utzoo!decvax!ucbvax!space
Thu Dec 17 04:17:16 1981
SPACE Digest V2 #62
>From OTA@S1-A Thu Dec 17 04:07:31 1981
SPACE Digest Volume 2 : Issue 62
Today's Topics:
Multiple-Laser Launching Systems
Skyhook Safety
Reluctant skyhooks
Re: Multiple-Laser Launching Systems
Re: Multiple-Laser Launching Systems
Re: Multiple-Laser Launching Systems
A few technical quibbles...
Project Orion and relatives
Re: SPACE Digest V2 #61
Checkoff on your tax form
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Date: 16 Dec 1981 1247-PST
From: Jim McGrath <CSD.MCGRATH AT SU-SCORE>
Subject: Multiple-Laser Launching Systems
To: lrc.slocum at UTEXAS-20
cc: space at MIT-MC
Good argument on the multiple-beam suggestion except that you did not
consider the most general case. ie in reply to:
Argument: well, the beam path will have moved (following the
launch vehicle). Counter-point: but not always significantly;
at sufficient distances and angles of incidence (beam path to
vehicle trajectory) the movement will be small, approaching
zero -- especially toward the laser end, where motion is
minimized, hence ohmic heating and consequent beam dispersion
is maximized where its effects are least desired (farthest
from the target).
I point out that the beam path changes because the payload has moved
(although if you send it straight up for some stupid reason this does
not have to be true) AND because you switch the origin of the beam!
ie you have a single laser with a complicated mirror system that
allows you to quickly redirect the beam to any one of several "firing
windows" on the ground. These windows can be separated by hundreds of
meters. The whole time the beam is under ground you keep it in
vaccum. Now the beam can follw QUITE different paths, even if the
payload it being launched straight up! And the heating of the air
near the firing windows is a rather small problem if the (relaxation
time needed for air to return to normal)/(number of firing windows you
use) is a small number.
The actual numbers you use here depend upon the relaxation time, the
degree of complexity of your mirror system (and cost) and the capability
of your central laser(s). Cost factors are difficult, but perhaps
someone could come up with that relaxation time and the laser design
(ie you can have a continous laser swapping between windows or a
laser which can send out bursts very quickly).
Jim
------------------------------
Date: 16 Dec 1981 1332-PST
From: Jim McGrath <JPM SU-AI AT>
Subject: Skyhook Safety
To: space at MIT-MC
cc: ota at S1-A, moravec at CMU-10A
I am still not really certain about skyhook safety. Remember, that
plane crashing into a skyhook (which would have a diameter on the
order of meters at most) at 500 mph would easily snap it if a
significant portion of the construction material could not take the
shear forces (thus potentially increasing the stress on the rest of
this weakened section). I even think that you could manage to
completely shear through the stalk with a single accident if the
diameter was a few meters.
Which brings me back to my original question: granted we can get
materials with high compression strength, can they also handle shear
forces like that? If my understanding of the present state of the art
in materials sciences is correct, these materials with great
compression strength are not equally impressive in handling shear
forces. Even if the material had the strength of steel, there could
still be serious consequences as the result of an accident with a
plane in low "orbit" (ie close to the ground, where the diameter of
the skyhook is also the smallest). Such an accident could lose you
the stalk (and yes, most of it will fall up - but I do not anticipate
that to harm many people (unless they are on the stalk at the time) -
the section falling down could cause damage in the immediate
vicinityof the anchoring).
All this does not mean you cannot build a stalk - only that the
engineering problems with such a structure should not be
underestimated. Which is why I do not think they are the natural step
after shuttle/laser launch, but that rather some orbital accelerator
concept will be implemented first.
Jim
------------------------------
------------------------------
Date: 16 December 1981 1743-EST (Wednesday)
From: Hans Moravec at CMU-10A (R110HM60)
To: McGrath at SU-AI, Space at MIT-MC
Subject: Reluctant skyhooks
It may be that a skyhook is harder to sell, but the reason, methinks,
is its strange and unusual nature, not the technical difficulties -
people haven't had a generation to ruminate the concept yet. Heavier
than air flight seemed that way in the late 19th century, and
interplanetary rockets must have seemed like pure lunacy in 1915.
Arthur Clarke suggests that the idea will become a reality twenty years
after people stop laughing.
Since you bring up Mars, by the way, it's nice to realize that Mars
is the best nontrivial place in the solar system to build a synchronous
skyhook, since it both rotates quickly, and has a shallow gravity well.
Any kind of skyhook for Mars can be made of steel, and is a piece of
cake with Kevlar.
As for shear strength under side impacts note that these days Kevlar
is the only material considered for bullet proof clothing. A quarter
inch of Kevlar weave will stop anything fired from a handgun.
another reference:
NASA technical memorandum TM-75174,
G. Polyakov, A Space "Necklace" About the Earth.
(translation of "Kosicheskoye 'Ozhere'ye' Zemli " in
Teknika Molodezhi, No. 4, 197, pp. 41-43)
------------------------------
Date: 16 Dec 1981 1703-CST
From: Jonathan Slocum <LRC.SLOCUM AT UTEXAS-20>
Subject: Re: Multiple-Laser Launching Systems
To: CSD.MCGRATH at SU-SCORE
cc: space at MIT-MC
In-Reply-To: Your message of 16-Dec-81 1447-CST
Re: beam path, I was considering each laser INDIVIDUALLY. The origin
of a particular beam does not change (by definition; I'm talking about
the location of a "nozzle", whatever its realization). Consider: a
laser fires, then shuts off while others are firing; sooner or later,
it must fire again. Through the same air that it fired thru before?
It depends on the diameter of the beam, the displacement of the nozzle
end since last fired, and the change in angular displacement to the
target. If the nozzle moves less than the beam diameter, the beam is
guaranteed to pass through at least some of the same air as last time.
Then, the smaller the change in angular displacement, the more "old"
air seen by the beam. No matter how you distribute the nozzles,
angular displacement will decrease with increasing target distance --
increasing the likelihood of problems -- but a wide distribution (more
than "hundreds of meters" apart) does help, within limits.
Increasing the number of lasers helps out in two ways: it increases
the available relaxation time, and angular displacement to the target
will have changed more between shots. I expect relaxation time to be
large (thunder lasts a long time), hence the number of lasers will
have to be large. If in addition they have to be widely dispersed AND
high up, we need lots of mountains.
Using mirrors instead of separate lasers is a neat idea, if it can be
arranged. It does keep laser costs down. But since every reflection
will introduce error -- which is multiplied by the next mirror -- the
tolerances will be small indeed. Astronomers and their optician
friends seem to have a good grip on most of the relevant factors. The
biggest problems would seem to be mirror coating and switching. The
mirror coating has to be awfully good. If switching is helped by
turning the laser off, OK; but if done by mirror motion alone, it gets
really tricky. Note that if lots of mountains are necessary, the
mirror proposal suffers a bit (no vacuum tunnels).
Using mirrors, of course, does not change arguments concerning beam
refraction. Simply replace "laser" with "mirror" and it all stands
except for details of the cost analysis.
------------------------------
Date: 16 Dec 1981 1604-PST
From: Jim McGrath <JPM SU-AI AT>
Subject: Re: Multiple-Laser Launching Systems
To: LRC.SLOCUM at UTEXAS-20
cc: space at MIT-MC
Agreed that the more lasers you have the better (although you
obviously reach a point of dimishing returns due to capital costs of
the lasers). If you employ a mirror system (and you are correct in
pointing out that there are severe switching and coating problems
here, although I do not think they are insolvable - lasers use mirrors
anyway in the generation of the beam, so something should be workable)
then, as far as the influence of the atmosphere is concerned, you can
make a single laser look like it is dozens of lasers hundreds of
meters (or even kilometers) separate. True, there is still a natural
spreading of the beam over meters of vaccum, but the effect can be
ignored at this level of discussion.
Now I am assuming that the column of heated air generated by the beam
is on the order of meters in diameter. Therefore and system with
different firing windows spread out over hundreds of meters should
generate beams whose heated air columns never coincide anywhere.
Now say the relaxation time is on the order of seconds. How long can
you fire a laser before thermal booming becomes a major facter? I
assume it is at least on the order of miliseconds. A dozen lasers,
with several dozen windows each, could handle this quite well.
Certainly better than hundreds of lasers!
Of course, the final launch configuration depends on a lot of factors.
But we could get high level land for a truely spread out launch site.
It ultimately depends upon the cost tradeoffs, which we cannot
evaluate exactly (although we can point out general relationships).
Thus this present discussion is reaching its limit unless people can
dig up hard numbers for things. Query: has NASA or anyone else done a
REAL study of this concept? (Maybe JEP has a pointer)
Jim
------------------------------
Date: 16 Dec 1981 1604-PST
From: Jim McGrath <JPM SU-AI AT>
Subject: Re: Multiple-Laser Launching Systems
Sender: CSD.MCGRATH at SU-SCORE
To: LRC.SLOCUM at UTEXAS-20
cc: space at MIT-MC
Reply-To: CSD.MCGRATH at SU-SCORE
Agreed that the more lasers you have the better (although you
obviously reach a point of dimishing returns due to capital costs of
the lasers). If you employ a mirror system (and you are correct in
pointing out that there are severe switching and coating problems
here, although I do not think they are insolvable - lasers use mirrors
anyway in the generation of the beam, so something should be workable)
then, as far as the influence of the atmosphere is concerned, you can
make a single laser look like it is dozens of lasers hundreds of
meters (or even kilometers) separate. True, there is still a natural
spreading of the beam over meters of vaccum, but the effect can be
ignored at this level of discussion.
Now I am assuming that the column of heated air generated by the beam
is on the order of meters in diameter. Therefore and system with
different firing windows spread out over hundreds of meters should
generate beams whose heated air columns never coincide anywhere.
Now say the relaxation time is on the order of seconds. How long can
you fire a laser before thermal booming becomes a major facter? I
assume it is at least on the order of miliseconds. A dozen lasers,
with several dozen windows each, could handle this quite well.
Certainly better than hundreds of lasers!
Of course, the final launch configuration depends on a lot of factors.
But we could get high level land for a truely spread out launch site.
It ultimately depends upon the cost tradeoffs, which we cannot
evaluate exactly (although we can point out general relationships).
Thus this present discussion is reaching its limit unless people can
dig up hard numbers for things. Query: has NASA or anyone else done a
REAL study of this concept? (Maybe JEP has a pointer)
Jim
-------
------------------------------
Date: 17 December 1981 00:20 est
From: Schauble.Multics at MIT-Multics
Subject: A few technical quibbles...
To: Space at MIT-AI
Mostly about the laser launch system.
Jim McGrath's points about the advantages of laser launch over laser
missle defense are very well taken. You CAN pick your launching
times, and that offers a LOT of advantages.
To all of you who are worried about selling the excess power to the
public power grid: That's the wrong way.
If you are going to build a nuclear power plant, then really take
advantage of it. Build an industrial park along with the power plant
thd the launch site. Populate the part with industries that use large
amounts of interruptable energy and large amounts of low-level heat.
There are many chemical processes that can use the low-level heat
effectively. And there are lots of uses for industrial electricity.
Someone mentioned electric furnaces for speciality steels, but there
is a much better one:
aluminum
Consider: it requires LOTS of electricity, is a high-value low-weight
product so you can afford to ship it from remote areas, and the pot
line can be shut down on almost zero notice without damaging the
process (something you can't do with an electric furnace).
A very large percentage of our aluminum is now made in Washington
state on third shift using the night power from the Columbia River
dams.
One of the aluminum companies (Alcoa? I really can't remember.) is
seriously studying building their own nuclear plant to run a very
large set of aluminum pots. They think it's justified just for the
aluminum. And they didn't even factor in co-generation and other uses
for waste heat.
There is also a study kicking around somewhere on exactly the subject
of having a nuclear plant power an industrial park, without extensive
connections to the main power grid. The conclusions were favorable.
The other point is that, while you need tremendous power to drive the
lasers, you only need it for 90 seconds. Clearly you don't design
your power plant to meet this peak. Rather, you design on some kind
of energy storage device and design your power plant to meet
something like the average need.
Superconducting coils and large flywheels come immediately to mind.
You then divert the output of your (much smaller) power plant for 8
hours or so to pump up your storage device and then let 'er rip. You
could, for example, not build your own power plant, but charge your
device during the night (keeping the power company happy since they
couldn't otherwise sell the power) and then launch at sunrise. Again,
you CAN pick your launch time.
The storage devices have been extensivelystudied, since power
companies want them for load-leveling. They seem practical, and
should produce something like a 90% in-out efficiency. Both flywheels
and superconducting coils can be discharged at very high rates.
And, of course, if you are using multiple lasers then you build a
storage device located with each one. Saves on power transmission
losses.
To Jim McGrath: I'm not sure why you think that the speed of light
would be a problem in an active stabilization system for the skyhook.
Any waves generated in the cable would travel at the speed of sound
in the cable. That has got to be several orders of magnitude slower
that light.
Personally, I like it. Shall we start a company and build one?
Paul
------------------------------
Date: 16 Dec 1981 21:50:29-PST
From: decvax!utzoo!henry at Berkeley
Subject: Project Orion and relatives
If anyone is interested in the details of nuclear pulse propulsion,
possibly the best place to start is the lead paper in the August 1979 issue
of the Journal of the British Interplanetary Society: "Nuclear Pulse
Propulsion: A historical review of an advanced propulsion concept". It
discusses everything from the original concepts to the recent schemes
based on beam-ignited microexplosions. About 1/3 of the 25-page paper
is the best technical (as opposed to project-history) discussion of Orion
I have seen, including an attempt at an analysis of the rather vague vehicle
descriptions in "The Curve of Binding Energy". The 97-item bibliography
might also be of interest.
------------------------------
Date: 17 Dec 1981 0314-EST
From: J. Noel Chiappa <JNC MIT-XX AT>
Subject: Re: SPACE Digest V2 #61
To: Space-Enthusiasts at MIT-MC
cc: JNC at MIT-XX
In-Reply-To: Your message of 16-Dec-81 0602-EST
I have this uncomfortable feeling that the report in Science is
correct. I'm goddamned if I'm going to sit still and let my tax dollars
be spent on everything EXCEPT the one thing I'd like to see them spent
on. If my granparents starve because of it, too bad.
-------
------------------------------
Date: 17 Dec 1981 0319-EST
From: J. Noel Chiappa <JNC MIT-XX AT>
Subject: Checkoff on your tax form
To: space at MIT-MC
cc: JNC at MIT-XX
Wouldn't it be neat if there were checkoffs on your tax forms
so you could send some of your tax money (or perhaps extra tax money)
to projects of your chosing, the way they have the election funding
thing now?
-------
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End of SPACE Digest
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