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Aucbvax.5453 fa.space utzoo!decvax!ucbvax!space Sat Dec 12 04:01:37 1981 SPACE Digest V2 #57 >From OTA@S1-A Sat Dec 12 03:51:56 1981

SPACE Digest Volume 2 : Issue 57

Today's Topics: Space Elevator space elevators Launching Laser Costs Atmospheric distortion and high power lasers cost of power for laser launch Penthouse please Laser launches from the Moon or in Orbit Penthouse, Yes Sir! Penthouse, P.S. Strong cables ----------------------------------------------------------------------

Date: 11 Dec 1981 05:55:28-PST From: CSVAX.wildbill at Berkeley To: space-enthusiasts@MIT-MC Subject: Space Elevator

The concept has been around for quite some time. I first remember encountering it about 10 years ago, at which time a friend and I did a calculation about how big it would have to be to support its own weight. As I recall, using steel as the material (as we did in our test case) requires that the cable be several times as big around as the earth at its widest point in order to be 1 inch around at sea level. There are obviously some problems here.

Arthur C. Clarke did an excellent treatment of this idea a few years back in his novel \\The Fountains of Paradise//. He avoided the problem by inventing a super-strong material with just the right properties for building such a device to keep the story going.

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Date: 11 Dec 1981 1106-CST From: Clyde Hoover <CC.CLYDE AT UTEXAS-20> Subject: space elevators To: space at MIT-MC

The idea of elevators into space is not new, I recall seeing it mentioned in a Sunday-suppliment magazine in the mid-60s. For a more proper treatment (rather than OMNI), read Arthur C. Clarke's "The Fountains of Paradise". -------

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Date: 11 Dec 1981 0954-PST From: Paul Dietz <DIETZ AT USC-ECL> Subject: Launching Laser Costs To: space at MIT-MC

That figure of $2500 per ton for th energy to launch a payload with a laser is probably low, because a gigawatt laser will consume a lot more than 1 gigawatt of power. Don't forget R&D costs, either.

To solve the turbulence problem we can put the laser on top of a mountain, just like a telescope. In this respect the launcher has an advantage over a laser weapon, because the weapon has to be able to work anywhere. A good choice for a launch site would be one of the Hawaiian shield volcanoes - there is very little turbulence there. -------

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From: BRUC@MIT-ML Date: 12/11/81 14:48:54 Subject: Atmospheric distortion and high power lasers

BRUC@MIT-ML 12/11/81 14:48:54 Re: Atmospheric distortion and high power lasers To: space at MIT-MC If a laser pulse is sufficiently short, the air molecules won't have time to move in order to affect the index of refraction. Atmospheric distortion really wouldn't be a problem. Absorption is a problem with visble wavelength light under certain weather conditions. Building it in the desert would be a good solution.

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Date: 11 December 1981 15:08 cst From: VaughanW at HI-Multics (Bill Vaughan) Subject: cost of power for laser launch Sender: VaughanW.REFLECS at HI-Multics To: Space-Enthusiasts at MIT-MC

You can't assume that big lasers are plugged into somebody's power line, and therefore you can't use $/KWH that are based on such an assumption.

Here's one way to do it, if you *insist* on using electrical rather than chemical energy:

Compute the size of a 1GWatt power plant. Figure out what it costs to build it. Assume some realistic interest rate. (No fair using 3 or 4 percent.) Add the costs to the cost of your laser. I hope you also used a realistic interest rate for the laser. I think you will find that your fuel costs are negligible by comparison with your interest costs.

Maybe you can get some of your costs back by agreeing to sell power to people who don't mind losing it for some time either side of a launch.

Seriously, peaking power is much more costly than baseline load, and you are talking about a gigawatt of peaking power.

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Date: 11 December 1981 18:02 est From: Tavares.Multics at MIT-Multics Subject: Penthouse please To: Space-Enthusiasts at MIT-MC In-Reply-To: Message of 11 December 1981 06:02 est from Ted Anderson

The idea isn't exactly new. Arthur Clarke wrote a medium-sized novel around this idea several years ago. More technology from the man who invented the value of the geosynchronous orbit-- perhaps the NEXT big value of the geosynchronous orbit?!

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Date: 11 Dec 1981 1452-PST From: Bob Amsler <AMSLER AT SRI-AI> Subject: Laser launches from the Moon or in Orbit To: space at MIT-AI

If atmospheric dissipation of laser's is the major problem to their successful use then wouldn't it be logical to propose them as a means of propulsion in space and from the moon, with the shuttle as the Earth-bound vehicle. Is the shuttle's design economical for OTHER than trips through the atmosphere?, i.e. would we be likely to use the shuttle for orbiting the moon or landing on it. Does No Air = No Shuttle for space travel? -------

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Date: 11 December 1981 2234-EST (Friday) From: Hans Moravec at CMU-10A (R110HM60) To: space at MIT-MC, Wedekind.ES at PARC-MAXC Subject: Penthouse, Yes Sir! CC: Hans Moravec at CMU-10A

Cables for travel to earth orbit are hardly more far out than chemical rockets or cannon. They become practical when bulk materials exhibiting one tenth the laboratory strength/weight ratios of single crystals become available. The best materials these days are Graphite composites and the synthetic Kevlar, both of which have about six times the strength/weight of steel. Another factor of six improvement would do it, well within the measured, not to mention theoretical, limits for normal matter. Chemical rockets have the same problem - only the most energetic fuels will do. The shuttle's main engines operate within a factor of two of theoretical limits.

The anchored cable you mention is the simplest, but also most expensive, of a whole class of cable based schemes. In principal you can build a cable to synch orbit (22,000 mi above surface) and beyond out of any material. The secret is to taper it so that the cross section at each level is just large enough to support the weight of the cable below. The cable starts thin on the ground, gets fatter as you go up, is thickest at synchronous altitude, and thins down again farther out. The problem is that the ultimate taper (ratio of cross section at synch height to that at ground) is exponential in the strength to weight ratio of the material. Using steel you would need a taper of about 10^40. Kevlar or graphite brings it down to about 10^7 (almost reasonable), and with another factor of six it becomes 10 and merely a large engineering project. (The mass ratio of a rocket that can leave earth is similarly sensitive to the specific energy of the fuel.) There are at least two novels centering on this concept - Arthur Clarke's "The Fountains of Paradise" and Charles Sheffield's "The Web Between the Worlds".

A very simple variant of the idea which is practical today is to have long cables spinning in orbit. A long tapered Kevlar cable orbiting the sun at the same distance as earth and spinning about its own axis could, on each half rotation, boost a payload massing about one hundredth of the cable's mass from a Venus/Earth minimum energy trajectory to one that takes it to Mars. The cable would lose orbital velocity in this maneuver, but would get it back when an equivalent mass used it to get back from Mars to Venus. Such inertial energy storage could make the energy cost of solar system commuting practically zero! The maneuver involves docking with the end of the cable, hanging on for about a half turn and letting go, with your velocity vector much changed. The docking with the end of the cable could be made as slow and low g as desired, because a cable with a given delta v capability can be built either short and fast spinning, for high turnover but tricky docking, or arbitrarily long and thin and slow. The mass remains unchanged as you excercise this tradeoff.

So in conclusion, another factor of five to ten improvement in high strength materials will make the space elevator concept in all its variants not only practical but practically humdrum. Ten to twenty years at the outside in my estimate, but I suppose it would not be conservative to count on it. (And we're all conservative, right?)

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Date: 11 December 1981 2237-EST (Friday) From: Hans Moravec at CMU-10A (R110HM60) To: space at MIT-MC, Wedekind.ES at PARC-MAXC Subject: Penthouse, P.S. CC: Hans Moravec at CMU-10A

Further reading available through this net address.

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From: FONER@MIT-AI Date: 12/12/81 01:16:34 Subject: Strong cables

FONER@MIT-AI 12/12/81 01:16:34 Re: Strong cables To: Space-Enthusiasts at MIT-MC I got a report about a year ago from DuPont on their interesting material called Kevlar, which is some five times stronger than steel for its weight and has some other interesting properties to boot.

They mentioned the "skyhook" idea, but commented that even though Kevlar was strong, it wasn't \that/ strong. They did, however, include some useful engineernig data on Kevlar (of course!) and on the skyhook idea (which was nice of them).

I'll dig the report out tomorrow and send any useful parts to the list.

<LNF>

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End of SPACE Digest *******************

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