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 ---------------------------------------------------------------------- Date: 16 Dec 1981 1247-PST From: Jim McGrath 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 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 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 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 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 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 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? ------- ------------------------------ End of SPACE Digest ******************* ----------------------------------------------------------------- gopher://quux.org/ conversion by John Goerzen of http://communication.ucsd.edu/A-News/ This Usenet Oldnews Archive article may be copied and distributed freely, provided: 1. There is no money collected for the text(s) of the articles. 2. The following notice remains appended to each copy: The Usenet Oldnews Archive: Compilation Copyright (C) 1981, 1996 Bruce Jones, Henry Spencer, David Wiseman.