Aucbvax.5487 fa.space utzoo!decvax!ucbvax!space Wed Dec 16 04:07:11 1981 SPACE Digest V2 #61 >From OTA@S1-A Wed Dec 16 03:59:33 1981 SPACE Digest Volume 2 : Issue 61 Today's Topics: Re: Linear accelerators as launchers. Skyhooks Planetary Science in extremis A few more comments on skyhooks multiple-laser launching systems Spaceports Elevators into Space Momentum ---------------------------------------------------------------------- Date: 15 Dec 1981 0855-PST From: Jim McGrath Subject: Re: Linear accelerators as launchers. To: ota at S1-A cc: space at MIT-MC My original message only assumed that accelerators be used as the "second stage", ie you get them up to suborbital speed and then accelerate them to orbital velocity. I did a short linear search of my library, but could not find the article. Any pointers from out there? I think the correct equation to use here is 2*A*L = Vf^2 - Vi^2 (Vf = final velocity, Vi = initial velocity). Then if we keep a= 100m/s^2 (10g), then we have for a final velociy of 8km/s (a couple of hundred miles up), then we have L Vi 1 km 7.93 km/sec 10 km 7.87 km/sec 50 km 7.32 km/sec 80 km 6.93 km/sec 125 km 6.24 km/sec 200 km 4.89 km/sec If we let A = 100g, then we get 1 km 7.87 km/sec 5 km 7.32 km/sec 10 km 6.63 km/sec 20 km 4.89 km/sec So we would need on the order of tens of thousands rings in order to accelerate from about 5km/sec. Two conclusions can be drawn - you want this system first for mass transport, not person transport, and it is easier to build than a skyhook by a couple of orders of magnitude. Thus it does appear to be an interesting step between rockets (or even lasers) and skyhooks. Jim ------- ------------------------------ Date: 15 Dec 1981 0917-PST From: Jim McGrath Subject: Skyhooks To: space at MIT-MC Hans - thanks for the references. Re skyhook stability - my comment on vertical stability was meant as a comment on principle - ie that you do have to be careful (ie anchor it, prevent the payload/structure mass from getting too high, etc...), and that puts greater demands on the engineering (it also sets you up for diasters - say you lose your space anchor - then the beanstalk comes crashing down) Once again, on the shear forces I was not so much thinking of operational problems, but potential diasters - like someone slamming into it in a passanger plane. The problem is that such diasters WILL happen, and that the potential loss of such an expensive structure in one will deter people from building them unless the engineering and safty precautions are VERY good. Thus I do not expect skyhooks within 20 years - more like 20 to 50 years (ie after we have a lot of space engineering experience and experience building VERY large structures). Jim ------- ------------------------------ Date: 15 Dec 1981 1222-EST From: MPH at MIT-XX Subject: Planetary Science in extremis To: space at mc This is an excerpt from an article in the December 18 issue of Science, "Planetary Science /in extremis/." Copyright AAAS, 1981. [Start of excerpt] The office of Management and Budget (OMB) wants the National Aeronautics and Space Administration to virtually cease its planetary exploration activities as of fiscal year 1983. The order was given to the space agency privately on 24 November. Although NASA will undoubtedly appeal the decision and try to negotiate a compromise, it has little time. The FY1983 budget must be ready for submission to Congress in January. The OMB proposal, as it stands, includes the cancellation of the Galileo orbiter/probe mission to Jupiter, which is already nearly built, and the Venus orbiting imaging radar (VOIR), which had been penciled into budget projections as a new start for 1984. The only mission that will /not/ be affected is Voyager 2, now on its way toward encounters with Uranus and Neptune. [End of excerpt] ------- ------------------------------ Date: 15 Dec 1981 1017-PST From: Ted Anderson Subject: A few more comments on skyhooks To: space at MIT-MC Note that a skyhook that looses its earth surface anchor doesn't fall down it falls UP. As to safety, I would like to emphasize here that a structure that is some 100,000 Kilometers long and weighs millions of tons is not likely to be inconvenienced by some thing as trivial as a C-5A running into it at 500 MPH. Though I don't deny that it might be possible to design a geosynchronous skyhook that would fail in this situation I think it would be hard to do. Keep in mind that it needs a strength reserve (strength in excess of that needed to maintain static equilibrium) to support the lifting of, say, a million tons per year to GEO or it wouldn't have been built in the first place. It is not likely to be a fragile object. ------------------------------ Date: 15 Dec 1981 1317-CST From: Jonathan Slocum Subject: multiple-laser launching systems To: space at MIT-MC As for JP's comment, I wasn't trying to nastily dampen enthusiasm for getting out there SOMEHOW, it just doesn't/didn't look like riding a laser beam thru the atmosphere would be viable. Question-raisers are not always enemies: maybe some are just ignorant friends willing to risk appearing like fools if it will stimulate discussion/learning. Or maybe I AM a fool, but a flat-worlder I am not! The multiple-beam answer to blooming is certainly thought-provoking. Each beam runs into the same problems I raised before, only (it is hoped) at a manageably small level. Ohmic heating will still take place, albeit to a smaller extent, and the air in the beam path will still expand. Argument: the beam will be off before the expansion gets going good. Counter-point: yeah, but the NEXT time the beam comes on the air will be in its expanding/ed state, and the next, and the next... 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). So we have the following variables: number of lasers (affecting the pulse length and beam deflection between pulses, hence ohmic heating potential); how widespread they are (affecting the extent to which they tend to share a common beam path, i.e., appear in effect like a single beam); how high they are (how much air they have to punch thru); and of course their individual power output & frequency. Without the expertise to run the numbers, I still suspect that inefficiencies at the target will require a large total beam power, which will require a LOT of (expensive?) lasers if the average per beam is to be kept sufficiently low. If high laser altitude and large numbers of them are also necessary, it will take LOTS of mountains to get around the path-sharing problem. Dispersing the lasers also tends to increase the laser-to-target distances (through the air). By the way, the hull of this vehicle is gonna have to ba a damn good mirror, lest it be subject to melting; what happens to the reflected energy? Does anybody REALLY want to watch this thing go up?? Cheers. -Jonathan ------- ------------------------------ Date: 15 December 1981 2202-EST (Tuesday) From: Hans Moravec at CMU-10A (R110HM60) To: space at MIT-MC Subject: Spaceports CC: Hans Moravec at CMU-10A The orbiting linear accelerator article (I thought both the article and the idea were extremely good) was Roger D. Arnold and Donald Kingsbury, The Spaceport, Part 1: Analog v99 #11 November 1979 pp 48:67 and Part 2: Analog v99 #12 December 1979 pp 61:77 They propose an accelerator length of 600 km subjecting payloads to 5g, with an active stiffening system on the structure. Neither the mass nor the complexity is obviously lower than a cable performing the same task: Imagine a cable in low earth orbit that spins in the plane of the orbit so that the spin just cancels the orbital velocity at the points where the cable tips come closest to the ground. The cable is like two spokes of a giant wheel that is rolling on the earth's surface at orbital speed. A flying machine can now jump up and grab the cable end at its lowest and slowest point (for a few seconds the tip is actually stationary with respect to the ground, just like the portion of the rim of a rolling wheel in contact with the ground is momentarily stopped). The cable can actually enter the atmosphere (and with terminal guidance and high precision, it could even kiss the ground), so the job of docking with it is simpler than for the linear accelerator spaceport. The payload then hangs on to the end, and lets the cable flip it around to be flung off at high velocity later. At the top of the swing the cable tip is moving at twice the (orbital) velocity of the cable's center of mass, and if the payload lets go then, it is sent off with a factor of more that sqrt(2) beyond escape velocity. The cable loses some orbital momentum in the process, wich it can regain from incoming payloads, or high specific impulse engines at its middle, just like the orbiting linac. Such a non-anchored skyhook can be build low and spinning fast, or long and orbiting high and turning slow. If you build one to orbit at synchronous height, it has most of the properties of the synchronous beanstalk. It turns out that there is a lower orbit which is optimum in the sense that it minimizes the taper required by the cable. The length of such an optimum cable is one third the diameter of the earth (this is a general principle; cute, huh?). So we have the cable about 4000 km long, with its center orbiting 2000 km above the surface. With a material that can make a beanstalk with a taper of 100, we can make an optimum rolling cable like this with a taper of only 10, using 100 times less material for the same payload capacity. The rolling cable can hoist 1/50 of its own mass on each touchdown. Such touchdowns happen every 20 minutes, in succession at six equally spaced points around the orbit. The cable is very long relative to the depth of the atmosphere, and because of the scale and the cycloidal shape of the tip trajectory, the cable ends appear to descend from the sky vertically on each touchdown, with a continuous upward acceleration of 1.4 g. They stab downwards into the atmosphere at a tame 2 km/sec, slow to a dead stop for an instant at their lowest point, and accelerate gently upwards to leave in the same way. The tip stays in the atmosphere five minutes each touchdown. The material of the cable (if graphite) has a tensile strength of at least 3 million pounds per square inch, so one or two square inches at the cable ends is certainly sufficient for most tasks. The average cross section would then be about five square inches. This gives the whole rolling skyhook somewhat the scale and geometry of a typical transatlantic telephone cable, except that the graphite is five times less massive than the copper and steel of the phone cable. It seems at least possibly cheaper to me than the accelerator, but cost analyses would have to decide. The big advantage of the accelerator is that it can be engineered entirely with known materials and techniques, while the cable awaits the next increment in high strength materials. Re: collisions with aircraft, I agree that most of the time a taut 3 million psi, inch diameter, cable would be to a slow moving aluminum plane much like a cheese cutter is to a piece of cheese. Almost all of the cable is above the atmosphere, however, and a collision at orbital velocity would be another matter. The hit probability is no greater than for a big satellite. The rolling cable is 4000 km long and about 5 cm in diameter. This gives it the same "frontal" surface area as a 500 meter diameter sphere. A collision would not be much of a disaster on the ground, because the small cable diameter insures that the cables burns up on reentry (though the sheet of flame across the sky as several thousand kms burn simultaneously should be interesting). Still the cost to the owner (or insurance) and to the payload on the cable at the time certainly make this event undesirable. Some kind of Norad (or coast guard) traffic control or monitoring would seem worthwhile. Given a few hours or days warning a skyhook can dodge a few kilometers, but it will probably be the least maneuverable object in earth orbit. It will probably have to be given right of way most of the time, just as law of the sea gives oil tankers right of way. Here are a few more skyhook references: Arthur C. Clarke, The Fountains of Paradise, Harcourt, Brace and Jovanovich, 1978. Charles Sheffield, The Web Between the Worlds, Ace SF, 1979. Charles Sheffield, How to Build a Beanstalk, Destinies Vol 1 #4, Aug-Sep 79, pp 41:68, Ace books. Charles Sheffield, Skystalk, Destinies Vol 1 #4, Aug-Sep 79, pp 7:39 Charles Sheffield, Summertide, Destinies Vol 3 #2, Aug 81, pp 16:84 ------------------------------ Date: 15 Dec 1981 22:29:14-PST From: decvax!watmath!bstempleton at Berkeley To: decvax!ucbvax!space@Berkeley Subject: Elevators into Space Correct me if I'm wrong, but might there be a problem with the fact that the Elevator is pulling a lot of mass into space against friction? As you pull the mass up, you give it momentum, and you thus give some downward momentum to the tower itself. All the momentum should be conserved at the end, but you keep pouring energy into the elevator which gets used up as friction, and the tower is a little bit lower every time. Perhaps the friction is very low, but over the whole 35,000 km it could add up. ------------------------------ Date: 15 Dec 1981 2338-PST From: HPM at S1-A Subject: Momentum To: space at MIT-MC No, such momentum loss is not a problem. The anchored cable-ballast system is ultra stable and self correcting - it is a weight at the end of a long string attached to an unstoppable spinning turntable. Any momentum loss by the cable (which must be by dropping off or picking up mass - as you say momentum is conserved in a closed system) causes the cable to lean forward or lag backward behind its anchor position. Its force vector is then no longer along a radius of the earth, but has a tangential component. This component taps the earth's angular momentum, a tiny fraction of which transfers itself to the cable until it straightens out again (this needs damping, otherwise you just get a 100,000km pendulum!). By riding loads up far beyond synchronous orbit you can actually get net energy from the system - centrifugal force will launch payloads for you, with energy that is drawn from the rotational energy of the earth. The non-synchronous rolling cable is on its own, however, and any momentum it loses in launching payloads must be regained sooner or later by landing other ones, or operating thrusters (or sails? what a kludge that would be), else the cable crashes. ------------------------------ 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.