Date: Thu, 8 Feb 1990 21:14-EST From: space-tech-request@cs.cmu.edu To: "~/st/lists/stdigest" Subject: Space-tech Digest #44 Contents: Jordin Kare Re: Launch Loop Peter Scott Re: Launch Loop David Birnbaum Re: Launch Loop Ralph Marshall Re: Launch Loop William Watson Re: Launch Loop Bob Munck Re: Launch Loop Lou Adornato Re: Launch Loop ------------------------------------------------------------ Date: Mon, 5 Feb 90 19:39:09 PST From: Jordin Kare To: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU, space-tech@CS.CMU.EDU Subject: Re: Launch Loop The "full orbit" launch loop has been proposed several times. I (re)invented it in about 1980, but later found a fairly detailed analysis, I think by Hans Moravec. It is simpler than the Lofstrom loop, but takes much more material, and would be much more subject to cutting by LEO debris/meteorites. Stabilizing it is also nontrivial, although not necessarily more difficult than for the Lofstrom loop. There are serious problems starting the thing up. However, the big problem is getting the mass up there in orbit to begin with -- at least the Lofstrom Loop gets built on the ground. Jordin Kare ------------------------------ Date: Sun, 4 Feb 90 14:50:49 PST From: Peter Scott Subject: Re: Launch loop To: space-tech@CS.CMU.EDU Um, one thing I don't understand about this scheme is how you propose to maintain the vacuum in a 2000km sheath when this ribbon is moving in one end and out the other at 14km/s. Since the ends of the sheath are in atmosphere, how can the ribbon enter the vacuum seal? Is it proposed that the vacuum be dynamically maintained, i.e., that the air is pumped out as fast as it gets in? That's all I can think of. I guess you could mount giant exhaust fans around each end, pumping the air out, and then have extractor stations at intermediate points. Since the ribbon doesn't present much of a cross-section, I assume that the main purpose of the vacuum sheath is to reduce friction rather than lower atmospheric heating. Although there must be some of both while the ribbon is travelling through the atmosphere. Does the sheath contain stator coils or does the ribbon stay clear of the walls by itself? Evn though there are no stresses on the ribbon while it is operating, there must be some when it is starting up. I bet those are considerable. Also the ribbon must be stressed at the stations. Picture the results of a break occurring as it leaves the second orbital station. I like the imagination, but I can't see anyone implementing this over an inhabited hemisphere. Peter Scott (pjs@grouch.jpl.nasa.gov) ------------------------------ [ The Lofstrom paper has a sheath along the entire loop, with pumping stations to maintain vacuum even in the presence of punctures. There are coils along the sheath, I think. The ribbon does have to be started up very slowly, since at that time (and only at that time) there is pressure between segments. As the loop accelerates, it is raised into position. --Marc ] ------------------------------ Date: Tue, 6 Feb 90 12:20:01 MST From: dbirnbau@NMSU.Edu To: space-tech@CS.CMU.EDU Subject: Launch loop questions I'm afraid I don't understand why we need to have the sheath open at all. Is there any reason we can't just close the loop off completely and have a sealed sheath all the way around? As far as I understand the concept, there's no real reason to have it open at all to the air; all interaction with the launch loop is magnetically coupled. I sure don't want to stick my hand in front of some meteoric speed iron! Secondly, firing this thing up is going to be a hell of a problem. If the sheath gets built on the ground, I don't see how to tilt the sheath upright and start firing iron through it. Also, we can't just start shooting the iron into the air and then building a sheath around it. So, how do we get it done; the chicken or the egg first? Something else I was hoping people can clear up for me is that if this loop is high enough and moving fast enough, then the rotation of the earth is going to cause some problems, but I don't want to sit here and calculate everything. Can this be ignored? As for excess energy and heat in the loop, any current picked up by or in the loop can be taken out through magnetic interaction at the bottom, with energy being maintained by a set of coils, loops, or whatever to keep the launch loop in equilibrium. If the loop generates power, great, we can use it to maintain the temperature and speed of everything. Last (but not least) to insure against catastrophic failure of the loop, we can put a huge lake of water wherever the iron is supposed to be coming down. The sheath is not going to appreciably alter the trajectory of this iron, so when the loop fails, all the iron lands in this lake and the water helps to absorb the shock, impact, etc. This was used in one of the Heechee novels by Frederick Pohl. David Birnbaum | "It shouldn't suprise anyone when the Networking, Computer Center | net messes up; the suprise is that New Mexico State University | the damn thing works at all!" ------------------------------ Date: Tue, 6 Feb 90 09:13:28 -0500 From: marsh@linus.mitre.org Message-Id: <9002061413.AA05499@darwin.mitre.org> To: space-tech@CS.CMU.EDU Subject: Launch loop Another thought about the space loop: a previous message indicated that the ribbon itself is under no stress, thus doesn't need to worry about extreme structural strength. However, this seems wrong to me, although I haven't done any math to back it up. If you are taking some of the energy out of the top of the loop to accelerate things, and adding new energy to the loop from the ground, it seems to me that the ribbon is going to be under some compression and tension. When energy is transferred to the objects in orbit, the ribbon will slow down somwhat (since as I understand it the only energy in the ribbon is kinetic), causing the faster moving ribbon coming up from the surface to compress and the slower moving ribbon that is being accelerated back on the ground to under go tension. While I don't know what the rates of acceleration involved are, it seems to me that the transfer of energy would have to be some non-trivial portion of the energy in the loop or you would be paying far too much for overhead. Does this seem like a plausible analysis? Ralph Marshall marsh@linus.mitre.org ------------------------------ [ A stretch totalling on the order of a percent is what Lofstrom envisioned, the slack being taken up by sliding joints between the segments. The overhead just has to be paid for, since you can't afford to have a big change in velocity or the entire trajectory would be shot. -- Marc ] ------------------------------ Date: Mon, 5 Feb 90 21:45:43 CST From: William Watson To: space-tech@CS.CMU.EDU Subject: Re: Launch loop Cc: tzone!watson@cs.utexas.edu Another problem I see with the launch loop concept, although probably small compared to those already aired, is that, as described, the loop is a huge single turn inductor. Due to it's size, this inductor would act as a hellacious antenna. I don't know what the aggregate level of electromagnetic fields is, but I would guess that it is on the order of several microvolts per meter, possibly greater than a millivolt/meter. While this might only add up to a few thousand volts over the length of the loop, it might do well to be included in any calculations. Further, the loop would have to be oriented so as to minimize cutting the magnetic field of the Earth, to avoid inducing further currents in the loop. Of course, if the segments of the loop are insulated from each other, the currents might not be a problem, but you might have to contend with induced voltages. Additionally, if you've got these nice conductive things sticking up through 80 km, the electro*static* fields may give you the willies. As I recall, these are on the order of several volts per meter, with extremely small current capability. Electrostatic motors have been run off of wires held aloft by balloons. Again, thse effects will likely be small compared to the forces required to run this monster. I'm not sure that I saw details of how the loop was to be suspended inside the vacuum tube, other than by alternating sections of (I presume) magnetic fields. If these fileds are static, then you've just described a huge generator. If they are AC fields, then the eddy currents you induce in the loop will cause heating of the iron. Since the iron is in a vacuum, this heat will tend to build up, possibly to the point of melting! On the logistical fron, just where do you suggest putting this thing? I can't really see anyone being any too thrilled about having this huge loop in their neighborhood. (These are pretty big neighborhoods here. 2000 km is 20% of the distance from the equator to the pole!) Would anyone feel comfortable with private planes flying through the loop? What kind of problems might the fast-moving, possibly magnetized chunks of iron induce in the avionics? If you want to keep planes out, how do you enforce the rule? How do you mark the area? How do you explain to the pilots that they have to take a 2000 km detour around your facility? As you can tell, I am an engineer by profession, so I'm not making grand statements about the beast, but am instead nibbling away at the edges. Unfortunately, I'm not enough of a magnetician to be able to answer a lot of the questions I just posed. Any takers? By the way, I assume that no one has any problem with trying to come up with the 4000 cubic meters of iron for the loop, or the materials for the vacuum tube. (No, not *that* kind of vacuum tube! This is 1990, after all! :-) William ------------------------------ [ I think magnetic forces are negligible compared to the other stuff going on, especially since the area surrounded by the loop doesn't actually move with respect to the Earth's magnetic field. Static shouldn't be a problem either. Logistics are another matter. --Marc ] ------------------------------ Date: Tue, 06 Feb 90 14:36:22 -0500 From: Bob Munck To: Space Technology Subject: Re: Launch Loop, Hula Hoop Reply-To: munck@mwunix.mitre.org Marc, My understanding of the Launch Loop is that it generates lift not (only) from the sheer inertia of deflecting the ribbon, but (also) from the fact that the eastbound segment is traveling above orbital velocity for its altitude/orbit. Given that, I don't think that the East and West LEO stations really have to be tethered to the ground, even though they're stationary with respect to it and therefore could be if stability necessitated it. The Hula Hoop (loop entirely around the earth, I assume moving faster than orbital velocity) has the obvious drawback of weighing on the order of 150,000 tons (for a 1x5 cm loop) and needing to be in orbit before it's usable. If we could put that much up, we probably wouldn't really need it. How about a 2.7 cm diameter Kevlar cable with a 1 cm iron core, breaking strength about the same as the Launch Loop (about 300T?)? That's down to a "mere" 50,000T in LEO. Does it really need that kind of strength? Would 50T be enough (cable weighs 8,000T)? That's down under 100 shuttle missions, a distinct possibility. The iron core could be discontinuous, say 1 cm pellets at 2 cm intervals. What's the tension on a cable around the Earth at (say) 50 km altitude rotating at 14 m/s (twice orbital velocity)? What happens if you hang (magnetically) a 5T payload carrier on it? According to Marc's figures in a previous message, you could hang a Kevlar 29 cable from the Hoop to the ground, but it would weigh 100T and barely be able to hold its own weight (with 4x safety factor). Of course, the spokes (cables from the Hoop to ground -- you did know that a bicycle hangs from its spokes, didn't you?) could *taper* from LEO to ground, allowing payloads of multiple tons. OK, where have I wandered? A 40,000 km Kevlar hoop (i cm diameter) with a (possibly discontinuous) iron core (j cm diameter) around the earth at k km altitude, traveling at x m/s. Stations at y km intervals capable of accelerating the hoop by magnetic coupling to the iron with power coming up a (superconducting) cable from the ground. Some of the stations would have an elevator capable of lifting an z ton payload and hanging it on the hoop, which would accelerate it up to orbital (or more) velocity. I like it. Possible values: i=1.5, j=0.6, k=75, x=15, y=2000, z=5. Starting up is fairly easy (but k might be low) and the broken cable mode flings cable outwards and drops stations straight down, relatively safe. What factors have I missed? How stable is it with wind and random payloads on the spokes? -- Bob Munck ------------------------------ Date: Tue, 6 Feb 90 12:47:02 CST From: Lou Adornato To: space-tech@CS.CMU.EDU Subject: Re: Launch loop .... Something no one has mentioned about this concept is that it could also be used to de-orbit something in a retrograde orbit. While this wouldn't pose that much of a benefit on Earth, where you can use aerobraking, it _would_ be a real benefit on the Moon, where the only alternative is a powered descent. IMHO, a launch/deorbit loop would be a much better alternative to a lunar colonization plan than a mass driver, because you wouldn't need fuel for launching _or_ landing. Lou Adornato | Statements herein do not represent the opinions or attitudes Cray Research | of Cray Research, Inc. or its subsidiaries. lfa@cray.com | (...yet) ------------------------------ End of Space-tech Digest #44 *******************