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Space Forum / Space Flight / March 2006Beanstalks...
A couple of questions on this space technology: 1)I was under the impression that carbon nanotubes, if manufacturable at reasonable lengths (~ a foot?) would make it possible to construct a beanstalk. Then I came across references saying that NO physical material would be able to take the stresses involved on Earth (though a moon or Mars beanstalk was possible). Which is true? Or are there different beanstalk designs which have orders of magnitude difference in the calculated forces, and are there reasons that the higher-stress version would be used? 2) Assuming you're building a beanstalk, what methods are likely to be used to anchor it?  SignatureSea Wasp /^\ ;;; Live Journal: http://www.livejournal.com/users/seawasp/ > 1)I was under the impression that carbon nanotubes, if manufacturable > at reasonable lengths (~ a foot?) would make it possible to construct > a beanstalk. Then I came across references saying that NO physical > material would be able to take the stresses involved on Earth (though > a moon or Mars beanstalk was possible). A lot of materials could be used, the taper factor required (the bit of the cable at geosynchronus altitude generally needs to be thicker than that at either end) makes the gathering of sufficient material to be problematic. An untapered cable needs a tensile strength comparable to the theoretical limit. > 2) Assuming you're building a beanstalk, what methods are likely to > be used to anchor it? Something along the lines of an oil rig has been suggested for early versions. Have a read of http://en.wikipedia.org/wiki/Space_elevator to get started, but note the section on the tensile strength required doesn't agree with section on calculating the taper. Anthony > A couple of questions on this space technology: > [quoted text clipped - 3 lines] > material would be able to take the stresses involved on Earth (though > a moon or Mars beanstalk was possible). You don't need long lengths of fiber. As you increase the fiber length, over a few nm, it starts to steeply rise, till you get to a few um, when it's not really rising much more. Consider cotton - the fibers are nowhere near a foot long, but it's strong enough. The rate at which it rises depends on the matrix which the nanotubes are in. > Which is true? Or are there different beanstalk designs which have > orders of magnitude difference in the calculated forces, and are there > reasons that the higher-stress version would be used? A non-tapered beanstalk on earth is not possible with any known material. A tapered one is possible, given certain constraints. The optimum tether is (pretty much) one with a given tension on the anchor point on the ground, with the material at (say) 90% of nominal breaking strain. As you rise up the cable, the cable has to support more of its weight in addition to the tension on the anchor, so needs to increase in crossection. (the tension on the cable is decreased when a load goes up it, and cannot fall below 0 if you want it stable) When you do all the maths, it turns out that the first bit of the cable tapers quite steeply, then the taper reduces as gravity falls off, with the cable reaching a maximum thickness at GEO (about 40000Km) and then gradually tapering out to a small counterweight. The further out you put the counterweight, the lower the mass of the whole system. The key to all of this is the taper ratio. (from memory), the maximum diameter is about 1.5 times the minimum diameter for theoretically optimal nanotubes. For 25GPa, it's 10 times, and for 12GPa, 100 times, and for 6 (the best of current non-nanofibers) it's about 1000. Elevators become theoretically practical (IMO) when the total payload that can be moved by the elevator in a year or two is about the same as its mass. If this can't be done, then the massive capital investment of launching it will only be repaid in dozens of years. If it can, then you end up with an elevator that can double in payload every few years, scaling up fairly rapidly to truly enormous sizes. There are other designs, but unfortunately, none gets significantly better than this - starting the tether from a balloon at 100Km only decreases the mass by a vanishingly small amount for example. Ian> for 6 (the best of current non-nanofibers) it's about 1000. Someone must have looked at having the base of the elevator up above the atmosphere by now. You could have the base moving at 1000 m/s relative to the equator, at a few hundred km altitude, which would make a pretty reasonable target for a 767 with rocket assist. The primary advantage is the lower orbit and thus the drop in length. 1000 m/s gets you a 30% drop in length. Since you get to higher v^2/r values at lower altitudes, I would think it would improve the taper value quite a bit, to something more like 30-50 for 6 GPa material. If people insist on having the bottom of the thing on the ground, the orbiting mode might be a cheaper way of lifting all that mass. Somebody wake me up when the taper value is around 10 for material that is already made in bulk, with safety margins. > Someone must have looked at having the base of the elevator up above > the atmosphere by now. http://members.aol.com/Nathan2go/SPELEV.HTM http://www.strangehorizons.com/2003/20030414/rope.shtml > You could have the base moving at 1000 m/s > relative to the equator, at a few hundred km altitude, which would make > a pretty reasonable target for a 767 with rocket assist. If you can get a 767 into vacuum, with safety margins, without building a virtually all-new vehicle, let me know. :) Mike Miller > If you can get a 767 into vacuum, with safety margins, without building > a virtually all-new vehicle, let me know. :) The NF-104 experience suggests that this might be possible. See www.nf104.com. Not easy, but possible. SignatureKevin Willoughby kevinwilloughby@acm.org.invalid In this country, we produce more students with university degrees in sports management than we do in engineering. - Dean Kamen Actually, building in both directions from LEO, with a platform at either end and your weaving/loom system in the middle, you'll gradually raise the orbit of the CG til it reaches GEO. All along the way, it would be practical at reducing necessary delta-v to orbit, thus slowly reducing launcher requirements and increasing launcher payload capacities. Eventually you would have a platform just above the atmosphere at the lower end of the cable, which SS1 type tourist buggies could reach easily. If by that point the CG is at GEO, then the platform is perfectly motionless wrt the earths surface below, and your SS1-class vessel can put all of its delta-v into reaching whatever altitude the platform is at, landing on it like an aircraft carrier, and dropping off and picking up passengers and/or cargo. This is the point at which things really start to get interesting. > Actually, building in both directions from LEO, with a platform at > either end and your weaving/loom system in the middle, you'll gradually > raise the orbit of the CG til it reaches GEO. All along the way, it However. this means lots of launches, or deliveries. If you can possibly launch the tether in one lump - with a very small payload - say a ton, and carry the rest of the tether up it, to strengthen it till you hit 100 tons payload (for example), then you don't need any launches at all. > Ian> for 6 (the best of current non-nanofibers) it's about 1000. > > Someone must have looked at having the base of the elevator up above > the atmosphere by now. If it were not connected to the earth then the change in the potential energy of the payload would be extracted from the kinetic and potential energy of the beanstalk, which would be self-defeating. As I understand it, the tether's CoG would be *ever* so slightly beyond geo, and the base of the tether would be under tension. This will cause momentum transfer from the earth's rotation, providing the launch energy. James M Its important to consider what is ment by Center of Gravity rather than the Center of Mass. An object the with dimensions of a space elevator does not really have a well defined IMHO CoG. That is gravity is not constant or even close to constant over the length of the cable, and then we also need to consider centrifugal force. Because engineers use both terms to mean the same thing i will assume that you mean the center of mass. In that case the center of mass is in fact quite a bit higher than geo. Its because gravity goes down proportional to r**2 while centrifugal force goes up proportional to r. Thus mass of the cabel closer to the earth tends to be pulled towards earth more than the mass futher out, and more mass futher out is needed to ballance it. OK i'm really not that good at explaining these things. But hopefully you get the idea. Greg [a pretty nice description of the mathematical mess that is involved in talking about the beanstalk's "Center of Gravity"] So, what are the arguments pro and con for having the high end of the beanstalk anchored to some Big Honking Rock (tm applied for) just beyond (originally _at_) geosynchronous, where the knitting machines can sit, verus running the beanstalk much farther out, and having a considerably smaller B.H.R., or just a garden variety space station, as the far point anchor, given that the beanstalk's stationkeeping and momentum losses for upbound cargo can be assisted by dragging a current-fed wire through the earth's electromagnetic fields anyway? Or am I again in muddled mode? xanthian, guessing that the momentum problem, which in the long term might balance out for upbound versus downbound cargo, still must be solved in the short term, where most cargo (almost all cargo?) is likely to be upbound. > So, what are the arguments pro and con for having > the high end of the beanstalk anchored to some Big [quoted text clipped - 3 lines] > farther out, and having a considerably smaller > B.H.R., There will almost certainly be a sizable counterweight and it only really makes sense when its quite a nit past GEO. The first reason to have a counterweight is dynamic stability, to keep everything from shaking itself apart. Its huge so destructive resonate modes will be measured in hours. There are a few reasons to put the weight way past apart from what i mentioned above. First would be weight, the further out the lighter the counterweight needs to be, but then the more cable. The second is usefulness. If the cable is long enough, releasing a payload at the end will have escape velocity. I don't know how long it needs for this though. Lots of people keep coming up with all these momentum and energy arguments. Assuming that there is no under-damped resonate modes, then any payload will be by nature a very small perturbation on the structure generally. Long term averages are not nearly as bad as many seem to think. The cable either extracts energy from the earths rotation or you need some low power thrusters or electrodynamic th ether. These are not problems compared to finding a material that you can make enough of and afford to launch the "bootstrap" mass. greg > There will almost certainly be a sizable counterweight and it only > really makes sense when its quite a nit past GEO. The first reason to > have a counterweight is dynamic stability, to keep everything from > shaking itself apart. Its huge so destructive resonate modes will be > measured in hours. Hmm, as with Niven's _Ringworld_, a beanstalk may need active countermeasures to keep it from yanking itself out of orbit. Would the much mentioned concept of using the earth's electromagnetic fields for marching satellites around also work for a beanstalk, so that lengths of powered cable up and down it's length could be tuned to counter and thus damp the resonances? xanthian. Fun off-topic factoid: about 32 years ago, I was driving a research ship to support el Nino research by putting down and picking up deep ocean buoys in 4000 meters of water. A "many metric tons"-stressed Kevlar(tm) cable that long is a very low period musical instrument, in the passing tidal current flows my passengers were studying. The deep infrasound it created turned out to attract huge schools of huge tuna, visible through the clear water 100 meters down as if they were fleets of trucks doing synchronized maneuvers, which infrasound-tropic fish the crew then gleefully harvested with hook and line, to barbecue on deck for lunch. Think of that next time you hear some stranger humming, or a cellist getting in tune. In sci.space.tech Kent Paul Dolan <xanthian@well.com> wrote: > [a pretty nice description of the mathematical mess > that is involved in talking about the beanstalk's [quoted text clipped - 8 lines] > B.H.R., or just a garden variety space station, as > the far point anchor, given that the beanstalk's The total system mass is considerably larger if you end it at a large rock, than that if you taper it out. Plus, if you taper it out, the very tip has a significant velocity, which can remove a fair bit of needed rocket propulsion of launched payloads, if timed right. The problem with the BHR approach, is that first you've got to get a really huge rock into orbit. Then there is the fact that doing this will take either large masses, or nuclear propulsion. If at all possible, the way I'd do it would be: Two 100T-GEO class launches. The first one has two massive reels of 20000Km of pre-tapered fiber. This is reeled out (both sides at once) slowly, and with luck and physics eventually orients itself the correct way. The other launch takes up a small SPS, and cargo handling hardware, which gets parked at the middle. Then you attach it at the bottom, and start sending up 1t payloads of new cable, and attaching it. I suspect the best way to do this is to have lots of ~10Km segments of cable, and a machine capable of going to one end, putting the new one in parallel with the old one, and then removing the old one. Or putting in a new segment, and winching the existing cable onto the ground to lower it all 10km below it. Gradually replace the segments with heavier ones, until you can launch hundreds of tons. > The problem with the BHR approach, is that first you've got to get a > really huge rock into orbit. I think pretty much everybody takes it for granted that the big rock which serves as a counterweight is in fact a NEA which has been captured into Earth orbit. I've always had the same view you have wrt the advantage of going with a greater length of cabling vs. a counterweight. I suppose the counterweight keeps getting brought up with the above assumption, i.e. every ton which is counterweight is a ton we didn't have to manufacture. (I know it's not a 1-1 ratio, but I think you get my point.) SignatureRegards, Mike Combs ---------------------------------------------------------------------- By all that you hold dear on this good Earth I bid you stand, Men of the West! Aragorn > So, what are the arguments pro and con for having > the high end of the beanstalk anchored to some Big > Honking Rock ... or just a garden variety space station, as > the far point anchor, given that the beanstalk's > stationkeeping I'm of the "big honking rock" fan club. The big honking rock doesn't need to be rock - it can be a metallic asteroid or some other valuable asteroid. If travel on the orbital elevator gets cheap enough, it might be worthwhile to deliver ore (or refined metals) from the anchor rock to Earth. As for making up momentum, putting the anchor rock beyond geosynchronous should result in the Earth's rotation replacing any momentum lost by the beanstalk. Mike Miller > > Ian> for 6 (the best of current non-nanofibers) it's about 1000. > > [quoted text clipped - 4 lines] > energy of the payload would be extracted from the kinetic and potential > energy of the beanstalk, which would be self-defeating. Not necessarily for the purposes of transporting construction materials, nor is it self defeating if the trade cycle sees equal masses travelling down the tether as up it. The tether robs rotational energy from the earth anyways, without being anchored, especially as most earth anchored plans involve it being anchored to a floating oceanic platform. This would be no different than anchoring the cable to a platform floating above the atmosphere. It is the tidal influence on the mass that causes the theft of Earth's angular momentum. > As I understand it, the tether's CoG would be *ever* so slightly beyond > geo, and the base of the tether would be under tension. This will > cause momentum transfer from the earth's rotation, providing the launch > energy. This would be useful in the case of a lunar L-1 tether. Given, however, that GEO tethers use floating ocean platforms, the CG imbalance is balanced back to GEO by the mass of the anchor platform. Having the anchor platform on the ocean or above the atmosphere is immaterial. The fact is that you can put energy into the system from solar and other sources by running a current through the tether to interact with the earth's magnetic field (as in Brin's "Tank Farm Dynamo" story), so any possible losses from moving more mass up the thether than down it can be made up for electrically. > Someone must have looked at having the base of the elevator up above > the atmosphere by now. You could have the base moving at 1000 m/s > relative to the equator, at a few hundred km altitude, which would make > a pretty reasonable target for a 767 with rocket assist. Yes, the are often called LEO elevators IIRC, but 767 with rocket assist?. Unfortunatly they still require quite good materials anyway to get a decently low speed (orbit velocity does not go down that quikly with height). You could run a GEO (GEO as in period and inclination) space elevator were the end still sits 1000km up or a little more. This could a simpler way to avoid debris, while the dV is still very modest for a rocket. The only thing that is a bit niceer on the specific strength of materials it rotavators. greg > > Someone must have looked at having the base of the elevator up above > > the atmosphere by now. You could have the base moving at 1000 m/s [quoted text clipped - 8 lines] > little more. This could a simpler way to avoid debris, while the dV is > still very modest for a rocket. Zubrin refers to it as a hypersonic skyhook. The important characteristic for a material is "characteristic velocity, U = SQRT(Strength / Density). http://www.islandone.org/LEOBiblio/SPBI1MA.HTM has a table. Kevlar has a U of 2.2km/s, and nanotubes in theory 15km/s according to this. According to Zubrin, kevlar is 1.2 to 1.6. A skyhook with a material with U of 2km/s, and tip velocity relative to Earth of 5km/s would have to mass 11 times its payload. > The only thing that is a bit niceer on the specific strength of > materials it rotavators. A problem with elevators and skyhooks is the need for an elevator with power source. This means that cargos would have to spend days travelling through the radiation belts. Rotovators get round this problem by flinging cargos through the radiaton belts in a matter of 10s of minutes. They also need no elevator mechanism, and can be lighter than an equivelant skyhook. > Zubrin refers to it as a hypersonic skyhook. Its not in the atmosphere, so this is a bad name for it (what is hypersonic speed when there is no speed of sound!). My understanding was that a hypersonic skyhook was infact a rotovator that had one end dipping into the atmosphere. But i don't want to get into the names of things. Lets just agree that names are not well defined at this point. I also agree that rotovators make a lot more sence. But still end up big by todays stardards and space junk is a big problem. Greg > A problem with elevators and skyhooks is the need for an elevator with > power source. This means that cargos would have to spend days > travelling through the radiation belts. "Days"??? Why "elevator"? Once you have a beanstalk, and can thicken it pretty much indefinitely using itself as a hoisting mechanism and (limited only by keeping its mass below the order of planetary size masses, so it isn't flinging the Earth itself around in its orbit) there really isn't much reason _ever_ to stop increasing its strength/diameter by using "free" power to run "cheap"(*) bulk-up loads of nanotube fiber up the beanstalk to help it gain mass) think of it as the base stratum for some (very vertical) construction, and run a railgun up it, whose mass could by then be trivial w.r.t. the mass of the beanstalk. Even half a G of continuous acceleration/deceleration gets stuff from here to there quite promptly, IIUC. xanthian. (*) Granted, after a while, you're going to need to mine a gas giant for the needed carbon for the nanotubes, or you'll come up short one ecosystem. > A non-tapered beanstalk on earth is not possible with any known material. That's not actually true. Blaise Gassend has shown that a non-tapered beanstalk is possible with nanotube material strength of about 65 GPa (which is the same target strength needed for a conventional beanstalk). A non tapered beanstalk has much less capacity though. But there is a big advantage during construction of a non tapered beanstalk- it's actually possible to create a loop out beyond GEO and back to the ground and spin it using a motor on the ground using untapered fiber. It turns out that construction is faster than the normal construction approach using laser powered climbers. You can exponentially increase the cable thickness from the ground; so basically you would be using carbon nanotube to lift more carbon nanotube, and with efficient/cheap mechanical power supply from the ground. on Wed, 15 Mar 2006 19:40:25 -0000, Ian Woollard <ian.woollard@gmail.com> sez: ` Ian Stirling wrote: ` > A non-tapered beanstalk on earth is not possible with any known material. ` That's not actually true. Blaise Gassend has shown that a non-tapered ` beanstalk is possible with nanotube material strength of about 65 GPa ` (which is the same target strength needed for a conventional ` beanstalk). A non tapered beanstalk has much less capacity though. Well, if "known material" is read as "known to be manufacturable to the required length" then AFAIK the statement is true. ` But there is a big advantage during construction of a non tapered ` beanstalk- it's actually possible to create a loop out beyond GEO and ` back to the ground and spin it using a motor on the ground using ` untapered fiber. It turns out that construction is faster than the ` normal construction approach using laser powered climbers. You can ` exponentially increase the cable thickness from the ground; so ` basically you would be using carbon nanotube to lift more carbon ` nanotube, and with efficient/cheap mechanical power supply from the ` ground. I'm trying to visualize what you are describing here. A pair of cable ends at the earth mounted next to each other and rotated about a common centre? Or a cable pulled through a pulley like a hoist? I assume the latter as the former doesn't make any sense to me, though "spin" doesn't quite seem like the right word for that - it would be parallel lines rather than a ring. Signature========================================================================== vincent@triumf[munge].ca Pete Vincent Disclaimer: all I know I learned from reading Usenet. > Which is true? It depends on what you assume about the properties of carbon nanotubes. It's one thing to make nanotubes a foot long. It's another to make a structure that maintains near-perfect nanotube properties for over 22500 miles. > 2) Assuming you're building a beanstalk, what methods are likely to > be used to anchor it? For an Earth beanstalk? An asteroid with a mass much greater than the beanstalk mass at an altitude above the beanstalk's center of mass (i.e., above geosynchronous orbit). Mike Miller > A couple of questions on this space technology: > [quoted text clipped - 3 lines] > material would be able to take the stresses involved on Earth (though > a moon or Mars beanstalk was possible). With a reasonable amount of tapering, carbon nanotube will do it. An untapered nanotube elevator would not work. No one would ever build an untapered elevator (until the day that buying a 50,000 km spool of quarter-inch unobtanium rope is cheaper than buying separate spools of 1/4, 3/16 and 1/8 inch rope and tying the ends together). > Which is true? Or are there different beanstalk designs which have > orders of magnitude difference in the calculated forces, and are there > reasons that the higher-stress version would be used? > 2) Assuming you're building a beanstalk, what methods are likely to > be used to anchor it? Just about anything would work. The tension at the ground end is very small (of order a few times the useful payload). You just have to hold on to it. One idea is to tie it to a ship, which lets you move the Earth end away from weather, and to set up a ~hundred km exclusion zone to keep out terrorists, wayward airplanes, etc. which is easier to do in mid-ocean than on land. You can even divide the tether at the Earth end and tie it down in several places, for redundancy. Good sources for information: http://www.liftport.com/ http://www.spaceelevator.com/ http://www.tethers.com/ SignatureDavid M. Palmer dmpalmer@email.com (formerly @clark.net, @ematic.com) > A couple of questions on this space technology: > [quoted text clipped - 10 lines] > 2) Assuming you're building a beanstalk, what methods are likely to >be used to anchor it? The requirements for a Space Elevator is currently being investigated at the Institute for Scientific Research. http://www.isr.us/research_es_se.asp Paul C james.moughan@sunderland.ac.uk wrote: >Not necessarily for the purposes of transporting construction >materials, nor is it self defeating if the trade cycle sees equal >masses travelling down the tether as up it. Do you mean by this that for every trip up (or down), an equal mass must be sent in the other direction to counter the stress created in the tether? BR |
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