Thanks, Chris et al. Who knew? Anyway, couple of thoughts. If the tether is made of carbon, that's more than a few dollars worth of carbon sequestration, esp if manufactured from air or waste CO2 such as what Newlight does: http://newlight.com/
Secondly, once one Sky Elevator is in place it can be used to spawn additional elevators, this time with manufacturing on the ground and at a fraction of the original cost. Lastly, with the tether(s) and ballast(s) whipping through the sky, there's got to be more than a few kW of static electricity generated. Harvest that energy (via insulated strands in tether) and you can replace more than a few coal fired power plants. OK, I understand that off-grid, ocean based tethering is best - no problem. Plug in my patented electrogeochemical cell* out there on the tether barge and generate C-negative H2 (energy carrier back to mainland), consume excess ocean/air CO2, and generate beneficial ocean alkalinity (mitigating ocean acidification). A grateful planet thanks us - no? *http://www.pnas.org/content/110/25/10095.full.pdf Thanks again, Greg -------------------------------------------- On Wed, 8/19/15, Chris Burgoyne <[email protected]> wrote: Subject: Re: [geo] space elevator To: "Greg Rau" <[email protected]>, [email protected] Cc: "Hugh Hunt" <[email protected]>, [email protected] Date: Wednesday, August 19, 2015, 8:06 AM This isn't really geoengineering but because this is a topic that keeps cropping up:- The space elevator, while superficially attractive, has major flaws. The fundamental problem is that it relies on balancing gravity and centrifugal force. This can only happen at one height, some 36,000 km above the earth's surface, which is of course where geostationary satellites are placed. So the centre of gravity of the system has to be at that height; any material below that will be hanging from that centre of gravity, and thus in tension, and there must also be a balancing amount of weight higher than that, which will be providing an outward tension. So we can then ask "How do you build it?". We have already shown that building towers upwards from the earth to even 20 km height is impractical, (http://www-civ.eng.cam.ac.uk/cjb/papers/cp94.pdf) and going even higher would be impossible. So the logic is that all the material for the elevator system would have to be taken up to Geostationary orbit (GEO) and some automated system would then have to extrude the tether system downwards until it reached the earth's surface, while at the same time building the counterweight upwards. I don't know the cost of putting material into GEO orbit but a quick browse on the web gives figures like $US 10,000 to 15,000 per kg to Low Earth Orbit, double that to Geostationary Transfer Orbit, and double that again to GEO. So you would have to carry up all the material for the tether and the factory at something like $US 50,000/kg. Given that it would be impossible to take the tether up in one go, you would have to build an automated factory to assemble it. This would need power, would need to be effectively autonomous, and given that you would be making a single tension element with no redundancy, flawless. The question of what material to use is also pertinent. The Free Length of a material (The length of itself that it can support) is a useful concept. For aramids like Kevlar it is about 200 km; PBO is about double that. Carbon nanotubes and graphene might get you to 5000 km but that relies on being able to convert the properties of a molecule into large rope-scale assemblies with no loss of strength. It is worth remembering that while the stiffness of a material is governed by the molecular bonds in the bulk material, the strength is governed by how effectively you can get stress in and out of the molecules. So it is the connections that matter, rather than the strength of the molecules themselves. Consider an aramid fibre; the individual molecules are linked together in liquid crystals; these form micro fibrils and then single filaments (which you can see) in the spinning and stretching process. The filaments are then assembled into yarns (which you can handle) which are then assembled into strands, sub-ropes and finally ropes. (I simplify considerably). So there are at least 8 levels of structure between the molecule and something you can use on a large scale. There is inefficiency in force transfer at every one of these levels, so the strength of the highest level element is much lower than the strength of the molecule. More realistic estimates of what will be possible at the engineering scale for all known materials, including graphene and other exotica, give free lengths <500 km, which is not far off what you can do now with materials like PBO, and nowhere near 36,000 km. The free length concept assumes that you have a uniform cross-section, so all is not lost for the tether because it can be shaped, but it would need to be a lot thicker at the top than at the bottom. Whatever is going to crawl up the tether would have to take account of that change in diameter. See the web page by my colleague, Hugh Hunt (http://www2.eng.cam.ac.uk/~hemh/space_elevator.pdf). Because the calculations depend exponentially on the ratio (density/strength), the mass of stuff to be lofted dramatically depends on the material you can make. If you could get a copius supply of "Unobtanium" you would "only" need a weight of 500 Te, but at $50,000/kg to orbit that's $25 billion just to get the stuff up there - probably double that for the factory as well. But then we have the problem of what do you do with the payload when you get it up there. Only if you take the payload right to the top will it stay there. If you let it go anywhere below the geostationary height it would fall straight back down to earth. You would have given the payload the height (potential energy), but not the speed (kinetic energy) needed to maintain it in orbit. For an orbit at a height of 200 km the potential energy is 2MJ/kg but the kinetic energy needed is 32MJ/kg. So you will only have provided 6% of the energy and will still have to carry up a rocket to provide the other 94%. What's the benefit? None of this takes into account Coriolis forces, which would move the tether sideways as weights go up and down, or the risks of satellites or space debris hitting the tether. Since the tether would be at very high stress (in order to keep weight down), and the space junk would be travelling at very high velocity, the effect of an impact on the tether would almost certainly be catastrophic. Imagine pulling a bungee cord taut and then cutting some of the fibres (don't try this at home). Even if the tether was not completely destroyed, cutting fibres would release huge amounts of strain energy that would send shock waves along the tether, almost certainly causing damage elsewhere. I believe the space elevator concept is an interesting piece of science fiction, and has a role in inspiring young minds to think of science for a career, but they would learn proper physics by considering why it wouldn't work, rather than just thinking that "they" (whoever "they" are) have solved the world's problems. A more relevant Wikipedia reference is https://en.wikipedia.org/wiki/Indian_rope_trick Chris Burgoyne Professor of Structural Engineering University of Cambridge On 18/08/2015 19:19, Greg Rau wrote: > In contrast to towers, what about this?: > "A space elevator is a proposed type of space transportation system.[1] Its main component is a ribbon-like cable (also called a tether) anchored to the surface and extending into space. It is designed to permit vehicle transport along the cable from a planetary surface, such as the Earth's, directly into space or orbit, without the use of large rockets. An Earth-based space elevator would consist of a cable with one end attached to the surface near the equator and the other end in space beyond geostationary orbit (35,800 km altitude). The competing forces of gravity, which is stronger at the lower end, and the outward/upward centrifugal force, which is stronger at the upper end, would result in the cable being held up, under tension, and stationary over a single position on Earth. Once the tether is deployed, climbers would repeatedly climb the tether to space by mechanical means, releasing their cargo to orbit. Climbers would also descend the tether to > return cargo to the surface from orbit.[2] " https://en.wikipedia.org/wiki/Space_elevator > > Greg -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. 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