Space Elevator Options Everyone’s pretty familiar with the idea of a space elevator, a big long chord stretching up from the equator to geosynchronous orbit and then a bit further where it ends in a counterweight. Here are some alternate options to play with for your setting. Multiple Tethers to the same orbital station: Just as guy wires often hold up objects you don’t have to have your elevator as a single tether going straight from the equator to a station above. You could have, say, three tethers rising up like the edges of a pyramid to a station above. These need not be on the equator, so New York, Paris, and Rio de Janeiro could share the same orbital hub, each with a tether rising at an angle to a station above the Mid Atlantic. This does require about 4 times as much tether as a single one would, and the tether has to be a bit stronger than the minimum needed for a tether for that world, but the mechanics work fine. Nor do the tether bases need to be equally spaced out to form an equilateral triangle, though that is best. You can connect more than three to the same station and a city could have more than one leaving it. They must have both northern and southern hemisphere base locations and can’t be on the other side of the planet from each other, otherwise you can put them almost anywhere. This has the upshot of allowing pre-existing cities to continue being ports and not need to be on the equator. It also means you don’t face local monopolies. New York could share one with Paris and Rio, but also share one with San Francisco and Lima in Peru. So the station can’t be strong-armed with tariff fees by its land-side counterpart because they can transport their cargo at least two other ports, likewise that station can’t do the reverse because a place can have a many more tethers to other stations. Nor do the stations have to be on the equator, with the example of the Lunar Space Elevator, which has a station the equator and on the north or south pole, dragging the counterweight in orbit slightly south and allowing you to fling things from the pole out into space. Before moving to the other variant, a quick note on tether length. This corresponds to the length to geosynchronous orbit. This length is controlled by only three things: Mass or density, the planets radius, and Day length. In reality it only depends on mass and day length but you have to subtract the planet’s radius from your tether length. For worlds pretty close to Earth’s Mass and Day Length this doesn’t vary terribly much but if the world’s mass or day length are getting to be outside a half or double the Earth Norm you should go through the math. Next up, Orbital Rings. Now there’s a classic example of one that is simply a ring at geostationary heights, where space elevators descend to earth’s surface like spokes on a wheel. There’s also a the equivalent of a rotating ring hab where you live on the underside and look up at Earth, held in place by centrifugal gravity, just like Niven’s Ringworld only smaller and with Earth at the center. But we’re interested in another sort. An Active-support orbital Ring. One hanging stationary above the ground just over the atmosphere. This sort would allow you to build buildings, supertall skyscrapers or arcologies, which had a far shorter elevator, only a few dozen kilometers rather than a few dozen thousand kilomneters, or even stretched up and physically connected to the ring. You build it over the atmosphere so you don’t have to deal with air friction and drag with your ships, the main fuel use for getting things into orbit. These seem to just hover in the air, supported by nothing, and in fact have normal Earth gravity on their tops, just like on the ground, maybe a percent less at most. Nor do they have align to the equator, but they do have to be circular. So what holds them up? Anti-gravity? Nope. An Active structure relies on basically holding something up by pushing on it constantly, an example would be holding a plate aloft by spraying the bottom with a hose. What’s done here though is to produce a giant circular, hollow ring that has objects moving inside it. Usually particles or small filings of magnetic material. To picture this imagine a long rubber hose spread out on the ground with both ends attached to a pump shoving the water along in one direction. This will shove the hose into a firm rigid circle. If you push on it there will be resistance. Flip it vertically and it will turn into an ellipse but on top you could hang weights and it would push it down more but it would stay up in the air (assuming you connected some guy wire to keep it from falling over). This is an actively supported structure. You extend this to a worldspanning scale, but substitute in magnets and things you can move with magnets for water. It will lose power, depending on how efficient your device is, but you attach a power plant or some solar panels along your orbital ring to keep it going. Inside the hose the matter is moving very fast, but the hose does not move and neither do the things attached to the outside of it. You string up to in parallel and lay a sidewalk of solar panels around them and you have a structure constantly hanging over the Earth. You attach some tethers or towers and run up more hoses, solar panels, and building materials. You could make one miles wide and several stories thick. You could have it at an angle to the equator or not. You could have two rings, one just a bit under or over the other, which physically connect, of the same width or not. You could have 3, 4, or a million. You could actually completely englobe a planet this way which is the usual basis for concept like Supramundane worlds where thin shell around a big gas giant with dirt on it apes a giant earth-like planet. And again the gravity is just a hair lower then on the planet below. The one downside is that if the hose is cut it will rapidly begin falling down while spraying matter out into deep space. So while you only need one or two small ones up to begin using them to pull stuff into space, you would probably want the structure to have many small hoses (magnetic conduits) rather a few big ones. Also as the mass of the total structure begins to grow in relation to the amount of mass actually spinning through those ‘hoses’ you simply increase the speed of the material in the hose. Youd also want to balance the weight out along the circumference of the ring. This, incidentally, requires no high-tech materials or physics-cheats. We could do one now but it’s monstrously expensive. Like building CERN only a hundred time bigger and in space. If your tech was good enough that you could make something the width of a fire hose with superconducting ring around it, and with a bit of stretch, you could hang the world-spanning circle from blimps then energize it and have it slowly rise and stretch into position. Then pull up more of them. You cnat just lay it across the planet because planets arent very smooth circles and it needs to maintain a very nearly perfect circle. So I hope that sparked some interesting story ideas for some of you, as always if you’ve got questions I’m glad to field them as much as I can.
Posted on: Sat, 30 Aug 2014 12:23:56 +0000
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