How do proponents of orbital towers (‘beanstalks’) propose to compensate for the pressure of the wind and the constant earthquakes caused by plate tectonics? Even a Richter 1 earthquake is going to have some effect on so tall a structure, isn’t it?
Remember that it wouldn’t be a tower as we normally think of one as the entire structure is in geosynch orbit. It’s really hanging down from space from a terrestrial POV. Most (99%) of the structure will be above the atmosphere so wind would probably be mainly a problem for climbers. Even if an earthquake shifted the ground station by x meters, I don’t see it having much effect on the overall integrity of a structure tens of thousands of miles in length. I think ice could be a bigger problem for the atmospheric portion as that could conceivably add a bunch of weight to a structure that depends on being light in order to support its own weight.
But wouldn’t earthquakes vibrate it like a violin string? Or are you suggesting that there be no direct contact with the ground? In which case, how do you keep the thing in place?
Why would such vibration be a problem?
As noted, a point below the Centre of Mass of the orbital tower is the bit in geosynchronous orbit - the tail reaching down and the extending counterweight at the top are not (a point made by Clarke in “The Fountains Of Paradise” when he has an orbital engineer forget this fact and fall from the counterweight). The lower end of the tower is tethered to the earth, but the whole structure is flexible, not rigid, so tectonic movements won’t impact the structure any more than the wind would. Equatorial Africa is pretty stable, but ocean platforms are another possibility for base stations.
The dynamics of the tower if the tethers at the bottom snap are complex. The CoM stays in orbit, but the bottom and top ends pendulum, probably moving in a circular motion. This sort of thing is described in “The Descent Of Anansi” (Barnes and Niven), where a descending shuttle carrying a large reel of space-manufactured monofilament is sabotaged. To control and slow down the shuttle for re-entry they anchor the filament to the shuttle bay and eject the massive spool. The spool and shuttle pendulum, controlling the shuttles descent until it can disconnect and re-enter safely. For an orbital tower, the problem will be re-aquiring the flailing end for re-tethering. Gravity will help pull the cable to the surface. The expectation would be that the atmospheric free end has some aerodynamic surfaces allowing control from the tower to manage wind forces and allow steering. I am not sure if this would be sufficient, however.
If the counterweight breaks free, then the tower collapses wrapping itself round the rotating planet. I think that this has also been described in “Red Mars” (Kim Stanley Robinson).
Si
Because they’re vibrations. They’ll transfer energy and move things. Things you don’t want moved, like the counterweight.
The counterweight of the beanstalk will have small station keeping rockets to dampen out unwanted vibrations, as well as attitude jets along the entire length of the structure. It is tempting to think of an orbital elevator as a skyhook, but it needs to be a much more complicated structure than that.
I’d be less worried about plate tectonics and more worried about this guy.
For a space elevator, the mass where it connects to the Earth is its thinnest point along its length. If you think of a bullwhip, the thin tip is the part that touches Earth. The thick part in your hand is in orbit. For a bullwhip, you whip it with your hand, and as the disturbance travels to the thinner end, it speeds up until it can break the speed of sound. Conversely, going the other way, if you shake the tip, the disturbance will slow down as it gets to the thicker part.
The same would happen with the space elevator, with the amount of movement getting smaller as the elevator cable gets thicker.
That said, I’d think they would want to take into account the possibility of Earthquakes, and at least analyze what happens, to see if they’d maybe need some kind of active system to hold the base stationary while the Earth shakes. This doesn’t sound like anything intractable to me, however.
ETA: I should mention, this is something actually being worked on for buildings. Here’s an example book, Active base isolation of buildings subjected to seismic excitations.
If the cable is made of carbon nanofiber (the only known material that has even a chance of being practical), it’ll just burn up in the atmosphere before it reaches the ground. It’d be a major financial loss to whoever has a stake in the structure, of course, but it wouldn’t be a disaster to anyone else. And even the financial loss wouldn’t be all that bad, since it makes no economic sense to build only one elevator: Once you’ve got the first one, it becomes far, far cheaper to build subsequent ones.
Walter White?
It’s still got to be thick enough to take the mountings for the elevators.
A company called LiftPort LLC attempted to actually start the process of creating a space elevator. They didn’t get too far, and ran out of money. They’re now on a shoestring, hoping to rebuild.
They did have a basic design, and their first steps were to work on creating the materials and technologies necessary.
Their plan was a carbon ribbon, which would be very thin and about 3 feet wide at the base. The bottom end would be attached to a floating platform on the Equator, in the mid-Pacific. That would insulate it from any seismic events.
Also, the ribbon would be designed to vibrate. They expected that satellites of various sorts would occasionally pose a danger to the ribbon, and they’d avoid this by moving the platform to cause a wave to travel up the string, so that it moves out of the way temporarily.
ETA: Looking at their website, it looks like the current design has the ribbon 5 meters wide.
The wind probably would pose a problem to a five-metre wide ribbon of ultralight material; this could be reduced by turning the ribbbon face-on to the wind, but this might turn the ribbon into a giant vibrating reed.
One way to reduce the effects of wind on the elevator would be to build a protective tower around the lower sections of the ribbon, so that the worst of the atmsopheric effects could be removed. Dani Eder has suggested building a tower about 15 kilometers tall to protect passengers and freight as they pass through the thickest part of the atmosphere.
15 km high structures supporting their own weight are a long way in the future, but to be honest so are space elevators. Arthur Clarke said the first one would be built ten years after everyone stops laughing; there’s still plenty of mirth attached to the concept, unfortunately.
Personally, I suspect that nanofiber will be developed for other purposes anyway: It’s just too practical a material, for all sorts of purposes, to just sweep under the rug. First we’ll see golf clubs and fishing lines and other luxury goods, then eventually someone will build a suspension bridge out of it, and then we’ll be able to start looking at all the (many, but comparatively small) other engineering challenges associated with a space elevator.
Since we’re talking about space tethers I have a question that’s been bugging me. What holds the anchor in orbit? I means the elevator is going to keep pulling down on the cable to pull its self up. Newton’s third law says the energy to pull the elevator up is gong to have to come out of the structure it is pulling on.
Seems like if you keep taking energy out of the anchor, then it will fall out of orbit.
There is a weight above geosynch height. The tidal force created by the different gravitational potentials along the ribbon keep it stretched tight. The climbers do take a bit of momentum out of the lower part of the ribbon, I suppose, but they give it back once they get past 23,200 miles up.
The tether itself holds the anchor in place. The tether is forcing the anchor to circle the Earth at a geosynchronous pace, one orbit every 24 hours, but the anchor, being at an altitude beyond GEO, “wants” to fly off in its own orbit. The tension in the tether holds it in place. In fact, if you were to ride a climber all the way out to the anchor, it would feel like you were descending the entire way with the Earth above you and the anchor below you. Your weight would gradually increase as well. The section at GEO orbit is the only place along the entire systems that experiences zero-gee, i.e. weightlessness.
Actually, it probably makes more sense to have linear and circular counterbalances working as tuned mass dampers to cancel out unwanted oscillation.
As for seismic issues, there is really no reason to have the bottom end of the tether rigidly attached to ground, and in fact it would be far more sensible to have a floating ground terminus that could allow for movement of the entire structure to a different longitude or (temporarily) to a different azimuth (to avoid hazard or provide a launch platform for off-ecliptic spacecraft.
Actually, while you are correct that the initial momentum transfer will pull the anchor down (toward the Earth) slightly, ultimately the energy of the rising payload will come directly from the Earth in terms of transfer of angular momentum, with the upper terminus returning to its original orbit. The anchor should actually be slightly beyond geostationary orbit so as to maintain a positive tension on the tether.
As for the hypothetical material used to construct the tether, I would agree with Chronos that not only will it be developed for other purposes first, but it isn’t even the limiting factor in constructing a tether. The infrastructure to deliver materials to orbit and support construction is probably a greater technical and logistical challenge, and would be a marcroengineering project that would dwarf any previous structure or effort.
Stranger
The proposals I’ve seen all involve bootstrapping off of the cable itself. You launch the initial cable using a manageably-small number of conventional heavy-lift rockets, enough to support a payload of a measly few kilograms. Then, you build a lightweight robotic climber with a spool on it that goes up that initial cable, and lays down a little bit more alongside it as it goes, thickening the cable by about a percent. Then you have another climber that’s 1% bigger than the previous one, and so on. A percent gain on each run might not seem like much, but it’s an exponential growth, and adds up quicker than you’d think.
Of course, the exponential growth need not stop once you finish your cable. Then, you can start using similar methods to build a second cable, then four cables, then eight, until you have as much redundancy as you want. And then, you manufacture a few more cables at Earth, and send them off to the Moon, Mars, and other solar system destinations. Within a few years after the first elevator, you’ve got cheap access to every solid body except Venus.