The last time I looked at this concept, it was still the Clarke/Heinlein/Asimov era of science fiction. QOQ, now we've actually made progress toward it.
I came across this article at the CBC's website. Really fascinating stuff. Not something I would expect to see within my lifetime, but really fascinating stuff.
But there's still something about the concept that my brain fundamentally balks at - just can't see that long a cable actually being feasible, carbon nanotubes or not. How durable is it? How do you repair it? How the hell do you even install it in the first place? How do you manage the torque and other forces involved? Not only is the Erp spinning, invoking gravitational and centrifugal/centrifical forces, but you've also got all that friction with the planet's atmosphere, at multiple levels. Even if the cable itself is strong enough to withstand these forces, you have a major potential weak link at the base. How are you going to anchor the platform in place, so that it's not dragged around in, or pulled out of, the water? What about waves, storms, and neap tides? The mathematics of the endeavor would be staggering.
Posted by thinkum at February 28, 2004 05:23 PMOh ye of little faith ...
I was not impressed with the quality of the story, first of all. They had the basic idea, but the whole thing was written with a "gee whiz" attitude that I, an engineer, really didn't care for. :)
Durability is the primary problem. The cable will have to resist tensile stresses that will be no joke. Believe it or not, carbon nanotubes should have that much strength with a comfortable safety margin.
Installing would be tricky, but essentially you start from geosynchronous orbit (the midpoint of the cable) and extrude both ends of the cable at the same time. It gradually dips into the atmosphere, and then down to the ground. Terminal guidance will be a problem.
Repair is possible because the cable will be multi-stranded. Individual strands can be replaced -- just send a car up and stop at the right place.
Torque isn't the problem; tensile stress is. Properly set up and balanced, the cable is literally in orbit around the plant, so the magnitude of torque is much, much less than the magnitude of tensile stress.
The centrifugal force balanced by gravity is what holds the whole thing up.
The cable would experience no more friction than an airplane. Remember, it's moving with the earth's rotation.
The anchor is a problem. I don't think it would be put in water; it should be tied to the ground. Exactly how a base strong enough could be constructed is something I don't know.
It's an elegant concept, and engineering can conceive of a way to do it. Lots of messy details remain to be resolved, of course.
There's an alternative that's probably more feasible. Instead of a stationary cable, you have one spinning. The midpoint is, again, geosynchronous orbit. The ends dip into the atmosphere, low enough that a plan can reach them. The plane docks with the end of the cable, and then rides out into space with a pretty healthy velocity. This will take more energy -- you have to spin the cable or cables, and keep them spinning -- but it could be solar powered.
Posted by: PyeCat at February 29, 2004 04:16 AMOkay, I get the repair part - sort of akin to how they fix the cable cars' cable in San Francisco.
By atmospheric friction, I was thinking more along the lines of weather, than of rotational forces - winds, etc.
I'm not clear on this "start in the middle" approach - if the upper end of the cable terminates in geosynchronous orbit, isn't the middle of the cable somewhere within the Earth's atmosphere? Until the lower end is anchored, I would think it would be a major navigational hazard. Even getting it anchored would be a headache - you would need some way to pull it taut, in line with the upper anchor. I have this mental image of the line streaming sideways as the jet stream blows it around.
What makes the spinning alternative more feasible? It sound a heck of a lot scarier to me...
Posted by: Thinky at March 1, 2004 12:56 AMThe upper end is not at the point of geosynchronous orbit. Geosynchronous orbit is the middle. The cable extends further out into space just as much as it extends down to the ground.
The spinning alternative is better because you don't have to anchor one end to the ground.
Posted by: PyeCat at March 5, 2004 02:26 AM"The upper end is not at the point of geosynchronous orbit. Geosynchronous orbit is the middle. The cable extends further out into space just as much as it extends down to the ground."
Um...cuz why? I'm not talking about the spinning approach, but the static version. Why would you want the cable to go off farther into the ether than the passenger terminal at the geosynch point?
"The spinning alternative is better because you don't have to anchor one end to the ground."
But you have to control an untethered cable sufficiently for it to make precise contact with the elevator car 100% of the time. People have to be able to get on and off this thing, and with the "lower" end untethered, that gets more complicated.
Posted by: Thinky at March 5, 2004 02:17 PM