November 02, 2008

Space Elevator -- dreams and doubts

James writes:

Just read your post RE "Rocket Girls". I have an oblique question.

I like to say that the materials needed to build space elevators would have to be incredibly strong and light. If it were even possible to create such material, then I can think of a thousand Earthbound applications that would make a butt-tonne of cash for the lucky corporation or inventor who first came up with the idea. Why build space elevators when there is such a rich vein to be tapped down here?

The example I like to use to the enthusiasts is body armor. If you have a material that can be used to build a space elevator, then T-shirts can be made to be bullet proof. When my tighty-whiteys can shed Magnum caliber pistol rounds, then I'll know that a space elevator is at least possible because we have something strong enough to build it out of.

But something happened that has made me think that I am terribly, horribly, completely wrong. It was an earthquake in Texas.

I always thought that inland Texas would be a great place to build a space elevator, if one was actually possible. It is near the equator, Texas has a lot of space industries already in place, and there is all that lovely empty desert if the cable goes snap and a few thousand miles of space elevator starts to fall down out of orbit.

But, of course, no place is immune to earthquakes. It seems to me that even a minor trembler would cause the end of a space elevator to jerk around pretty fierce. It would be like God deciding to play Indiana Jones with the space elevator as his bullwhip. And that would mean that the materials used would have to be stronger than I thought. Maybe a bullet proof T-shirt is sort of optimistic. Maybe we could use a few layers of T-shirts to armor tanks.

Just a thought. I'm hardly an engineer by any stretch of the imagination, the nearest being shooting a few holes in stuff. But it seems reasonable to me.

Texas isn't remotely near the equator. Houston is at 30 degrees north latitude. The likelihood is that the base of a space elevator really would have to be on the equator, or within a degree or two of it, mainly because I can't think of any way to connect the cable to an anchorage which is a long way away from the equator. (I can't figure out how to connect the cable at all, but that's a different question.)

Earthquakes don't really move the ground all that far, but it is a potential problem. Any kind of relatively rapid (e.g. cm/s) movement of the anchorage is a serious problem because it can temporarily increase the tensile force on the cable by a hell of a lot, which might cause the cable to part. So tectonic stability is certainly one of the desirable features of the location of the anchorage.

But there are places which are tectonically stable. Earthquakes is actually one of the less serious problems. There are much worse ones.

Coming up with an appropriately strong fiber to do the job is another of the issues, because we don't have anything yet that not only can support the stresses involved but also can survive without wear as the elevator crawls up and down it.

My main problem with the concept has to do with what engineers refer to as "dynamics". I would want to see some serious simulation work done on lateral stresses on the system and on oscillation modes, because my opinion is that what you'd eventually get is the counterweight (up in space) swinging back and forth, which would increase stress on the cable, place serious stress on the anchorage, and just be a bad thing in nearly every way.

Just for drill, of course, they really ought to look at how the system would be affected by severe wind storms, by rain, by lightning strikes, and what would happen if an aircraft collided with the cable (accidentally or on purpose). Also whether the cable would vibrate in a steady wind, and what that would do to the system. (See "Tacoma Narrows Bridge".) Both lateral vibration and torsional vibration modes could be important.

And after an appropriate fiber shows up, a few decent environmental aging experiments on the stuff would be nice before you commit to building with it. Will the physical properties change after 15 years of exposure to rain (in the atmosphere) or hard UV (out in space)?

And I'd like to see some studies done on the after effects of a cable break, no matter why that happened. Like what happens when thousands of miles of cable fall from the sky and strike near, or not so near, the anchorage. (The safety issues are the reason why, were I the one tasked with building it, I'd put the anchorage someplace like Jarvis Island, an itty bitty rock surrounded by ocean. But only, of course, if I could convince myself that the footing there was strong enough, which it probably wouldn't be since it's made of coral. The Galapagos would probably be a better choice. Or one of the islands off the west coast of Indonesia.)

So far as I know no serious work has been done on the dynamics of the system, and until it is I won't take the concept seriously. In fact, no one seems to be paying any attention at all to potential failure modes. And it looks to me like nearly every failure mode is potentially catastrophic. I don't like catastrophic failure in systems which contain this much energy.

The reason I haven't seen such a study is that such studies would be a waste the time and money until a synthetic fiber with the right properties appears and is being manufactured in quantity, which isn't even on the horizon yet. (Back in the day a lot of people who were fans of this concept thought that Kevlar was going to be it, but it isn't good enough. Now you hear them muttering about "carbon fibers" but that's just religious catechism.)

There are a lot of other questions that no one has answered. Moving the elevator up from the ground to geosynchronous orbit involves a huge amount of energy even if you're not wasting any of it on making steam and blowing it out a nozzle. Unless the elevator is dragging a power cord after it (yeah, right), then it's going to have to carry something that can produce enough electrical energy to run the motors that make it crawl up. You're gaining a lot of potential energy and a lot of kinetic energy. How do you carry that much energy inside the elevator? That's a really big unanswered question; I've never seen anyone even speculate about it.

And how do you utilize it? A space elevator won't be immune to the Second Law of Thermodynamics; utilizing that much energy is going to produce a lot of heat, and most of that will happen outside the atmosphere. How do you cool it?

Turning that around, the downward side of the cycle is even worst. All the kinetic energy and potential energy you gained when you lifted has to be shed somehow. Where does it go? How do you get rid of it?

Pretty much the only answer is "big radiators". The good news is that you can control the speed of ascent/descent to limit the energy release rate so that it doesn't overwhelm your cooling system -- but I want someone to figure out the details of that cooling system and its capacity, and then to figure out how slow the elevator has to be in order to avoid melting. Why? Because if it takes two weeks for each elevator ride then the elevator doesn't solve the problem of bulk delivery of cargo to orbit. The cooling problem could impose such a prohibitive capacity bottleneck as to render the entire system concept pointless. (And a severe capacity bottleneck could also render the system more expensive per delivered ton than disposable boosters, once you amortize the construction cost of the system.)

Also I want to know a lot about that cooling system for other reasons: how heavy is it? Once you've put that power supply and the cooling system into the elevator, have you completely consumed your mass budget? How much is left for payload?

How effective is that cooling system? Can it keep at least some part of the elevator cool enough for passengers?

comments disabledI've got lots of other questions about the concept (like exactly how you connect the cable between the anchorage and the counterweight), but doubters like me are about as welcome to this party as someone with audible flatulence. We spoil the mood.

UPDATE: And yeah, I asked a bunch of questions and then blocked comments. That's because I've been through this. Someone who is a fan will focus in on one small piece of what I've said and criticize it, and then self-satisfiedly proclaim that they've shot down all my issues and demand that I confess my sins and become one of the faithful.

The last time I wrote about this, the flood of outraged letters became so great that I didn't even bother finishing it up.

UPDATE: I tell you, for some kinds of problems, spreadsheets are just the coolest thing since sliced bread. I popped mine open and did some calculations on just how big the energy problem (and cooling problem) would be.

If I didn't louse up (which I have been known to do) then if 1 kg is boosted to geostationary orbit, it gains 350.7 kilojoules of potential energy and 3.39 megajoules of kinetic energy.

If the elevator plus its payload weighs 10 metric tonnes (which ain't very big) then it gains 3.5 gigajoules of potential energy and 33,863 gigajoules of kinetic energy.

If the lift time to orbit is 2 hours, then the average power is 5.1 megawatts. Figure a 50% inefficiency from the Second Law of Thermodynamics and you double that to about 10 megawatts.

The number isn't exactly right; my calculation of potential energy doesn't take into account that the pull of gravity declines with altitude, so my number is a bit high. (I didn't feel like mucking with differential equations to get an exact number, OK?)

Anyone care to explain what power source you'll be using to pack 67 terajoules into appreciably less than 10 metric tons?

UPDATE: If there were a cable fall, Coriolis effect would cause the cable to fall to the east of the anchorage, so the Galapagos islands, and the western Indonesian islands, would be poor choices for the anchorage. It looks to me like Waigeo island is just about the only good choice. And it isn't a very good one, because that's prime typhoon territory.

UPDATE: James posted about this. It turns out he was hoping I'd talk about how futuristic technology would affect every day life. Maybe he'd like this post from a couple of years ago.

UPDATE: The Brickmuppet responds.

So does Avatar.

Posted by: Steven Den Beste in General Anime at 11:19 AM | No Comments | Add Comment
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