介绍:
Lecture4-Structural Engineering(Carbon Nanotubes)
Narrator
Listen to part of a lecture in a structural engineering class.
Professor
Today let’s begin to look at structural engineering in the Space Age. Uh, new problems…new possibilities mean we can think in new ways, find radically different approaches. So let’s consider…uh, well, what would you say is the biggest obstacle today to putting structures, equipment, people …uh, anything really, into space?
Student
Well, the cost, right?
Professor
Exactly. I mean, just taking the space shuttle up and back one time is hugely expensive. Uh, why?
Student
I guess a lot of it is for fuel, right? To…to get the rocket going fast enough.
Professor
OK. Fast enough to…
Student
To escape Earth’s gravity.
Professor
Good. So we are burning up an enormous amount of fuel at every launch just to get the rocket up to what’s known as escape velocity. Now, escape velocity is around 11 kilometers a second, pretty fast. But do we really have to go this fast?
Student
Well, yeah. I mean, how else can you, um…escape? I mean, that’s the whole point of escape velocity, right? Otherwise gravity will pull you back down to the Earth.
Professor
Actually, that’s a common misconception. Escape velocity is simply the speed of an object that’s …uh, let’s say, shot out of a cannon the minimum initial speed so that the object could later escape Earth’s gravity on its own. But that’s just if there’s no additional force being applied. If you keep on supplying force to the object, keep on pushing it upward. It could pull away from Earth’s gravity at any speed.
Student
Even really slow? So you’re saying …like, if you had a ladder tall enough, you could just climb into space?
Professor
Yeah! Uh, well, theoretically. I mean, I can see some practical problems with the ladder example. Uh, like you might get just a little bit tired out after the first few thousand kilometers or so, uh, especially with all the oxygen tanks you’ll have to be hauling up with you.
No. I was thinking more along the lines of an elevator.
Student
Wait! You are serious?
Professor
Sure. An elevator. That’s a new idea to most of us, but in fact it’s been around for over a century. If we could power such an elevator with solar energy, we could simply rise up into space for a fraction of the cost of a trip by rocket or shuttle.
Student
But wait, elevators don’t just rise up. It have (sic ) to hang on some kind of wire or track or something.
Professor
Uh, true. And for decades that’s exactly what’s prevented the idea from being feasible or even just taken seriously. Where do we find the material strong enough yet lightweight enough to act as a cable or track. I mean, we are talking 36,000 kilometers here. And the strain on the cable would be more than most materials could bear.
But a new material developed recently has a tensile strength higher than diamond, yet it’s much more flexible. I am talking about carbon nanotubes.
Student
OK. I’ve read something about carbon nanotubes. They are strong, alright, but aren’t they just very short little cylinders in shape?
Professor
Ah, yes. But these cylinders cling together at a molecular level. You pull out one nanotube or row of nanotubes, and its neighbor’s come with it, and their neighbors, and so on. So you could actually draw out a 36,000-kilometer strand or ribbon of nanotubes stronger than steel, but maybe a thousandth the thickness of a human hair.
Student
OK. Fine. But what’s going to hold this ribbon up and keep it reach enough to support an elevator car?
Professor
Well, we definitely have to anchor it at both ends. So what we need is a really tall tower here on the ground right at the equator and a satellite in geostationary orbit around the Earth. There’s a reason I mentioned that figure of 36,000 kilometers. That’s about how high an object would have to be orbiting straight up from the equator to constantly remain directly above the exact same spot on the rotating planet Earth. So once you are in this geostationary orbit right over the tower, just lower your carbon nanotube cable down from the satellite, tether it to the tower here on Earth. And there you have it!
Student
So you really think this is a possibility? Like, how soon could it happen?
Professor
Well, the science fiction writer Arthur C. Clarke talked about building a space elevator back in the 1970s. And when someone asked him when he thought this idea might become a reality, his reply was, “Probably about fifty years after everybody quits laughing.”
http://discovermagazine.com/2009/jul-aug/09-ways-carbon-nanotubes-just-might-rock-world
