Ok, I’ve heard a whole bunch of times the ‘goal’ of a space elevator as the end all be all of getting things into space. Yet automatically I can think of two major problems with it in direct contradiction to specific claimed benefits.
Supposedly it would allow for a cheap “launch”. I dont see this. Yes you save on payload wait, etc, but you still have to provide energy somewhere in the system, and I don’t even want to guess at the cost in construction, so the first question is would it really save money/resources? How long would it take to ‘pay’ for itself?
Better for the environment (this is the stupid argument that set off the question). Yes, you wouldn’t need environmentally damaging fuels but answer me this, just how much ecological disaster would be created by this thing? I’m thinking it would seriously affect weather patterns if nothing else. You’d have a fairly large structure being dragged through the atmosphere at the speed of earth’s rotation. Would this not create significant heat? Wouldn’t it affect air movements as its dragged along?
Cost of construction, maintenance costs, etc. Your guess is as good as mine, we don’t even have the materials yet. When it comes to the energy one can imagine a solution like in an elevator of a lesser scale, where you have a counterweight. Balance loads going off earth with loads going down to earth and you only need the energy to make one trade place with the other. Get the energy from solar panels at the top end, and you’re way better off than with a chemical rocket.
It will be gigantic in length, but not width. And it’s not being dragged through the atmosphere at the speed of earth’s rotation, because the atmosphere mostly follows the earth’s rotation. Thus, no significant creation of heat, and no change in weather patterns.
The savings in energy would be enormous. A lot of the oomph in modern launches is required to get the ship – and the enormous amount of fuel – through the non-recoverable friction of the atmosphere into orbit. Yopu have to expend a lot of fuel very fast in order to be efficient. Then you have to burn even more fuel to rapisly accelerate all the fuel. If, on the other hand, you go up a space elevator you can go up slowly, at a fractioon of the speed and a fraction of the energy/fuel cost. Betterf still, most of that energy is recoverable when you come down, all of the gravitational potential energy that’s being held in your elevator plus payload being converted into kinetic energy of velocity as you go down, which you can convert into electrical/chemical potential energy by hooking your wheels up to a dynamo and battery charger. So you win both ways – you cvan use relatively slow electrically-driven (or diesel driven, if you want) power to go up into orbit, and not have to burn toxic rocket fuels in the process, and you can come down leiseurely , without danger of burning up, or losing radio communication due to MHD effects or problems due to lost tiles (and store a huge amount of energy) on the way down.
Think of it like a cable car, with two cars. As car #1, carrying a new satelite goes up, the car#2 carrying valuable moon rock comes down. Next time, car#1 goes down, and car#2 goes uop. As long as the loads going up and down balance, the energy requirement is minimal.
Most of what goes up into space has to come down eventually. Men, old batteries, feces, broken equipment, empty containers, completed experiments, pretty much everything you take up, except structural stuff. Moon rocks are not likely to become a large element in comparison to trash for a long time.
But gaining energy from the return trip in any amount is a big huge saving when compared to taking up enough fuel to deorbit all that mass to bring it down in shuttles.
Think of it like regenerative braking - you expend a whole bunch of energy getting the material into orbit, but when you bring it back down, you have to brake all the way, and you can recapture the energy. The only real energy cost then is friction (both on the tether and air friction) and the energy losses in converting the energy to motion and back again.
In fact, if you were to use an elevator just to get mass down to earth, you could actually turn it into an energy source. You’re converting the potential energy of an object in orbit into usable energy through regenerative braking.
Besides, the problem with going into space isn’t the amount of energy required, but the fact that in a rocket you have to take it with you. Over 90% of a rocket’s weight is typically fuel. That means you have to burn fuel to lift fuel, just so you have the fuel with you so you can burn some more. That’s what’s really expensive about sending stuff into space.
With a space elevator, the only stuff you send up is the stuff that belongs there. You cut the energy costs right off the bat by 90%.
Put a single nuclear plant at the bottom of the elevator, and you could move as much material back and forth into space as you could possible want indefinitely.
As an extreme example, the Saturn rockets used in the moon program were one-shot disposable machines that weighed over two thousand tons but could only lift a little over 100 tons to orbit. Even a piddly little crane can lift 100 tons using maybe electricity worth only a few tens of dollars. Give engineers some sort of semi-fixed point to push or pull against and they can do amazing things. Doing it all using Action/Reaction is much more complicated.
For a variety of reasons, a counterballanced cable car system (or really any cable powered elevator system) would not be the way to go. Some sort of “crawler” that could climb the cable would make more sense.
The atmosphere is rotating along with the Earth as is the cable. As far as weather is concerned, a space elevator would just be a static pole with a relatively small cross section.
The reason that the shuttle (or anything else coming out of orbit) produces so much heat is that it is traveling thousands of miles an hour relative to the Earths atmosphere in order to maintain orbit.
The space elevator cable on the other hand only has a stable orbit at the geosynchronous counterweight. That means that every inch of the rest of the cable, which is forced to travel at that speed is under constant strain to move to a more stable orbit.
As a car ascends the cable, would it not pull the counterweight out of orbit & create slack at the ground end? Since you can’t push on a rope (one of the few things I remember from my Mechanical Engineering classes), I don’t think the effect is countered the way down. So would you constantly have to bring fuel up (or have electically powered ion thrusters) to keep the counterweight in place?
citybadger, the elevator would be designed such that there is tension on the ground anchor. When a car goes up, that tension would decrease, but (assuming the thing is designed right) would not go to zero, so there would be no slack.
Piper, the simplest answer is that it’s held up by centrifugal force. It’s like when you have a weight on a string and spin it around above your head, except that here it’s being spun by the rotation of the Earth. Yes, I’ve just invited a bunch of folks into the thread to say that centrifugal force is wrong and you shouldn’t discuss it, but just ignore them. It’s a perfectly valid way to do the calculations, and in this case, the simplest to explain.
Isn’t a space elevator just a geosynchronous satellite? That is, it’s in orbit, and the cable has nothing to do with it “staying up”? If the cable was suddenly severed, the orbital portion wouldn’t go flying off, as a weight on a string would, it would just hang there.
In the book 3001, Arthur C. Clarke (or whoever :rolleyes: ) postulated the existance not only of space elevators, but a solid geosynchronous-orbital ring.
This system would make obsolete all sattelites, rocket launches, and take over most radio transmission (save for local transmission, and near the poles.)
Perhaps that will give an indication of cost/benefit.
I know that we probably shouldn’t get our scientific information from novels, but Kim Stanley Robinson goes into considerable detail about the physics and the logistics of building a space elevator (both on Mars and on Earth) in his Martian trilogy.
There seems to be a strong consensus that Robinson did very solid scietific research for his books, and that most of the science stuff contained therein is either currently accurate, or at least plausible based on our current knowledge.
If the orbital portion were just hanging there, there wouldn’t be any tension on the tether. I think if the cable broke, the top portion would drift away slowly (actually, it would enter an eliptical orbit with perigee at geostationary altitude) and the bottom part would come down quickly.
Wouldn’t there be a tendency for a payload going up the elevator to lag behind the rotation of the cable? Something on the ground at the equator is travelling eastward at about 1,000 miles per hour. By the time it gets to the top, it will have to be travelling much faster. The elevator lifts it up, but what gives it its tangential energy?
(I imagine the tether would be flexible, and that an object being raised would bend the tether such that climbing it would provide both height and tangential velocity. Even if we can develop the materials, it still sounds like it’s going to be very, very complicated.)
An object in geosynchronous orbit is in free fall at a rate that keeps it directly above one place on the equator. That part of the “space elevator” which is in geosynchronous orbit would therefore be in free fall. Anything below that would inevitably be in a non-free-fall orbit. The solution is therefore a Beanstalk longer than Clarke-orbit elevation, with the “counterweight” portion further out thant Clarke orbit undergoing centrifugal force sufficient to counteract the pull of gravity on that part of the Beanstalk below Clarke orbit, producing a net effect of a long, skinny satellite in Clarke orbit. (Note that anything elevated beyond Clarke orbit would achieve a significant portion of its launch velocity simply by being flung from the Beanstalk at that point, giving further savings.)