Is this structurally possible?? re: space cable (1 Viewer)

[Cees Alons]

What would be holding it up?


Cees

 

[cafink]

Gravity and centrifugal force.

 

[BrianW]

This is, in most cases, just another way of saying that the structure would simply be in (geostationary, in this case) orbit.

But when it comes to the Space Elevator, there is an overriding notion that centrifugal force on the cable, not orbital mechanics, is what keeps the structure in place, and that orbital mechanics need not enter into the calculations.

That's the idea, but the entire structure would simply be in orbit and be subject to the same orbital mechanics as any other satellite. Centrifugal force on the cable would not be sufficient to overcome changes in the structure's angular momentum (as would occur when things ride up and down the cable) to keep the structure in the same spot.

So if you move a mass up the cable, the structure will lose angular momentum and begin to drift westward, despite the cable's pull. Likewise, if you move a mass down the cable, the satellite will gain angular momentum and begin to drift eastward. Enough drift, and the structure will eventually fail.

So with the Space Elevator, more than ever, what goes up must come down.

 

[nolesrule]

Momentum has two components, not just mass. Couldn't adjustments to velocity also conserve angular momentum?

 

[Edwin-S]

 

[Eric_L]

Years ago we had a similar thread. We also had participation from some people employed in the space program. (I miss those days - the forum seemed to have so much of a larger membership - you could count on finding an expert in our midst no matter what the topic...)

Here is a link to it;

http://www.hometheaterforum.com/htf/...ick-pants.html

Essentially it was determined that the engineering obstacles are far more complex than setting a big giant space yo-you spinning around the earth might seem...

 

[Cees Alons]

Supposing it's basically "hanging", the forces at the location where it's fixed, or if it isn't: at its center of mass which is in geostationary orbit, are of course roughly equal to the whole weight of the structure. the material must be able to stand that force. Else it would eventually break like a strand of spaghetti and the lower parts would fall to Earth. That aspect alone might jeopardize the whole plan.


Cees

 

[BrianW]

Sure. But that's exactly the problem. Objects on the equator on Earth's surface are moving west-to-east MUCH more slowly than an object in geostationary orbit. When you try to move a mass up the cable, it tries to maintain the velocity it had on Earth's surface. Consequently, as it moves up the cable, it will drag behind the satellite, slowing the satellite down as the elevator's angular velocity speeds up. It's like you're forcing the geostationary satellite to pull a tree stump from a bog with a tow rope.

So when you send mass up the cable, you're not just sending it straight up. Before "launch" at the Earth's surface, the elevator is moving west to east at a little over 1000 MPH due to Earth's rotation. But the geostationary satellite is moving west to east at nearly 7000 MPH. And you're depending on the cable not just to support the weight of the elevator mass, but to accelerate the elevator to an additional 6000 MPH eastward direction just to be able to catch up with the satellite.

Once the elevator reaches geostationary orbit, it will have robbed angular momentum from the entire structure, causing the whole thing to start drifting to the west. And it won't stop drifting until you lower the elevator back down to Earth, transferring angular momentum back to the structure. And if you don't do that, the westward drift will eventually wrap the cable around the globe and cause catastrophic failure, which would be a bad thing.

And that's from just one, single elevator ride.

You could indeed make velocity adjustments, but that would take rockets, which would require more mass in fuel, which requires more fuel to compensate for the extra mass, and so on, which is what is altogether the thing we're trying to avoid with the space elevator in the first place.


There's just no getting away from orbital mechanics, transfer of angular momentum, and tidal forces. Even the Moon itself is not free of this angular momentum exchange. It's robbing angular momentum from the Earth, moving ever farther away, and slowing the Earth's rotation. And there's not even a cable attached between the Earth and the Moon.

 

[Al.Anderson]

As far as angular momentum goes, they could have another cable and *moveable* counterweight extending from the terminal point of the elevator. As the elevator goes up they could bring down the counterweight. Power would be solar genereted so there wouldn't be the problem of fuel. Engineering nightmare aside, I think that would acount for the physics aspect of the mometum shift.

 

[BrianW]

That's a great idea!

I was thinking they could outfit the toilet facilities with an orbit-stabilizing poo railgun, but your idea is way better!

 

[Dheiner]

Now, there's a phrase you don't hear every day!

 

[TonyD]

I guess i'm not comprehending the entire theory of a cable extending from anywhere on the earth into the sky/atmoshpere and not having some type
of lag like a kite would lag behind the kite flyer.

the end of the cable certainly wouldn't be straight up off the earth?
Or has this already been mentioned?

 

[ChristopherDAC]

Because the cable is stretched between a point on the ground, at the equator, & a point in geostationary Earth orbit which remains exactly above that point on the ground, the cable runs vertically. As a matter of fact it is possible to show that, if it did not run straight up & down but rather curved, it would be essentially impossible to build with any known or conceivable technology, rather than just nearly impossible.

This is complicated by factors such as the tendency of satellites in orbit to wander due to irregularities in the Earth's gravity, which could deflect the cable in different directions at different altitudes. There is, however, no reason for the cable to "lag", because it is not being towed, either by the ground or by a satellite.

 

[McPaul]

Alright, I have a question about this orbit stabilizing poo railgun. (see? I used it in a sentence)

I'm no genius on space travel, but is there not a lot of heat buildup where the atmosphere of our earth meets the limits of space? Is that not the whole reason they put so many insulating panels on our space shuttles? Surely this heat buildup will melt any such elevator, will it not? I can see if the elevator moves due to the lag mentioned above, the friction generated would be immense, would it not?

Also, what would happen to our astronauts when travelling up and down the elevator. Wouldn't there be an enormous amount of pressure crushing down on them as they go down the shaft? How could we protect our astronauts from this and how would this affect them?


orbit stabilizing poo railgun!
(this is really something that Stewie should have in his arsenal, yes?)

 

[RobertR]

The heat buildup you mention is caused by friction with the atmosphere due to the high speed of reentry. If the elevator isn't moving that fast, the friction causing all that heat isn't there. Why would the atmosphere "crush" people? It's just normal atmospheric pressure.

 

[BrianW]

They'd probably get bored out of their skulls listening to that awful music.

 

[ChristopherDAC]

The pressure difference between sea level on Earth and empty space is only as much as the difference between the surface of the water and a 30 foot depth. Descending cargo or personnel would have to have roughly the consistency of popped popcorn to be "crushed" by that type of change, or to be made to explode going upward. Considering that people and most cargo would normally be sealed inside pressure-tight cars, they would probably feel less discomfort than you do riding in an airliner or a high-speed elevator in a skyscraper.

As far as heat and temperature, while the upper layers of the atmosphere have a very high temperature as physicists use the term, they are not "hot" because the density is so low that they contain very little heat (just as there is less heat in a balloon full of air at 100 degrees than in the same-sized balloon full of water at the same temperature).
The insulation on spacecraft is required because, in order to get down to the ground, a spaceship must get rid of a very large amount of energy, both the kinetic energy stored in the high speed of its movement and the potential energy stored in its height above the Earth. They normally get rid of this energy by bouncing air molecules off the spacecraft, thus dumping it into the atmosphere as heat (thermal energy), but this requires some way of makuing sure that the heat is carried away by the air instead of into the spacecraft. In the space elevator, the energy lost by a car going downwards would be transferred gradually to the cable itself, to be drawn out again by a car going upwards.

 

[todd s]

 

[Rex Bachmann]

Off-topic (but not irrelevant!)

Sorry, I have to do this. It's driving me nuts! :frowning:


Uh, the word is "structurally" (structural + -ly).

Will no mod change this? :frowning:

 

[Cees Alons]

Cees