More on Space Elevators

finally writes "Space elevator news is being reported on Space.com and Yahoo. I, for one, am really excited about the project. I was wondering if any of the broad range of talents and skills that we have here have thought of doing a sort of open source assistance to this project by means of donating time and knowledge." We did a big story last week on this space elevator conference.

Comments

From the mouth of the master...

[Will_Malverson]

Arthur C. Clarke said that we'd build a space elevator about 50 years after everyone stopped laughing. With stories like this, I think it's safe to say that we've stopped laughing.

Why do they have to hang straight down?

[Mick D.]

I have a pretty decent knowledge of the physics involved in the construction and use of a Spave elevator. I know that they need to orbit in Geosync orbit so the cable doesn't change length. I know that the cable is actually hanging down to the Earth from the center and being pulled away from the Earth out at the tip of the cable beyond Geosync orbit.

But, the one thing that has always bugged me has been why the cable itself has to hang straight down from orbit. This places the "Ground" based end in much less practical areas. Couldn't there be multiple cables hanging down and strung like Christmas lights to northern and sorthern areas that would be more likely to use them. They would need to balance the weight to keep the orbiting cable in the correct spot, but with 4 or more anchor points it would provide multiple ways to orbit and redundency(sp) in case of catostrophic failure.

Any insight in this would be helpful. As far as I can tell the only problems would be the added weight and thus tension, but I haven't done the math and don't know how much stronger the cable material would need to be.

Re:Why do they have to hang straight down?

[WolfWithoutAClause]

But, the one thing that has always bugged me has been why the cable itself has to hang straight down from orbit.

The trouble is that the combination of rotation (which pulls at 90 degrees to the axis of the earth) and the earths gravity (which pulls directly towards the earths center)- the combination ends up pulling the end weight so that it is above the earths equator. The cable below that goes from the attachment point on the earths surface up to there.

You can move the endweight only a small amount from the earth end- the radius of the earth is only 6700 km, but GEO is 38000km, so the geometry for moving the end weight around doesn't add up.

So basically, the endweight is in the plane of the equator. So the cable comes off at an angle from the earths surface- and heads off to the weight beyond GEO. At the equator the angle is 90 degrees. But as you go north or south, the angle is lower, and the angle means that the tether is longer and weighs more, as it droops under gravity (there's little rotation force at low altitude to compensate, so it does it quite a lot.)

So if you go very far north you find that the cable leaves the earths surface horizontally... there's no point in going further north than that. Exactly how far north this happens depends on how heavy the cable is, and how much tension there is in the cable at ground level. So you can increase the tension and pull it up off the ground again. But by doing so, you are losing payload by doing this- the extra tension to make this work could be used to lift payload up the tether.

It's a bit oversimplified, but that's the main idea. You can do it, but it's probably not worth doing it.