Future of Space Elevator Looks Shaky

Slashdot ^ | 12/9/8 | kdawson

[Clint Williams]

lurking_giant writes

"In a report on NewScientist.com, researchers working on development of a space elevator (an idea we have discussed numerous times) have determined that the concept is not stable. Coriolis force on the moving climbers would cause side loading that would make stability extremely difficult, while solar wind would cause shifting loads on the geostationary midpoint. All of this would likely make it necessary to add thrusters, which would consume fuel and negate the benefits of the concept. Alternatively, careful choreography of multiple loads might ease the instability, again with unknown but negative economic impacts."

[RightWhale]

isn’t the Coriolis force a fictitious force?

Sure, but it is the sideways force upon you when you move from the equator to the pole and is a major force in atmospheric modeling.

[Question_Assumptions]

"The Ringworld is unstable! The Ringworld is unstable!"

[KevinDavis]

[Aussiebabe]

Somehow this failure must be connected with global warming in the future.

[Oztrich Boy]

Also, you need to realize that the reason a space elevator would work at all, is because the center of mass of the system is in a geostationary orbit. If you attach something massive to the tether and start cranking it up, the center of mass will be displaced downward, and thus the tether would want to move forward. The response is to crank something upward to keep the center of mass approximately the same; or to fire thrusters to adjust.

Two words: Ballast Weight.

Increase it slightly so the centre of mass is always above geostationary orbit position. Tension on the cable is slightly greater, but it's always stable.

[Oztrich Boy]

Solution might be to build a smaller scale elevator first on the Moon if you don't trust your computer modeling on the Earth one..

A Moonstalk would have lower stress on the cable, but because of the moon's lower rotation, it would have to be on much larger scale (maybe 4 times as long as an Earthstalk) - and the lower cost of lifting mass of the surface by reaction drives or surface catapult means it's not the economic way.

Now a MarsStalk is another matter: both shorter and lower stressed than an Earthstalk.

[r9etb]

Increase it slightly so the centre of mass is always above geostationary orbit position. Tension on the cable is slightly greater, but it's always stable.

Let's look at that. As the CM is above GEO, it will want to fall back. The tether has fixed length, so it will pull the CM downward. The net effect would most likely be that there's a slight catenary curve to the tether, with the CM at GEO, and you're back to the same situation. And that's just with the system itself -- once you add moving masses along the tether, the whole thing gets more dynamic.

[Oztrich Boy]

Let's look at that. As the CM is above GEO, it will want to fall back

Not if it's moving faster the orbital velocity for the altitude (as it would be)

[Tramonto]

I’m no physicist, but isn’t the Coriolis force a fictitious force? Its only an observation of behavior by the reference of being in a rotating environment?

Im no physicist either but there conservation of angular momentum. This means that as the elevator moves up, its angular momentum will make it slow down relative to the rotation of the earth. Its what drives the Coriolis effect and what makes an ice skater spin faster when they pull in their arms and legs.

[r9etb]

Not if it's moving faster the orbital velocity for the altitude (as it would be)

I don't think so. The CM is still in orbit. FOr orbits, faster=higher=backward. I don't think a tether will change that.

[Oztrich Boy]

FOr orbits, faster=higher=backward. I don't think a tether will change that.

A tether stops the faster=higher.

[r9etb]

A tether stops the faster=higher.

I don't think so. The tether isn't stiff in all directions -- the CM would move back and down.

[Tramonto]

I don't think so. The tether isn't stiff in all directions -- the CM would move back and down.

If the tether is long enough to have the mass farther out than geosynchronous orbit, it should maintain tension on the tether. The orbiting mass would not move down at all when the elevator goes up.

[JohnBovenmyer]

I hadn't thought far enough ahead when I said a Moonstalk would be economical for moon transportation. It still might be economical as part of the R&D budget for an Earthstalk if you either couldn't work out the engineering without it, or more likely sell it politically without a demonstration first. A Marstalk would be smaller and thus cheaper depending on how much that distance increases your costs. But heck, if we're talking Earthstalks, Moonstalks or Marstalks we're already dealing with astronomical numbers so lets have some fun. Still, if you want to try small scale engineering mock up first, instead leaping orders of magnitude in engineering without testing and with inevitable literal Chicken Little politics, and are willing to go to Mars, there is a better option. Build a Deimos stalk. Its rotation is synchronous with Mars and its surface gravity is only 0.0004 g. If my math was correct Deimos surface to stationary orbit is only 21km. The lengths and material strengths involved would make location the only issue. You could probably prefabricate most of it on Earth. It wouldn't be have direct economic benefit for transport on or off Deimos, but you could work out the bugs without anyone crying Chicken Little. The rest, as they say in DC, is only money. After building it the Earthstalk would be a much easier political sell.

[HamiltonJay]

Yes, but the masses they are talking about climing up and down the elevator are nowhere near large enough to have massive impacts... plus in some designs that I saw there are 2 cars, one climbing up the ribbon while the other climbs down.

If this is true, then its just a matter of adjusting the decent of the empty car with the loaded car to keep the center of gravity on the ribbon constant. Not a simple task, but not an impossible task either.

[Tramonto]

It would be cool if they had one of these in Alaska, it would be going up at a low angle along the earth.

[SunkenCiv]

Hindoooo rope trick. Arthur C. spent way too much time in the east.

[HamiltonJay]

Everything I read or saw on it was the plan was to place it along the equator. I’m not sure how it would work being at a northern latitude.

[KevinDavis]

I was kinda skeptical about an idea of a Space Elevator.

[SunkenCiv]

They can’t put a man on the moon using a space elevator, no wonder this [fill in the blank] doesn’t work.

On a side note... poor Steve Tyler... for the rest of his life, he’ll either have to take the stairs, or deal with “love in an elevator” jokes...