The Space Elevator

[onejayhawk]

I'm, uh, not sure what you're going for there. You might have a different conception of what I'm used to. Do you have a good schematic of what you're thinking of? Centrifugal force is a major component of the idea, except with the Moon Elevator.

No it isn't. It centripital force was important, you could have it at any altitude, but the strength of the support ribbon and the buttressing of the base would be much greater.

Here the weight of the ribbon alone is the issue. Even the climber is negligible in comparison. That is why a naturally stationary orbit is important. All you have to do is tie two fixed points together. Maybe a clothes line is a better analogy.

Whatever you are going to build the cable thingy out of its going to have to have an organic component. It has to be alive. Dynamic. Self replicating because it will have sense stress and adapt.

What do we have along those lines to work with?

The repairs will be done by the climber as it goes through a trek. Continual monitoring of the entire 35,000 km stretch is a major design issue.

J

 

[El_Machinae]

Again, I'll need a schematic. I have no idea what you're trying to say.

 

[onejayhawk]

Again, I'll need a schematic. I have no idea what you're trying to say.

Try considering a satellite in stationary orbit and the point below it on the ground. Those are two relatively fixed points. The elevator is a thread connecting the two point. It does not hold them together, merely connects. They continue the same without it.

J

 

[peter grimes]

You do not need a large mass at the top, much less an asteroid. This is not a string which is pulled tight by centripital force. The "string" is gravity. The tether is more akin to a climbing/rapelling line on the face of a cliff.

J

I think you might be wrong about this.

The counterweight has to have enough mass at geostationary orbit such that center of mass *Gravity!* of the entire of the (Ribbon + Orbital Anchor) system is also at geostationary orbit.

If the center of mass *Gravity!* of the connected (R + OA) is at a lower altitude, then a higher angular momentum is required to maintain orbit. The higher angular momentum translates into a higher velocity relative to the surface of the planet - and that means your ribbon will flex either east/West at different altitudes. That's catastrophic.

My rudimentary understanding of orbital mechanics suggests that the center of mass *Gravity!* of the system has to be at geostationary altitude.

http://www.braeunig.us/space/orbmech.htm

Very basic site, but helps understand some of this.

 

[IAM]

Since this thread started I have been reading articles and listening to many opinions about possible space elevators. I am more convinced now that we are further away than I use to think. Issues with cost, design, location etc. As someone suggested earlier maybe Doing a moon elevator first is a good option. Seems like a permanent moon base on the far side of the moon would allow us to test the concept in a lower gravity environment that is terrorist free. A permanent observation post on the far side of the moon also has the advantage of using the moon to block 'noise' from the earth creatures.

 

[El_Machinae]

Try considering a satellite in stationary orbit and the point below it on the ground. Those are two relatively fixed points. The elevator is a thread connecting the two point. It does not hold them together, merely connects. They continue the same without it.

J

Right, if you have such a set up, the least amount of force from a climber would pull the satellite down from geostationary orbit (a teeny bit, yeah) and then its velocity would be such (with the spinning of the Earth) that its orbit would never recover.

The 'bob' needs to be slightly beyond geostationary orbit (or further), so that its speed would continue to provide the centrifugal force required to offset any force imposed by a climber.

 

[Silurian]

last years thread


http://forums.civfanatics.com/showthread.php?t=496258&highlight=Nairobi

 

[onejayhawk]

Right, if you have such a set up, the least amount of force from a climber would pull the satellite down from geostationary orbit (a teeny bit, yeah) and then its velocity would be such (with the spinning of the Earth) that its orbit would never recover.

The 'bob' needs to be slightly beyond geostationary orbit (or further), so that its speed would continue to provide the centrifugal force required to offset any force imposed by a climber.

You are correct. This is an oversimplification. The mass of the ribbon is not negligible. The center of gravity is at stationary orbit, not the station. This does put a strain on the ribbon, though centrifgal force is not correct nomenclature. IIRC you could do without a orbital platform by increasing the ribbon length to about 100,000 km.

My point is that the load on the ribbon is designed to be as low as possible, with very little upward pull. This is very different from swinging a weight on a string.

Does anyone know what ascent rates they anticipate?

J

 

[Leifmk]

My rudimentary understanding of orbital mechanics suggests that the center of mass of the system has to be at geostationary altitude.

Center of gravity, really, not center of mass. Those two will not be in the same spot in an object on such a scale that gravity varies significantly across the length of it.

 

[onejayhawk]

last years thread
http://forums.civfanatics.com/showthread.php?t=496258&highlight=Nairobi

I offer no opinion on "should". I think the first two will either be Atlantic and Asian archipelago or a single mid Indian ocean, in the collection atolls there.

Center of gravity, really, not center of mass. Those two will not be in the same spot in an object on such a scale that gravity varies significantly across the length of it.

This is true. I used incorrect form. The distance to the local center of mass/gravity, ie the Earth's core, is not negligible.

J

 

[peter grimes]

Good catch Leif. I've corrected above

 

[El_Machinae]

My point is that the load on the ribbon is designed to be as low as possible, with very little upward pull. This is very different from swinging a weight on a string.

We're still talking past each other. What is keeping the ribbon 'up' if we keep tugging it downward?

 

[onejayhawk]

We're still talking past each other. What is keeping the ribbon 'up' if we keep tugging it downward?

It's a satellite. The same thing that keeps all of them up.

This satellite has a tail that drags the ground. That's why you start at the top and run the ribbon down.

J

 

[peter grimes]

We're still talking past each other. What is keeping the ribbon 'up' if we keep tugging it downward?

What is "centripetal force?"

 

[onejayhawk]

http://en.wikipedia.org/wiki/Centripetal_force

Centripetal force is the pull toward center, rather like gravity. It is used in free fall to simulated gravity. Centrifugal force is the force tending away from center. The two forces typically balance, equal and opposite.

With a satellite, the centripetal force is not like gravity, it is gravity. There is a well understood relationship between speed and the height of the orbit. Loosely, faster means higher. Higher means further around the circle, meaning longer orbit time.

We choose the one that takes 24 hours, which is very high and fast. Since the ribbon reaches all the way to the surface, parts are traveling much faster than other parts. If you ever played Crack the Whip, think of the person at the end of the line. This causes a strain on the ribbon, which is why the linear strength per weight is so important.

What is important to understand is that the analogy of a ball on a string has a big flaw. The string provides all the centripetal force keeping the ball in it's orbit. With the elevator, gravity does that. The ribbon's load is close to zero at the ground. It's more like the climbing ropes in a gymnasium, dropping down from the ceiling.

J

 

[El_Machinae]

It's a satellite. The same thing that keeps all of them up.

This satellite has a tail that drags the ground. That's why you start at the top and run the ribbon down.

J

Right, and when you attach a 1000 lb car to climb up the ribbon, that will be 1000 lbs of force pulling down on the satellite. It would force it downwards over time, unless there was a mechanism by which the force (keeping the satellite in space) was 'recharged'. This outwards pull is maintained by having the bulk of the mass outside of geosynchronous orbit but tethered to Earth to keep it from flying away. A mass going at 'geosynchronous' velocities, but higher that GEO orbit would fly off into space.

As soon as the majority mass falls below GEO, it would be going too slowly to stay up.

 

[Cutlass]

Right, and when you attach a 1000 lb car to climb up the ribbon, that will be 1000 lbs of force pulling down on the satellite. It would force it downwards over time, unless there was a mechanism by which the force (keeping the satellite in space) was 'recharged'. This outwards pull is maintained by having the bulk of the mass outside of geosynchronous orbit but tethered to Earth to keep it from flying away. A mass going at 'geosynchronous' velocities, but higher that GEO orbit would fly off into space.

As soon as the majority mass falls below GEO, it would be going too slowly to stay up.

If that's true, then the anchor on Earth can't be a ship, because the space anchor would pull the ship around.

 

[El_Machinae]

It would, yes, but then there would be a relative amount of inertia in a ship. Anchors, for example, would slow the dragging due to the SE tension.

 

[Cutlass]

You also have to consider wind. While in general the mass of air is moving with the planet, there's still winds pushing on the tether all the time. That would mean both that a high strength tether would be needed, and the masses on both ends would have to be great enough so that the wind couldn't move them much.

 

[peter grimes]

my comment was intended as a Jeopardy answer. Apologies