Replies

Wow - there is a lot of misconceptions floating around here. Verifying the basic feasibility of a space elevator was one of my homework assignments when I got my degree in astronautical engineering, so let me see if I can work on a few of the bigger points-

1. The total system will be in geosynchronous orbit, with the center of mass at the geosync point. Imagine an asteroid in orbit at geosync, stable at one point over the equator. Now reel cables out both up and down, at the same rate, until one touches the ground. The entire system is both in orbit around the earth and stable over one point- because its a geosynchronous orbit.

2. Just because the elevator crosses all the intermediate altitudes, does not mean it needs to achieve orbital velocities at those altitudes. The system is in orbit at its center of gravity - that's all that matters. There are plenty of large satellites in orbit right now which would be tumbling if this were not the case. In fact, the tendency of a tall/long spacecraft to self align with the vertical is called gravity gradiant stabilization, and is used to help keep some satellites stable.

3. All the talk about the payload mass throwing the system off or atmospheric effects bending the cable are ignoring the sheer size/mass of the system. Think about a 50cm cable, 62000km long. Assuming the cable is only 1/2 the density of steel, this is 6.2 million tonnes. A 100ton payload is just not going to have that big effect - and the control system would actually control the timing and speeds of the payloads, both up and down, to control the orbital perturbations of the moon, etc.

Hope this clears some stuff up.

Just double checked my math, and that should read 36.5 million tonnes.

Also, this example cable will mass about 200kg/m, so a 100 ton payload would shift the center of mass by about 500m, or 1/120,000 of the total.

Finally, as someone else mentioned earlier (and as Kim Stanley Robinson wrote about in his Mars trilogy) the best way to get the required material into orbit would be to start with an asteroid, which will probably mass somewhere on the order of 1 billion tonnes. Again, very able to withstand a little weather or asymmetry.

There are plenty of large satellites in orbit right now which would be tumbling if this were not the case

Your one of few here who have given this any serious thought, but you fail to mention that the satellites you describe are fairly 'rigid' bodied. This 'cable', no matter how strong, to be practically 'unreeled', will have flex properties. It also has mass as you describe, and orbit dynamics of large flex structures, as well as mag field interaction (if it is conductive at all), will force all kinds of dynamics besides gravity on it.

This is one of many ideas, but its not the winner. I will bet on it.

Thanks for that. We needed it.

I do have some questions.

First, I assumed the the center of mass would have to be beyond GS Orbit, to provide tension and force to oppose the forces involved in lifting the load. With the center mass directly at GS, this obviously isn't the case.

What then would be used to oppose the forces of lifting the payload? Or is the shear mass of the assembly enough to resist movement by any force imposed from lifting the pay load?

Once the payload was up to is traveling speed along the elevator, there would be no acceleration thus no thrusting force, but there would have to be acceleration to get it up to speed. Also, I would think moving the load through the atmosphere would have a slight effect as aerodynamic forces work against the load as it moved up. Wouldn't that over time, after thousands of lifts, tend to pull the assembly out of perfect GS orbit? And then, wouldn't an equal amount of force, be needed to move it back? If so, how may lifts could be performed before making a correction?

Or can the acceleration of the payload be done slow enough to prevent that? If so, how long would it take to get a load up?

Sorry, I have some more.:)

At what point does the load become self motivating, if you will. I would think once past geosynchronous, it would be on its own being "flung" out along the elevator by the orbital velocity of the assembly. By the time it reached the end, the velocity would be well over standard geosynchronous velocity. If you wanted to stop it before it went off the end, how would you do that? That's a lot of energy to convert, absorb or dissipate.

Hope I'm not bugging you too much.

Another thought just occurred to me.

It seems you could use the breaking action beyond its center of mass to provide any correcting thrust needed to counteract the force used to accelerate the load up to speed. If that's the case, you could use as much acceleration as you wanted to get the load up to speed, and use the breaking at the other end to compensate for it.

Does that sound halfway plausible? Would you even want or need to stop the load anyway?

"Think about a 50cm cable, 62000km long. Assuming the cable is only 1/2 the density of steel, this is 6.2 million tonnes."

Lift that to geosychronous orbit!

According to the link in post #185, the ribbon cable will only weigh 1000 tons. A 100 ton rising payload is gonna bend that sucker like a banana.

Did some more reading. It seems my assumption about the cable being held in tension by a weight at above geosynchronous orbit was correct.

Never mind all the questions then as they're irrelevant in that type of setup.

Thanks anyway.