August 28, 2012

Different Space Elevator Deployment to work with 42 GPa materials

Exponential Tethers for Accelerated Space Elevator Deployment (10 pages)

Reel-to-Reel buildup is the absolute best choice for strengths greater than 72 GPa. At 65 GPa it is nearly identical to climber-based buildup with a three day climber interval. This is remarkable, given how close 65 GPa is to the critical strength of 63 GPa. If climbers can only depart every three days, redeploy and splice is the method ofchoice from 42 GPa to 67 GPa.

An exponential space elevator is a space elevator with a tether cross-section that varies exponentially with altitude. With such an elevator it is possible to reel in tether material at one end of the elevator while reeling out at the other end, without changing the overall taper pro file. I show how to use this property to build up or clone a space elevator much more e fficiently than with standard climber-based methods.

Edwards, like his predecessors, considers that the tether has a cross-section that depends on altitude. Indeed, to maximize the payload, the cross-section of the tether should be chosen so that it is fully loaded along its full length. Indeed, if some parts of the tether were not fully loaded, material could be removed from those parts, resulting in a lighter elevator with a greater lift capacity. We shall refer to these tethers as uniform-stress tethers. They are skinny at the surface of the Earth, increase in cross-section with altitude up to GEO, after which they decrease in cross-section.





Uniform stress-tethers are ideal for lifting payloads, they are not necessarily the best tether pro le to use while building up the elevator. Indeed, the build up of a space elevator is limited by how fast one can get mass o ff the ground onto the elevator. During build-up, the mass that needs to be lifted is essentially made up of ultra-strong tether material. This is very di fferent from the type of mass that is to be lifted once the elevator is in general use (satellites, probes, ...). In Edwards' proposal, climbers are used to lift both types of mass. But while the tether is on the climber, it is just dead weight; its strength is wasted.

Instead of lifting ribbon materials using climbers, we shall lift ribbon material by adding material to the bottom of the elevator. Think of a kite. If you want to lift string o the ground, you can either have a roll of string lifted along the kite's string with a little climber device, or you can simply let out more string; the kite will rise to maintain tension in the string. In the latter case, as material is lifted, it is also providing strength to help lift the material below it. When this principle is applied to space elevators, we shall see that mass can be lifted much faster than is possible using climbers.

We shall now look at ways in which exponential tethers can be used to build up a space elevator. First we shall look at reel-to-reel buildup in which a tether can be deployed by reeling in material at the counter-weight and letting material out at the anchor. We will then discuss an alternative method, the pull-down build up, where both reels are on the ground and the counterweight is a simple pulley. Next we will see how the material that is accumulated at the counter-weight could be used to build a second elevator. This will lead to the idea of a breeder which can be used to rapidly produce two similarly sized elevators from one. Finally, by merging the two resulting elevators one elevator can be caused to nearly double in cross-section in a single step, leading to vastly improved tether growth rates; this is the redeploy and splice buildup method.

We have just seen that is possible to use material spooled up at the counterweight to build a new elevator. Better yet, it is possible to use that material to further build up an existing elevator. This leads to a very fast build up method. This method remains competitive even for much weaker tether materials. This is in sharp contrast with climber buildup methods that suff er signifi cantly if the tether strength is reduced.

In this method, tether material that gets spooled up at the counterweight is redeployed and spliced to the existing tether, resulting in a tether nearly twice
as wide as before. Buildup proceeds as follows:

1. Reel up enough tether material to reach from anchor to counterweight and to provide additional mass to counterweight.
2. Cut the tether at counterweight, and attach it to a new spool.
3. Reel up enough tether material to reach from anchor to counterweight.
4. Attach the of the end of the tether on the old spool to the tether connected to Earth.
5. Pull attachment point back down. The counter- weight splices the material from the two spools together as they are reeled out.


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