October 30, 2003

Elevator Follies

Steven Den Beste has a remarkable mind but sometimes he goofs. His recent article on space elevators is one such instance.

First of all, kevlar just doesn't cut it as a space elevator material and hasn't been seriously considered in years, ever since carbon nanotubes were discovered in the early 90s and their superior theoretical strength properties were documented. This establishes little except that SDB hasn't recently done a literature search. No biggie in itself but this means he's likely been unaware that Los Alamos recently sponsored a conference on space elevators along with the Institute for Scientific Research (in fact, it's the 2nd annual one).

He also probably could profit by looking up the liftport group who are raising funds and conducting research to create a space elevator over the next two decades. The Liftport FAQ discusses the problem of sideways movement in the cable during its week long lift time. The answer, in general terms, is to fire a horizontal rocket when appropriate. From the FAQ marked frequent misconceptions

When an elevator ascends the ribbon, it must be accelerated eastward because the Earth's rotation represents a larger eastward velocity the higher you go. The required eastward force on the ascending elevator would have to be provided by a corresponding westward force on the ribbon, possibly requiring rockets at intervals along the cable. If you go through the math quantitatively, the angular momentum for the climbers requires a few newtons of force over the one-week travel time, and we do that easily with our many tons of material in the anchor and the counterweight. The additional angular momentum will eventually be recovered from that of the entire Earth. The quantities really are tiny, but just to be complete, a climber going up pushes the entire elevator slightly to the east, causing it to lean. However, the ribbon recovers for the same reason that it stays up in the first place. Centripetal acceleration is acting on the counterweight pulling it outward, and the lost angular momentum is replaced very quickly (essentially as fast as it is lost). The ribbon will never loose enough angular momentum to even deflect a single degree, let alone fall. The extra angular momentum is stolen from the Earth's rotation; we will have to worry about this effect slowing down the Earth and making the day longer if we ever decide to ship Australia into space.

I look forward to hearing further posts from SDB on the space elevator. Hopefully, they'll be better documented.

Posted by TMLutas at October 30, 2003 06:49 PM