Re: Naive questions about a space elevator

On Mon, 06 Nov 2006 03:46:14 +0000, Orval Fairbairn wrote:

In article <pan.2006.11.06.03.08.42.937059@xxxxxxxxxx>,
sal <pragmatist@xxxxxxxxxx> wrote:

I have two naive questions about a space elevator to which I haven't
seen clear answers. Both have to do with the bottom 0.5% of the cable:
the part in the atmosphere....

[ snip ]

The biggest question is, "How do you impart and remove velocity going up
and coming down?" The energies simply don't match!

Well, yeah, you need to add a delta-v of about 5,000 MPH to get to
geosynchronous orbit. And since the cable is so long, the tension is
essentially vertical at each point even if the cable wanders around a lot,
so you don't get a lot of help with horizontal thrust from the cable
unless it's seriously bent -- one would not expect the cable to act very
"stiff". This suggests (to me) that thrusters of some sort might still be
needed, which would reduce the win of the cable but probably won't
entirely eliminate it.

And, of course, if the counterweight breaks off, the cable will not fall
straight down -- that 5000 MPH delta-V will assure that it winds itself
around the Earth once or twice as it falls, which could be interesting
from the POV of the insurance company. But again that's one of the
obvious in-your-face problems which needs to be addressed -- it's one of
the first things most people think of when they hear about an S-E, I
suspect.


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