Naive questions about a space elevator

sal 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. (I hope this isn't too off-topic for a
_policy_ newsgroup...)

1) How is the cable expected to handle tropical storms? Is it
believed that such a structure could ride out hurricane force winds
without turning into Galloping Gertie? Or is it assumed that, since the
elevator will touch down either on the equator or very near to it,
hurricanes won't be an issue, since they normally don't form closer than
about +/- 300 miles from the equator (due to lack of Coriolis effect)?

The 300 mile "restricted zone" for hurricanes sounded good until I
realized massive storms can migrate to the equator, even if they can't
form there, and even if they're doomed by crossing the equator they still
might be able to cause significant havoc to a ground station on the
equator.

One issue with riding out strong winds, of course, is that the
tension vector is almost straight up, even if the the cable has been
pulled far off to one side: at the top of the atmosphere we're already
more than 99 percent of the way down. This would seem to suggest that
the cable will not be very "stiff" in response to horizontal
wind loading.

2) What's the current story on the atmospheric E and B fields? I seem
to recall a shuttle experiment with a tethered satellite failed due to
high electrical tension along the cable. Now, as I understand it,
that was most likely due to the earth's B field (which the shuttle
cuts across at high speed), which would presumably not be an issue
for something stationary WRT the Earth's surface. But the atmosphere
also has a significant (vertical) E field. I've seen speculation
(elsewhere) that this would be a problem for an elevator; I've speculated
privately that this could be a great resource for an elevator to tap (if
the voltage isn't too impossibly high). Does anyone here know the correct
story on this? Is it even an issue?

No, the biggest problem is a sheer strength of materials problem. If
you had these materials building an SSTO wouold be trivial. I did in
fact suggest running aircraft on the "rubber band" principle. This,
rightly, was poo pooed. However look at the tremendous amount of
elastic energy present in the strain. One almost feels that you could
launch satellites using a crossbow.

- Ian Parker

In article uGn5h.1593$fk2.466@trndny02,
Martha Adams wrote:
The problem I do not hear anyone talking about is, how do you
consistently and efficiently grow nanotubes up to a few cm long? If
we can learn to do that, seems to me, the rest of it is (relatively)
easy.

Not quite, although that would help. The central problem is, how do we
*join* nanotubes into a useful engineering material? No matter how long
you can make the nanotubes themselves, you have to be able to join them
well, because practical structural materials need fault tolerance -- the
ability to transfer loads around a break in a fiber. If you can join them
efficiently, the nanotubes themselves don't have to be terribly long.

Unfortunately, this is seriously hard. For fundamental reasons, nanotubes
are slippery. It's hard to get them to stick well to a "matrix" material
without messing up their structure enough to ruin their strength.

The good news is that lots of people are working on it. Nanotube-based
materials would have *many* uses even if they weren't quite good enough to
build space elevators. (One of the economic problems of the elevator is
that its competition isn't today's rockets, it's the rockets you could
build with nanotube-material structures!) There is an immense amount of
money to be made with even partial solutions to the problem.

The bad news is that all those people have been working on it for several
years now, and progress is slow.

My guess for the #2 major problem: achieving an adequate
energy density of the laser light that powers the elevator's motors,
without melting the hardware.

Not much of an issue. Good solar cells are 50%+ efficient at converting
laser light at a well-chosen wavelength, so the power density doesn't have
to be all that high -- a few times sunlight at most.

Re airplanes again, have you thought what you might do with such
a laser if you spotted a known hostile airplane coming up over the
horizon?

Sorry, won't work. We're not talking about huge laser powers here. The
power densities are orders of magnitude short of what you'd need to make a
good weapon (and the laser output would be continuous, where a laser
weapon almost certainly wants to be pulsed).

Besides, the dominant problem will not be known-hostile aircraft, but
aircraft whose intentions are unknown.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

(Henry Spencer) wrote:

The bad news is that all those people have been working on it for several
years now, and progress is slow.

It's a good cooling-off exercise to go back and look at some of the
breathless articles from the 1960s, soon after the first findings on
the strength of carbon fibers/whiskers. We were supposed to have the
Gibraltar Bridge and the mass-produced 200-pound car chassis well
before this. Yet carbon fiber is still a relatively low-volume niche
engineering material, cranky and labor-intensive to work with.

CNTs are stronger still. But even if synthesis challenges are
overcome, there are plenty of process-engineering challenges on the
way to an affordable bulk engineering material.

Monte Davis
http://montedavis.livejournal.com