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.
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