From: Robert J. Bradbury (bradbury@aeiveos.com)
Date: Tue Sep 25 2001 - 17:27:54 MDT
Hal wrote:
> And for long distances, the problem is the necessary tunneling depth.
> A train from NY to LA would be over 250 miles deep at the center.
> The deepest hole which has ever been dug was less than 10 miles
> deep, and that was a massive effort drilling straight down.
Its far worse than that, as you go deeper it gets much warmer.
I believe the deepest mines now have problems with plastic
deformation of the rocks and explosions where the pressure
can expel weakly bonded rocks. So you have to line the tunnel
with a strong material. Your limit with even nanotechnology
will be the point at which relatively common materials hit
their property limits. Titanium carbide melts at 3413K,
it probably starts losing its material strength at 60-70%
of that value (2000-2400K). I don't know how fast the
temperature rises with depth, but this is going to set a
hard limit on how deep you can go without a requirement for
active cooling. The tunnel cooling towers are going to make
nuclear reactor cooling towers look like midgets. Though
if you do it right, you can produce a lot of energy this way.
Its nonrenewable over the long run of course.
The other thing to consider is earthquake fault lines.
An fault line shift across a tunnel is going to be
very bad for trains approaching it. It probably requires
a double shell tunnel that allows shifts in the alignment
of the outer shell without affecting the inner shell.
That is going to require active repair of the outer
shell following any earthquake.
Bottom line -- I don't think this will work for the Earth
until it has cooled to a relatively solid mass. It could
work for the moon however.
Robert
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