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.require "memo.pub[let,jmc]" source;
Here's another possible dynamic compression member.

.cb A Dynamic Casing for a Hole through the Center of the Earth


	A hole through the center of the earth would have lots of
uses.  It would provide a fine track for a mass driver or one could
even imagine dropping a rocket through the hole and having it fire
at the center of the earth giving it a much lower escape velocity.
However, we would have to get the exhaust gases out of
the hole before we could fire another rocket.

	The main problems in maintaining the hole are pressure and
temperature.  The temperature problem may be the easier of the two,
because the heat conductivity of rock is low enough so that we might
be able to pump the eat out of the hole as fast enough to keep the
temperature down.

	As for the pressure, we propose exploring the idea of a
"dynamic casing".  This is a casing that spins fast enough to
counteract the pressure with centrifugal force.  Dimensional
computations show that the velocity of spin is independent of
the diameter of the hole.  It depends on the density of the
casing, the fraction of the radius of the casing taken up
by the spinning wall and the depth in the hole.
It also depends on the radial distribution of the earth's mass.
Casual computation suggests that the casing would have to
spin at about 4000 meters per second at the center of the earth.

	The immediate question is whether there is a way of making
a frictionless bearing between a spinning inner casing and a
stationary outer casing.  If the heat removal took place at
ambient pressure in the outer casing, the space between the
inner and outer casings could be cold if this would help.