From: Jeff Davis (jdavis@socketscience.com)
Date: Sun Feb 27 2000 - 23:13:51 MST
Friends,
Following up on Robert's "The Tsiolkovski Tower Re-examined" post, there's
a little related idea I wanted to run by you guys.
In the August (I think) 1965 issue of Scientific American there was an
article on "trains" running in evacuated tubes inside of underground
tunnels. I loved the idea 35 years ago, and I've thought about it often
since then. What appealed to me was the elegance and efficiency. A
cylindrical train car, like a piston in a cylinder, leaves the station with
a boost of atmospheric pressure behind it (airlocks close behind it
sequentially to keep the air from filling a significant length of the tube.
The air between airlock doors and station exit doors is then pumped out.)
Then the train accelerates downhill by gravity and some form af additional
propulsion, perhaps linear induction (electrical), until it reaches its
cruising depth and speed. Then it glides along to its next stop where it
is decelerated by a similar process in reverse. To whatever extent you can
minimize the train-to-tube friction--by maglev, for example--you get a free
ride. It's very much like a spacecraft in orbit, and, in fact, at orbital
velocity, it actually is in orbit, the passengers are weightless and the
train car does not push down of the bottom of the tube. Over the years of
playing with this I discovered that at escape velocity--about 25K mph--the
centripital force would be exactly one gee directed upward against the top
of the tube. This means that as the train accelerates from zero to 17.5K
mph (orbital velocity) and then from there to 25K mph, a passenger in the
train would experience a gradual lessening of the downward force of gravity
until it reached zero at 17.5 K mph, then, if the train rotated 180 degrees
in the tube and continued to accelerate, the passenger would experience a
gradual increase on the "downward" force, until at 25K mph, he would be
sitting with his or her butt pressed into the seat just like any present
day train passenger. (This example is simplified for explanatory purposes,
and is only accurate for a tube circling the planet at the equator.
Elsewhere there are side thrust forces due to coriolis effects and due to
tube curvature in any turns.)
Now, I thought to myself, as a result of seeing the train as being "in
orbit", that you could actually use this as a launch method if you could
get the end of the tube--or, in train parlance, the end of a "spur" of the
"line" up above the atmosphere. (You could have an airlock-type closure on
the end of the tube, so that you didn't really have to go all the way up
the "hard" vacuum. You could open the airlock briefly so the space train
could exit and then afterwards close the airlock door behind it.)
Depending on certain small details, such as how precisely tangentially you
exit the tube, you may have to adjust your orbit a bit with ye olde
conventional thrusters to avoid plunging back into the atmosphere half an
orbit later. Compared to the launch energy however, this adjustment is
very, very small.
Okay. So now, how to get the tube up there, some hundred miles or so.
Here's where things get fun. I started out with the old "mountainside
ramp" idea, trying to take advantage of mother nature and Mount Everest for
the first six miles. But clearly those first six miles are nothing
compared to the last ninety-four, so I abandoned that idea right away.
Then, using a seven to one incline, I imagined a kind of roller-coaster
dorsal-ridge truss-structure ramp, the end of which would be a hundred
miles up, the base at the end two hundred miles wide, and the dorsal ridge
707 miles long (base 700, height 100, hypotenuse 707). (This is the
closest I come to a comparison with the Tsiolkovski Tower.) It was at this
point that I had my little Eureka moment.
Throw away all of that massive truss structure. Throw it ALL away.
Dispose entirely of the idea of a compression loaded structure. Support
the launch tube entirely in tension, and not sky hook from above, but
rather sky hook from below. Picture a seven hundred mile long conical
balloon with a hemispherical base of radius one hundred miles. Slice it in
half axially, and lay it down on its cut side in the mid-Pacific. With the
cut edges anchored, and the balloon inflated, and the tube attached to the
dorsal ridge, you have your launch ramp. And with airlocks to allow access
to the interior of the ballon, you would have very little ocean surface
area obstructed.
(Places Nobel prize back on mantlepiece, draws blinds and dances little
dance of self-adulation, as dog, lying before fire, looks on wondering,
"What's gotten into him?")
Best, Jeff Davis
"Everything's hard till you know how to do it."
Ray Charles
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