From: Carl Feynman (carlf@atg.com)
Date: Mon Mar 03 1997 - 04:27:53 MST
Forrest Bishop wrote:
>>As this is an "open-air" design, the ends only have to come up from the
cylinder wall
>>about 150 kilometers, and can be very thin near the top, as will be the
enclosed
>>atmosphere.
>
I wrote:
>That gives an atmospheric half-life of only 60,000 years.
>
Forrest Bishop wrote:
>[[Only 60,000 years eh?. BTW, I just guessed at the height. Did you use a
>planetary or planar gas kinetic equation for this?
Here's how I did it. My trusty chart of the 1958 ICAO Standard Atmosphere
says that about 10^-10 of the mass of the atmosphere is above 150 km. I
assumed that all the air above that height was expanding freely horizontally
into vacuum, which would give it a velocity equal to the speed of sound, or
282 m/s at that altitude. Thus, 10^-10 of the atmosphere disappears in a
time equal to the time required for sound to cross the width of the ring.
The rest is multiplication. (hmmm... missed a factor of two there; should
be half the width of the ring, because it's spilling off both sides.)
Obviously the dynamics of the gas is a little more complex than this. But I
don't think I'm off by more than a small factor.
Zoom,
--CarlF
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