On Sat, 26 Apr 2008 16:37:27 +0100, Dr J R Stockton
<jrs@[EMAIL PROTECTED]
> wrote:
>In rec.arts.sf.science message <00d45da9-d14d-41e4-82ad-fc6675a1caa3@[EMAIL PROTECTED]
>g2000hsh.googlegroups.com>, Fri, 25 Apr 2008 09:16:59, Brian Davis
><brdavis@[EMAIL PROTECTED]
> posted:
>>
>>Let’s say you have a person (named, let’s say, “Callie”) standing in
>>the middle of a large airlock (10 [m] long by 3[m] by 3[m]). The bad
>>girl opens the large doors at the end, “blowing the lock” (it starts
>>at 1 [Atm]). What happens to the helpless heroine?
>
>At worst, approximately, and assuming a heroine of only moderate size
>(i.e. not a plug) : since the molecular speed is about the speed of
>sound, the energy can only accelerate the gas to about the speed of
>sound, 330 m/s. The heroine, being around a thousand times more dense
>than air, will be accelerated to about a thousandth of that, around a
>foot per second.
>
>A worst case approximation is that a transition between 10^5 Pa and 0 Pa
>propagates past her at 330 m/s. So, per square metre, she gets 10^5 N
>for a duration of T/330 s, where T is her thickness in metres. Her mean
>density will be, of course, 1000 in SI units, so per square metre her
>mass is 1000*T; so her change in speed will be 10^5 * T/330 / 1000*T,
>which is about 0.3 m/s.
Ah, so if I hang a sheet of tissue paper just inside the airlock of an
O'Neill habitat, and open the door, it won't go anywhere, right? Because
all it will experience is an infinitesimal moment of acceleration as the
transition between atmosphere and vacuum propagates past its negligible
thickness?
I'm thinking that's not right. I'm also thinking that a propagating
transition between atmosphere and vacuum would represent a violation
of the law of conservation of mass.
What actually propagates, is a transition between air at 10^5 Pa, and
air at 5.28x10^4 Pa moving outwards at 310.42 m/s. And that transonic
wind condition, remains even after the transition has passed - for as
long as it takes for the transition wave to reach the farthest wall
of the chamber behind our heroine, and as long beyond that as it takes
for the wind to actually empty the chamber.
If the geometry is cylindrical, I get for a standard heroine in a
standard atmosphere, a net velocity of 1.8 m/s per meter length of
air-filled volume behind her. That's in the low-velocity limit; as
she herself approaches transonic velocity downstream, the force will
decrease and her own velocity will asymptotically approach 310.42 m/s.
If the geometry is not cylindrical, it gets rather more complicated
of course.
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chamber behind her.
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