Date: 13 Jan 90 07:51:39 GMT
From: att!watmath!watserv1!utgpu!utzoo!henry@ucbvax.Berkeley.EDU  (Henry Spencer)
Subject: Re: Airlocks & Life support

In article <11704@csli.Stanford.EDU> jkl@csli.stanford.edu (John Kallen) writes:
>I've often wondered what happens tro the air in the airlocks when 
>cosmonauts perform EVAs...

On all US spacecraft, and I'm pretty sure on all Soviet ones, it's simply
dumped.  So far, the mass costs of doing something about it are too high
to be worth it, given that EVAs are infrequent and air compressors are
heavy.

>... Is air sucked out of the airlock prior to the
>opening of the outer hatch before EVA exit, or is it dumped in space when
>the hatch opens? 

It's vented through a valve before opening the hatch.  A reasonable fraction
of one atmosphere exerts a good many thousands of pounds of pressure on
a normal-sized hatch; one does not want to just unlatch something holding
back that much force.  Some of the hatches are designed to exploit that
pressure, in fact, to give a better seal:  if the hatch opens inward,
air inside holds the hatch very firmly against the gaskets.  (This is
also a minor safety feature, in that the hatch cannot be opened by
accident with air inside.)  As I recall, both hatches on the shuttle
airlock open inward.

>... Is the
>evacuation of the airlock the activity that makes EVAs so lengthy timewise?

No.  The EVAs themselves are lengthy because work in free fall while wearing
a clumsy suit is difficult.  Preparation for them is lengthy because keeping
the suits' clumsiness to a minimum requires running with an absolute minimum
of pressure in them.  To do that, one has to run the suits on something
approaching pure oxygen.  For various reasons, current preference is to
run spacecraft cabins at near one atmosphere and use a near-normal mix
of oxygen and nitrogen.  The transition from a high-pressure atmosphere
rich in nitrogen to a low-pressure one requires lengthy pre-breathing
of pure oxygen to get the nitrogen out of the astronauts' bodies.  (The
alternative is a strong possibility of decompression sickness, "the
bends", potentially crippling or fatal, as that nitrogen comes out of
solution as bubbles.)  There has been a lot of talk about high-pressure
suits, but getting adequate joint flexibility is difficult.

>... how often does MIR need to be "refueled"
>with air when it is manned? How long can they survive only by recycling with 
>LiOH(I think?) before they absolutely need more oxygen...

LiOH takes CO2 out of the air, but it does not restore oxygen.  A constant
trickle of oxygen is required.  Without it, survival time is determined by
how much oxygen there is in the air filling Mir... which is not a lot,
sizable though Mir is.
-- 
1972: Saturn V #15 flight-ready|     Henry Spencer at U of Toronto Zoology
1990: birds nesting in engines | uunet!attcan!utzoo!henry henry@zoo.toronto.edu