John Schilling wrote:
(Henry Spencer) writes:
Derek Lyons wrote:
It's also possible to design an emergency escape suit *that doesn't
require prebreathing*. Get in it, seal it, and pop out the airlock.
Prebreathe while dropping the pressure down to a level where you can
move the suit, complete the remainder of the escape sequence.

Unfortunately, starting from the station's normal 14.7psi atmosphere, suit
prebreathing even on an emergency basis takes about four hours, which is
kind of a long time to just float in the suit. (Preplanned spacewalks use
less prebreathing time than that because those guys live in reduced
pressure, with increased oxygen content, for 12+ hours first.)


My references indicate that the Russians use a thirty-minute prebreath from
one standard atmosphere to Orlan suit pressure. Admittedly, the Orlan is a
higher-pressure suit than NASA's designs, and the Russians are willing to
risk higher R values, but it would seem that in an evacuate-the-station
class emergency even NASA ought to be able to stretch the margins enough
to allow a similar half-hour prebreath.

Which still leaves you with that first half an hour to deal with, and my
references may have overlooked an oxygen-rich pre-prebreath such as you
allude to, but I'm skeptical of the four-hour figure as a hard limit for
emergency operations.

If you pass 1.6 by very much you get bent. There seems to be good
hypobaric evidence for that.

One of the things which is not well mapped right now is what level of
bent is acceptable for emergencies. The bends are not an immedately
fatal or incapacitating incident. Bubbles take time to form under most
circumstances, and you can still function, in moderate to extreme pain,
if you have a moderate case of the bends. Or so I have heard; I have
not had the pleasure myself.

Normal operations have to be designed to avoid it as much as possible,
because no matter how much you can tough through it, it is going to
be a risk and risk permanent damage if it happens to you. Emergency
ops and procedures can tolerate trading risks off, on a one time basis.


-george william herbert