If maintenance of normal respiratory function were the only
consideration, a cabin atmosphere of about sea-level composition and
pressure might be an ideal and straightforward choice for manned
spacecraft. In fact, this atmosphere has been used in the manned space
flights conducted by the U.S.S.R. No other atmosphere has been shown to
be more satisfactory from the physiological point of view, and the
tedious respiratory studies which should accompany the use of other
atmospheres can be avoided. Nevertheless, the formidable problems of
spacecraft design and the necessary precautions for safeguarding the
crew from accident require that other atmospheric compositions and
pressures be considered. For example, if a cabin at 1-atm pressure were
decompressed to space suit pressure (0.3 atm), the occupants would
develop decompression sickness; i.e., "bends."
Several engineering considerations argue for low cabin pressures and
pure oxygen composition. Among these are structural design, weight of
atmospheric gas storage and control equipment, and the difficulty of
contriving pressure suits which allow operation at pressures near one
atmosphere. Such departures from the normal human gaseous environment,
however, require the demonstration of an acceptable level of safety and
physiological performance.
The limits of the composition and pressure of acceptable cabin
atmospheres are then set by—
(1) A pure oxygen atmosphere at a pressure which will provide an
alveolar oxygen partial pressure equal to that provided by air at
sea level
(2) A mixed gas (oxygen and inert gas) atmosphere having a pressure
and composition that will allow decompression to the highest
acceptable suit pressure without the risk of bends
A numerical value for the lower limit (1) is approximately 0.2 atm of
pure oxygen. The upper limit (2) is determined by the operating pressure
and composition of the space-suit atmosphere and may be of the order of
0.5 atm for a cabin atmosphere of 50 percent oxygen. It is necessary to
determine the astronaut’s ability to survive and perform his duties in
any atmosphere selected.