The severe limitation in weight and power available to the spacecraft designer for support of human function has dictated optimization of the atmospheric environment from both the physiologic and the engineering points of view. Selection of space-cabin atmospheres has therefore become a major question in the planning of manned space programs. The following is a presentation of the significant factors in this man-machine interaction which defines the selection process.The basic physiologic and engineering requirements for selection of space-cabin atmospheres have recently been reviewed in detail (RoT~, 1964a(RoT~, , 1964b(RoT~, , 1965(RoT~, , 1966a. Much of the material to be presented is condensed from these studies. The most prominent factors to be considered are:1. Total pressure; 2. Partial pressure of oxygen; 3. Fire and blast hazard; 4. Diluent inert gas factors; 5. Humidity and temperature control; 6. Carbon dioxide control; 7. Toxic contaminants; 8. Dusts, aerosols and ions; 9. Circulation of the atmosphere; 10. Minimization of weight, power, complexity and cost. Both from engineering and physiologic points of view, all of these factors are tightly interrelated. Each will be dissected in an attempt to clarify the basic issues and unknowns which we still face.
PressureTotal pressure limitation in a space cabin is more dependent on the engineering realities than on the physiologic. The practical upper limit may be considered the sea level pressure of 760 mm Hg or 14.7 psia. A higher pressure offers no advantage, and in fact, great disadvantage from a physiologic as well as engineering point of view. As will be demonstrated below, the sea level atmosphere we breathe on earth presents the virtue of familiarity along with many physiologic and engineering disadvantages.
