According to Johnston, "one of the most complex development problems, if not the most complex problem, to be resolved in manned space flight is the life support of man in space for prolonged periods."12 Life-support requirements for manned spaceflight include food, water, and atmosphere at a satisfactory pressure and composition to maintain blood-oxygen levels. To maintain a livable environment, the metabolic products of carbon dioxide, heat, and water must be controlled. Systems must be provided to collect, store, and treat human body wastes. Adequate protective systems must be devised to enable the astronaut to withstand the flight stresses—stresses expected in routine operations and those imposed by complex emergency situations.
For all system development, including life systems, certain design requirements existed, the prime one being to provide be necessary equipment in the minimum volume with the minimum weight. System reliability had to be provided in terms of the total mission reliability factor. As mission time increased, the system required revision to permit crewmen to "troubleshoot" malfunctions and to make in-flight system repairs. The systems had to be designed to withstand both the natural and the induced environmental conditions including vacuum, acceleration, heat, and radiation. Finally, they had to be revised to integrate with other spacecraft systems to allow usage of common supplies and to serve dual purposes.13
These were the problems that faced design engineers in the fall and winter of 1958-59 as they began the development of the Mercury life-support systems.
12. R. S. Johnston and E. L. Michel, "Spacecraft Life Support Environment," NASA Fact Sheet 115, Manned Spacecraft Center, Dec. 1962. See also C. F. Gell, E. L. Hayes, and J. V. Correale, "Developmental History of the Aviator's Full Pressure Suit in the U.S. Navy," J. Aviation Med., vol. 30, no. 4, Apr. 1959, pp. 241-250.