The environmental control system developed in Project Mercury could be considered as two subsystems, the cabin system and the pressure-suit control system.
The primary function of the environmental control system was to provide
a livable gaseous environment for the astronaut. A basic requirement was
to provide a 28-hour flight capability based on an oxygen consumption of
500 cc./min at standard temperature and pressure (STP) and a maximum cabin
leakage rate of 300 cc/min STP. Four pounds of oxygen were needed to meet
this requirement, although actually the Mercury system was to be supplied
with 8 pounds to provide for complete redundancy. The next requirement
was a cabin pressurization level of 5 pounds per square inch absolute (psia)
with pure oxygen atmosphere. This pressure level was chosen as the best
compromise to provide (1) necessary oxygen partial pressure, (2) efficient
use of supply for emergency modes of operation, (3) a pressure offering
small differential change during cabin decompression emergencies, and (4)
a level for which decompression sickness would be minimal.
A closed-type environment was selected to conserve oxygen and thus reduce the oxygen weight and volume required. The astronaut at all times would wear a full-pressure suit to provide emergency decompression protection. The cabin system controlled the pressure between 4.0 and 5.5 psia. The heat-exchanger system was design for an astronaut metabolic heat production of 500 British thermal units per hour (Btu/hr).
The decision to use a 100-percent, oxygen atmosphere at 5 psia was based upon both engineering and physiological considerations. From the engineering viewpoint, the system incorporated the factors of simplicity, minimal weight, and reliability. Physiological considerations involved the requirement to prevent bends in the event of emergency decompression, and maintenance of an adequate oxygen partial pressure. The pressure-suits would operate at a pressure of 4.6 psia following cabin decompression.
Originally it was contemplated that the pressure-suit system would be maintained with pure oxygen and that the cabin would be enriched with pure oxygen at launch to provide a cabin atmosphere of approximately 66 percent oxygen and 33 percent nitrogen. This was to allow the visor of the pressure suit helmet to be opened in flight. One of the major reasons for selecting the oxygen-nitrogen mixture was the fire-prevention consideration. During the early ground tests of the system, however, it was found that nitrogen gas could concentrate in the pressure-suit circuit since the flow of oxygen into the suit was initiated by a slight negative pressure on a demand regulator. Consequently, cabin atmosphere was changed to 100 percent, oxygen and special emphasis was placed on material selection and quality control to eliminate the potential fire hazard. 14
The pressure suit was a backup system to the cabin atmosphere. Oxygen was forced into the suit at a torso connection by a battery-powered electric blower. In the, suit body cooling took place and a mixture of carbon dioxide, water vapor, and oxygen was produced. This gas mixture left the suit by a helmet connection and entered a physicochemical treatment cycle. Odors were removed by activated charcoal, carbon dioxide was removed by the chemical absorption of lithium hydroxide, and heat was removed by a water-evaporative heat exchanger. The water vapor condensed in the heat exchanger was removed by mechanical separation. Oxygen pressure was maintained in the pressure suit by a demand regulator which metered oxygen from a 7,500-psi oxygen supply. The operation time for the system would be dependent upon the system consumables: oxygen, coolant water, lithium hydroxide, and electrical power. The design was based on a carbon dioxide production rate of 400 cc./min.15
A closed-type environmental control system meeting these requirements was developed by the AiResearch Manufacturing Division of the Garrett Corp. (under a McDonnell Aircraft, Corp. subcontract). This system was located under the astronaut support couch, and the astronaut was clothed in a full-pressure suit to provide protection in the event of a cabin decompression. The cabin and pressure suit were maintained at 5 psi in normal flight with 100 percent oxygen atmosphere. Although the system was designed to control the environmental conditions automatically, manual controls were provided for use in the event of automatic-control malfunction.16
The manned development tests for the cabin system were conducted in December 1959 at the AiResearch Manufacturing laboratories. By that time the Mercury pressure-suit and the environmental control suit functioned as a unit. In October 1960, a pressure-suit control system was installed in the Johnsville centrifuge, and test were made under both manual and emergency conditions. At that time it became apparent that the system would support the astronaut in orbital flight.17 This phase is discussed later in the chapter.
The pressure-suit circuit provided breathing oxygen, maintained suit pressurization, removed metabolic products, and, through positive ventilation, maintained gas temperatures.
The single-piece pressure suit itself was developed by the U.S. Navy, NASA, and the B. F. Goodrich Co. The Navy Mark IV was chosen as the basic suit, with modifications as requirements were clarified.
15. Johnston reported (ibid.) that these oxygen consumption and carbon dioxide production rates originally established for Mercury had not been exceeded, and that flight data had been determined grossly at 360 cc/min.
16. See, for example, Stanley C. White, Richard S. Johnston, and Gerard J. Pesman, "Reviews of the Biomedical Systenis Prior to the MR-3 Ballistic Flight," an undated manuscript circa winter 1959-60. See also Richard S. Johnston, "Mercury Life Support Systems"; Anton A. Tamas. "Toxicological Aspects of Closed Atmospheric Systems"; William R. Turner, "Regenerative Atmosphere Systems for Space Flight"; and E. L. Hayes and Roland A. Bosee, "Development and Evaluation of Bio-Astronautic Life Support Systems," all in Life Support Systems for Space Vehicles, SMF Fund Paper No. FF-25, IAS, Jan. 1960.
17. See also William S. Augerson, James
P. Henry, et al., "Project Mercury Life Systems Aspects of Third Mercury-Aviation
Medical Acceleration Laboratory Centrifuge Program," NASA Project Mercury
Working Paper No. 187, Apr. 20,1961.