The environmental control system for Mercury, logically divided into the cabin and suit subsystems, grew directly out of previous aviation experience in maintaining men and machines at high altitudes. McDonnell had to seal hermetically the pressure vessel within prescribed limits; a subcontractor developed the dual air-conditioning system. Because the clothing needed for space travel turned out to be unavailable from the shelves of government issue, another subcontractor was called upon to make a full-pressure suit that would in effect be a secondary cabin.
When McDonnell and STG engineers first considered the problems of the pressurized cabin, they sought the experience of the foremost company of industrial specialists on the subject. AiResearch had grown since the 1930s into the Manufacturing Division of the Garrett Corporation, the Nation's primary supplier of the needs of the pressurized flight industry.5 In January 1959 the three groups began to discuss the most realistic design criteria for ambient and partial gas pressures, air and water regeneration methods, thermostats, and heat exchangers. R. A. Fischer, Edward H. Olling, and Richard C. Nelson of Garrett, Herbert R. Greider, John R. Barton, and Earl A. Reed of McDonnell, and Stanley C. White and Richard S. Johnston of STG were the principal designers of this system.
While the process of fabricating the pressure-vessel shell by the fusion-welding techniques of William Dubusker and his production engineers was cut and tried on the factory floor, the important question of cabin atmosphere gas composition was being debated by physicians and physicists. Should the cabin air and pressure imitate "sea level" air mixtures of nitrogen and oxygen, or should the space cabin endorse the experience of aviation and use at highest altitude whatever would guarantee oxygenation?6 Stanley White championed the latter position forcefully, in response to rather late outside criticism that "shirtsleeve" environmental  air might be preferable. John F. Yardley and Barton, Faget and Johnston agreed emphatically that a five-pound-per-square-inch pressure of pure oxygen would be far more practical for saving weight, controlling leakage, and avoiding the extremely difficult problem of providing reliable oxygen partial-pressure sensors. Faget explained STG's choice:
The most important consideration in choice of a single gas atmosphere is reliability of operation. If a mixed gas atmosphere were used, a major increase in complexity in the atmospheric control system and in monitoring and display instrumentation would have resulted. Furthermore, the use of a mixed gas system would have precluded the use of simple mechanical systems for a great number of these functions which in itself would have decreased the reliability of performance.7Reduced to practice, these designs had evolved into hardware for three spherical oxygen bottles, tested at 7500 pounds per square inch, with simple regulator valves, a lithium hydroxide canister to remove carbon dioxide and odors, an evaporator heat exchanger (its water would boil around 35 degrees F at a 100-mile altitude), and a simple pulsating-sponge water removal system, all to be located beneath the astronaut's legs. Blowers, a fan, snorkels, and plumbing were also included to make the capsule livable under the extremely diverse conditions existing before, during, and after an orbital mission. The most novel parts of this system were the high-pressure oxygen bottles, the use of lithium hydroxide, and the "sponge squeezer" to collect perspiration and respiration water vapor from the cabin atmosphere. Cleanliness in the manufacture of these components was so important that AiResearch built the first "surgery," or "white room," for Mercury fabrication in the summer of 1959.8
McDonnell and AiResearch engineers consulted the voluminous literature on aeromedicine before imposing STG's specific requirements on top of the state of their art. One of the best independent guides to that state was a report prepared in mid-1959 by A. B. Thompson of Chance Vought Astronautics, entitled "Physiological and Psychological Considerations for Manned Space Flight." Thompson compiled a consensus on environmental parameters derived from a wide number of sources; then he presented these factors systematically in the order of their occurrence on a typical orbital mission. Concerning the internal atmospheric environment, he drew heavily from submarine, as well as aviation, practice and expressed particular concern over abnormal toxicities peculiar to space conditions. Regarding temperature tolerance, Thompson wrote:
Man can exist and carry out simple tasks in environmental temperatures from - 40° to 140° if suitable clothing is worn for the low, and if humidity is kept at 30-50% for the high. Time of exposure to high temperatures should be well below man's tolerance limits. Up to 160°F can be withstood for 20 minutes. Such temperature highs are possible at reentry into atmosphere. Insulation, double walls, cabin temperature and atmosphere cooling should limit the heat of cabin to less than 140°F even when skin temperature of the vehicle is much higher.9 Between John Barton of McDonnell and Edward Olling of AiResearch, the system specifications for environmental control began to emerge in mid-1959, subject to continuous reappraisal as other systems also took shape. Their original set of design parameters rather arbitrarily selected 400 British thermal units per hour for one man's average heat production rate over 28 hours, and an ambient pressure of 5 pounds per square inch circulating through the cabin, with a breathable supply of oxygen at the partial pressure of 3.8 pounds. An assumed oxygen consumption rate of 500 cubic centimeters per minute allowed a slight margin for suit leakage. Setting the average rate of perspiratory and respiratory water production at 6 pounds per day dictated the weight and size of their system's hardware.
Particularly knotty for the development of the active air-conditioning system and the passive insulation to control the cabin temperature was a problem that Barton described in terms of applied thermodynamics:
Studies of launch, orbit and reentry heating effects disclosed that the insulation requirements for the cabin side-walls for the orbit and reentry phases were diametrically opposed. In orbit it is desirable to lose heat from the side-walls and during reentry it is necessary to prevent the entry of heat. The reentry phase, being more critical, dictated the side-wall insulation. In orbit, the insulation becomes an almost perfect heat barrier and dictates that the cabin cooling be primarily accomplished by the cabin heat exchanger.10At the end of July 1959, Barton and Frank G. Morgan, Jr., met with 18 STG engineers, including all the astronauts, to describe the basic designs and developmental problems, especially leaky instrumentation fittings, for the system now known as "the ECS." Faget, White, William K. Douglas, William S. Augerson, and Robert B. Voas, and the ECS systems engineers, Richard Johnston, Frank H. Samonski, and Morton Schler, all warned that the design parameters were set too low. They demanded larger margins of at least 1000 British thermal units per hour for astronaut heat generation, at least 7 pounds per day assumed water production, and certainly no less oxygen pressure in the suit than in the cabin.11 Greider and Barton warned the astronauts to learn early and thoroughly the symptoms of hypoxia in themselves so they could take action soon enough to ensure an emergency oxygen supply. Otherwise probe sensors of some sort in the nostrils or the lungs might be necessary.
McDonnell hurried the building of a "man-rating" environmental system test chamber through September 1959, so that a reliability test program for each subsystem could be conducted, complete systems tests could be scheduled, and astronaut familiarization training could begin as soon as possible. By the end of the month, Gilbert B. North, as McDonnell's test astronaut, had endured so many failures or inadequacies in the bench testing that STG sought the aid of physiologists from Duke University School of Medicine and from the Navy Air Crew Equipment Laboratory in Philadelphia to help speed the man-rating of the environmental control system. At the end of January 1960, neither the cabin nor the suit environmental control system had passed its test to operate as designed for 28 hours.  Richard Johnston reported that experience with the system was still "rather meager." He urged aeromedical investigators to provide more "realistic metabolic data" for his engineers to use in system redesign.12
Difficulties with the body ventilation and post-landing snorkel ventilation subsystems continued troublesome through 1960. Extensive testing at AiResearch and intensive manned tests at McDonnell beginning in June slowly eradicated most of the "bugs" plaguing the reliability of the environmental control system. A robot "crewman simulator," designed primarily by Charles F. Jahn and Eugene Wulfkehler at McDonnell, served to calibrate the physical parameters for average human inputs and outputs to this closed ecological system. Then, too, Gilbert North and Herbert Greider learned to outwit the peculiarities of the mechanisms to avoid hypoxia, dysbarism, and hyperventilation. The initial manned tests of the ECS hardware were endured by McDonnell volunteers; occasionally the Mercury astronauts would observe. Gas analysis problems delayed the accumulation of reliability records and the verification of certain operational procedures, such as ground purge and ground cooling, until early 1961.13
4 See Frank H. Samonski, Jr., "Project Mercury Environmental Control System Technical History," MSC, Crew Systems Division report No. 63-34, Nov. 14, 1963, for a most thorough topical overview of this subject.
5 Seymour Chapin, "The Pressurized Flight Industry in the Southwest Since 1930," paper, Pacific Coast Branch meeting, American Hist. Assn., Los Angeles, Aug. 26, 1964. See also Irwin Stambler, "Environmental System for Mercury Capsule is Simple, Rugged," Space/Aeronautics, XXXII (July 1959), 42-45.
6 A. D. Catterson, interview, Houston, Oct. 23, 1964; memo, Stanley C. White, "Present Status - Major Systems: Environmental Systems," Feb. 1959; memo, Gerard J. Pesman to Tech. Assessment Committee, "Meeting with McDonnell Aircraft Corporation to Discuss Environmental Control System for the Manned Spacecraft Capsule," Jan. 23, 1959. Although STG decided this question in favor of the latter alternative in 1958, the possibility of a change existed throughout 1959. See also William K. Douglas comments, Aug. 17, 1965.
7 Ms., Maxime A. Faget and Aleck C. Bond, "Technologies of Manned Space Systems," 55. For an excellent view of the controversy about mixed gas versus 100 percent oxygen systems, see Eugene B. Konecci, "Soviet Bioastronautics - 1964," paper, National Space Club, Washington, D.C., Dec. 15, 1964. The Soviet choice of a near-sea-level environment was wise for several reasons, explained Konecci, but he added that the danger of decompression sickness may retard extravehicular operations and therefore "may prove to be the Achilles heel in their program."
8 Edward H. Olling, interview, Houston, Sept. 14, 1965. See also Olling's Ms. paper, "Design Solutions and Test Results for the Life Support System for Project Mercury," Sept. 1960. Control of atmospheric dust and debris at the micron level was apparently first applied in the aerospace industry at the AiResearch factory in Los Angeles a few weeks before similar arrangements were made at the McDonnell plant in St. Louis.
9 A. B. Thompson, "Physiological and Psychological Considerations for Manned Space Flight," Report E9R-12349, Rev., Chance Vought Aircraft Inc., Dallas, July 7, 1959, 165.
10 Ms., John R. Barton, "Systems Engineering Considerations in Designing and Testing the Life Support System for Project Mercury," Oct. 14, 1960, 4.
11 Minutes, Jack A. Prizzi, "Meeting with McDonnell - Environmental Control System - July 30, 1959, at Space Task Group," Aug. 17, 1959. Cf. memo, White to Chief, Flight Systems Div., "Approval of Specification Control Drawing No. 45-83700, Revision S," Aug. 7, 1959. Eventually capsule No. 7 was standardized on the ECS assumptions of 500 cc./min. oxygen consumption rate, 300 cc./min. cabin leakage rate, and 500 B.t.u./hr. body heating rate.
12 Memos, Richard S. Johnston to Chief, Flight Systems Div., "Test Program - Environmental System Trainer," July 27, 1959; and "Report on Trip to ACEL to Discuss Installation of ECS Trainer in Altitude Chamber," Sept. 21, 1959; letter, Robert R. Gilruth to Chief, Bureau of Aeronautics, Dept. of the Navy. "Test Program for Environmental System Test Vessel," Sept. 28, 1959; memo, Charles D. Wheelwright to Chief, Flight Systems Div., "Trip Report," Sept. 29, 1959; Johnston, "Mercury Life Support Systems," paper, 28th annual meeting, Institute of Aeronautical Sciences, New York City, Jan. 25-27, 1960, 15.
13 Memos, Samonski to Chief, Flight Systems Div., "Developments in the Environmental Control System Testing Program at McDonnell," June 13, 1960; and "Progress of Manned ECS Tests at MAC," July 25, 1960; Ms., Johnston, "The Control and Measurement of the Mercury Capsule Environment," paper, Fifth National Symposium on Space Electronics and Telemetry, Washington, Sept. 19-21, 1960; J. A. Maloney and F. G. Richardson, "Test of a Life Support System under Simulated Operating Conditions," McDonnell Aircraft Corp., March 1, 1961, 21-23.