THE OXYGEN QUESTION

THE deaths of the astronauts brought to a head longstanding differences between advocates of pure-oxygen atmospheres for spacecraft and those who favor a two-gas system. The fire hazard inherent in a pure-oxygen system had discomforted space officials for years. In 1962, two crewmen in a space-cabin simulator at San Antonio were overcome by fumes from an instrument-panel fire but were rescued without serious injury. The same year, four men in an oxygen-filled test chamber in Philadelphia suffered second-degree burns when a short circuit in a lighting fixture caused a fire.

A 1964 NASA report on fire and blast hazards in spacecraft atmospheres noted that materials which were not highly combustible in a normal atmosphere erupted into flames during the Philadelphia blaze. Critics' suspicions seemed tragically justified last week when two airmen perished in a fire that flashed through the pure-oxygen atmosphere of a sealed test chamber at the U.S. Air Force School of Aerospace Medicine at San Antonio. The difficult decision now facing NASA is whether or not to continue to provide American astronauts with a pure-oxygen atmosphere.

The initial choice of oxygen was not made lightly. NASA scientists were aware that a two-gas system, one that would supply an earthlike atmosphere of roughly 20% oxygen and 80% nitrogen, would substantially reduce--but not eliminate entirely--the risk of catastrophic fires. It would also do away with some of the known, adverse physiological effects of exposure to pure oxygen: eye irritation, hearing loss, clogged chest, and possibly other painful symptoms not yet known to doctors.

On the other hand, an oxygen-nitrogen system has serious drawbacks for space flights. The additional storage tanks, valves, tubing and instruments necessary to blend and monitor a two-gas atmosphere would add an estimated 500 lbs. to a spacecraft the size of the Apollo.

Unlike a pure-oxygen system, which requires a cabin pressure of only 5 lbs. per sq. in., a two-gas system would have to approximate the sea-level pressure on earth--14.7 lbs. per sq. in.--to ensure that enough oxygen reached the astronauts' lungs. If a small meteorite should puncture the skin of a ship containing a nitrogen and oxygen atmosphere and cause rapid decompression, the astronauts on board would develop a painful and perhaps fatal attack of the "bends"; nitrogen dissolved in their bodies would come out of solution, forming gas bubbles in tissues.

The Russians, with their huge booster rockets, have been less concerned about weight; they have employed a two-gas system from the beginning of their manned-space program. It has proved awkward in at least one of their space missions. Before Cosmonaut Aleksei Leonov could leave Voskhod II for his space walk, he had to breathe pure oxygen (to rid his body of dissolved nitrogen and avoid the possibility of bends). He then entered an air lock, sealed his suit, gradually lowered its pressure to about 3 lbs. per sq. in. (so that it would be less inflated and more flexible) and only then was able to open the outer hatch and step into space, still breathing pure oxygen. By contrast, U.S. astronauts, always breathing oxygen at reduced pressures, can step directly out of their cabin into space.

Having considered all of these possibilities, NASA decided in the late 1950s that a space-mission failure was more likely to occur because of the added complexity and weight of a two-gas system than because of the fire hazard of a pure-oxygen system. Designers spared no efforts to fireproof the Mercury, Gemini and Apollo spacecraft. All electrical wiring was coated with noncombustible materials. Devices capable of sending out sparks were placed in sealed boxes. Space suits, seats, instruments and cabin walls were all designed to avoid the generation of static electricity.

For more than seven years, during 1,024 hours of successful space flight and thousands of ground tests in pure-oxygen atmospheres, NASA's reasoning seemed sound. There was no apparent need for conversion to a two-gas system--a conversion that would require the complete redesign of the spacecraft and could set the Apollo program back two years. But in 14 terrible seconds at Cape Kennedy, NASA's carefully considered decision has been thrown open to question.

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