Like all earlier Soviet manned spacecraft, Buran used a mixed oxygen/nitrogen atmosphere very similar in composition and pressure to what we breathe on Earth. NASA did not introduce the oxygen/nitrogen mix until the early 1970s on Skylab, having used 100% oxygen atmospheres on Mercury, Gemini, and Apollo. A 100 percent oxygen atmosphere allows for the construction of lighter vehicles and obviates the need for spacewalk pre-breathing, but, on the other hand, significantly increases the fire hazard, as vividly demonstrated by the Apollo-1 fire in 1967.
Buran had three subsystems to provide the crew with breathable air both in standard and emergency situations. These were the Pressurization and Depressurization System (SNiR), the Gas Composition System (SGS), and the Personal Life Support System (ISZhO). The SNiR maintained cabin absolute pressure between 93.3 and 107.3 kilopascals (kPa), supplying oxygen and nitrogen to the cabin from tanks situated in the mid fuselage. As on the Shuttle Orbiter, the oxygen was stored cryogenically in the tanks of the fuel cell system. The SNiR would pump up to 1.5 kg of air into the crew module per day to compensate for routine loss of cabin air and would also repressurize the airlock after spacewalks. The system was automatically activated whenever cabin pressure sank to 98.7 kPa and would then repressurize it to a level of 101.3 kPa. It could also be operated via manually controlled valves. The SNiR was also designed to respond to various emergencies. It could replace the cabin air after a fire or a malfunction of the carbon dioxide removal system and in case of cabin depressurization due to a micrometeorite or space debris impact would blow air into the cabin to give the cosmonauts more time to don pressure suits. If Buran was to have re-entered with a depressurized cabin, the SNiR would have opened a valve to allow outside air to stream into the cabin and minimize pressure differences.
The SGS maintained oxygen partial pressure between 18.7 and 29.3 kPa, making sure that oxygen levels never exceeded 40 percent to limit the fire hazard. The system kept carbon dioxide partial pressure below 1.07 kPa. This was accomplished with regenerators in which CO2 reacted with potassium superoxide to produce oxygen, which was then recirculated to the cabin air. The ratio of absorbed CO2 to regenerated oxygen was roughly the same as the respiratory quotient of a human being— that is, the ratio of the volume of carbon dioxide released to the volume of oxygen consumed by the body. The regenerators also had filters to remove trace contaminants from the cabin atmosphere. Similar C02 removal systems had also flown on earlier Soviet piloted spacecraft.
Depending on crew size and mission duration, Buran would have needed to carry 6 to 18 regenerators on a single flight. The crew’s responsibility was to regularly rehook flexible hoses between cabin ventilators and the regenerators as the potassium superoxide ran out. The Shuttle Orbiter has usually relied on non-regenerative lithium hydroxide canisters for C02 removal, as many as 30 of which may be needed on a single flight. NASA did install a regenerative carbon dioxide removal system on the Orbiters Columbia and Endeavour for Extended Duration Orbiter missions, but it did not produce oxygen as a byproduct of the chemical reaction.
The ISZhO was primarily designed to provide life support functions to a full pressure suit that the crew was supposed to wear during critical mission operations such as launch, docking, undocking, and re-entry. Called Strizh (“Swift”—the bird), the suit was derived from the Sokol (“Falcon”) pressure suits worn by Soyuz cosmonauts and adapted to be used in conjunction with ejection seats. The system could operate either in an open-cycle or closed-cycle mode. With the loop open, the suit was ventilated with cabin air, which was then released back into the cabin via the helmet (if that was open) or through pressure regulators (if the helmet was closed). With the loop closed, oxygen was supplied to the suit from the fuel cell liquid-oxygen tanks or (if that didn’t work) from back-up gaseous oxygen tanks. There were also small portable oxygen containers that could sustain a crew member for 20 minutes. After having passed through the suit, the air moved through a contamination control assembly to remove carbon dioxide and other gases and through a unit that cooled the air and removed the moisture. Finally, the gas was enriched with oxygen and recirculated through the suit. The main operating pressure of the suit was 440 hecto – pascals (hPa), but could be manually reduced to 270 hPa. A single ISZhO unit formed a ventilation loop for two suits.
The system automatically switched from open loop to closed loop in the event of cabin depressurization or when smoke or other harmful substances were detected in the crew module. The closed-loop mode could also be manually activated by the crew. If the crew members were in shirtsleeves during cabin depressurization, they were able to individually don the Strizh within five minutes, with the SNiR supplying enough air to the cabin to keep them alive during that time (assuming the leak wasn’t too big). Since as many as 12 hours could elapse between depressurization and an emergency landing, the Strizh also had a waste collection and water supply system. The suits were put to the test in 1990-1991 at a vacuum chamber of the Air Force Scientific Test Institute in Akhtubinsk, when test engineers wore the suits for up to 18 hours, including 12 hours in a mode simulating a depressurized cabin. Unlike the Strizh suits, the pressure suits worn by Space Shuttle astronauts only provide protection during launch and entry, not during in-orbit emergencies.
An additional task of the ISZhO was to support a cosmonaut clad in an Orlan spacesuit during pre and post-spacewalk operations in the airlock, thereby increasing the resources of the suit during the spacewalk itself. More particularly, the system was used to feed oxygen to the suits, to cleanse and cool the air circulating in the suit, and provide water to the cooling garment. The ISZhO was also used to dry the spacesuits in preparation for the next spacewalk.
For unmanned missions the oxygen content in the cabin atmosphere was supposed to be lower to reduce the fire hazard. For instance, Buran had a 90 percent nitrogen/10 percent oxygen atmosphere on its one and only mission in 1988.
The SNiR and SGS were developed by the NPO Nauka organization in Moscow, while the ISZhO and associated pressure and spacesuits were products of the Zvezda organization in Tumilino just outside Moscow .