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Atmosphere revitalization (SOGS)

This page briefly describes part of the Russian segment Environmental Crew and Life Support Systems of the International Space Station: atmosphere revitalization (SOGS), generating oxygen and filtering carbon dioxide.

Onboard air for the ISS’s human occupants must be artificially generated and recycled, and monitored for purity. The Russians developed this technology on the Salyut and Mir space stations, and it’s really rather clever.

Oxygen supply system

This consists of the Elektron unit, two solid-fuel oxygen generators (TGK) and deliverable oxygen from the Progress cargo spacecraft.

Elektron
«Электрон»

The Elektron is the main source of oxygen, and produces oxygen via a simple process: the electrochemical decompostion of water to separate the oxygen and hydrogen that water is comprised of (H2O). In simpler terms, an electric current passed through water breaks the water down into its atomic components (hydrogen and oxygen). An electrolysis unit breaks down (decomposes) the water in a potassium hydroxide solution (30% in solution). The resultant oxygen is released into the Service Module; the hydrogen is vented into space. Water is pumped from a supply tank to the Elektron device, which resembles a tall silver cylinder about the size of a human torso, and is located behind panels 429 & 430 in Zvezda (on the starboard side of crew quarters, just forward of the galley table)

The Elektron, like most other equipment, is controlled by the onboard computer system; crews interact and monitor it via a laptop, as well as the Elektron’s manual controls.

TGK
ТГК

Two solid fuel oxygen generators (TGK, Твердотопливный Генератор Кислорода, Tverdotoplivnyi Generator Kisloroda) provide back-up oxygen generation if the Elektron fails. They provide oxygen by heating a solid oxygen compound (either lithium perchlorate mentioned below, or potassium perchlorate, mentioned in the LSS-ISS) which decomposes into oxygen and potassium chloride. The candles are also referred to by the acronym SFOG by NASA.

The solid oxygen compound used was lithium perchlorate (LiClO4) in the old-style TGK, and potassium perchlorate (KClO4) in the new design. The change in compound was because it was the lithium type that caught fire onboard the Mir Space Station. The compound is contained in briquette form in a “candle”, which generates 600 liters of oxygen by thermal decomposing (burning), enough to support one crewperson for one day, and takes up to 20 minutes to decompose. The reaction temperature is 450-500°C which generates about 800W of heat.

The old design, which was manually ignited by rotating a driving handle and could fail to ignite on occasion, was replaced by an electrically-fired device in 2006, as noted in the 4 October On-Orbit Status:

In the SM, FE-1 Tyurin completed the scheduled removal and replacement of one of the two SFOG (Solid Fuel Oxygen Generator, TGK) ignition “boxes” with the new electrically-ignited type and connected the associated electronic control unit (BU TGK). Afterwards, he successfully burned a “candle” as a test, which yielded ~1.6 mmHg of O2. (The SFOGs, on standby as oxygen sources as backup to the Elektron and Progress-stored O2, generate O2 by decomposing cartridges of solid potassium perchlorate (KClO4) into potassium chloride (KCl) and O2 when heated at 400°C. The “old style” SFOGs were mechanically ignited with a percussion cap.)

  • More details (LSS ISS) (note that this describes the older version of the TGK)
  • NASA: Public Lessons Learned Entry: 1143 – Utilization of Russian-Built SFOG May Pose a Safety Hazard Aboard International Space Station (ISS) Given Past Problems With the Solid Fuel Oxygen Generator (SFOG).

Air purification system

This is designed to remove carbon dioxide and gaseous trace contaminants from the atmosphere. The system includes the following:

Vozdukh
«Воздух»

The Vozdukh (meaning “Air”) removes carbon dioxide and other gaseous trace contaminants from the Station’s atmosphere. CO2 is absorbed by molecular sieves consisting of Zeolite, a solid porous absorbent material. (This can be regenerated by exposure to vacuum.) The Vozdukh can be operated in manual or automatic modes. A series of valves channels the air; these can be quite noisy and emit a loud screech about every 10 minutes or so (as Expedition 1 found out to their displeasure). If the Vozdukh is off-line, CO2 absorbent canisters are utilized as a back-up. The Vozdukh is located behind panel 425 on the starboard side of Zvezda (just forward of the Elektron).

The Vozdukh’s emergency vacuum valves are tested monthly – they vent carbon dioxide filtered from the sieves into the vacuum of space, so it is obviously important that they close properly! From 23 April 2004 On-Orbit Status:

In another joint walkthrough with Padalka, Kaleri completed the periodic (monthly) functional closure test of the Vozdukh CO2 removal system’s emergency vacuum valves (АВК, AVK), last time done: 4/9). (The AVKs are critical because they close the Vozdukh’s vacuum access lines in the event of a malfunction in the regular vacuum valves (БВК, BVK) or a depressurization in the Vozdukh valve panel (БОА, BOA). Access to vacuum is required to vent carbon dioxide (CO2) during the regeneration of the absorbent cartridges (ПП, PP). During nominal operation of the Vozdukh, the AVK valves remain open.)

Carbon dioxide absorbent canisters

The lithium oxide-based canisters of the Service Module Zvezda provide a backup means of removing CO2 from the atmosphere of habitable compartments. There are two canisters; fans push air through them and the lithium oxide absorbs the carbon dioxide from the atmosphere. The CO2-removal capacity of one canister is 1600 liters (56.5 ft³).

Trace contaminants control unit (БМП, BMP)

This removes contaminants from the atmosphere. It consists of two regenerable activated-charcoal cartridges, a cartridge containing a catalytic oxidizer, a filter, a fan, and valves. Atmosphere purification can be performed in one of two modes of operation: by using both charcoal cartridges simultaneously or by using only one cartridge at a time. The charcoal beds absorb high molecular-weight trace contaminants. The catalyst oxidizes carbon monoxide to carbon dioxide, and the hydrogen to water. The charcoal cartridges must be regenerated every 20 days by exposure to vacuum for a 12-hour period; they are heated to 200°C during the process.

Harmful impurities filter

This, the FVP, ФВП, absorbs harmful gases (acetone, ammonia, hydrogen sulfide, carbon monoxide, hydrocarbons, etc.) from the Station’s atmosphere. It is installed in Zarya behind panel 411. The filter is made up of two parts: a replaceable cartridge containing a chemical sorbent and activated charcoal, and a permanent catalyst (for oxidizing CO to CO2).

The ФВП can be activated via ground commands. It was activated prior to the arrival of Expedition 1 and then until the main elements of the SOGS were activated. It was then to be used to purify Zvezda’s atmosphere in the event of an off-nominal situation.

Other systems

Gas Analysis System

This continuously monitors partial pressures of oxygen, carbon dioxide, water vapor and hydrogen content in the atmosphere. There is one analyzer in Zvezda, and one in Zarya.

Pressure Integrity Monitoring System

This monitors the total atmospheric pressure in the Russian Segment’s compartments, and transmits a warning signal to the Caution and Warning panel if the pressure drops below acceptable levels (i.e. if there is a hull breach).

The Interface Pressure Integrity Monitors are used to pressurize and equalize the atmosphere in the docking chambers, such as during a Soyuz or Progress docking.

Temperature and humidity control

The atmosphere in the ISS’s pressurized modules is equal to that at sea level. The atmospheric control elements maintain the air’s composition at the required percentages of nitrogen (78%) and oxygen (21-40%). Liquid-air heat exchangers provide temperature and humidity control. Temperature is generally kept around 23°C; humidity around 10%.

Fire detection and suppression

All three Russian modules have smoke detectors. Zvezda has a “Signal” infrared smoke detection system, while Zarya’s system consists of 10 induction smoke detectors (which use particle detectors). Portable fire extinguishers are provided, which produce a non-toxic foam. These cannot, however, be used in the U.S. segment as the foam conducts electricity and poses an electric shock hazard from equipment there carrying a voltage of 120 Vdc (Russian segment is 28 Vdc). Rebreather-type gas masks are provided for protection against smoke inhalation; these produce oxygen via a chemical reaction for up to 140 minutes. From the 9 August 2002 On-Orbit Report:

A new Flight Rule is being written to prohibit use of Russian fire extinguishers (PFE) in the US segment (USOS). Onboard Emergency Books are being modified accordingly. Since the Russian PFEs use a foam extinguishant (akin to water in terms of conductivity), they pose an electrical shock hazard from equipment carrying the USOS voltage of 120Vdc, as compared to the Russian segment voltage of 28Vdc. (Recent testing has shown that the foam is capable of transmitting 500 milliamps (mA); 40 mA poses a serious shock hazard.)

  • This schematic diagram shows the locations of these (the forward end of the module is to the viewer’s right, starboard at bottom, port at top – the overall view is looking down on the module).
  • More details (LSS ISS)
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Photo gallery

Sergei with the Elektron

Sergei Krikalyov prepares to wrangle with the Elektron.

Aleksandr near the Vozhdukh

Aleksandr Kaleri working on the Vozdukh CO2 scrubber.

Sergei with the Vozdukh

Sergei with the Vozdukh on STS-98, prior to its installation in the ISS in 2001.