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TheEnvironmental Control and Life Support System (ECLSS,ee-kliss) is a critical component of theInternational Space Station (ISS), responsible for maintaining a safe and habitable environment for crew members, similar tothat of Earth, with an air pressure equivalent to sea level. Maintaining an Earth-like atmosphere enhances crew comfort and safety, and is significantly safer than a pure oxygen environment.
The various subsystems of the ISS ECLSS regulate atmospheric pressure,control temperature and humidity,remove carbon dioxide, manage oxygen and nitrogen levels, provide ventilation,treat sewage, and generatepotable water.
The system was jointly designed and tested byNASA'sMarshall Space Flight Center,UTC Aerospace Systems,Boeing,Lockheed Martin, andHoneywell. In addition to its primary functions, the ECLSS serves as aproof of concept for future, more advanced life support systems intended for deep space missions.[1]
The ISS has two water recovery systems.Zvezda contains a water recovery system that processes water vapor from the atmosphere that could be used for drinking in an emergency but is normally fed to theElektron system to produceoxygen. The American segment has a Water Recovery System installed duringSTS-126[2] that can process water vapour collected from the atmosphere and urine into water that is intended for drinking. The Water Recovery System was installed initially inDestiny on a temporary basis in November 2008[2] and moved intoTranquility (Node 3) in February 2010.[3]
The Water Recovery System consists of a Urine Processor Assembly and a Water Processor Assembly, housed in two of the three ECLSS racks.[4]
The Urine Processor Assembly uses a low pressure vacuum distillation process that uses a centrifuge to compensate for the lack of gravity and thus aid in separating liquids and gasses.[5] The Urine Processor Assembly is designed to handle a load of 9 kg/day, corresponding to the needs of a 6-person crew.[2] Although the design called for the recovery of 85% of the water content, subsequent experience with calcium sulfate precipitation[3] (in the free-fall conditions present on the ISS, calcium levels in urine are elevated due to bone density loss) has led to a revised operational level of recovering 70% of the water content.
Water from the Urine Processor Assembly and from waste water sources are combined to feed the Water Processor Assembly that filters out gasses and solid materials before passing through filter beds and then a high-temperature catalytic reactor assembly. The water is then tested by onboard sensors and unacceptable water is cycled back through the water processor assembly.[4][5]
The Volatile Removal Assembly flew onSTS-89 in January 1998 to demonstrate the Water Processor Assembly's catalytic reactor in microgravity. A Vapour Compression Distillation Flight Experiment flew, but was destroyed, inSTS-107.[5]
The distillation assembly of the Urine Processor Assembly failed on 21 November 2008, one day after the initial installation.[2] One of the three centrifuge speed sensors was reporting anomalous speeds, and high centrifuge motor current was observed. This was corrected by re-mounting the distillation assembly without several rubber vibration isolators. The distillation assembly failed again on 28 December 2008 due to a high motor current and was replaced on 20 March 2009. Ultimately, during post-failure testing, one centrifuge speed sensor was found to be out of alignment and a compressor bearing had failed.[3]
Several systems are currently used on board the ISS to maintain the spacecraft's atmosphere, which is similar tothe Earth's.[6] Normal air pressure on the ISS is 101.3 kPa (14.7 psi); the same as at sea level on Earth. "While members of the ISS crew could stay healthy even with the pressure at a lower level, the equipment on the Station is very sensitive to pressure. If the pressure were to drop too far, it could cause problems with the Station equipment."[7]
TheElektron system aboardZvezda and a similar system inDestiny generate oxygen aboard the station.[8]The crew has a backup option in the form of bottled oxygen andSolid Fuel Oxygen Generation (SFOG) canisters.[9]Carbon dioxide is removed from the air by theVozdukh system inZvezda. One Carbon Dioxide Removal Assembly (CDRA) is located in the U.S. Lab module, and one is in the US Node 3 module. Other by-products of human metabolism, such as methane from flatulence and ammonia from sweat, are removed byactivated charcoal filters or by the Trace Contaminant Control System (TCCS).[9]
Carbon dioxide and trace contaminants are removed by the Air Revitalization System. This is a NASA rack, placed inTranquility, designed to provide a Carbon Dioxide Removal Assembly (CDRA), a Trace Contaminant Control Subassembly (TCCS) to remove hazardous trace contamination from the atmosphere and a Major Constituent Analyser (MCA) to monitornitrogen,oxygen,carbon dioxide,methane,hydrogen, andwater vapour. The Air Revitalization System was flown to the station aboardSTS-128 and was temporarily installed in theJapanese Experiment Module pressurised module. The system was scheduled to be transferred toTranquility after it arrived and was installed during Space ShuttleEndeavour missionSTS-130.[10]
The Oxygen Generating System (OGS) is a NASA rack which electrolyses water from the Water Recovery System to produce oxygen and hydrogen, like the RussianElektron oxygen generator. The oxygen is delivered to the cabin atmosphere. The unit is installed in theDestiny module. During aspacewalk,STS-117 astronauts installed a hydrogen vent valve required to operate the OGS.[11] The OGS was delivered in 2006 bySTS-121, and became operational on 12 July 2007.[12] From 2001, the US orbital segment had used oxygen stored in a pressurized tank on the Quest airlock module, or from the Russian service module. Prior to the activation of the Sabatier System in October 2010, hydrogen and carbon dioxide extracted from the cabin was vented overboard.[5]
In October 2010, the OGS stopped running well due to the water input becoming slightly too acidic. The station crew relied on theElektron oxygen generator and oxygen brought up from Earth for six months. In March 2011,STS-133 delivered the repair kit, and the OGS was brought into full operation.[13]
The Advanced Closed Loop System (ACLS) is anESA rack that converts carbon dioxide (CO2) and water into oxygen and methane. The CO2 is removed from the station air by an amine scrubber, then removed from the scrubber by steam. 50% of the CO2 is converted to methane and water by aSabatier reaction. The other 50% of carbon dioxide is jettisoned from the ISS along with the methane that is generated. The water is recycled by electrolysis, producing hydrogen (used in the Sabatier reactor) and oxygen. This is very different from the NASA oxygen-generating rack that is reliant on a steady supply of water from Earth in order to generate oxygen. This water-saving capability reduced the needed water in cargo resupply by 400 liters per year. By itself it can regenerate enough oxygen for three astronauts.[14]
The ACLS was delivered on theKounotori 7 launch in September 2018 and installed in theDestiny module as a technology demonstrator (planned to operate for one to two years).[15] It was successful, and remains on board the ISS permanently.[16]
ACLS has three subsystems:
The NASA Sabatier system (used from 2010 until 2017) closed the oxygen loop in the ECLSS by combining waste hydrogen from the Oxygen Generating System and carbon dioxide from the station atmosphere using theSabatier reaction to recover the oxygen. The outputs of this reaction were water and methane. The water was recycled to reduce the total amount of water carried to the station from Earth, and the methane was vented overboard by the hydrogen vent line installed for the Oxygen Generating System.[17]
Elektron is a Russian Electrolytic Oxygen Generator, which was also used onMir. It useselectrolysis to convert water molecules reclaimed from other uses on board the station into oxygen and hydrogen. The oxygen is vented into the cabin and the hydrogen is vented into space. The threeElektron units on the ISS have been plagued with problems, frequently forcing the crew to use backup sources (either bottled oxygen or the Vika system discussed below). To support a crew of six, NASA added the oxygen generating system discussed above.
In 2004, theElektron unit shut down due to (initially) unknown causes. Two weeks of troubleshooting resulted in the unit starting up again, then immediately shutting down. The cause was eventually traced to gas bubbles in the unit, which remained non-functional until aProgress resupply mission in October 2004.[18] In 2005, ISS personnel tapped into the oxygen supply of the recently arrived Progress resupply spacecraft when theElektron unit failed.[19] In 2006, fumes from a malfunctioningElektron unit prompted NASA flight engineers to declare a "spacecraft emergency". A burning smell led the ISS crew to suspect anotherElektron fire, but the unit was only "very hot". A leak of corrosive, odorlesspotassium hydroxide forced the ISS crew to don gloves and face masks. It has been conjectured that the smell came from overheated rubber seals. The incident occurred shortly afterSTS-115 left and just before arrival of a resupply mission (includingspace touristAnousheh Ansari).[20] TheElektron did not come back online until November 2006, after new valves and cables arrived on the October 2006 Progress resupply vessel.[21] The ERPTC (Electrical Recovery Processing Terminal Current) was inserted into the ISS to prevent harm to the systems. In October 2020, theElektron system failed and had to be deactivated for a short time before being repaired.[22]
TheVika or TGK oxygen generator, also known as Solid Fuel Oxygen Generation (SFOG) when used on the ISS, is achemical oxygen generator originally developed byRoscosmos forMir, and it provides an alternate oxygen generating system.[23] It uses canisters of solidlithium perchlorate, which decomposes into gaseous oxygen and solid lithium chloride when heated.[23] Each canister can supply the oxygen needs of one crewmember for one day.[24]
Another Russian system,Vozdukh (RussianВоздух, meaning "air"), removes carbon dioxide from the air with regenerable absorbers of carbon dioxide gas.[25] An incident occurred in 2018 when one of the two Vozdukhs (also known as SKVs) deactivated without a command but was reactivated a little while later.[26]
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Temperature and Humidity Control (THC) is the subsystem of the ISS ECLSS which maintains a steady air temperature and controls moisture in the station's air supply. Thermal Control System (TCS) is a component part of the THC system and subdivides into the Active Thermal Control System (ATCS) and Passive Thermal Control System (PTCS). Controlling humidity is possible through lowering or raising the temperature and through adding moisture to the air.[citation needed]
Fire Detection and Suppression (FDS) is the subsystem devoted to identifying that there has been a fire and taking steps to fight it.
'The OGA over the past six months has not been running well because the water that's been fed to it is just slightly too acidic,' [station Flight Director Chris] Edelen said. 'We've done some pH tests on the OGA and the engineers have determined that because the pH is not quite balanced right, there was a breakdown in the reaction beds inside the OGA such that material is being released and that material can clog the little holes, the little pores, which can decrease the effectiveness of the OGA at generating (oxygen).'
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