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The Shuttle retirement in 2010 will force the ISS program to reconsider how to supply the Station with nitrogen and oxygen for six to ten more years beyond 2010. The major options for post-Shuttle retirement resupply are resupply via transfer vehicle, the use of small Intervehicular Activity (IVA) high pressure tanks, “stockpile” enough gas to support International Space Station (ISS) through end of life, or generate the necessary gases onboard the Station. The method chosen to sustain the ISS will serve as a building block for producing new minimally dependent environmental control and life support systems for future manned missions to the Moon, Mars and beyond.

The Nitrogen System aboard the International Space Station (Station) continues to maintain Station total pressure and support several ongoing scientific and medical tasks. This paper addresses elevated leakage in the Nitrogen System, behavior during events such as nitrogen usage in other parts of the Station, and describes behavioral changes of the nitrogen Regulator/Relief Valve (regulator) since the activation of the Nitrogen System in 2001.

A significant drop in cabin pressure was detected in the International Space Station (ISS) in late December of 2003 and continued through mid-January of 2004. Analytical methods discussed will define the calculations required to determine the cabin air mass via total pressure, nitrogen partial pressure, oxygen partial pressure, carbon dioxide partial pressure, and water vapor partial pressure. Operational and analytical issues will be outlined and discussed as well as describing the leakage cause.

International Space Station (ISS) Crewmembers perform one of three denitrogenation protocols prior to performing Extravehicular Activities (EVAs) using the International Space Station (ISS) Airlock. The three denitrogenation protocols are: a) Exercise, b) Campout, and c) In-suit. EVA gas usage is categorized into Denitrogenation, Extravehicular Mobility Unit (EMU) oxygen use during EVAs, and air loss gas usage. The amount of gas usage depends on the denitrogenation protocol that is used. Each protocol's gas usage will differ as a result of different requirements of denitrogenation and EMU support. Flight data is correlated with theoretical values when it is available. The correlation to flight data provides a validation of the analysis data. Theoretical and actual gas usages from the ISS were calculated for EVAs out of the Airlock during Stage 7A to Stage 11A. Components of denitrogenation and EMU support gas usage are included.

The International Space Station (ISS) nitrogen is used to maintain total pressure within the cabin. Nitrogen is also required to support on-board experiments and medical equipment. Nitrogen is stored on the ISS Airlock in two tanks. The Progress also can bring up pure nitrogen, but usually just brings nitrogen up in the form of air. A proper balance must be maintained to ensure all users have the required amount of nitrogen to support their needs. The ISS oxygen is used to maintain oxygen partial pressures within the cabin. Oxygen is also required to support medical equipment as well as emergency masks and Extra-Vehicular Activity (EVA). Oxygen has many different sources. The main metabolic oxygen source is the Elektron, a water electrolyzer. Oxygen candles (Solid Fuel Oxygen Generators or SFOGs and BOCS) can supply metabolic oxygen as an alternative to the Elektron. Additionally, oxygen is stored in two tanks on the ISS Airlock as well as brought up in Progresses.

The International Space Station (ISS) loses cabin atmosphere mass at some rate. Due to oxygen partial pressures fluctuations from metabolic usage, the total pressure is not a good data source for tracking total pressure loss. Using the nitrogen partial pressure is a good data source to determine the total on-orbit cabin atmosphere loss from the ISS, due to no nitrogen addition or losses. There are several important reasons to know the daily average cabin air loss of the ISS including logistics planning for nitrogen and oxygen. The total average daily cabin atmosphere loss was estimated from January 14 to April 9 of 2003. The total average daily cabin atmosphere loss includes structural leakages, Vozdukh losses, Carbon Dioxide Removal Assembly (CDRA) losses, and other component losses.

The International Space Station (ISS) Airlock Crewlock can be depressurized via various methods. The ISS Airlock is divided into two major sections, the Equipment Lock and Crewlock. The Equipment Lock, as the name indicates, contains the equipment to support EVA activities including Extravehicular Maneuvering/Mobility Unit (EMU) maintenance and refurbishment. The Equipment Lock also contains basic life support equipment in order to support denitrogenzation protocols while the Airlock is isolated from the rest of the ISS. The Crewlock is the section of the Airlock that is depressurized to allow for Extravehicular Activity (EVA) crewmembers to exit the ISS for performance of EVAs. As opposed to the Equipment Lock, the Crewlock is quite simple and basically just contains lights and an assembly to provide services, oxygen, coolant, etc, to the EMUs. For operational flexibility, various methods were derived for Crewlock depressurization.

The on-orbit oxygen and nitrogen supply for the United States On-Orbit Segment (USOS) of the International Space Station (ISS) is provided in tanks mounted on the outside of the Airlock module. Gasses are supplied, for distribution to users within the USOS, via pressure regulators in the Airlock. The on-orbit storage can be replenished with gas that is scavenged from the Space Shuttle, or by direct replacement of the tanks. The supply and distribution system are described in this paper. The users of the gasses are identified. The system architecture is presented. Operational considerations are discussed.