CHAPTER 19 SYSTEMS: ENVIRONMENTAL CONTROL AND LIFE SUPPORT SYSTEM—SUPPORTING THE HUMAN ELEMENT OF THE ISS 338 Catalytic Oxidizer Temp Sensor RPS Fan SD MCA RPCM TCCS CDRA Charcoal Bed Assembly Figure 3. CDRA and the TCCS in the Atmosphere Revitalization Rack—schematic (left) and actual rack before launch (right). Also seen in the drawing are the MCA, the Remote Power Control Module (which powers the rack), and a fan that helps blow smoky air over the smoke detector to determine the presence of fires as well as to provide cooling. The Rack Power Switch allows the crew to power off the entire rack quickly in the event of a fire. Another tool in keeping the air clean is the Trace Contaminant Control System (TCCS). The TCCS (Figure 3) can remove most of the more than 200 possible contaminants on the ISS, including hydrocarbons, ketones, silicones, aldehydes, sulfides, and inorganics. The TCCS works mainly by pulling air through various filter beds that consist of activated charcoal, a catalytic oxidizer, or LiOH. The TCCS can even absorb small amounts of ammonia, but it would be inadequate in the event of an Interface Heat Exchanger (IFHX) breach. Once the filters have absorbed as much contaminants as they can hold, they are disposed of and new ones are installed. After providing the crew with the proper atmosphere needed to live and work, the next priority of the ECLSS is to offer support for scientific payloads. Nitrogen lines are plumbed throughout the Laboratory module and can be fed into payload racks. Nitrogen is usually used to purge some other gas from an experimental rack, but nitrogen may have other uses determined by the researcher. Also, two types of vacuum lines are available for the payloads. One type is the Vacuum Resource System (VRS), which is an open line to space. Although the space around the ISS is not a perfect vacuum, owing to small amounts of gas (mostly O2), the pressure is about 10 to 100 billion times lower than at the surface of the Earth and is as good or better than the best vacuum chambers on Earth. If an experiment requires vacuum to operate, it would be connected to this system to allow a constant vacuum, as needed. This will occur after the experiment itself has undergone careful analysis to ensure it cannot easily leak. If an experiment uses chemicals or gasses that need to be removed after a trial run (e.g., during combustion research), these