International Space Station (ISS) Major Constituent Analyzer (MCA) On-Orbit Performance 2006-01-2092

This paper summarizes the first 5 plus years of on-orbit operation for the Major Constituent Analyzer (MCA). The MCA is an essential part of the International Space Station (ISS) Environmental Control and Life Support System (ECLSS). The MCA is a mass spectrometer instrument in the US Destiny Laboratory Module of the International Space Station. The MCA provides critical monitoring of six major atmospheric constituents (nitrogen (N2), oxygen (O2), hydrogen (H2), carbon dioxide (CO2), methane (CH4) and water vapor (H2O)) sampled continuously and automatically in all United States On-Orbit Segment (USOS) modules via the Sample Distribution System (SDS). Sample lines have been routed throughout the U.S. modules with valves to facilitate software-automated sequential sampling of the atmosphere in the various modules. Continuous readout of the partial pressures of these gases is critical to verifying safe operation of the Atmosphere Revitalization (AR) system, Atmosphere Control System (ACS), and crew safety for Airlock Extravehicular Activities (EVAs). The system also supports dedicated rapid sampling of any one location, primarily the Joint Airlock atmosphere during crew preparations for EVAs.

Initial MCA on-orbit activation in February 2001 and subsequent operations of the MCA are discussed, including operational anomalies and their resolution. Early on-orbit operations indicated that the extensive Built-In-Test (BIT) capabilities of the MCA may have been unnecessary given the unique aspects of the ISS microgravity environment and long-term unattended operation.

The MCA utilizes an Orbital Replaceable Unit (ORU) modular architecture. The performance of each ORU is discussed along with comparison of operating life against predictions. Projections of future service life are presented and life-extending options are proposed.

An MCA accuracy assessment was performed with very favorable results which NASA Mission Operations Directorate (MOD) has exploited for increased operational flexibility and more efficient use of consumables. The accuracy study is continually updated as more operational experience is accumulated.