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Spacecraft Fire Safety

The importance of spacecraft fire safety can never be over emphasized and early detection of smoke particles becomes the critical step of fire prevention in space vehicles. Design of fire detector systems requires understanding of the background conditions and the expected range of events to be detected. Detectors for terrestrial fires were developed using the results of extensive study of terrestrial fires. Unfortunately, there is no corresponding data set for spacecraft fires and consequently current spacecraft fire detectors were developed based upon experience from terrestrial systems. Given the large changes in fire behavior seen in reduced gravity, it is reasonable to expect that the signatures and detection requirements of these fires will change. The absence of buoyant flow in low-gravity increases the residence time in microgravity fires and increases the transit time from the reaction zone to the detector. The materials used in spacecraft are different from typical terrestrial applications where smoke properties were previously evaluated. Very significantly, the limited options available to respond to a spacecraft fire increase the importance of detecting the fire in an earlier stage of its development than is typical for terrestrial applications. All of these effects can be expected to change the smoke particle size distribution. The objective of this work was to make sufficient measurements of smoke from spacecraft fires to enable improved design of future detectors.

The Smoke Aerosol Measurement Experiment Re-flight (SAMER) is part of a series of efforts by NASA researchers and scientists at USRA's National Center for Space Exploration Research (now known as Advanced Research Associates). Previous projects studying smoke in space include Comparative Soot Diagnostics Experiment (CSD) and the Smoke Aerosol Measurement Experiment (SAME). The SAME experiment sought to avoid the problems experienced by the CSD experiment by obtaining the particulate size statistics on-orbit with a reduced dependence upon sample return to Earth. This is a challenging endeavor because existing aerosol instrumentation is typically large, incompatible with spacecraft experiment constraints, and may require substantial sample return to Earth. An alternative approach was employed that used three discrete instruments to measure separate moments of the size distribution. When combined, these moments provide useful aggregate statistics of the size distribution. The measurements were made using smoke generated by overheated spacecraft materials in much the same manner as the CSD experiment however the sample temperature, flow field, and particle aging time were more rigorously controlled. The target of SAMER project was to produce data in time to support Orion Environmental Control and Life Support System (ECLSS) design. SAMER experiments were conducted on-board International Space Station from July through September, 2010, using 36 Samples plus 30 reloads of 5 materials: silicone rubber, Teflon, Kapton, lampwick and Pyrell.