“smoke:” Characterization of Smoke Particulate for Spacecraft Fire Detection

INTRODUCTION The “Smoke” experiment is a flight definition investigation that seeks to increase our understanding of spacecraft fire detection through measurements of particulate size distributions of preignition smokes from typical spacecraft materials. Owing to the catastrophic risk posed by even a very small fire in a spacecraft, the design goal for spacecraft fire detection is to detect the fire as quickly as possible, preferably in the preignition phase before a real flaming fire has developed. Consequently the target smoke for detection is typically not soot (typical of established hydrocarbon fires) but instead, pyrolysis products, and recondensed polymer particles. At the same time, false alarms are extremely costly as the crew and the ground team must respond quickly to every alarm. The U.S. Space Shuttle (STS: Space Transportation System) and the International Space Station (ISS) both use smoke detection as the primary means of fire detection. These two systems were designed in the absence of any data concerning low-gravity smoke particle (and background dust) size distributions. The STS system uses an ionization detector coupled with a sampling pump and the ISS system is a forward light scattering detector operating in the near IR. These two systems have significantly different sensitivities with the ionization detector being most sensitive (on a mass concentration basis) to smaller particulate and the light scattering detector being most sensitive to particulate that is larger than 1 micron. Since any smoke detection system has inherent size sensitivity characteristics, proper design of future smoke detection systems will require an understanding of the background and alarm particle size distributions that can be expected in a space environment. Prior studies of spacecraft aerosol particulate include (Urban et al. 1997 and Liu et al. 1991). Urban et al. (1997) measured the STS and ISS detector response to various smoke particulate sources. In general the size of the smoke particulate was found to be larger in low gravity and the apparent size of the particles in smokes where the particulate was primarily liquid drops was found to increase to the point that the STS detector was essentially unable to detect the smoke. Liu et al. (1991) measured the mass concentration and particle size distribution of the cabin air on one shuttle flight. Two particle size distributions were measured and were found to have the mass median particle size greater than 100 microns. This is to be compared with their values of about 1 μm to 3 μm for ambient aerosols. The large difference results from the larger particles rapidly settling out at 1 g. The mass concentration of the particulate was found to be approximately 55 μg/m in the shuttle, versus 11 μg/m for 1-g ambient conditions. The ideal approach would be to obtain complete particle size distributions for all of the smokes of interest. Unfortunately, the instruments that provide data of this type (classifiers or