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This paper presents a fire suppression analysis for the Altair project. The architecture of the Altair systems relevant to fire safety is briefly reviewed. This is followed by an outline of a fire safety analysis of the spacecraft including an outline of a probabilistic risk analysis (PRA). The particular emphasis of this analysis is the change in risk as the vehicle moves to lower pressure, higher operating voltage and increased oxygen mole fraction. The analysis shows that all of these changes increase the likelihood and intensity of a fire. The paper then outlines the options for a suppression system followed by a trade analysis of the different options. The candidate systems include inert gas agents (nitrogen, carbon dioxide and helium), water-based systems (spray, mist and foam) and chemically active agents. Chemically active agents are included for reference purposes since they are not likely candidates for the Altair vehicles.

This paper discusses the unique aspects of fire suppression in reduced (micro- and Lunar/martian) gravity environments. It builds on a trade study conducted by the Fire Prevention Detection and Suppression group at the NASA Glenn Research Center that examined the efficacy of fire suppressants in reduced gravity. The first part of the present paper reviews the differences in flame characteristics between terrestrial and extraterrestrial fires and how these characteristics change the action of a fire suppressant. Special emphasis is placed on enriched oxygen ambient environments, a condition that will routinely exist on future spacecraft and extraterrestrial habitats. The most important difference between normal gravity and reduced gravity fires is the increase in the minimum suppressant concentration (for gaseous agents in a total flooding application) required to extinguish a fire in reduced gravity compared to normal gravity.

This paper discusses the current state of technology in reduced gravity fire suppression. The focus is on the unique issues associated with the CEV and future spacecraft including operation in reduced gravity and enriched oxygen ambients. Inert gas agents such as carbon dioxide, nitrogen and helium have different minimum extinguishing concentrations (MEC) in microgravity compared to normal gravity; in most instances the MEC in microgravity being higher than in normal gravity. This means that designs based on terrestrial standards will not offer the same factor of safety in microgravity. The results also show that the MEC is a strong function of ambient oxygen concentration in reduced gravity (as expected).