Pressure Effects on the Self-Extinguishment Limits of Aerospace Materials

The Orion Crew Exploration Vehicle Module (CM) is being designed to operate in an atmosphere of up to 30% oxygen at a pressure of 10.2 psia for lunar missions. Spacecraft materials selection is based on a normal gravity upward flammability test conducted in a closed chamber under the worst expected conditions of pressure and oxygen concentration. Material flammability depends on both oxygen concentration and pressure, but since oxygen concentration is the primary driver, all materials are certified in the 30% oxygen, 10.2 psia environment. Extensive data exist from the Shuttle Program at this condition, which used essentially the same test methodology as the Constellation Program is currently using. Raising the partial pressure of oxygen in the Orion CM immediately before reentry, while maintaining the total cabin pressure at 14.7 psia, has been proposed to maximize the time the crew is able to breathe cabin air after splashdown. Leak testing the CM with ambient air at a maximum pressure of 17.3 psia has also been recommended. No data exist to assess how high the cabin oxygen concentration can be at 14.7 psia or 17.3 psia. Re-testing a large number of materials at these pressures would incur significant cost. However, since the maximum oxygen concentration (MOC) at which a material will self-extinguish has been determined for a variety of spacecraft materials as a function of pressure, an alternative is to use existing data to estimate the MOC at 14.7 psia and 17.3 psia. This paper will examine this data and present an analysis to determine the oxygen concentrations at these increased pressures that will result in self-extinguishment of a material. This analysis showed that the oxygen concentration for the Orion CM at 14.7 psia cannot be set higher than 25.6% without potentially invalidating the materials flammability certification in 30% oxygen at 10.2 psia for some materials. Materials certified under these conditions would still be self-extinguishing in ambient air at 17.3 psia.