Flame extinction of thermally thick PMMA in Earth gravity versus microgravity: A comparison of Earth and on-orbit space experiments

Future spacecraft may operate at sub-atmospheric pressures and elevated oxygen concentrations to reduce preparation time for extra-vehicular activities. It is important to understand fire behavior and the fire risk of materials brought aboard a spacecraft. The limiting oxygen concentration (LOC) is one of the parameters used to determine the flammability of solid fuels. This work aims to compare the LOC for opposed flame spread extinction of polymethyl methacrylate (PMMA) in sub-atmospheric microgravity and Earth-gravity environments. Experiments with cylindrical PMMA samples are conducted in a combustion chamber in the Integrated Combustion Rack (CIR) on the International Space Station (ISS), as well as on laboratory facilities on Earth, at sub-atmospheric pressures with a forced flow of 10 cm/s opposed (counter-current) to the flame spread direction. The oxygen concentration in the chamber decreases naturally at a constant ambient pressure as a result of the PMMA combustion until flame extinction. Results show the LOC in microgravity is lower than at Earth gravity, meaning that flame spread can be sustained at lower oxygen concentrations in reduced buoyancy. The findings confirm that flammability test methods conducted in normal gravity and atmospheric pressure are not conservative enough for the conditions future spacecraft are expected to operate in.