The Effect of Gravity on Flame Spread over PMMA Cylinders

Fire safety is a concern in space travel, particularly with the current plans of increasing the length of the manned space missions, and of using spacecraft atmospheres different than in Earth, such as microgravity, low-velocity gas flow, low pressure and elevated oxygen concentration. In this work, the spread of flame over a thermoplastic polymer, polymethyl methacrylate (PMMA), was conducted in the International Space Station and on Earth. The tests consisted of determining the opposed flame spread rate over PMMA cylinders under low-flow velocities ranging from 0.4 to 8 cm/s and oxygen concentrations from 15% to 21%. The data show that as the opposed flow velocity is increased, the flame spread rate first increases, and then decreases, different from that on Earth. The unique data are significant because they have only been predicted theoretically but not been observed experimentally before. Results also show that flame spread in microgravity could be faster and sustained at lower oxygen concentration (17%) than in normal gravity (18%). These findings suggest that under certain environmental conditions there could be a higher fire risk and a more difficult fire suppression in microgravity than on Earth, which would have significant implications for spacecraft fire safety.