UC Berkeley

While fire behavior on Earth has been studied for centuries, fire research in microgravity conditions is still in its infancy. The frequency of human space travel and length of individual missions have resulted in a greater need to understand microgravity fire safety. There is particular concern about reducing the flammability risk inside a spacecraft, as the consequences of a fire could be catastrophic in an enclosed environment where egress is difficult or impossible.The flammability of a material is greatly influenced by the environmental conditions that surround it. Spacecraft environments generally feature reduced ambient pressures, low velocity flows, and potentially elevated oxygen concentrations, and understanding how these conditions affect combustion is an important aspect of flammability assessments. This work aims to compare the flame spread and limiting oxygen concentration (LOC) for opposed flame spread extinction of polymethyl methacrylate (PMMA) in sub-atmospheric microgravity and earth-gravity environments under the influence of an external radiant heat flux, using ground and ISS experiments. The radiant heating ranges from 0 to 3.3 kW/m2 and the subatmospheric pressures tested are as low as 40 kPa. Special attention is paid to the influence of a diminishing oxygen concentration on the flame spread rate and process of flame extinction. Experiments show that the ambient pressure and external heat flux have parallel effects on the rate of flame spread over a solid combustible. Reducing the pressure reduces the flame spread rate and increases the LOC. At the same time, reducing the external radiant flux also reduces the flame spread rate and increases the LOC. It appears that an increased external radiant flux influences flame spread more strongly, and it counteracts the deterring effect of reducing the ambient pressure. The LOC in microgravity is far lower compared to normal gravity, emphasizing the gravity of this research.