Experimental Results on the Effect of Surface Structures on the Flame Propagation Velocity of PMMA in Microgravity

Materials foreseen for the design of manned spacecraft must pass the NASA-STD 6001B Test 1 regarding its fire hazard. During this qualification test in 1g conditions, a flat sample with fire protected edges is placed vertically in a quiescent environment, and ignited at its lower end. To pass the test, it must extinguish within 150 mm propagation length. Even though PMMA does not pass this test, it is extensively used for scientific investigations because of its repeatability and use in previous studies. Systematic ground tests of generic geometries have revealed that almost any realistic machined geometry like sharp or rounded edges, fins or grooves may lead to a rise in flame propagation velocity up to a factor of four related to the flat standard sample. For the first time, the flamed spread over a structured, thick PMMA sample of 290 x 50 mm was examined in microgravity (3x10-5g0) under concurrent flow of 0.20 m/s onboard Orbital ATK’s re-supply spacecraft Cygnus. The results were compared to the behavior of a similarly-sized flat sample. Just as in 1g, it was found that vertical structures promote faster flame spread compared to a flat sample but to a lesser degree than what is observed in 1g. While the structured sample burned 70% faster than the flat sample in 1g, this difference was reduced to only 32% in microgravity. Both samples burned drastically slower in microgravity: 23 times slower for the structured sample and 18 times slower for the flat sample. In 1g the pyrolysis front rapidly spreads along the surface and takes advantage of improver in depth heat transfer afforded by edges but, in microgravity, the burning mostly confined to the leading edge which has the best supply of oxygen. Finally, the microgravity flames produced more smoke and exhibited a larger preheat area.