Experimental Verification of Material Flammability in Space

The flammability in microgravity of three US-furnished materials, Delrin, polymethylmethacrylate (PMMA), and high-density polyethylene, was determined using a Russian-developed combustion tunnel on Mir. Four 4.5-mm-diameter cylindrical samples of each plastic were ignited under concurrent airflow (in the direction of flame spread) with velocities from no flow to 8.5 cm/s. The test results identify a limiting air-flow velocity V(sub lim) for each material, below which combustion ceases. Nominal values are V(sub lim) < 0.3 cm/s for Delrin, 0.5 cm/s for PMMA, and 0.3 to 0.5 cm/s for polyethylene. These values are lower than those obtained in prior ground testing. Nevertheless, they demonstrate that flow shutoff is effective for extinguishment in the microgravity environment of spacecraft. Microgravity test results also show that the plastic materials maintain a stable melt ball within the spreading flame zone. In general, as the concurrent flow velocity V decreases, the flame-spread rate V(sub F) decreases, from an average (for all three materials) of V(sub F)= 0.5-0.75 mm/s at V = 8.5 cm/s to V(sub F)= 0.05-0.01 mm/s at V = 0.3-0.5 cm/s. Also, as V decreases, the flames become less visible but expand, increasing the probability of igniting an adjacent surface.