NASA is working to develop (or extend) a normal-gravity test (or set of tests) that can be used to screen materials for use in space exploration environments. One candidate test method uses the Equivalent Low Stretch Apparatus (ELSA). In this test, gravity effects are minimized by burning the material in a ceiling fire configuration, so the hot air cannot rise. The “stretch”--the velocity gradient near the fuel surface--is created by an air jet blowing up toward the fuel surface. Stretch is varied by varying the air jet velocity.
Flame shortly after ignition of a fuel sample in ELSA. The flame is beneath the fuel sample.
Developed at the NASA Glenn Research Center, ELSA is a modified version of the standard cone calorimeter test method. It uses an oven coil wound into an open-tipped cone shape to radiatively heat a fuel sample up to its ignition temperature. In ELSA, the cone is underneath the sample, with the air jet blowing through the center of the cone.
Because ignition delay times are shorter at lower stretch (or lower g), the normal cone calorimeter test overestimates the ignition time in microgravity, lunar gravity, and Martian gravity. To date, ELSA data with multiple fuels have confirmed that ELSA can simulate key features of the flammability behavior of low-stretch materials. Comparison with available low-gravity data is good.
The critical heat flux necessary for ignition decreases with stretch rate (org level). At zero stretch, any heat flux that is just slightly more than the experimental heat losses, such as radiation from the surface, is sufficient to ignite the material. The heat flux needed for ignition is lowest in microgravity.
Regression rates (burning rates) for polymethyl methacrylate (PMMA, or Plexiglas--Rohm and Haas Company) increase with heat flux and stretch rate, but the low-stretch flame with external radiation is very fuel rich, and nearly an order of magnitude more CO is produced (1000 ppm) than in the normal cone configuration.
These results suggest that the current standard cone calorimeter test used to evaluate materials for use in spacecraft (NASA-STD-6001, test 2) overestimates the ignition times and underestimates the toxic products in a reduced-gravity environment.
Ignition delay for Kydex 100 decreases with increasing heat flux, as shown here. In the inset, ignition delay decreases at lower stretch rate (i.e., lower gravity) at 25 kW/m2.
Olson, S.L., et al.: An Earth-Based Equivalent Low Stretch Apparatus for Material Flammability Assessment in Microgravity and Extraterrestrial Environments. Proceedings of the Combustion Institute, vol. 30, issue 2, 2005, pp. 2335-2343.
Olson, S.L.; Beeson, H.; and Haas, J.P.: Experimental Demonstration of an Earth-Based Equivalent Low Stretch Apparatus to Assess Material Flammability for Microgravity & Extraterrestrial Fire-Safety Applications. The 3rd Joint Meeting of the U.S. Sections of The Combustion Institute, Chicago, IL, 2003.
Find out more about this research:
Glenn’s Spacecraft Fire Safety program:
http://microgravity.grc.nasa.gov/life/firesafe.html
ELSA video:
http://exploration.grc.nasa.gov/combustion/web/vid_elsa.htm
Last updated: November 2, 2007
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