Fire in space:

results from STS-107 - Columbia's final mission

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Abstract

“Flame balls” are steady, convection-free, spherically symmetric flame structures that were originally predicted by Zeldovich in 1944 but not seen experimentally until 40 years later in short-duration drop tower experiments. Flame balls represent the simplest possible interaction of chemistry and transport in flames and thus bear a similar relationship to combustion research that the fruit fly does to genetics research. With this motivation, space flight experiments on flame balls in weakly burning flames in hydrogen-oxygen-inert and methane-oxygen-inert mixtures were conducted on the STS-107 / Columbia Space Shuttle flight. A total of 39 tests were performed. Most tests (by design) produced only 1 flame ball, though one test intentionally designed to produce a large number of flame balls resulted in 9 balls. Over half of the science data was downlinked during the mission, resulting in minimal loss of science despite the loss of Columbia and its crew. Among the accomplishments of the experiment were

• The weakest flames ever burned, either in space or on the ground. The weakest flame balls produced about 0.5 watts of thermal power. By comparison a birthday candle produces about 50 watts of thermal power.
• The leanest flames ever burned, either in space or on the ground. The leanest hydrogen-air test points contained 3.2 mole percent H₂ in air (equvalence ratio < 0.079).
• The longest-lived flame ever burned in space (81 minutes)

Several totally new results were found, including

• Oscillating flame balls that were predicted theoretically but heretofore never observed experimentally.
• For some tests, particularly in methane-oxygen-sulfur hexafluoride mixtures, flame ball drift not related to gravitational disturbances nor interactions with other balls or walls. This was a completely unexpected and as yet unexplained result.

The data obtained during the mission will keep combustion scientists busy for many years to come and will help lead to the development of cleaner, more fuel-efficient engines as well as improved methods for spacecraft fire safety assurance.