Paul D. Ronney, Principal Investigator
University of Southern California
This summary was written before the loss of Columbia.
Click here to read about my perspective on the loss of Columbia and its crew
Click to view crew tribute produced by NASA-Glenn
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| 1 flame ball moving in corkscrew pattern | Another corkscrew test | 2 large flame balls |
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| 3 flame balls |
9-ball test (Camera 1 view) Only first 15 minutes of test shown; we won’t know which one is Kelly until post-mission. |
9-ball test (Camera 2 view) Only first 15 minutes of test shown; we won’t know which one is Kelly until post-mission. |
Click on image to see view movie
Purpose
The objective of the SOFBALL experiment was to study weakly burning flames in hydrogen-oxygen-inert and methane-oxygen-inert mixtures in a configuration called “flame balls” that were originally predicted by the Russian physicist Ya. B. Zeldovich in 1944 but not seen experimentally until 1984 in short-duration drop tower experiments conducted by Prof. Ronney. Because flame balls are steady, convection-free, spherically symmetric and occur in fuels with simple chemistry, they represent the simplest possible interaction of chemistry and transport in flames. In this sense flame balls bear a similar relationship to combustion research that the fruit fly does to genetics research.
Summary of results
A total of 39 tests were performed in 15 different mixtures, resulting in a total of 55 flame balls, of which 33 were named by the crew. The total burn time for all flames was 6 1/4 hours. Since flame balls are extremely sensitive to gravitational acceleration, all tests were conducted during orbiter free drift periods. Microgravity levels were measured using OARE. The quality of the microgravity was found to be excellent (average accelerations less than 1 micro-g for most tests).
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 burned contained about 8% of the chemically balanced mixture. By comparison, the lean limit for gasoline-air mixtures in an internal combustion engine is about 70% of the chemically balanced mixture.
- The longest-lived flame ever burned in space (81 minutes)
Several totally new results were found, for example
- Oscillating flame balls (see plot below) that were predicted theoretically about 15 years ago by Prof. John Buckmaster at the University of Illinois and Dr. Guy Joulin of CNRS in Poitiers, France, 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.
Parting notes
- When the Gods want to punish you they answer your prayers. It will take me years to analyze all of the data obtained on STS-107.
- Flame balls live by the old stage performer motto – “leave ‘em wanting more…” Several tests were expected to last for more than an hour, but (until the last test) none lasted more than 25 minutes because of the mysterious drift. The very last test produced 9 flame balls initially (below right) (a large number of balls was expected from this test) that extinguished one by one until only one flame ball, named “Kelly” by the crew, remained. Unexpectedly, Kelly survived an astounding 81 minutes, seemingly immune to drift, until the test was extinguished due to operational reasons (it was still burning at the time).
- Kelly could be nicknamed “Magellan,” who also set out to circumnavigate the globe but was extinguished before completing the journey. (It takes 90 minutes to orbit the earth).
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| Radiometer signal showing flame ball oscillations | Image of flame balls in a hydrogen-oxygen-sulfur hexafluoride mixture at 3 atmospheres pressure. |