In the end, the flame extinguished itself only when the oxygen generator had emptied. Over the next several hours, the station’s life support systems cleared Mir’s atmosphere of all smoke, and the crew escaped the incident without significant damage to either themselves or the station’s structure.
Playing with fire
OK, so we’ve established that sealing up these gaps in our knowledge about fire’s behavior is clearly important. Now, how exactly do scientists do that?
Well, in 2008 NASA created their Combustion Integrated Rack (CIR) and sent it to the ISS. Used to safely fiddle with controlled combustions in microgravity, its hardware includes a 26-gallon combustion chamber and five different cameras that have been put to good use in thousands of tests over the past 15 years.
Many of those tests were part of the Flame Extinguishment Experiment, or FLEX, which began about a year later. As the name suggests, these revolved around putting out fires in space and ultimately improving fire suppression systems on future spacecraft. With the help of CIR, researchers aboard the ISS would ignite tiny droplets of either heptane or methanol fuel and record the results.
Daniel Dietrich, a scientist at NASA’s Glenn Research Center, told the administration that “one of the biggest discoveries, not only in the microgravity program, but in probably the past 20 [to] 30 years of combustion research has been during the FLEX experiments on the space station.”
The discovery in question? After certain liquid fuels are extinguished in space, they spontaneously reignite. In these cases, the subsequent flame — called a “cool flame” — burns at lower temperatures and is invisible to the naked eye.
Scientists aren’t sure exactly why this happens, but from a practical point of view, we could use such low temperature combustions hypothetically to produce fewer air pollutants in diesel engines back on Earth. We’re far from that reality, though research conducted in June 2021 took another great leap when it repeated the phenomenon using gaseous fuels, rather than liquid ones.
Bringing the flames home
Arguably the best part of studying flames in space, however, is that the lack of gravity just … makes things simpler, in a lot of ways. Whereas the candle on your coffee table may flicker, for example, as a result of buoyancy-driven instability, this isn’t the case in microgravity.
Another series of NASA studies, the Advanced Combustion via Microgravity Experiments (ACME), took advantage of this to really dig into what makes a good flame — one that is efficient but doesn’t give off a ton of pollutants like soot. Beginning in 2017, ACME scrutinized more than 1,500 flames in the CIR, over the course of more than four years.