Brand new findings on fire safety in space
For the fifth time, a team of 25 international scientists used the Northrop Grumman CYGNUS supply vehicle for the International Space Station to conduct experiments with large fires in space. This time, amazing data was transmitted from the return journey to Earth that even the combustion researchers could not have predicted: in weightlessness, a flame tends to spread in the opposite direction to the air flow. Accordingly, trying to blow out a flame on a spacecraft would be a really bad idea – if the blowoff speed is not high enough.
The experiment, which was conducted on January 6, 2021, as part of the SAFFIRE-V campaign, lasted 23 hours and was intended to test how a fire behaves in a crewed spacecraft. Since open fire in space, even when ignited in a controlled manner, always poses a major risk, the research team has been using the CYGNUS cargo craft for this purpose since 2016. It delivers supplies to the space station and burns up on return into Earth's atmosphere. In the phase between undocking and before reentry, samples are ignited, data is recorded, and then transmitted to Earth. ZARM's portion of the experiment consists of a structured acrylic glass fuel sample (PMMA). The sample is 40 cm wide, 20 cm long, and 10 mm thick, with ribs of varying widths on it, blown by a continuous stream of air (20 cm/s) designed to simulate the ventilation and oxygen supply system of a spacecraft. The pressure is about 70 percent of normal atmospheric pressure with an elevated oxygen concentration of 26 percent – simulating the conditions envisioned on future crewed exploration missions.
An unintended hardware failure led to what may be the most enlightening discovery of all the SAFFIRE project experiments to date. The first test on the structured PMMA fuel sample was supposed to have ignited its upstream end so that the resulting flame would have spread concurrently, meaning in the same direction as the flow. At the completion of this test, a second test was to ignite the downstream end of what should have been the partially-burned fuel sample. The scientists actually expected more compelling results from the first test, but the igniter circuit failed. This left a pristine sample for the second test and set the stage for some very novel findings. In the second test, a flame was successfully ignited on the downstream end of the fuel sample establishing opposed-flow spread, namely flame spread in the direction opposite the flow. Contrary to normal gravity results on Earth, in zero gravity, the flame spread rapidly along the ribs of the fuel by propagating toward the air flow. Since the PMMA fuel sample was still intact due to the failed first ignition, this behavior could be observed very clearly and over the entire sample length.
Several conclusions can be drawn from the observations. The most basic one is that a flame in zero gravity tends to propagate against the air flow – the exact opposite of what we intuitively expect to happen. Why is this the case? Under normal gravity, hot gas expansion caused by the flame produces lighter, less dense regions which rise upward and draw fresh oxygen-rich air into the flame from the bottom and the sides. The side entrainment of oxygen into the tall fire plume facilitates growth of the flame. In microgravity, however, there are no weight differences and therefore no buoyancy. The hot gas areas also expand, but they remain stratified. Without any air flow at all, the flame would even suffocate in its exhaust gas. However, due to the existing air flow in the experiment, only the flame base – where the air flow first meets the flame – is sufficiently supplied with oxygen. Accordingly, the flame is most active here and propagates surprisingly fast towards the air flow.
Another observation may prove even more relevant for future astronauts. In microgravity, the flame burning rate is often limited by its rate of oxygen uptake. Without buoyancy, the oxygen transport is only through diffusion and forced flow. This means that excess fuel vapor may be produced by the heated fuel which is not consumed by the flame. Coupled with the reduction in natural convective cooling of the hot sample, this amount of excess fuel can be significant. Thus, a lot of combustible but unburned flue gas escapes the flame and may accumulate elsewhere in the system, especially on the fuel side of the flame. This further means that the slightest disturbance to the stratification could oxygenate this hot, unburned flue gas and would result in dramatic deflagrations. Such a backdraft also exists at ground level, but it requires incomparably stronger disturbances to trigger it. This phenomenon is best known in connection with fires in closed rooms. Here, a layer of hot flammable gases also forms under the ceiling and opening a door or bursting a window can mix in new oxygen and cause a deflagration. The research team also observed this reaction during the previous flight, where the air flow was apparently switched on again too soon after the sample extinguished. Only after this latest test the team has an explanation for the dramatic end to the SAFFIRE-IV experiment.
What does this mean for the behavior of a space crew in the event of a fire? A crew member moving around in a fire scenario, or even attempting to extinguish the fire with a jet (of whatever), can trigger the sudden combustion of all accumulated smoke gases. Thus, when selecting firefighting technology, one will have to evaluate quite differently whether active intervention can be useful at all.
The SAFFIRE project is funded by NASA’s Human Exploration and Operations Directorate. The team consists of researchers from the United States, Europe, Russia and Japan.
Brand new findings on fire safety in space - SAFFIRE V (video): youtu.be/-wPH_KoQzZs
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