NASA NEW LAB

In today’s A Lab Aloft guest blogger, Sandra Olson, Ph.D., reveals some of the mysteries of how flames burn in microgravity, as well as how flame studies on the ground and aboard the International Space Station help with fire suppression and safety in space.

Whether dropping through a hole in the ground as part of a drop test or zipping through space aboard the International Space Station, flames behave in fascinating ways in microgravity! In the Zero Gravity Research Facility, or ZGRF, at NASA’s Glenn Research Center, I get to study solid fuel combustion behavior first hand. ZGRF is a historic landmark and the deepest drop tower in the world with a freefall of 432 feet. Drop test experiments, like the one pictured below, look at material flammability during the brief, 5.18-second period of microgravity achieved as the sample package falls.

During a Zero Gravity Research Facility tour, Facility Manager Eric Neumann (far left) shows International Space Station Program Scientist Julie Robinson (front center) and her colleagues one of the drop packages used in the facility. The top of the white vacuum drop shaft is in the background. (NASA/Marvin Smith)

The drop test was remotely run from the ZGRF control room. Controllers activated the miniature wind tunnel apparatus to establish a spacecraft ventilation flow environment, then ignited the material and dropped the experiment. Once the sample releases into freefall, the experiment is completely automated. The drop vehicle lands in the catch-bucket at the end of the 5.18 second test.

Experiment images (left) and catch-bucket facility images (right) appear on the ZGRF control room screen. (NASA/Marvin Smith)

We have performed many drop tests studying how materials burn in microgravity compared to how they burn in normal gravity, or 1g. What we have found is that many materials actually burn better in the spacecraft flow environment than in 1g. This is because on Earth the buoyant flow—created when less dense materials rise within greater density environments—is strong enough to blow the flame out with oxygen reduction. In low ventilation, however, the slow flow provides the oxygen at an optimum rate, so the flame can survive to lower oxygen levels than in 1g. To learn more about the concepts of microgravity and combustion in the space environment, watch this "NASA Connect" video.

A flame burning in microgravity at the end of a 5.18-second drop from the Zero Gravity Research Facility. The material for this test was cotton fabric burning in 5 centimeter per second air flow, which is the typical International Space Station atmosphere. Crew clothing is often made of cotton. (NASA)

Enhanced flammability in space was recently proven in longer duration burn experiments aboard the space station as part of the Burning and Suppression of Solids, or BASS, investigation. For this study, the crew of the space station gets to play with fire. As a co-investigator, I get to observe via video on the ground and directly talk to the crew as they ignite a flame in the controlled area of the Microgravity Science Glovebox, or MSG, filming the behavior of the burn.

After his recent return to Earth, Astronaut Don Pettit, who worked on the BASS flame study in space, testified to a Senate subcommittee about the investigation and the importance of combustion experiments in microgravity.

“If you look at fire, fire and its either discovery or learning how to tame fire is what literally brought us out of the cave and allows us to have our civilization in terms of what we know now,” said Pettit. “Fire gives us our electricity. Fire allows us to have vehicles, airplanes and cars, and machines. It literally turns the wheels of our civilization...space station now offers us the ability to dissect deeper down into what the processes are in combustion… by looking at it in an environment free from gravity, free from the gravitational-driven convection. And this allows us to look at things and figure out what’s going on at a level that you could never see without taking it to space…and what we found is that things are more flammable than what we thought.”

(Left) Astronaut Joe Acaba runs BASS in the Microgravity Science Glovebox, or MSG. (Right) Astronaut Don Pettit holds up a burned acrylic sphere to show the science team on the ground how a fine layer of soot coats the wake region of the material, while the front part of the sphere looks like a meteorite with the surface marred with many craters. (NASA)

These experiments so far have confirmed that when the air flow is turned off, the flame extinguishes rapidly as it runs out of oxygen, with no fresh air flow. The MSG provides an enclosed work area, sealed to contain fluids, gasses and equipment for the safe running of combustion experiments. The crew views the burning material through the front window. The flame can be seen through this window in the picture with Joe Acaba (above). You also can see Don Pettit working on a previous run of BASS aboard station in this video.

This finding reaffirms the space station fire alarm protocol to turn off any forced air flow in the event of a fire alarm. Surprisingly, though, when the astronauts used a small nitrogen jet built into the flow duct for fire suppression testing, the flame did not go out when the air flow was turned off, if the nitrogen jet was on. In fact, the flame appeared to get brighter. Researchers intend to continue to study this unexpected discovery in which the nitrogen jet was able to entrain air all by itself, as the finding has important implications for gaseous fire suppression systems like the
CO₂ suppression system currently employed on station.

Acrylic sphere burning as part of the Burning and Suppression of Solids, or BASS, investigation aboard the International Space Station. (NASA)

BASS results also catch the attention of future spacecraft designers. One of the sample materials burned in BASS is acrylic, also called Plexiglas. This material is under consideration for spacecraft windows because of its excellent strength, mass and optical properties. However, it also burns quite well in the space station air environment. BASS payload summary reports mentioning acrylic have spurred a number of recent inquiries to the investigator team about the flammability of this material. After all, you don’t want your spacecraft windows to catch on fire!

A wax candle flame in very low air flow is nearly spherical with an inner sooty layer near the wick, and an outer blue layer. This blue is due to chemiluminescence, which is when a chemical reaction emits light. (NASA)

The BASS investigation has direct applications to spacecraft fire safety and astronaut wellbeing. A combustion experiment, BASS was jointly designed by scientists and engineers at NASA and the Universities Space Research Association, or USRA. BASS operations are scheduled to begin again aboard the space station in the spring of 2013.

The best part of my job as a researcher is the thrill of discovering new phenomena unique to microgravity. It is exciting to work with something as beautiful and powerful as fire, especially in these unique microgravity environments. The fire images have inspired me to create art images from them. 

2009 Art “Fire's Ribbons and Lace”
The delicate and fractal nature of charring cellulose is amplified here in repeated magnified images of a flame spread front over ashless filter paper. (Sandra Olson)

2011 Art “Flaming Star”
Microgravity flames converging toward the center of the starburst ‘implode’ against an outflow of wind, creating a diffusion flame ‘supernova.’ (Sandra Olson)

The more we understand the behavior of flames with given materials and conditions, the better prepared we will be to harness their potential and contribute to fire safety in future space exploration. What’s next will depend on what we discover from these ongoing tests, building on the knowledge already gained from these important combustion studies.

Sandra Olson, shown here with the microgravity wind tunnel drop apparatus.

Sandra Olson, Ph.D., is a spacecraft fire safety researcher at NASA’s Glenn Research Center, as well as the project scientist and co-investigator for the BASS investigation. She has a B.S. in Chemical Engineering and a M.S. and Ph.D. in Mechanical Engineering. She has worked at NASA since 1983, most of that time studying microgravity combustion.