Reno native and triathlete Sara McAllister has a lot going for her these days. The newly minted Berkeley mechanical engineering Ph.D. and current post-doc not only successfully participated in some 16 triathlons–including a grueling half-Iron Man Aquabike race, she also recently appeared on the History Channel series “The Universe,” published 13 academic papers on her NASA-funded research, and taught an upper level undergraduate “Fundamentals of Combustion” ME 140 class, which she called “a unique and eye-opening experience.”
For the last two-and-a-half years, McAllister has been the lead graduate student in the NASA-funded UC Berkeley Microgravity Combustion Processes Lab in Hesse Hall, an ongoing collaboration between NASA, Berkeley mechanical engineering professor and associate dean of the Graduate Division Carlos Fernandez-Pello, and his team of graduate students — this year, five of them women.
Following President Bush’s January 2004 directive to complete the International Space Station, develop a new manned space exploration vehicle, and return to the moon “as the launching point for missions beyond,” the Berkeley team was tasked with building an experimental apparatus to test the flammability of materials in space environments. It’s one piece of a focused effort to reevaluate existing parameters and generate data that could lead to a new set of guidelines for preventing fires in space in next-generation spacecraft.
For years it was widely assumed that fires in space would naturally suffocate themselves due to the lack of buoyant air currents in the absence of gravity. “But that’s not always the case,” says Professor Fernandez-Pello. “We now know that combustible materials flame up more easily in a spacecraft environment.” Hot air does not rise in space, so the air around a piece of smoldering material stays put, insulating it, and causing it to heat up. What’s more, the low-velocity air from an air conditioner on a spacecraft helps fan these flames.
It adds up to this, says McAllister: Where there’s a high concentration of oxygen [so astronauts can breathe] and low air flow, materials are even more
The danger of fires in space has long been on NASA’s radar. There were the near misses aboard the Mir space station in 1997 when a faulty oxygen supply ignited, endangering the six-man crew, and aboard Apollo 13 in 1970 when an oxygen tank exploded. But the 1967 fire in the command module of Apollo I during a test and training exercise, which killed all three astronauts, was perhaps the greatest fire mishap in NASA’s history. It’s widely believed that among the many factors contributing to the disaster was the 100 percent oxygen used for the test (we breathe 21 percent oxygen at sea level), and the presence of flammable materials — some of them unexpected culprits, like Velcro — in the cockpit.
Space fires may begin when electrical cables, circuit boards, or combustible materials overheat and begin smoldering. “Smoldering is a weak process,” says the professor. “It can go undetected for a long time, but once smoldering materials reach elevated temperatures and receive additional oxygen, they suddenly flare up. Yet, past understanding of smoldering is based almost entirely on how it behaves in normal gravity.”
So, Fernandez-Pello’s question was how to design and run a set of experiments in a near-zero gravity environment without stepping too far outside Earth’s comfortable atmosphere.
Film director Ron Howard solved it for Apollo 13, and so did Professor Fernandez-Pello, aboard the NASA-owned KC-135 aircraft based at Ellington Field in Houston, used to train astronauts. The fifth such craft since NASA began flying microgravity missions in 1959, the KC-135 is a big plane, the military version of a Boeing 707. NASA calls it the “Weightless Wonder,” but it’s better known as the “Vomit Comet.”
Its flight trajectory follows a series of parabolic loops, unusual for such a bulky aircraft, ascending to 30,000 feet before freefalling to level off before the next steep climb. At the top of the loop, passengers experience a 30-second burst of weightlessness, barely enough time, in the Berkeley flammability team’s case, to run an experiment and grab their data. Then up it goes again, 10 times in a row, 40 loops in a day’s work, plastic baggies at the ready.
These onboard experiments worked well enough, but were costly and had to be complemented with supporting experiments on the ground that allowed for adequate time to take accurate measurements. NASA had already built a mock-up of a vessel capable of holding a vacuum to conduct combustion experiments in the Space Station. It was there that the Berkeley team would start.
McAllister took charge of the new equipment setup at the heart of this round of experiments, the so-called FIST — Forced Ignition and Flame Spread Test —apparatus. One FIST apparatus already existed, equipped with a small-scale wind tunnel where combustible materials are exposed to heat in differing air flow velocities. The new FIST apparatus to be set up by McAllister would be a pressure chamber able to hold a vacuum to perform experiments in varying atmospheric pressure.
“I spent a lot of time planning and thinking things through for the FIST pressure chamber, figuring out how to get a power plug inside a chamber when you have to have it completely sealed off,” says McAllister, who is entirely comfortable building and fixing complicated devices, a legacy of long hours spent with her father learning to adjust the valves and change the link pins in the front end of her VW bug. “It hadn’t been done before, so nobody knew how to do it and one of us had to sit down and figure it out. You have to learn how to do it the hard way, building it from the ground up. It was my struggle and Professor Fernandez-Pello let me struggle with it, and I’m grateful for that. If he had stepped in, I wouldn’t have learned how to do it. And since I created the setup, if something broke or went wrong, I knew how to fix it.”
The challenge McAllister faced was to fit the original FIST combustion wind tunnel, initially designed for the International Space Station, into the pressure chamber and incorporate piping for oxygen and nitrogen intake and instrumentation wiring to conduct the flammability experiments, all this while keeping the chamber completely sealed off and capable of holding a vacuum.
M.S. student Sarah Scott, also part of the flammability team, machined several of the metal structures in the apparatus, including heater mounts, sample holders, and the fire suppression system. “Sara and I would discuss what needed to be held or mounted and I’d think about the holds and the space allowed and what would be the most efficient way to go about making it, and then I’d scrounge around the lab for metals and make it at the machine shop.”
“The result is an innovative, finely tuned experimental apparatus that is the first of its kind,” says Fernandez-Pello. “It’s relatively easy to use and capable of providing interesting data on the flammability of materials in low pressure and varied oxygen-concentration environments.”
The beauty of the FIST pressure chamber is that it allows the team to simulate the space exploration atmosphere to test flammability of materials in near-space conditions, dovetailing neatly with NASA’s longstanding plan to retire the aging Space Shuttle next year. Work on its replacement vehicle, the “Constellation,” is ongoing and will include new parameters set for the craft’s interior atmosphere.
While current spacecraft operate at sea-level conditions, and the Apollo missions used low cabin pressure (34 percent of the pressure at sea level) and 100 percent pure oxygen, next-generation spacecraft, including the Constellation, have settled on what engineers now call the Space Exploration Atmosphere: 30 to 32 percent oxygen with air pressure more like Denver’s, but somewhat higher than that used for the Apollo missions. It’s with the exploration atmosphere, chosen as a compromise between astronaut comfort and material flammability, where Fernandez-Pello’s students, including Sara McAllister, Sarah Scott, and Sonia Fereres have worked in sync with NASA’s schedule.
“With the pressure chamber, we have the flexibility of changing the ambient atmosphere conditions when we’re running tests, increasing or decreasing nitrogen or oxygen as we need to, raising or lowering air pressure to test how flammable the materials are,” says doctoral student Sonia Fereres, a native of Spain. “This device gives us the first indicator of how easily a material would ignite aboard a future spacecraft.”
Current experiments in the lab are now focused on polymers, such as polypropylene-glass composites, Plexiglas, and other plastics used in the transportation industry. Plexiglas, or PMMA, short for its cumbersome chemical name, is the material they’ve worked on most recently. “It’s what we use to test the flammability of solid fuels in Space Exploration Atmosphere,” says Fereres. “We know how PMMA ignites in a normal atmosphere when heated; now we want to know how it will ignite in a low-pressure, high-oxygen atmosphere when exposed to high heat.”
To quantify PMMA’s flammability, the team measures the time it takes from the moment you start heating a sample until it ignites, as well as the ignition temperature, this in varying atmospheres, according to McAllister. “We’re trying to isolate the effects of a low pressure and increased oxygen environment, like the one you’d find in a next-generation’s spacecraft’s interior.”
“The beauty of the FIST pressure chamber is that it allows the team to stimulate the space exploration atmosphere to test flammability of materials in near-space conditions…”
After two years of experiments, McAllister’s results were striking, even unexpected: that the time for ignition was decreased by as much as 27 percent in an exploration atmosphere. Reduce air pressure, increase oxygen concentration, and ignition occurs more quickly.
“Before we did the experiments, we actually had a bet going in the lab. Some, including myself and Carlos, thought that the ignition time would increase because chemical reactions slow down as you reduce the pressure. But solid ignition is a pretty complicated event and there was a lot more going on than just the chemistry.”
McAllister leaves the Berkeley lab this spring, more than satisfied with her Berkeley experience. “I started out in heat-transfer for my M.A., but my interests were always in combustion,” says McAllister, who, on hiking trips, is the one looking at the camping stove to figure out the combustion process there. “So when there was an opening at the FIST lab, I took it. It was a natural extension of working on cars for me to get into combustion. The research in Carlos’ lab is great. It’s experimental work, which I’m totally committed to doing. It’s hands on, fixing things, touching things. I can’t imagine working on a computer all day. And it’s so great to be able to say my project is funded by NASA.”
The flammability team will press on with their experiments, branching out from McAllister’s ease-of-ignition research, to work on ways to quantify how solid fuels burn, flames spread, the flammability of fabrics used in space suits, and materials’ toxicity. As always, they’ll maintain their monthly contact with NASA scientists and engineers, discussing test results, submitting reports, planning future experiments, their ongoing work adding to the massive data NASA is gathering for future missions.
“We know that a fire in space does not have to be large to have dire consequences,” says Fernandez-Pello. “Our objective is to help NASA assess the hazard of fires in space, and to provide them with information to screen and rank materials’ flammability to help NASA decide which materials they can allow onboard a spacecraft.”
—By Nancy Bronstein