Preventing the fire next time
Berkeley researchers accept a bit of nausea to help future space travelers avoid a far more deadly threat
| 05 March 2003
The dangers confronting astronauts and other travelers in space are clearer than ever these days, as the tragic accident involving the space shuttle “Columbia” shows. Whatever the causes of that accident are proven to be — if indeed they can ever be positively determined — it’s likely that they relate to the complexity of the aircrafts and their operation, and the unique conditions surrounding high-speed re-entry into the earth’s atmosphere.
A life-threatening fire aboard a spaceship still in earth orbit, or venturing into deep space, is another danger that NASA engineers must consider. Such a fire would likely involve different factors (and possibly different materials) than the one that doomed “Columbia,” and would have unique characteristics if only because the oxygen necessary to sustain such a fire and its toxic products would be contained within the spacecraft’s internal environment. But that doesn’t mean that the likelihood of such a fire is in any way reduced; in fact, some researchers predict that the chances of a severe, even tragic fire occurring on a spaceship outside earth’s atmosphere are high. The odds, they say, are particularly high for spacecraft on long missions, such as the 10- to 20-year missions anticipated for NASA’s International Space Station, or a manned mission to Mars.
In a spacecraft’s small cabin, a fire could rapidly use up all available oxygen, while flames, smoke, and smoldering could destroy the computers and navigational equipment. What’s more, without gravity — and the buoyancy it causes — smoke doesn’t rise to activate a smoke detector’s alarm; nor are fire extinguishers particularly effective, because in the weightless atmosphere foam just scatters about.
NASA has long been concerned about the dire consequences of fire aboard a spacecraft. But until recently the agency operated under the assumption that, since fresh air plays a greater role in flammability on earth than in space (because an air current will fan a fire, not suppress it), materials that are not flammable on earth would also not burn in space. Based on that assumption, the agency has analyzed the flammability of the materials used for spacecraft interiors only in earth’s atmosphere, where the conditions that affect flammability can be remarkably different than those in space.
New parameters for safety in space
Five years ago, NASA called on Carlos Fernandez-Pello, Berkeley professor of mechanical engineering and director of the NASA-funded Microgravity Combustion Laboratory, to develop a methodology for testing the flammability of the materials used aboard spacecraft and, for the first time, to perform those tests under zero-gravity conditions. What Fernandez-Pello found defines a new set of parameters for fire safety in space. “After conducting the first tests in zero gravity, we were all surprised to find out that materials ignite more easily and burn faster in spacecraft than in earth’s gravity,” he says.
A variety of factors influence how fire behaves in space. Because there is no gravity, the fire does not induce buoyant air currents. “If you think of a fire in earth’s normal gravity conditions,” Fernandez-Pello says, “you can see that the buoyancy-induced air has two roles.” First, he explains, it cools the burning material by drawing in colder air, which tends to suppress the fire. Conversely, the cooler air brings fresh oxygen, fanning the fire. “Our job is to find out if conditions in space would favor the cooling factor or the fresh oxygen factor, because that’s what determines flammability.”
The first step was to look for ways to replicate zero gravity’s conditions in earth’s atmosphere, a feat they could accomplish with an extraordinary aircraft called the KC-135, a plane able to follow a parabolic flight pattern at an altitude of 30,000 feet. At the peak of each of its roller-coaster-like parabolas, zero gravity is momentarily achieved inside the craft.
Affectionately known as the “Vomit Comet” for obvious reasons, the KC-135 doubled as a film set, providing “Apollo 13” film director Ron Howard with authentic weightless scenes for actor Tom Hanks and his crew.
When the plane is used as a laboratory, researchers aboard the KC-135 strap their feet to stay put, and try to keep a calm stomach. “We do 10 parabolas in a row and then the plane levels out, and then another 10, for a total of 40 in a day,” Fernandez-Pello says. “That’s where the airplane got its name — and it’s why they give us little plastic bags.”
As the aircraft descends from the parabola and gravity kicks in again, passengers usually hit the floor with a bang. “You get used to it,” Fernandez-Pello says with his trademark grin. “It’s actually a fantastic experience.”
Beyond those visceral challenges is another: the data must be collected at just the right moment in the parabolic loop to take advantage of zero gravity. “For no more than 20 or 30 seconds,” says Fernandez-Pello, “we have a chance to measure the flammability of materials as if we were in space.” To that end, the team used a new testing device developed in Fernandez-Pello’s lab, called the Forced Ignition and Spread Test (FIST), a small wind tunnel equipped with an external radiant heat flux, or very intense flame.
Materials mimicking those used aboard a spacecraft are placed inside the wind tunnel and exposed to both the radiant heat and the kind of air currents present in a spacecraft, allowing researchers to calculate just how quickly each one ignites. Fernandez-Pello’s team is now testing acrylic plastics, blended polypropylene with fiberglass composites, as well as the laminated epoxy glass often used in circuit boards. They have been surprised to learn that many of the materials used in today’s state-of-the-art spacecraft actually ignite as much as 50 percent faster in zero gravity conditions than on earth. “It turns out that the cooling effect of air currents is much more important on earth than we realized,” Fernandez-Pello says.
This revelation is crucial because a fire in space is much more likely to occur than our current sci-fi visions of space travel would have us believe. Spacecraft contain abundant combustible materials, from paper, clothing, and plastics to circuit boards and electrical cables. Back in 1997, a faulty oxygen-supply unit caused such a dangerous fire aboard the Mir space station that the six-man crew had to don gas masks and prepare for an emergency escape. “Spacecraft designers need accurate information to know which materials to use where,” says Fernandez-Pello. “We can’t build spacecraft out of steel, right? So we really do have to know which materials are flammable and which are not.”
A previous version of this story appeared in “Forefront,” the thrice-yearly magazine of the College of Engineering.