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Fire aboard spacecraft:
The devil's in the atmosphere
By Susan Davis
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. While there have been no fatalities from fires
aboard a spacecraft recently, some researchers predict that there’s
an extremely high probability of a severe, even tragic fire occurring
on a spaceship. 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.
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| Fire
is an extremely dangerous and very real possibility aboard
the International Space Station. Several incidents of overheated
and charred cables and electrical components have already
occurred on the Space Shuttle, the craft used to transport
equipment and personnel to the space station. |
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
the weightless atmosphere just scatters the foam about.
NASA has long been concerned about the dire consequences of fire
aboard a spacecraft. But until recently, the agency has 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, not suppress a fire), materials that are not flammable
on earth would be the same in space. Based on that assumption,
the agency has only analyzed flammability of the materials used
for spacecraft interiors in earth’s atmosphere, where the
conditions that affect flammability can be remarkably different
than those 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," says Fernandez-Pello.
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."
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Riding
the notorious "Vomit Comet," where Fernandez-Pello
and his research team have a 30-second window to run their
flammability experiments, requires efficiency,
pluck, and a strong stomach. |
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 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: that 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, we have a chance to measure the flammability of materials
as if we were in space," Fernandez-Pello says. 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 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 poly-propylene 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.
"Spacecraft designers must have accurate information so they
know which materials to use where," Fernandez-Pello says.
"We can’t build spacecraft out of steel, right? So
we really do have to know which materials are flammable and which
are not."
Author Susan Davis,
whose father helped design the Apollo fuel cells, is a Bay Area
writer and editor. Davis has written on environmental issues for
Intel Corporation, Lawrence Berkeley National Laboratory, and
The Nature Conservancy. She co-authored The Sporting Life, a book
on the physics of sports, and is currently working on a book about
the natural history of rabbits.
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