HOUSTON - NASA is more confident in its
ability to make minor in-flight repairs to a shuttle's heat shield after a
successful test during a Wednesday
spacewalk outside the Discovery orbiter, according to one mission manager.
Tony Ceccacci, lead shuttle flight director for Discovery's
STS-121 mission, said preliminary results from today's spacewalk by astronauts Piers
Sellers and Michael Fossum are positive, but their repair technique will
likely be confined to the limited types of repairs they performed.
During a
seven-hour and 11-minute spacewalk, Sellers and Fossum
tested application methods for a sticky, black material dubbed NOAX - short for
non-oxide adhesive experimental - designed to fill in cracks and gouges to the
carbon composite panels that line a shuttle's nose cap or wing leading edges.
They applied the NOAX with spatulas and caulk gun-like tools to squares of
reinforced carbon carbon (RCC), the same material
used to protect the shuttle nose and wings from searing reentry temperatures.
"I don't
think you can ever certify them, I think you can get confidence in these
specific [damage types]," Ceccacci said during a
mission update here at NASA's Johnson Space Center (JSC). "We expect some good
data out of this."
Today's
spacewalk expands on some initial
tests conducted during NASA's STS-114 astronauts in July 2005 to determine the
basic behavior of NOAX under actual flight conditions.
"For
STS-114, the conditions that we did the NOAX repair was more of a science
project to see if we could throw it on the piece of RCC and determine what the
results were," Ceccacci said. "For 121, these guys
went through a lot of work to determine exactly how you would repair [damage]."
Instead of
spending one hour testing two heat shield repair methods like their STS-114
predecessors, Sellers and Fossum spent a full three
hours solely on testing the NOAX material, Ceccaci
said. The astronauts perched themselves on the space station's robotic arm and
a shuttle foot restraint in an attempt to recreate the types of positions that
would likely be used in the event of an actual repair, he added.
Of prime
importance were the surrounding temperatures at the time of the repair tests,
NASA officials said.
Ground
tests have shown that the deal temperature to begin RCC repairs with the NOAX
material is around 140 degrees Fahrenheit, with today's target temperatures
targeted between 35 and 100 degrees Fahrenheit (1.6 to 37 Celsius).
"We want to
start at a higher temperature and work our way down," said Tomas
Gonzalez-Torres, lead spacewalk officer for Discovery's STS-121 mission.
The method
allows astronauts to vent - or outgas - the NOAX by spreading it thin like a
pancake to bake out volatiles and reduce the amount of bubbling that occurs
during application, Gonzalez-Torres added.
Too many
bubbles can lead to voids in the NOAX repair that could sabotage the fix by
creating a direct path to a shuttle heat shield's damaged area, NASA officials
said.
Sellers and
Fossum hit a peak temperature of about 93 degrees
Fahrenheit (33 degrees Celsius) and low of 25 degrees Fahrenheit (3.8 degrees
Celsius) during their NOAX test, as they worked to time their activities within
the sunrise-sunset cycle of each orbit,
NASA said.
Astronauts aboard Discovery and the ISS see one sunrise and one sunset every 90
minutes.
Of
spatulas and tethers
With the
end of today's spacewalk, Sellers and Fossum have finished
a marathon session of three extravehicular activities (EVAs)
that began with their first
orbital work session on July 8.
"I'm so
excited to be complete with all three EVAs,"
Gonzalez-Torres said, crediting the deft handling of the ISS robotic arm by
STS-121 mission specialists Stephanie Wilson and Lisa Nowak for giving their
spacewalking counterparts extra time for a get-ahead task. "This one, just like
the other ones, was very successful."
With the
complexity of each spacewalk, Ceccacci said the small
hitches - some tether issues for Fossum, Sellers' jet
backpack glitch on Monday and a lost spatula that drifted away from the
astronauts this morning - aren't too bad.
"If that's
all that happens, we're happy," Ceccacci said.
"You're kidding yourself if you think nothing like that is going to happen."
In their first
spacewalk, they bounced, pushed and pulled from the end of Discovery's
combined robotic arm and inspection boom, a 100-foot (30-meter) mechanical
appendage - to prove its potential as a repair platform for orbiter heat shield
repairs. Initial results from that test
were positive, with Ceccaci and other NASA
officials saying they are now confident the boom could be used to hold
astronauts near a shuttle's fragile but precious heat tiles and RCC panels.
On Monday,
the Sellers and Fossum were
at it again, this time performing vital
repairs to restore the space station's Mobile Transporter, a sort of
railcar platform, to full operations by installing a backup power and data
cable reel. The fix was not
only effective, but immediately put to work as ISS managers moved the
Mobile Transporter to a new worksite just before today's spacewalk.
"We did
that to get configured for the late inspections that we're going to be doing in
a couple of days," Ceccacci said.
As a side
note, NASA's spacewalk has placed Sellers among the Top 10 spacewalkers of all
time, and within the U.S. space agency's Top 5 bracket, with
a grand total of 41 hours and 10 minutes over six EVAs.
The all-time record holder is Russian cosmonaut Anatoly Solovyev
(77 hours and 41 minutes over 16 spacewalks) with NASA astronaut Jerry Ross (58
hours and 18 minutes) a distant second.
Fossum,
who made his spacewalk debut - and his first spaceflight - with Discovery's
current mission, racked up 21 hours and 29 minutes of orbital work.
Discovery's
STS-121 astronauts are set to have most of the day off on Thursday, with their
ISS Expedition 13 counterparts slated for a light day as well, Ceccacci said.
About the
only thing on Thursday's docket is some preparatory work to ready Discovery's
cargo pod - the Italian-built Multi-Purpose Logistics Module Leonardo - for its
Friday unberthing from the ISS. The cargo module will
carry more than 4,000 pounds (1,814 kilograms) of unneeded equipment, tools,
trash and other items when Discovery returns to Earth Monday.
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