March 20, 2008

Astronauts to Test Shuttle Heat Shield Fix in Spacewalk

HOUSTON - Two astronauts will float
outside the International Space Station (ISS) Thursday armed with a space age
caulk gun and its goo-like ammo to test their effectiveness in shuttle
heat shield repairs.

Spacewalkers Robert Behnken and Mike
Foreman will begin their orbital work at 6:28 p.m. EDT (2228 GMT) to
demonstrate a technique for fixing dinged shuttle heat shield tiles.

"While we've been able to test this
material on the ground, we have not been able to test it both in weightless and
the vacuum of space at the same time," Foreman said from orbit late Wednesday
during a series of televised interviews. "We'd like to know how well we'd be
able to use it in case we ever need it for tile repair."

Foreman and Behnken will devote the
bulk of their planned 6 1/2-hour spacewalk to testing the shuttle repair tool,
known to NASA as the Tile Repair Ablator Dispenser (TRAD), and its gooey filler

NASA developed the device and tile
filler after the tragic 2003 loss
of the shuttle Columbia
and its seven-astronaut crew. That shuttle
was destroyed during atmospheric reentry due to a hole in its left wing-mounted
heat shield.

"It's kind of like a big, fancy
caulk gun," said Mike Moses, NASA's lead shuttle flight director for
Endeavour's flight, adding that the device mixes two different materials into a
thick, sticky - but heat-resistant - paste that hardens as it cures.
"Toothpaste is really the best way I'd describe it."

About the size of a small,
round vacuum cleaner
, the TRAD device contains two materials that combine
into an ablative filler inside the gun-like applicator as an astronaut squeezes
it into a dinged, gouged or otherwise damaged shuttle

The cylindrical container fits under
a spacewalking astronaut's life support and emergency rocket pack. It is much
slimmer than an earlier incarnation, which was mounted onto an extra backpack
and tended to induce bubbles into the heat shield repair goo, Moses said.

During today's test, Foreman will
squeeze the material intentionally damaged sample tiles, some of which mimic
actual damage sustained
by Endeavour
on its last flight in August 2007. The samples will be
returned to Earth in the shuttle's payload bay for analysis.

NASA engineers hope the test will
help them better understand how the material cures under real space conditions,
and whether bubbles could cause an effect like that of rising bread, where the
ablative filler swells above the rim of a damage site and generates hotter
temperatures downstream.

"That's probably worse than the hole itself," Moses said in a briefing here at the Johnson
Space Center, adding that engineers are confident the goo, itself, will perform
as designed. "This test is more to refine our technique of applying it."

NASA initially hoped to test the
TRAD gun and goo during an October 2007 shuttle flight, but deferred the demonstration
to repair a torn solar wing outside the space station.

The space agency wants to gain
experience with the repair method before the planned Aug. 28 launch of the
shuttle Atlantis on the last flight to overhaul
the Hubble Space Telescope
. Unlike ISS construction flights, Hubble-bound
astronauts cannot seek safe haven aboard the space station because the Hubble
observatory is in a different orbit.

NASA will prepare a second shuttle
to serve
as a rescue ship
should an emergency occur. But the space agency also wants
to give the Hubble servicing crew as many in-flight repair tools as possible.

Since NASA returned its shuttle
fleet to flight in 2005, the agency has tested carbon composite patches and a
gray, heat-resistant primer to repair damaged shuttle tiles, as well as a
putty-like black paste to mend dings in an orbiter's vital wing and
nose-mounted heat shield panels.

"So it's going to be really valuable
to actually have a test of this in case we ever need it in our bag of tricks
for a shuttle repair," Behnken said late Wednesday of the TRAD goo gun.

Behnken and Foreman are also expected
to replace a broken station circuit breaker during today's spacewalk, as well
as give a running description of what they see as they perform the tile repair
test. The excursion is the fourth of five spacewalks for Endeavour's STS-123 astronaut
crew to deliver a new crewmember, Japanese module and massive Canadian robot to
the space station.

"Mike's going to be our play-by-play
guy and Bob will be the color commentator," Moses said.

NASA is broadcasting Endeavour's
STS-123 mission live on NASA TV. Click
for's shuttle mission coverage and NASA TV feed.