NASA will strip away cracked layers
of insulation from its next space shuttle fuel tank to fly, a top shuttle
official said Friday.
Wayne Hale, NASA's space shuttle
program manager, said an X-ray inspection of brackets on the shuttle
Discovery's external fuel tank found cracks in an underlying layer of thermal
protection that could lead to similar debris shedding events like that which
dinged the space
shuttle Endeavour's underbelly during its Aug. 8 liftoff, he added.
"Clearly, this could lead to a
shedding of foam debris, along with this heavier weight SLA, which we now know
could have a debris transport path to the underside of the orbiter," Hale
told reporters in a Friday teleconference. "Therefore, we've decided that
this is an unacceptable situation."
The cracks run a few inches long in
a composite material known as super-lightweight ablator (SLA), which sits
between the fuel tank's exterior foam and aluminum bracket base, Hale said.
Engineers at NASA's Kennedy Space
Center in Florida will now pry the foam insulation and underlying
corkboard-like ablator, which is composed of silicone resins and cork, from at
least four of five aluminum brackets securing a liquid oxygen feed line to
Discovery's fuel tank hull.
The brackets will then be coated
with only the lighter, less dense foam insulation when the shuttle launches
toward the International Space Station (ISS) on Oct. 23, Hale said, adding that
the modification is expected to take up to nine days.
Hale said that removing the ablator
will strip away about the equivalent of a ping pong ball's worth of material
from each fuel tank bracket.
"So we're not talking about a
large mass," he added.
NASA believes the cracks in the
ablator may have been caused during the fuel tank's manufacturing process at
the agency's Michoud
Assembly Facility in New Orleans, Louisiana.
From ding to Discovery
NASA began inspecting Discovery's
fuel tank after a baseball-sized piece of debris weighing about 0.021 pounds
(about one-third of an ounce) fell from a bracket on the shuttle Endeavour's
external tank about one minute after it launched towards the ISS on NASA's
STS-118 mission.
The debris, which likely had some
SLA or ice attached, ricocheted off a metal strut to carve a 3 1/2-inch by
2-inch (9-centimeter by 5-centimeter) gash
across two of the heat-resistant tiles along Endeavour's undercarriage. The
debris strike penetrated completely through one tile to expose a slim area of
underlying felt material.
The dinged tiles posed no risk to
the safe return of Endeavour's seven-astronaut crew, though NASA engineers
conducted an in-depth analysis to determine whether to fill in the divot during
a risky spacewalk to avoid additional structural damage that would require
lengthy repairs after landing. After six days of study, mission managers
decided no such repair was necessary.
Endeavour landed
safely on Wednesday, with its dinged tiles experiencing some
slight erosion but no major additional damage, though engineers will
scrutinize the ding to continue heat shield improvements, NASA said..
Hale said that additional X-ray
inspections on the fuel tank for the planned Dec. 6 launch of Atlantis, which
will deliver the European Space Agency's Columbus laboratory to the ISS, also
turned up cracks in the ablator coating on some of its brackets. Those areas
will also require repair, he added.
NASA initially began using the
super-lightweight ablator material on shuttle fuel tanks to guard sensitive
areas against high heating for during launch, when the orbiter and its fuel
tank fly through the Earth's atmosphere at supersonic speeds. But the agency
has since found that the extra measure is not required for the current launch
trajectories now flown by three-orbiter fleet, Hale said.
Final fix ahead
NASA has kept a close watch on the
health of its orbiter heat shields and the amount of debris shed by a shuttle's
external tank during liftoff since 2003, when a chunk of foam insulation breached
the shuttle Columbia's heat shield during launch and led to the loss of the orbiter and its
crew during atmospheric reentry.
"We are making a continuing
effort to eliminate areas or risk as we have the resources available to do
it," Hale said.
NASA engineers were already planning
to replace the foam-covered aluminum brackets shuttle fuel tanks with
titanium ones, which are less susceptible to ice formation, when Endeavour was
dinged during liftoff. The first flight with that new upgrade is slated to
launch in April 2008.
Hale said that current projections
for Discovery's fuel tank fix should allow the orbiter to launch its STS-120
mission to deliver the new Harmony connecting node to
the ISS around Oct. 23. But if more time is required, NASA will not flinch from
delaying the flight a few days and readjusting plans for the December mission
and later ones as needed, he added.
"The point is, we will take the
amount of time that we need to get this repair done properly," Hale said.
"We will not rush."
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