The STS-120 crew walks out for launch rehearsal. In the left row are: (from front) pilot George Zamka and mission specialists Stephanie Wilson, Daniel Tani, and Scott Parazynski. In the right row are: (from front) commander Pam Melroy and mission specialists Doug Wheelock and Paolo Nespoli.
Credit: NASA/Kim Shiflett.
CAPE CANAVERAL In an engineering showdown next week, shuttle managers will recommend launching Discovery on Oct. 23 despite concerns raised by an independent safety group about tiny cracks in critical wing panels.
NASA shuttle chief Wayne Hale will tell agency leaders that the defects are too small to allow hot gases to burn through the composite carbon covers and destroy the spaceship during atmospheric re-entry.
And he'll note that new testing techniques developed after the 2003 Columbia accident show the cracks have not grown despite repeated exposure to re-entry temperatures as high as 3,000 degrees Fahrenheit.
"As it stands right now, based on the discussions and the detail that Wayne Hale heard yesterday, he said the risk as laid out is acceptable to proceed with the launch," Kyle Herring, a spokesman for NASA's Johnson Space Center, said Thursday.
However, experts from the NASA Engineering and Safety Center, an independent advisory organization established after the Columbia accident, on Wednesday recommended that NASA replace the panels as a precaution.
"They felt that the prudent thing to do would be to stand down and just swap out the panels to be sure," Herring said.
Doing so would force NASA to roll the shuttle back to its assembly building, disconnect Discovery from an external tank with attached solid rocket boosters, and return the orbiter to its processing hangar.
Launch of the U.S. Harmony module and the time-critical construction of the International Space Station would be delayed at least two months.
NASA would be hard-pressed to launch 13 outpost assembly flights and a Hubble Space Telescope servicing mission before September 2010, a presidential deadline for station completion and shuttle fleet retirement.
NASA Administrator Mike Griffin and top agency executives will make a call during a Flight Readiness Review on Tuesday at Kennedy Space Center.
Shuttle orbiter wings each are outfitted with 22 composite carbon panels that protect them from extreme temperatures during atmospheric re-entry.
The U-shaped panels are coated with silicon carbide to prevent oxidation, which reduces their structural integrity.
The manufacturing process exposes the panels to temperatures up to 3,000 degrees, and as the silicon carbide cools, small defects called "craze cracks" form.
The cracks similar to scratches in the clear coat of an automobile finish are filled with sealant.
Inspections after NASA's first post-Columbia test flight in July 2005 a mission flown on Discovery showed that the sealed cracks in some cases could open and expand during flight.
A new technique that employs a heat source to detect small defects was used to perform the tests.
Underlying composite carbon on one of Discovery's wing panels sustained enough damage to prompt NASA to replace it.
Two other tiny cracks on the orbiter's right wing and one on its left were discovered.
The panels nonetheless were deemed safe to fly and NASA formed an internal team to try to pinpoint the cause of the problem.
In the meantime, engineers continued post-landing inspections to see whether the small cracks expanded during flight.
Discovery flew twice in 2006 and the cracks have not changed.
Still stumped on the root cause, shuttle managers asked the NASA Engineering and Safety Center to study the matter.
The request came prior to a shuttle mission in June.
Discovery rolled out to launch pad 39A on Sept. 30.
The independent experts circulated a draft report Oct. 5.
A formal presentation was made during a shuttle program review Wednesday.
The root cause still is unknown, so the group contends NASA cannot adequately estimate the risk posed by the small cracks.
They said the prudent course is to replace the three panels.
The experts on the internal shuttle team recommended flying "as-is."
NASA leaders are pondering three options.
- Launching Discovery as scheduled in 11 days.
- Rolling the shuttle back to its hangar to replace the suspect panels.
- Ordering more testing and analyses in a bid to prove Discovery is safe to fly "as-is."
More tests and analyses, however, could be inconclusive.
Hale will recommend flying "as-is" and continuing tests.
"That's basically Option One," Herring said. "You fly and continue to try to understand what causes this phenomenon."
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