This story was updated at 5:41 p.m. EST.
Engineers investigating a debris shedding problem with NASA's shuttle fuel tanks have found a series of hairline cracks in the same area where foam popped free during the July launch of the Discovery orbiter, agency officials said Tuesday.
A total of nine cracks - only two of them visible on the surface - were detected along a protective foam ramp on NASA's External Tank 120 (ET-120), one of several under scrutiny at the agency's Michoud Assembly Facility in New Orleans, tank officials said during a briefing at the agency's Johnson Space Center in Houston.
"We're still trying to figure out what this means," said NASA's John Chapman, external tank project manager at Marshall Space Flight Center in Huntsville, Alabama. "I wouldn't consider that a Eureka [moment] or smoking gun at all."
Originally tapped to fuel Discovery's STS-114 liftoff but later replaced, ET-120 was one of three tanks sent back to Michoud from NASA's Kennedy Space Center in Florida and the only one of them to hold the supercooled liquid fuel used during shuttle launches.
Shuttle program manager Wayne Hale added that, while the engineering assessment of the cracks is still pending, it appears that the thermal and pressurization changes involved in fueling the tank during two tests may be one potential cause.
"It does appear that that's a factor," he told reporters.
Hale said that if everything proceeds as expected, NASA could be ready to launch Discovery on STS-121, the agency's second return to flight mission, in May 2006, but stressed that the external tank work - not a schedule - comes first.
Preventing the loss of potentially harmful chunks of external tank foam during a shuttle launch has been a major focus of NASA since the loss of the Columbia orbiter and its seven-astronaut crew during reentry Feb. 1, 2003.
A 1.37-pound chunk of foam critically damaged Columbia's heat shield at launch leaving it vulnerable to the searing hot atmospheric gases during its descent, investigators later found.
NASA spent two and a half years and some $200 million to reduce the amount of large foam debris during shuttle launches, but was surprised when an onboard camera caught a one-pound chunk fall from a protective ramp thought to be safe.
The cracks on ET-120 were found on the same ramp - known as a Protuberance Air Load (PAL) ramp - designed to protect fuel length cabling from the aerodynamic pressures of launch.
Hale said that while initial shuttle fuel tanks designed in the late 1970s required PAL ramps, newer containers have been improved and strengthened over time. Plans are underway to streamline foam applications on the PAL ramp - which consists of 22 pounds of insulation foam - as well as strip it from future tanks altogether, he added.
"It is possible that we may get there for the first flight," Hale said, referring to STS-121. "It is more likely that it may take us until the fall to complete that work."
While engineers continue their work on shuttle fuel tanks, other workers are preparing the orbiters themselves for flight.
Steve Poulos, head of NASA's orbiter project office, said that engineers have determined that faulty stitching caused a nose-mounted thermal blanket to balloon outward during Discovery's STS-114 flight.
"We've inspected 486 blankets on both Discovery and Atlantis," Poulos said, adding that 40 blankets will be replaced aboard Discovery and 60 on Atlantis."
Shuttle workers have also pored over Discovery's heat-resistant reinforced carbon carbon (RCC) panels that its nose and wing leading edges to determine which need replacement or repair.
Poulos added that new adhesion processes are also in hand to glue ceramic cloth gap fillers between the heat tiles that line a shuttle's underside.
During Discovery's STS-114 flight, orbital photographs showed two gap fillers jutting out of the orbiter's tile-lined belly, raising concern that they may lead to hotter reentry temperatures along the shuttle's aft. STS-114 astronaut Stephen Robinson plucked the offending gap fillers from Discovery's hull during the flight's third spacewalk.
"At the end of the day, we will have checked every gap filler on the vehicle," Poulos said.
- NASA Tackles External Tank Tests, Aims for May '06 Shuttle Launch
- Special Report: NASA's STS-114 Return to Flight Shuttle Mission