Engineers investigating the debris-shedding problem with NASA’s shuttle fuel tanks found a series of hairline cracks in the same area where foam popped free during the July launch of the shuttle Discovery, agency officials said Nov. 22 .
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 John Chapman, external tank project manager at NASA’s Marshall Space Flight Center in Huntsville, Ala. “I wouldn’t consider that a Eureka [moment] or smoking gun at all.”
The external tank in question, ET-120, originally was supposed to fuel Discovery’s liftoff, but it 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 might be one potential cause of the cracking.
Hale said that if everything proceeds as expected, NASA could be ready to launch Discovery on 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 re-entry Feb. 1, 2003.
A 0.62-kilogram 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 shed during shuttle launches, but was surprised when an onboard camera caught a 0.45-kilogram chunk of foam as it broke off of a protective ramp previously thought to be safe from foam shedding.
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 under way to streamline foam applications on the PAL ramp — which consists of 10 kilograms 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 also have pored over Discovery’s heat-resistant reinforced carbon-carbon panels that line its nose and wing leading edges to determine which need replacement or repair.
Poulos added that new adhesion processes also are 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 re-entry 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.