A fuel leak and crack on the space shuttle Discovery's huge external tank has forced NASA to call off any attempts to launch before Nov. 30 – the latest in a series of delays for the spacecraft's final voyage.
Credit: NASA/Troy Cryder
A misaligned seal on the space shuttle Discovery's external fuel tank may be to blame for the potentially dangerous leak of hydrogen gas that thwarted NASA's attempt to launch the spacecraft last week.
NASA engineers discovered the misaligned seal after inspecting the site of Discovery's fuel tank gas leak while the spacecraft sits atop its seaside launch pad at the Kennedy Space Center in Florida. The leak forced NASA to cancel Discovery's Nov. 5 launch attempt.
The hydrogen gas leak was located at the ground umbilical carrier plate (GUCP), which is an attachment point between the external tank and a 17-inch (33-centimeter) pipe that carries gaseous hydrogen safely away from the shuttle to a flare stack, where it can be burned off.
NASA is hoping to complete the work and try once again to launch what will be space shuttle Discovery's final flight on Nov. 30. NASA is retiring its three-shuttle fleet next year and Discovery's upcoming delivery trip to the International Space Station will be its last.
Inspecting shuttle Discovery
NASA technicians worked overnight to remove the component, and initial inspections were performed on the faulty seal and its corresponding connector before they were sent to labs for more thorough examinations. Data collected from the initial inspections will be evaluated today. [GRAPHIC: NASA's Space Shuttle – From Top to Bottom]
Similar leaks have occurred during launch preparations for two previous shuttle missions, both in 2009. Both of these were caused by slight misalignments of the carrier plate itself, but this new leak was caused by a different issue — a misalignment of the flight seal inside the ground umbilical carrier plate.
This could account for why the leak cropped up earlier in the tanking process, and leaked significantly greater amounts of hydrogen, than the two previous events, NASA spokesman Allard Beutel said.
"They found that the flight seal was not properly aligned when they opened up the GUCP," Beutel told SPACE.com. "Last year the issue was a slight misalignment of the whole carrier plate. This is very different."
Based on early evaluations, the team should be ready to install a new flight seal and umbilical carrier plate tomorrow (Nov. 12), Beutel said.
Shuttle fuel tank cracks eyed
Engineers also removed damaged foam insulation on Discovery's external tank that cracked during initial loading operations for the shuttle's most recent launch attempt on Nov. 5.
While removing the cracked foam layer that covers the tank, two new fractures were found yesterday (Nov. 10) on a section of the tank's aluminum skin.
The two 9-inch (23-cm) cracks were located at a site called the "stringer," which is an aluminum strip that provides structural integrity to a part of the fuel tank known as the intertank — the partition between the liquid oxygen and liquid hydrogen tanks.
The fractures were found underneath the area of cracked foam, and early analysis suggests they were the root cause of the foam damage. The cracks in the stringer likely pushed the metal outward to sufficiently dislocate the foam, NASA officials said.
"Even though the foam didn't come off, the crack underneath propagated up, so the cracks in stringer led to the crack in the foam above it," Beutel explained. "They understand the stresses that go on at cryogenic temperature levels while loading the tank. It's a known failure method."
Shuttle repair options weighed
Engineers are now evaluating images of the cracks to determine the best repair options that can be performed with Discovery remaining at her seaside launch pad at Kennedy Space Center.
Shuttle technicians have repaired similar cracks on external tanks by removing the fractured aluminum and replacing it with a twice-as-thick stringer section, before replacing the foam. But these repairs have never before been executed at the launch pad.
Previously, these types of cracks were addressed during the external tank production phase at NASA's Michoud Assembly Facility in New Orleans, La., where space shuttle fuel tanks are built.
Engineers determined that cracks in a single stringer would still leave the intertank with enough structural capability for launch and ascent. But while the foam was firmly attached to the damaged structure, launching the vehicle in that configuration could present potential hazards, NASA officials said..
NASA officials stressed, however, that the cracked area is clearly visible during pre-launch inspections, and would have been detected by the final inspection team before liftoff.
Other fixes for Discovery
With the Discovery's launch delayed, NASA teams now have the ability to perform more thorough examinations of the shuttle and its hardware components. As a result, engineering teams have opted to remove and replace a cockpit panel circuit breaker that caused a separate launch delay early last week.
Shuttle technicians encountered an electrical glitch with one of the orbiter's backup engine controllers on Nov. 2. The hiccup was thought to have been caused by transient contamination, and after 24 hours of tests and evaluations, the problem did not return.
"It cleaned itself out, basically – it never showed up again. But, now that they have some extra time, the engineers can go through a full series of tests," Beutel said. "At the time, we wouldn't have gained confidence in the system by switching them out. In fact, you ran the risk of introducing errors because you couldn't thoroughly test them."
Discovery's STS-133 mission managers are evaluating the data to determine the schedule of repairs and the next possible launch opportunity for the orbiter. Engineering teams will meet Friday to discuss the status of the work currently under way.
After Discovery's final flight, NASA plans to retire the shuttle — along with the rest of the agency's shuttle fleet — in 2011.
The next possible time that Discovery can attempt a liftoff to the International Space Station is Nov. 30 at 4:02 a.m. EST (0902 GMT). Discovery is scheduled for an 11-day supply mission to the space station to deliver a humanoid robot helper for the station crew and a new storage room for the orbiting lab.