Baffling Shuttle Cracks Found on Discovery

NASA Discovery Space Shuttle

A collection of cracks on a region of Discovery’s external fuel tank that separates internal oxygen and hydrogen propellant containers have appeared.

The mystery over the underlying cause of the four cracks in adjacent “intertank” stringers has had Discovery’s final flight, an 11-day assembly mission to the International Space Station, on hold since early November. A launch attempted last Nov. 5 was scrubbed by an unrelated hydrogen leak at the fuel tank’s Ground Umbilical Carrier Plate, a vent line fixture. At the time, the external tank was undergoing cryogenic shock as it was filled with liquid oxygen chilled to -300F and hydrogen propellant at -423F.

The launch has since slipped from Dec. 3 to no earlier than Dec. 17-20 and perhaps January or late February. Meanwhile, NASA is performing an intense fault-tree analysis, with a strong suspicion the cracks were the result of defects or damage to the two 21-ft.-long aluminum-lithium stringers during manufacture or tank assembly. The fissures were found on Nov. 5 below a worrisome 20-in.-long crack that formed in the insulating foam that jackets the tank.

Shuttle Program Manager John Shannon said on the eve of a Thanksgiving break in the analysis that “It’s not a design issue.” Shannon said “These stringers, how they are designed and how they are attached, the loads they would take during this initial cryo loading and also during launch, they have plenty of design margin. A properly assembled stringer in the expected flight environment will not crack.”

A ring connecting the top of the 108 fuel tank stringers is designed to shrink slightly in response to the low temperature of the liquid oxygen.

Nonetheless, there was unexpected damage. And if there were defects in or damage to the stringers that escaped NASA’s attention before Discovery reached the launch pad, shuttle managers cannot be assured there is not more that could crack and unleash a chunk of foam in flight.

Forty-three tanks have been constructed with the lighter alloy, requiring just more than 4,600 stringers. So far, 31 cracks have been found, including those on Discovery.

“All of those have been known assembly issues,” Shannon said of the previous cracks, which were traced to misalignments of the stringers as they were fastened to the tank or to mishandling in which the fragile stringers struck or were struck by other hardware. Discovery’s cracks were the first found and repaired at the launch pad using techniques previously employed only at the production plant.

Bill Gerstenmaier, NASA’s associate administrator for space operations, claims that the ongoing detective work is immune to schedule and budget pressure.

“We would like to have found the most probable cause by now,” Gerstenmaier said. “This is turning out to be a lot more complicated from an analysis standpoint, and it does not lend itself to a very easy answer.”

 

-aviationweek.com

– aerospaceguide.net

Space Shuttle Discovery Launch Postponed

Due to a hydrogen gas leak detected while filling the external tank, NASA postponed space shuttle Discovery’s launch to the International Space Station. The next launch attempt could be no earlier than Monday, Nov. 8 at 12:53 p.m. EST.

The leak, detected early Friday morning while the shuttle’s external fuel tank was being loaded, was at the Ground Umbilical Carrier Plate, or GUCP, an attachment point between the external tank and a 7-inch pipe that carries gaseous hydrogen safely away from Discovery to the flare stack, where it is burned off.

The next window for Discovery’s liftoff is Nov. 30 through Dec. 5 if a Monday launch is not possible. The 11-day STS-133 mission will deliver the Permanent Multipurpose Module (PMM) to the station. The PMM, which was converted from the multi-purpose logistics module Leonardo, will provide additional storage for the station crew and experiments may be conducted inside it, such as fluid physics, materials science, biology and biotechnology.

STS-133 patch

STS-133 is the final shuttle mission planned for 2010, Discovery’s 39th flight and the 35th shuttle mission to the station.

The flight also will transport critical spare parts and the Express Logistics Carrier 4 (ELC4) to the station. ELC4 is an external platform that holds large equipment. Robonaut 2, or R2, will be the first human-like robot in space when it flies on Discovery inside the PMM to become a permanent resident of the station. The mission will feature two spacewalks to do maintenance work and install new components.

Commander Steve Lindsey leads the veteran crew, which includes Pilot Eric Boe and Mission Specialists Alvin Drew, Tim Kopra, Michael Barratt and Nicole Stott.

 

-planenews.com