Leaking vent line attachment disassembled, apparent seal problem found (UPDATED)
Editor's note...
By WILLIAM HARWOOD
CBS News
KENNEDY SPACE CENTER, FL--A leaking hydrogen vent line attachment fitting on the side of the shuttle Discovery's external tank was removed and disassembled overnight, revealing an unevenly compressed internal seal. The quick-disconnect hardware also may have a less concentric fit than pre-fueling measurements indicated. An analysis is underway to determine if the defects are responsible for the gaseous hydrogen leak that grounded Discovery last Friday.
A replacement ground umbilical carrier plate, or GUCP, was previously fit-checked on Discovery's tank during its assembly and measurements indicated a better fit. Engineers plan to carry out a detailed comparison of the two mounting plates before deciding how to proceed.
But barring any other surprises, they likely will be cleared to install the replacement GUCP on Discovery's tank. A meeting is scheduled for Friday to discuss an expected fueling test to make sure the vent line is leak free under cryogenic conditions.
In the meantime, another team of engineers is refining plans for repairing a structural brace on the side of the tank that experienced two cracks during Discovery's fueling Friday. The defects were found after engineers cut away cracked foam insulation covering a vertical stringer, part of the ribbed structure of the compartment that separates the liquid hydrogen and oxygen tanks.
Based on past experience with a similar crack on external tank 138, scheduled for use by the shuttle Atlantis next summer, engineers believe they can repair Discovery's tank, ET-137, by attaching a so-called doubler on a replacement section of bracing. Fresh foam insulation can then be applied and shaped to fit.
In a statement posted on NASA's web site Thursday, the agency said the cracks may be the result of using a lightweight aluminum-lithium alloy in the latest generation of external tanks.
"While the material is stronger and lighter in weight, it is more brittle and has resulted in increased cases of cracked stringers in the areas noted," the statement said. "The cracked stringer section matches well with the area of foam that cracked, which indicates the stringer crack was the root cause of the foam crack.
"Analysis shows the intertank has sufficient structural capability for the launch and ascent environments with the cracks in a single stringer," the statement said. "Although the foam was firmly attached to the damaged structure, it is undesirable to launch in this configuration. This area is clearly visible during the pre-launch inspections and would have been detected by the final inspection team."
To fix the cracks, engineers likely will cut out the damaged section of the stringer and replace it with another piece that exactly matches the removed section. The doubler then will be attached to add additional strength followed by an application of BX foam. The insulation will require four days to cure.
"Exact schedules and repair options still are being evaluated," NASA said. "Engineers will meet Friday to discuss the status of the three repair activities underway, the external tank stringer and foam repair, the ground umbilical carrier plate and the cockpit panel L4 removal and replacement."
The latter is a circuit breaker panel in Discovery's cockpit that may be responsible for a subtle electrical glitch blamed for an earlier launch delay last week. While the system appears sound, NASA managers decided to replace the panel while the other repair work is underway.
The engineering community is hopeful the repair work can be completed before the opening of the year's final shuttle launch window Nov. 30.
- Posted at 09:25 AM, 11/11/10: Leaking vent line attachment disassembled, apparent seal problem found
- Updated at 12:25 PM, 11/11/10: Foam repair plan refined; adding NASA statement; new photos
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| A leaking hydrogen vent line quick-disconnect fitting is shown being removed from the shuttle Discovery's external tank. An initial inspection revealed apparent problems with an internal seal that might explain the leak that led to a launch delay last Friday. (Photo: NASA) |
CBS News
KENNEDY SPACE CENTER, FL--A leaking hydrogen vent line attachment fitting on the side of the shuttle Discovery's external tank was removed and disassembled overnight, revealing an unevenly compressed internal seal. The quick-disconnect hardware also may have a less concentric fit than pre-fueling measurements indicated. An analysis is underway to determine if the defects are responsible for the gaseous hydrogen leak that grounded Discovery last Friday.
 |
| The suspect quick-disconnect fitting after removal from an umbilical plate. (Photo: NASA) |
A replacement ground umbilical carrier plate, or GUCP, was previously fit-checked on Discovery's tank during its assembly and measurements indicated a better fit. Engineers plan to carry out a detailed comparison of the two mounting plates before deciding how to proceed.
But barring any other surprises, they likely will be cleared to install the replacement GUCP on Discovery's tank. A meeting is scheduled for Friday to discuss an expected fueling test to make sure the vent line is leak free under cryogenic conditions.
 |
| A technician works at the site of cracks found in Discovery's external tank. After cutting away foam insulation, engineers found two cracks in an underlying stringer. (Photo: NASA) |
Based on past experience with a similar crack on external tank 138, scheduled for use by the shuttle Atlantis next summer, engineers believe they can repair Discovery's tank, ET-137, by attaching a so-called doubler on a replacement section of bracing. Fresh foam insulation can then be applied and shaped to fit.
In a statement posted on NASA's web site Thursday, the agency said the cracks may be the result of using a lightweight aluminum-lithium alloy in the latest generation of external tanks.
"While the material is stronger and lighter in weight, it is more brittle and has resulted in increased cases of cracked stringers in the areas noted," the statement said. "The cracked stringer section matches well with the area of foam that cracked, which indicates the stringer crack was the root cause of the foam crack.
"Analysis shows the intertank has sufficient structural capability for the launch and ascent environments with the cracks in a single stringer," the statement said. "Although the foam was firmly attached to the damaged structure, it is undesirable to launch in this configuration. This area is clearly visible during the pre-launch inspections and would have been detected by the final inspection team."
To fix the cracks, engineers likely will cut out the damaged section of the stringer and replace it with another piece that exactly matches the removed section. The doubler then will be attached to add additional strength followed by an application of BX foam. The insulation will require four days to cure.
"Exact schedules and repair options still are being evaluated," NASA said. "Engineers will meet Friday to discuss the status of the three repair activities underway, the external tank stringer and foam repair, the ground umbilical carrier plate and the cockpit panel L4 removal and replacement."
The latter is a circuit breaker panel in Discovery's cockpit that may be responsible for a subtle electrical glitch blamed for an earlier launch delay last week. While the system appears sound, NASA managers decided to replace the panel while the other repair work is underway.
The engineering community is hopeful the repair work can be completed before the opening of the year's final shuttle launch window Nov. 30.