Atlantis fueling test points to source of sensor trouble
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: December 18, 2007
Precisely timing how electrical pulses moved back and forth through suspect engine cutoff sensor wiring during a fueling test today indicates intermittent open circuits that grounded the shuttle Atlantis on Dec. 6 and 9 were caused by problems in a critical-three part "feed-through" connector. The connector carries sensor data through the wall of the ship's external fuel tank.
While engineers do not yet know what will be required to fix the problem - or whether Atlantis can meet a no-earlier-than Jan. 10 launch date - shuttle Program Manager Wayne Hale said he was relieved today's tanking test exonerated the sensors themselves and isolated the problem to an area that is accessible at the launch pad.
"Exactly what we've got to do and where in this three-part connector we have to do it is a little bit of work ahead of us," he said. "I'm just pleased as punch we know it's in the connector and not some other place in this 100 feet or so of wiring and sensors and electronic boxes (that were in question) so we know what area to concentrate our efforts."
During fueling for a Dec. 6 launch try, engine cutoff - ECO - sensors Nos. 3 and 4 "failed wet" about 35 minutes after they were submerged in liquid hydrogen. After the launch was called off, another sensor that shows when the tank is 5 percent full failed wet. After the tank was drained, ECO sensor No. 1 failed wet as well.
During fueling for a second launch try Dec. 9, ECO sensor No. 3 failed wet 24 minutes after the others were submerged. In both cases, the sensors returned to normal operation after the tank was drained and temperatures rose.
During today's tanking test, sensors 2 and 3 showed open circuit failure indications for a few seconds before returning to normal operation. Sensor 1 failed shortly thereafter and stayed that way for the duration of the test. Sensor 2 worked properly throughout the tanking test but sensor 3 failed several hours after recovering from its initial open circuit indication.
The wiring to all four ECO sensors, as well as the 5 percent level sensor, pass through the same feed-through connector.
"Today we had problems on three of those sensors and we captured the data," Hale said. "And the data is indicating we have a problem at what we call the feed-through connector that leads the wires from the inside of the liquid hydrogen tank in the cryogenic fluid to the exterior of the tank. That's a multi-part connector and our time domain reflectometry instrumentation has indicated that is where our open circuits are occurring at these very cold temperatures.
"The team is looking at all of this data, all the recorded voltages, this TDR instrumentation, there is literally gigabytes worth of data that was collected today. They'll be going over this data very carefully for the next few days, trying to make sure they properly interpret the results. The preliminary indication is we have a problem at this connector."
Engineers plan to brief shuttle managers Wednesday on details of the tanking test, what the TDR data indicate and what repair options might be feasible.
"I do not have any information about a launch date today," Hale said. "Frankly, we are still in the midst of troubleshooting. ... Where the troubleshooting and replacement and repair work leads us will determine what the launch date's going to be. We are not going to be driven by schedule on this one. We need to get to the bottom of this, fix it and make sure it's fixed once and for all and then we can fly safely through the rest of the program, at least in this area."
The connector in question can be accessed at the launch pad but how much time it might take to replace suspect components will depend on whether the fault is found to be inside or outside the tank.
"Some timelines have been developed to change out those parts that can be reached from the outside, and they are on the order of a week to 10 days kind of work," Hale said. "However, the part that's difficult to get to is the socket connector on the inside of the tank and that would be more invasive. You would have to go inside the tank through the manhole cover we've got at the bottom or some other access point and that obviously would be a longer-term operation."
No additional tanking tests are planned in the near term, although engineers are subjecting test equipment to cryogenic conditions in the laboratory to learn more about how the connector functions when it is chilled to ultra-low temperatures.