BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: December 9, 2007
Dashing hopes of finally launching shuttle Atlantis on a critical space station assembly mission today, one of the four low-level engine cutoff - ECO - sensors in the hydrogen section of the shuttle Atlantis' external tank failed to perform properly during initial tests after being submerged in supercold propellant. NASA managers said Saturday they would proceed with launch today if and only if all four sensors and associated instrumentation worked flawlessly and at 7:25 a.m., today's launch attempt was officially called off.
Fueling began at 5:56 a.m. and by 6:48 a.m., the loading process transitioned to "fast fill." A few minutes later, launch control commentator George Diller reported what engineers initially believed was good news.
"The liquid hydrogen sensors have been cycled and they are all confirmed to be working properly," he said. "Once again, at this time, all four liquid hydrogen sensors are working. This was somewhat predicted based on past tanking attempts when we had a problem with the sensors and then tanked a second time and the sensors have worked properly. So right now, this is consistent with our experience base. And all four hydrogen sensors are currently operating."
But moments later, sensor No. 3 apparently failed.
"One of the liquid hydrogen sensors, No. 3, has failed," Diller said. "Standing by for further action. ... This is not good news."
Engineers continued filling Atlantis' external tank to collect additional data. NASA has a flight rule that permits a launch if three of four sensors are working properly. But during fueling for an initial launch attempt Thursday, three of the four sensors, including No. 3, did not respond properly. Because the system is suspect - and because repairs are not possible before the end of the current launch window Dec. 13 - NASA's Mission Management Team required all four sensors and associated instrumentation to be working properly today for the countdown to proceed.
"As planned, we came in and picked up with our tanking operations," Launch Director Doug Lyons explained later. "There's a sequence of events we go through, we chill the lines down and we get into a slow fill to condition the facility lines as well as the orbiter. And ultimately, we get into what we call fast fill. And that's where we've been experiencing these failures.
"So we were going through that process, we had gotten to fast fill. There are two things we look at: the voltages on the circuit, and those were all good on all four circuits, and we also look at the discretes, whether they're indicating wet or if they're indicating dry. And all of them went wet as expected. Once we were in fast fill for roughly 10 minutes or so, we went and did system checkout and that's when we send simulated dry commands to the sensors sequentially and see if they go from wet to dry. And if they do operate in that manner, that's what we're expecting and that's a good checkout.
"So we were watching, of course, with great interest in the firing room," Lyons continued. "We started the test and sensor number 1 went dry. Sensor number 2 went dry. Sensor number 3 - and if you recall, 3 and 4 were the ones that failed last tanking - 3 went dry and 4 went dry. All the voltages were reading good values as well. So it looked like we had a good system and of course, the firing room, we were very excited and felt like we had a good system and we'd be ready to go fly today.
"We continued to monitor the system and two or three minutes after this test went on, they were all dry and we were keeping them in that condition. And then we saw sensor number 3 go from dry to wet, which was a failure. At that point, based on our revised LCC (launch commit criteria), which calls for four-of-four sensors, we were scrubbed for the day."
Additional checks were made and in each case, he said, sensor No. 3 remained in the wet state, indicating an open circuit.
"On Thursday, sensors 3 and sensor 4 failed," Lyons said. "In this case, only sensor 3 failed. But they failed in generally the same time frame and in the same manner. We have seen in the past when we've had an open circuit and we've come back and tanked, the sensor corrected itself and operated properly. And that's what we did see with sensor number 4, but we just didn't have any joy with sensor number 3."
Lyons would not speculate on what mission managers might decide about where to go from here. A Mission Management Team meeting was scheduled to begin at 9 a.m.
"We do have multiple options and folks have been thinking about what ifs," he said. "We'll put something together and be ready to implement it coming out of that meeting."
The goal of Atlantis' mission is to deliver the European Space Agency's Columbus research lab to the international space station, along with French astronaut Leopold Eyharts. Eyharts will replace Expedition 16 flight engineer Dan Tani and oversee the activation of the Columbus module.
Three spacewalks are planned to prepare Columbus for installation; to install two experiments on its hull; and to replace a spent nitrogen tank used to pressurize the station's ammonia cooling system. The nitrogen tank and a failed gyroscope will be moved back to the shuttle for return to Earth.
But NASA only has until Dec. 13 to get Atlantis off the ground because of temperature and power issues related to the angle between the sun and the plane of the space station's orbit. The window reopens Dec. 30, but any delay past Dec. 13 would push the flight into January to avoid countdown software issues due to the end-of-year rollover.
Engineers cannot access all of the ECO sensor circuitry at the launch pad, raising the possibility of a rollback to the Vehicle Assembly Building to remove the orbiter from the external tank for more extensive troubleshooting. But no such decisions have been made as of this writing.
The ECO sensors are part of a backup system intended to detect low fuel levels in time to prevent the shuttle's main engines from draining the tank after some other problem that might cause the powerplants to use up hydrogen faster than expected. Running out of fuel during engine operation likely would trigger a catastrophic failure.
In the wake of earlier problems with the cutoff sensors, NASA implemented a flight rule change that would permit a countdown to proceed if three of four sensors were operating normally and certain other conditions were met.
But engineers do not yet know what caused two sensors to "fail wet" during fueling Thursday or why a third sensor did the same during de-tanking operations. Because the system is suspect, the flight crew office proposed proceeding with launch Sunday if, and only if, all four sensors are operating normally.