Fuel sensor scrubs Atlantis launch
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: September 8, 2006; Updated with details from news briefing
After a dramatic, down-to-the-wire debate, NASA's Mission Management Team called off the shuttle Atlantis' countdown today and delayed launch at least 24 hours because of concern about an apparently faulty low-level hydrogen fuel sensor in the ship's huge external tank.
"OK, Brent, we gave a lot of thought to the ECO sensor issue today and what we decided to do as a team is follow the launch commit criteria as written. And so we're going to detank this vehicle, come back in tomorrow, fill it back up and see how they behave and see if we can get more confidence in that system for tomorrow's attempt," Launch Director Mike Leinbach told commander Brent Jett and his five crewmates during a final "hold" at the T-minus nine-minute mark.
"So we appreciate you guys getting on board today," Leinbach said. "Tomorrow's weather should be just as good as it was today if not better. We all feel good about the attempt today, we didn't get there because of the ECO sensors."
"OK, Mike, we understand and we concur 100 percent," Jett replied from Atlantis' flight deck. "That whole plan was given a lot of thought by a lot of smart people, under not having the pressure of a vehicle on the pad. So it's the right thing to do."
Problems with ECO sensors bedeviled NASA last year during the ramp up to the first post-Columbia mission, prompting one scrub and intense analysis. After the flight, engineers traced the problem to a suspect connection between sensors and electrical cables in a specific batch of ECO sensors manufactured in the late 1990s.
The sensors in a tank used by the shuttle Discovery last month were replaced with a set thought to be fault free and those sensors worked as expected. The sensors in Atlantis' tank also were replaced after detailed inspections identified the best available sensors.
It is not yet clear whether the problem seen today, when hydrogen ECO sensor No. 3 "failed wet," involved the sensor, its wiring or an avionics box aboard the shuttle that reformats and routes the data to flight computers. But the timing of the failure indicates it more likely involves the sensor itself and not electronics aboard the shuttle.
Twenty four propellant sensors are used in the shuttle's external tank, 12 each in the oxygen and hydrogen sections. Eight are used in each tank to measure the amount of propellant present before launch. Four ECO sensors in each tank are used in flight as part of a backup system intended to make sure the ship's engines don't shut down too early, resulting in an abort, or run too long, draining the tank dry with potentially catastrophic results.
NASA's original launch commit criteria required three operational ECO sensors for a countdown to proceed. But in the wake of the 1986 Challenger disaster, the LCC was amended to four-of-four because of concerns two sensors could be knocked out by a single failure in an upstream electronic black box known as a multiplexer-demultiplexer. The single-point failure later was corrected, but the four-of-four launch rule remained on the books.
Because of ECO sensor problems going into me first post-Columbia mission, NASA managers ultimately developed an "exception" to the four-of-four rule that would permit a launch if A) a hydrogen sensor failed wet; and B) engineers could show the problem didn't originate in the multiplexer-demultiplexer avionics system.
As originally written, the flight rule exception called for standing down a day. A second launch try could then be made depending on an analysis of the way the sensor failed and how it behaved during a second fueling. With one sensor failed wet, two more ECO sensors would have to fail wet to pose the threat of running the tank dry.
Many observers were surprised NASA's Mission Management Team held open the option of launching today for so long. The flight rule was intended to give engineers a way to press ahead with a launch in the case of a single sensor failure, but only after taking time to make a thorough analysis, away from the pressure of a launch countdown.
Asked why NASA pressed so hard today, shuttle Program Manager Wayne Hale seemed surprised the question was asked.
"What are we going to do, go look for a tee time? I mean we are here to launch the vehicle, so it always, always makes sense to look at the situation you've got. Now, you do that with a great deal of trepidation ... because you're in the heat of the launch and you can get carried away by emotion.
"There's no point in knocking it off early, even though coming in the door this morning, most of us thought we were not going to launch today," Hale said. "So I'm not a bit surprised that's the way it turned out."
It seemed to be a close thing, however. Sources said the majority of the engineering community favored pressing ahead with launch. But a representative of the flight crew operations directorate at the Johnson Space Center voted against proceeding. Other non-voting engineers and mangers also voiced reluctance to proceed.
In the end, Mission Management Team Chairman LeRoy Cain called off the countdown.
"In this case, this engine cutoff sensor No. 3 failed to what we call a wet condition," he said. "And for that condition, we had previously established launch commit criteria that said if we had one of the four sensors fail that way , we would detank, come back the next day and retank, and evaluate the situation as to whether or not there were any changes and whether we'd be safe to fly. That is exactly the case we had today.
"At the end of the day, we had some folks who did not feel like we should press on and launch today and that we should follow the procedure we had previously established, for good reason. So we decided that was the approach we'd take."
The behavior of the three presumably good ECO sensors will be critical to any decision to launch Saturday.
"Overnight, the engineers are going to look very carefully at how all the other sensors look during the detanking when they go dry," Hale said. "And if everything is performing as we expect, and if we just have one sensor continue to be a bad actor, we'll launch tomorrow."
While engineers were close to developing a technical rationale for flying today, "we felt it was more prudent to do what we had planned to do in our calmer moments and review the data," Hale said. "We'll be back tomorrow. The weather's probably going to be good for us tomorrow and we'll give it another shot and hopefully, be successful."
If the shuttle doesn't get off the ground Saturday, the long-awaited and oft-delayed mission almost certainly will slip to late this month - or next - because of a daylight launch constraint and conflict with a Russian mission to rotate crews on the international space station.
A launch past Saturday is not believed to be an option. NASA presumably could launch after a Soyuz carrying the returning station crew lands Sept. 28. But that would require managers to relax a current requirement to launch in daylight for photo documentation of the shuttle's external tank and heat shield.
The next lighted launch opportunity after Saturday is Oct. 26. But Hale would not speculate on how all of that might play out, saying only that he expected to launch Saturday.
The official crew patch for the STS-115 mission of space shuttle Atlantis to resume orbital construction of the International Space Station.
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