Ultimate test for shuttle sensor fix comes Thursday
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: February 5, 2008
LeRoy Cain, manager of shuttle integration at the Kennedy Space Center, said today he is confident the redesigned connectors intended to fix on-going problems with low-level fuel sensors in the shuttle Atlantis' external tank will work properly Thursday when the ship is fueled for takeoff. But engineers will be paying close attention to the sensors and if any problems develop that might cast doubt on the fix, the shuttle will remain on the ground.
"We expect to see the system work perfectly normally and we expect to see completely nominal results when we ... cover up the sensors and they're initially wetted (with super cold liquid hydrogen rocket fuel)," Cain said. "If they see any failures, we'll respond to those per the pre-planned contingency procedures. But I expect to see a completely nominal system, I have very high confidence that the resolution we came to here on this system is very solid."
The space shuttle's external tank is equipped with a variety of propellant level sensors, including four at the bottom of the hydrogen section that are known as engine cutoff - ECO - sensors. The ECO sensors serve as a backup system to make sure a shuttle doesn't run out of fuel while the engines are still running because of some other problem.
During attempts to launch Atlantis on Dec. 6 and 9, multiple ECO sensor circuits failed to work properly. Subsequent analysis, including results of a fueling test Dec. 18, indicated the problems were the result of temperature-induced gaps in the pins and sockets of a connector that routes sensor data out of the tank to the shuttle.
The solution was to solder the connector pins and sockets together, eliminating any possibility of more open circuits when the hardware is chilled to liquid hydrogen temperatures.
NASA managers are so confident the fix will work, they agreed to use the normal launch commit criteria, which calls for three of the four ECO sensors to be operational for a launch to proceed. But Cain said any failure Thursday will be closely scrutinized to make sure it's not something that could affect the other sensors or is the result of a problem with the redesigned connector.
"What we have put in place from a launch commit criteria standpoint is we can go fly safely with three of four functioning LH2 engine cutoff sensor circuits," Cain said. "In that case, however, the one failure that we've suffered, we need to understand it well enough to be able to say it's not potentially a generic problem or it's not some problem that could somehow be associated with this failure mode that we think we just fixed.
"So, if the one failure that we see somehow potentially indicts the failure mode that we believe we just mitigated with this new connector hardware, then we're not going to be 'go' to launch in that case. But if it's a problem that associated with an MDM (multiplexer-demultiplexer), a multiplexer card or some other part of the system that we can positively identify that ... would not indict this fix we just put in place, then from an overall safety of the system ... standpoint, then we've said we're good to go with three of four. But it has to be in those circumstances."
And only one sensor can fail. If two circuits malfunction for any reason, launch will be scrubbed.
"We'll be keeping a close eye on our ECO sensors," said Launch Director Doug Lyons. "The program's given us clear direction on our LCCs and so we would be no go at that point. ... We're done with two (failures)."
Atlantis' countdown began Monday afternoon, setting up a launch attempt at 2:45:25 p.m., roughly the moment Earth's rotation carries pad 39A into the plane of the international space station's orbit. Forecasters are predicting a 60 percent chance of stormy weather from a passing cold front, but conditions are expected to improve to 80 percent "go" Friday and Saturday.
Engineers plan to begin pumping liquid hydrogen into the external tank around 5:20 a.m. Thursday. The four ECO sensors will "go wet," or be submerged in hydrogen, around 6:05 a.m. Shortly thereafter, engineers will send commands to simulate "dry" sensors and then monitor the circuits to make sure they respond properly. Additional tests are planned throughout the countdown with a final check during a hold at the T-minus nine-minute mark.
The goal of the delayed mission is to deliver the European Space Agency's Columbus research module to the international space station. Alan Thirkettle, space station program manager for the European agency, said it is "terribly important" to get Columbus into orbit, but he said he agreed with the decision to delay the flight when the ECO sensors acted up in December.
"If we'd launched in December, we wouldn't have anything to look forward to on Thursday," he joked. "So, you have to take the positives on these things! We've indeed waited a long time for this launch and it's just going to make it all the better when it gets up there and it works.
"We know the business that we're in. We know the people that we're dealing with, we know the systems that we're dealing with. It's terribly important for us to fly, but it's even more important for us to fly safely. And that's always been the line that we've taken. It would not have been prudent to fly in December, the right decisions were taken at the right time. Hopefully on Thursday we'll get through and we'll get the mission we've all been looking for."