NASA troubleshooting possible engine cutoff sensor problem
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: September 8, 2006
Engineers fueling the shuttle Atlantis for launch today are troubleshooting a possible problem with a hydrogen engine cutoff - ECO - sensor in the ship's external tank. During a test, hydrogen ECO sensor No. 3 indicated it was wet when it should have indicated dry. The three-hour fueling procedure is continuing while engineers look into the issue and launch remains on track for 11:40:37 a.m.
NASA's Mission Management Team is assembling early to discuss the issue, according to NASA spokesman Bruce Buckingham. Because of problems during the first post-Columbia launch campaign, a launch rule modification is on the books that permits a flight to continue with three of four operational ECO sensors. But that rule, in its original form, also called for a 24-hour delay to fully assess the matter and as of this writing, it's not clear how today's countdown might proceed if the problem cannot be resolved.
Problems with hydrogen ECO sensors delayed the first post-Columbia mission last year. After the flight, engineers traced the problem to a suspect connection betweem sesors 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 hydrogen sensors in Atlantis' tank also were replaced.
The ECO sensors are part of a backup system that ensures the shuttle's main engines shut down normally before the tank runs out of fuel. NASA launch rules require all four ECO sensors to be operating before a launch can proceed.
But going into last month's flight, NASA managers developed rationale for an "exception" to the four-of-four flight rule that would permit a second launch attempt the next day if engineers could show the fault was in the shuttle's electronics system and not the sensors themselves.
Two of the four hydrogen ECO sensors would have to "fail dry" to trigger a premature engine shutdown. Three sensors would have to fail wet for the engines to run the tank dry. The three-of-four rationale was developed to cover a failed wet case. No such rationale was developed for the failed dry case.
Updates will be posted here as information becomes available. In the meantime, detailed background on ECO sensor operation is available here.
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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