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Debate about what to do with the ECO system caused managers to totally reverse positions within 24 hr.—swinging first toward flying without relying on half of the sensors, to the current assessment that the whole ECO system must be functioning and fully redundant. Some top managers initially eyed flying with no ECO sensors operational, given that they believed new data indicate the system has always been unreliable...
“It seems to me likely that we have been flying the entire history of the shuttle program with a false sense of security and that we never had reliable protection from a [catastrophic] liquid hydrogen low-level engine cutoff,” says Wayne Hale, shuttle program manager...
The problems with Atlantis and/or its Lockheed Martin external tank (ET) are now considered so fundamental to program safety, that the difficulties have also halted any option of alternate flight operations with Endeavour or Discovery, says Hale.
The STS-122 ECO sensor problems with Atlantis will delay launch of the European Space Agency Columbus module until at least sometime in January.
“And until we come to the bottom of this mystery, we are in no better shape to launch any other orbiter,” says Hale.
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 in each tank, known as engine cutoff - ECO - sensors, are part of a backup system intended to make sure the ship's engines don't run too long, draining the tank dry with potentially catastrophic results, after other problems that might prevent an on-time shutdown.
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 was corrected during Discovery's last overhaul, but the four-of-four launch rule remains on the books.