www.cbsnews.com/ breaking news update excerpt
. . .Barring a delay for the Air Force Atlas 5 Rocket carrying a missile (Friday, May 6), Endeavour's launch could move to May 8 or, if the Air Force launch slips a day, to May 10. A May 9 launch date is not available for the shuttle because undocking from the International Space Station would come on the same day a Russian Soyuz is scheduled to depart.
Endeavour was grounded Friday during the final hours of the countdown because of telemetry indicating multiple fuel line heaters used by one of the shuttle's three hydraulic power units were not activating normally. The heaters are needed to keep the lines from freezing and possibly rupturing in flight.
The shuttle is equipped with three auxiliary power units, providing the hydraulic muscle to move the ship's engine nozzles, wing elevons, rudder, tail fin speed brake, body flap, landing gear brakes and nose wheel steering system. The shuttle can safely fly with a single APU, but flight rules require full redundancy for a countdown to proceed.
Likewise, each of the shuttle's three APUs is equipped with redundant heater "strings" and only one channel is required for normal operation. But again, the flight rules require redundancy to protect against a subsequent failure that could knock the system out of action.
Early Saturday, engineers ruled out a problem with the fuse panel in the shuttle's cockpit that routes power to the APU circuitry. That left two possible culprits: one or more faulty heater control thermostats or the aft load controller assembly, or ALCA-2, avionics box the heater circuitry runs through.
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