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Space shuttle Endeavour launch delayed further BY WILLIAM HARWOOD STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION Posted: May 1, 2011 
Engineers have traced an electrical problem blamed for grounding the shuttle Endeavour Friday to an avionics box in the ship's engine compartment, officials said Sunday. Replacing the box will delay launch past Monday, but NASA managers have not yet determined when another attempt might be possible.
Whether the Air Force might agree to delay the Atlas launch to make way for Endeavour is not yet known, but work to replace and retest the avionics box is expected to take several days. It appears unlikely NASA could complete that work in time for a launch by May 4, the last available day before the Atlas cutout. Barring a delay for the Atlas, 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 commander Mark Kelly and his crewmates -- pilot Gregory H. Johnson, Michael Fincke, Gregory Chamitoff, Andrew Feustel and European Space Agency astronaut Roberto Vittori -- flew back to Houston early Sunday aboard a shuttle training aircraft to await developments. "Things happen fast," Johnson said in a Twitter update. "We are now all aboard an STA for return to Houston. Be back in a few days. More to follow." 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. To find out if a faulty thermostat was to blame, engineers working in Endeavour's cramped engine compartment Saturday afternoon sprayed compressed air on APU No. 1's B-channel heater thermostats to lower their temperature enough to find out whether they would cycle on or not. They did not, but that could have been the result of a wiring problem or a bad connector. Additional tests were carried out overnight and no such problems were found. Engineers met early Sunday and recommended replacing the ALCA-2 box. It's not yet clear how long that work might take. "Due to additional troubleshooting required for APU 1 heater issue, Monday's launch attempt has been scrubbed," the launch team was told early Sunday. "Currently in work to secure from launch countdown operations at this time. No new launch date has been determined."
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