NASA moves Endeavour date amid ongoing repair work
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: May 6, 2011
KENNEDY SPACE CENTER, FL--Engineers have not yet pinned down the cause of a short circuit that vaporized fusing in a heater control power switch April 29, delaying launch of the shuttle Endeavour on its 25th and final flight. But a replacement power switching box has been installed and NASA managers have approved a plan to replace wiring between the box and the heaters in question to clear the way for a second launch attempt.
For a launch at 8:56 a.m. EDT (GMT-4) on May 16, Endeavour's countdown would begin around 7 a.m. on Friday, May 13. The shuttle would dock with the International Space Station around 6 a.m. on May 18, undock on May 27 and land back at the Kennedy Space Center around 3:20 a.m. on May 30.
NASA managers are holding open an option to extend the mission by two days to give the shuttle crew time to help out with needed station maintenance. Complicating mission planning, three of the space station's six crew members -- Dmitry Kondratyev, Paolo Nespoli and Catherine Coleman -- are scheduled to return to Earth May 23 aboard a Russian Soyuz spacecraft, leaving the lab understaffed until three replacements arrive June 9.
The primary goals of Endeavour's flight are to deliver a pallet of spare parts and to install a $2 billion particle physics experiment on the station's power truss. The Endeavour astronauts also plan to stage four spacewalks to perform critical maintenance, but the launch delay and Soyuz departure are expected to result in a major flight plan revision and the EVA schedule is not yet clear.
Endeavour was grounded during the final hours of a launch countdown April 29 when one of two "strings" of fuel line heaters used by one of the shuttle's three hydraulic power units failed to power up normally. While the system can safely operate with a single string, a subsequent failure could result in a fuel line rupture, knocking auxiliary power unit No. 1 out of action and possibly contaminating the aft engine compartment with toxic hydrazine.
At the time of the failure, engineers believed there were three possible explanations: trouble with a fuse panel in the shuttle's cockpit that routes power to the APU system; a problem with the aft load control assembly, an electrical distribution box upstream of the heaters; or trouble with downstream heaters or thermostats.
Subsequent testing showed the cockpit fuse panel was working normally and that commands were reaching ALCA No. 2 as required. Additional tests indicated the thermostats in question were healthy but not getting power from the distribution box. As a result, ALCA No. 2 was removed from avionics bay No. 5 in Endeavour's aft engine compartment for a detailed failure analysis.
Engineers quickly found vaporized fuse elements in a so-called "hybrid driver" circuit responsible for routing power to APU 1's B-string fuel line heaters. But it was not immediately clear whether the blown circuitry was the result of a problem inside the power box or the result of a short in external wiring.
At launch pad 39A, meanwhile, a replacement ALCA was installed, but fuel line heater power was not immediately reconnected to make sure whatever caused the initial problem did not damage the replacement box.
Engineers met Thursday and discussed three options: replacing the wiring between ALCA No. 2 and the heater thermostats; replacing all the wiring, thermostats and heaters downstream of ALCA No. 2; and pressing ahead "as is" with the replacement ALCA and the original wiring.
They opted to replace the wiring between ALCA No. 2 and the heaters. The original power line will remain in place but will not be plugged in to protect against the possibility of a short. With a replacement ALCA and new downstream wiring, engineers are hopeful the heaters will work normally.
MISSION STATUS CENTER