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Failure analysis indicates possible external short circuit
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: May 4, 2011


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Engineers inspecting a suspect electrical distribution box removed from the shuttle Endeavour after a launch scrub last week have found blown internal fusing that could be the result of an external, yet-to-be-identified short circuit somewhere in the ship's complex electrical system, officials said Wednesday. But they cautioned that troubleshooting is not yet complete and a problem with the box itself has not yet been ruled out.


The new ALCA sits on the bench before installation into Endeavour's aft compartment. Credit: NASA
 
Hoping for the best, engineers installed a replacement aft load control assembly switching box in Endeavour's engine compartment, but initial testing has been modified to make sure the new box is not inadvertently damaged by whatever caused the presumed short Friday.

Endeavour was grounded when telemetry indicated one of two redundant "strings" of fuel line heaters used by one of the ship's three hydraulic power units had not activated as required. Testing showed no obvious problems upstream of an aft load control assembly switching unit that routes power to the system and no obvious problems with the downstream heaters or thermostats.

On Saturday, engineers decided to replace ALCA No. 2 on the assumption that something had failed inside the box, preventing power from reaching the heaters. The box was removed from avionics bay No. 5 in Endeavour's engine compartment Monday, clearing the way for installation of a replacement.

The ALCA is one of three in the aft compartment that routes power to nine critical subsystems, including the hydraulic system, from three major circuits, or buses.

On the assumption a fault would be found in the suspect power box, NASA managers said Monday Endeavour's launch would be on hold until at least May 10 to give engineers time to install the replacement ALCA, retest downstream systems and to ready the shuttle for another three-day countdown.

If additional testing confirms an external short -- and if engineers are not able to quickly find and fix whatever might have caused it -- launch likely will face an additional delay. Endeavour's current launch window extends through May 29. The next available window opens June 19 based on temperature constraints related to the space station's orbit.

Only two fights remain on NASA's shuttle manifest, Endeavour's mission and a final flight by the shuttle Atlantis that is currently scheduled for launch June 28. Endeavour's launch delay likely will trigger a slip for Atlantis, but managers have not yet made any official changes.


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VIDEO: FAILED ALCA-2 BOX REMOVED FROM ENDEAVOUR PLAY | HI-DEF
VIDEO: WORKING ON OLD AND NEW SWITCHBOXES PLAY | HI-DEF
VIDEO: LAUNCH OF ENDEAVOUR DELAYED FURTHER PLAY

VIDEO: PRESIDENT OBAMA ARRIVES AT THE CAPE PLAY | HI-DEF
VIDEO: TOUR OF ATLANTIS AND ASTRONAUT MEETING PLAY | HI-DEF
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VIDEO: POST-SCRUB NEWS BRIEFING PLAY
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VIDEO: SCRUB DECLARED FOR APU NO. 1 HEATER GLITCH PLAY | HI-DEF
VIDEO: ASTRONAUTS LEAVE QUARTERS BEFORE SCRUB PLAY | HI-DEF
VIDEO: CREW GETS SUITED UP FOR LAUNCH PLAY | HI-DEF
VIDEO: TIME-LAPSE OF PAD GANTRY RETRACTING PLAY | HI-DEF

VIDEO: ENDEAVOUR'S PRE-FLIGHT CAMPAIGN PLAY | HI-DEF
VIDEO: PAYLOADS' PRE-FLIGHT CAMPAIGN PLAY | HI-DEF
VIDEO: PREPARING ENDEAVOUR FUEL TANK FOR LAUNCH PLAY | HI-DEF

VIDEO: COUNTDOWN UPDATE AND WEATHER BRIEFING PLAY
VIDEO: THE PRE-LAUNCH NEWS CONFERENCE PLAY
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VIDEO: PRE-FLIGHT INTERVIEW WITH GREG JOHNSON PLAY
VIDEO: PRE-FLIGHT INTERVIEW WITH MIKE FINCKE PLAY
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VIDEO: PRE-FLIGHT INTERVIEW WITH DREW FEUSTEL PLAY
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VIDEO: ASTRONAUT CHAT AT THE PRESS SITE PLAY | HI-DEF
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VIDEO: CANISTER ROTATED UPRIGHT FOR PAD PLAY | HI-DEF
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VIDEO: AMS PAYLOAD PUT INTO TRANSPORTER PLAY | HI-DEF

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VIDEO: ENDEAVOUR ARRIVES ATOP PAD 39A PLAY | HI-DEF
VIDEO: ROLLOUT FROM VEHICLE ASSEMBLY BUILDING PLAY | HI-DEF

VIDEO: HOISTING ENDEAVOUR TO TANK AND SRBS PLAY | HI-DEF
VIDEO: ORBITER GOES VERTICAL PLAY | HI-DEF
VIDEO: ENDEAVOUR LEAVES PROCESSING HANGAR PLAY | HI-DEF

VIDEO: PAYLOAD MEDIA DAY PLAY | HI-DEF
VIDEO: THE ALPHA MAGNETIC SPECTROMETER PLAY | HI-DEF
VIDEO: PAYLOAD REMOVED FROM C-5 PLAY | HI-DEF
VIDEO: AMS ARRIVES FROM EUROPE PLAY | HI-DEF

VIDEO: EXPRESS LOGISTICS CARRIER NO. 3 PLAY | HI-DEF
VIDEO: HIGH-PRESSURE OXYGEN GAS TANK PLAY | HI-DEF
VIDEO: PUTTING STATION ANTENNA ON CARRIER PLAY | HI-DEF

VIDEO: TANK UNLOADED TO ASSEMBLY BUILDING PLAY | HI-DEF
VIDEO: THE EXTERNAL FUEL TANK ARRIVES PLAY | HI-DEF
VIDEO: BOOSTER SEGMENT MOVED TO VAB PLAY | HI-DEF
VIDEO: KENNEDY SPACE CENTER SRB CEREMONY PLAY | HI-DEF
VIDEO: ENDEAVOUR FITTED WITH MAIN ENGINES PLAY | HI-DEF
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MORE: STS-134 VIDEO ARCHIVE
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