NASA Thursday decided to delay the Sept. 16 launch of the space shuttle Endeavor so critical electrical wiring can be checked. The inspection was prompted by a potentially dangerous short circuit during last month's launch of the shuttle Columbia's. The exam to make sure Endeavor's wiring is sound will delay the flight by approximately three weeks.
At the same time, engineers will conduct similar inspections in the cargo bay of the shuttle Discovery. It is scheduled for blastoff Oct. 14 on a quick-response mission to install new gyroscopes and other equipment on the Hubble Space Telescope.
Donald McMonagle, director of shuttle integration at the Kennedy Space Center, said NASA probably would stick with the current launch sequence. Under that scenario, Endeavour would take off whenever it is ready - probably during the first week in October - pushing back the Hubble servicing mission to late October or early November.
Columbia's launch July 23 was marred by two unrelated malfunctions: An engine nozzle leak and a short circuit five seconds after liftoff that knocked out two of the shuttle's six main engine control computers.
Each main engine is equipped with two such controllers and each is powered by a separate AC bus, or circuit, to prevent a single electrical problem from shutting down both controllers.
During launch, a short in the AC-1 phase A bus caused voltage to drop from 115 volts to just under nine volts for a half second. That caused a circuit breaker in the crew module to pop open, cutting power to the primary controller on Columbia's center engine and the backup controller on the right engine.
Only one controller is required to keep an engine running, but the short circuit left both power sources without any redundancy. Another failure would have triggered an engine shutdown, which in turn could have forced the crew to attempt a risky emergency return to Earth.
Post-flight inspections revealed the short was triggered by arcing between a chafed wire and a screw head near Columbia's aft bulkhead on the left side of the orbiter. Laboratory analysis indicates the wire's insulation was damaged by mechanical impact, not by vibration or any other age-related problem.
McMonagle said work platforms in that area eight flights ago were attached to mounting points very close to the chafed wires. The most likely scenario is that a technician stepped on the cable tray, nudged the wires in some fashion, or perhaps dropped something on them.
Slight damage also was found in the insulation of a wire directly across the bay from the one that caused the short circuit. The damage was much less severe - no bare wire was exposed - and McMonagle said, "We were not on the verge of having a second short."
However, finding a second are of damage implied similar problems might be present on other shuttles that, until now, had escaped detection. Columbia and Atlantis, scheduled for launch in December, already have been inspected. Today's decision affects only Endeavour and Discovery.
By William Harwood