After Friday’s launch scrub, Kennedy Space Center technicians searched for the cause of a failure in a heater circuit associated with Endeavour’s hydraulic power system. The failure was found to be in a power circuit in a switchbox in the shuttle’s aft compartment.
Managers and engineers are developing a schedule to remove and replace the switchbox and retest the new unit. That work will delay Endeavour’s launch until at least May 8.
The shuttle has three Auxiliary Power Units (APUs) that provide hydraulic power to steer the vehicle during ascent and entry. The hydrazine fuel lines on each APU have two heater circuits that prevent the fuel from freezing while the shuttle is in space.
NASA launch commit criteria and flight rules require all three APUs and heater circuits to be operational for liftoff.
Endeavour’s six astronauts have returned to NASA’s Johnson Space Center in Houston for several days of additional training.