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.
The failure appears to be a power problem within the aft load control assembly-2 (ALCA-2), a box of switches controlling power feeds. "That basically means the power is not getting out to the heaters that weren't working on launch day," said Space Shuttle Program Launch Integration Manager Mike Moses. The plan is to remove and replace the box, but that work and related testing will take several days to complete. Once the new box is installed, the team must verify it's working properly — at least a two-day process — and perform forensics on the failed box.
At NASA Kennedy Space Center's Launch Pad 39A, a technician makes his way across a platform in space shuttle Endeavour's aft section as work begins to remove and replace the aft load control assembly-2 (ALCA-2). Photo credit: NASA/Kim Shiflett
"We can tell you, pretty much, that it's not going to be any earlier than (May 8)," Moses said. "We're really not even setting the schedules today. There's still a whole lot of short-term work that has to be done."
"Responding to problems is one of the things we do best around here, and the team always likes a good challenge," said Shuttle Launch Director Mike Leinbach. "I'm sure we're going to be really glad when Endeavour's finally on orbit, but right now, the team is upbeat and ready to execute."