Shuttle
launch director Mike Leinbach stated that Endeavour's launch will be no
earlier than Monday at 2:33 p.m. EDT. Engineers need that time to
troubleshoot an issue that resulted in today’s launch scrub.



During
today’s countdown, engineers detected a failure in one of two heater
circuits associated with Auxiliary Power Unit (APU) 1. Heaters are
required to keep the APUs’ hydrazine from freezing on orbit. Attempts to
activate the heater were not successful and engineers now believe the
problem might be associated with a Load Control Assembly, which is a
switchbox, located in the aft end of Endeavour, or an electrical short
in the wires leading into or out of the switchbox.



Endeavour’s
external tank will be drained, technicians will enter Endeavour’s aft
compartment overnight and by tomorrow afternoon, will put in a platform
to gain access to the avionics bay where the Load Control Assembly is
located. Once there, they will assess whether they need to remove and
replace the switchbox, or fix an electrical wiring short.



Because of this, Leinbach said there will be a minimum 72-hour scrub turnaround.



Engineers
and managers will meet this afternoon to further refine their
troubleshooting plan. NASA has scheduled a news conference at 4 p.m. to
discuss today’s scrub and the plan.