Endeavour launch not earlier than Monday.

The launch of Endeavour on the next-to-last mission in America's space shuttle program was delayed at least 72 hours on Friday due to a problem in one of the craft's auxiliary power units.

Auxiliary power unit No. 1, one of the three units which supply hydraulic power to steer the shuttle’s main engines in flight and control flight surfaces and other critical functions during re-entry, developed a problem managers believed could not be resolved in time for the scheduled 3:47 p.m. liftoff.

Two heaters, required to keep the unit's turbine engine thermally conditioned, failed.

Shuttle Launch Director Mike Leinbach said managers believe the problem involved a thermostat on one of the fuel lines feeding the unit's heaters.

"We need to keep those lines warm to prevent them from freezing on orbit," Leinbach said. "We have two thermostats, two heaters for each fuel line for each APU. On auxiliary power unit number one of those two heater units failed and the troubleshooting we did on it proved it was a hard failure. We were not able to get it to come to life no matter what we did."

"We tried to let the line cool down just by normal means and to see if the thermostat on that heater would kick in. That did not happen. We tried to command the heater from the cockpit of the orbiter. That did not happen either."

Managers believe the problem lies with one of the load control assemblies or switchbox that points toward a short in the box or one of the lines leading to or from the box.

They were to meet at 2:30 p.m. and hold a 4 p.m. press conference to lay out their solution which would involve NASA and contractor crews erecting platforms to gain access to the aft compartment of the orbiter at the launch pad to perform detail examinations and work toward a solution by Saturday afternoon.

Managers could elect to either replace the box or repair lines where a short might have occurred. The solution could mean a turnaround longer than 72 hours.

The mission of Endeavour on STS-134 will fly no earlier than 2:33 p.m. Monday from Kennedy Space Center Launch Pad 39B on a planned 14-day flight to the International Space Station.

The mission drew intense interest that transcended it being Endeavour's 25th and final flight since President Barack Obama and his family were to attend. Also, Arizona congresswoman Gabrielle Giffords, wife of Endeavour Commander Mark Kelly, was in attendance four months after being critically wounded in a Jan. 8 assassination attempt in Tuscon.

The scrub was announced just as the van carrying the six-member crew to the launch pad was leaving the Launch Control Center after a stop over observers called the first in the shuttle's history. Normally the van stops on the main road leading to the launch pad to drop off the chief astronaut and the pilot who flies the weather reconnaissance aircraft before continuing to the pad.

This time, however, the van pulled into the LCC parking lot and stopped for several minutes before leaving and turning left to head back to the crew quarters at the Operations and Checkout facility.
-Mark DeCotis, Cape Canaveral