Engineers troubleshoot electrical glitch (UPDATED)

Editor's note...
  • Posted at 03:24 PM EDT, 04/30/11: Engineers troubleshoot electrical glitch
  • Updated at 05:55 PM EDT, 04/30/11: Thermostats fail to respond in initial low-temperature tests; more troubleshooting planned
CBS News

KENNEDY SPACE CENTER, FL--Engineers troubleshooting an electrical glitch that grounded the shuttle Endeavour Friday have verified that a cockpit fuse panel is working normally, officials said Saturday, indicating the problem likely is either an open circuit in a hydraulic system fuel line heater thermostat or trouble inside an avionics box in the shuttle's aft engine compartment.

Initial thermostat tests were consistent with a problem in the avionics box, but engineers could not immediately rule out a connector problem or some other wiring issue elsewhere in the system.

If the problem can be isolated to an open circuit in a suspect thermostat, a replacement possibly could be installed in time to support a second launch attempt Monday, at 2:34:00 p.m. EDT (GMT-4). But if the problem is traced to the aft load control assembly -- ALCA-2 -- avionics box that routes power to the heaters, launch likely would be delayed until at least May 8, after the planned May 6 launch of an Atlas rocket carrying an Air Force missile early warning satellite.

And that assumes the problem can be traced to a fault in ALC-2.

The problem cropped up Friday during the final hours of Endeavour's countdown to launch on its 25th and final mission. Telemetry showed that multiple fuel line heaters used by auxiliary power unit No. 1 were not activating normally. The heaters are needed to keep the lines from freezing and possibly rupturing in flight.

The shuttle is equipped with three APUs, providing the hydraulic muscle to move the ship's engine nozzles, wing elevons, rudder, tail fin speed brake, body flap, landing gear brakes and nose wheel steering system. The shuttle can safely fly with a single APU, but flight rules require full redundancy for a countdown to proceed. Likewise, each of the shuttle's three APUs is equipped with redundant heater "strings" and only one channel is required for normal operation. But again, the flight rules require redundancy to protect against a subsequent failure that could knock the system out of action.

To find out if a faulty thermostat was to blame, engineers working in Endeavour's cramped engine compartment Saturday afternoon sprayed compressed air on APU No. 1's B-channel heater thermostats to lower their temperature enough to find out whether they would cycle on or not. The initially tested thermostats did not respond, but that could be the result of a wiring problem or a bad connector. Additional tests were planned overnight.

If no obvious culprits are found -- and engineers were not optimistic -- the problem likely is the result of a fault in ALCA-2, a box that contains multiple electronic switches for a variety of subsystems. Replacing ALCA-2 is a complex job requiring extensive retest.

NASA managers and engineers plan to review the troubleshooting Sunday morning to determine a course of action. But without an obvious "smoking gun," the chances of a launching Monday appear slim.

Endeavour's countdown currently is holding at the T-minus 11-hour mark. If a Monday launch is possible, the countdown would resume at 10:09 p.m. Sunday. Fueling would begin at 5:09 a.m. Monday and the crew would begin strapping in at 11:14 a.m. to await launch at 2:34:00 p.m.

The Spaceflight Meteorology Group at the Johnson Space Center is predicting scattered clouds at 3,000 feet and winds out of 100 degrees at 14 knots with gusts to 20, violating NASA's crosswind limits for an emergency landing.

The forecast for Tuesday calls for acceptable weather while the outlook for Wednesday calls for high crosswinds and a chance of showers within 20 nautical miles of the runway.