Why Challenger's engine shut down

When one of the shuttle Challenger's engines shut down six minutes after liftoff recently, NASA technicians were quick to blame a pair of faulty temperature sensors in one of the engine's high-pressure fuel pumps. Indeed, agency officials say the sensors have displayed problems for years during ground tests, and researchers have been trying to develop replacements.

But engineers had been confident that the sensors would not pose a major problem in flight, because each sensor was always paired with a backup, and monitoring computers were programmed to recognize when a sensor was malfunctioning. But on flight 51-G a highly unlikely sequence of events apparently conspired to overcome the safety measures.

At the heart of the sensor is a wire of tungsten and rhodium that alters its ability to conduct electricity as temperature changes. Computers determine when the wires break, thus becoming unable to conduct electricity and sending an erroneous signal of infinite temperature.

Challenger's computer, however, apparently turned to the second sensor just as some insulating compound around a few wires leading to the sensor began to deteriorate. Instead of a sudden of leap to ``infinite temperature,'' which never occurs in nature, the sensor registered a gradual drift toward dangerously high temperatures. So the computer shut the engine down.

NASA officials say the incident has caused them to speed development of new sensors. They plan to have them installed, they say, in time for Discovery's scheduled Aug. 24 launch.