scope:

Description of Driving Event:

The External Tank (ET) hardware incorporates five categories of instrumentation, sensors, and associated cabling, where the instrumentation data is recorded in the orbiter for transmittal to ground stations. The electrical system provides the propellant level, among other functions. The instrumentation is comprised of 38 sensors which control ullage temperature and pressure, liquid level, LH2 depletion, and vent valve position. During pre-launch propellant loading of the STS 51-L mission, a HIM failure caused a two hour, 20 minute delay while repairs were made. A HIM is used to interface the ET sensors with the launch control computer systems, where it is analyzed by systems engineers. The HIM failure resulted in the loss of all fire detection and hazardous gas measurements, which are required during propellant loading to provide adequate visibility of propellant leaks and fire.