scope:

Description of Driving Event:

On March 25, 1993 an Atlas/Centaur launch vehicle (AC-74, Atlas1) inserted a Hughes UHF followon spacecraft into orbit with an apogee altitude of 4,967 nmi instead of the targeted nominal of 9,098 nmi. Although functioning normally, the satellite was declared a loss by the Navy due to the unacceptable orbit.

The investigation team concluded that the precision regulator in the booster engine system failed. The output pressure of this regulator controls the output power level of the booster engine, which caused the decline in thrust. This failure was caused by the stem screw rotating and decaying the output of the precision regulator.

This precision regulator had been reworked twice during its build to repair excessive vent port leakage. The first authorized rework required loosening and retorquing the set screw. The set screw has been redesigned so that it is custom made for each mission. The length of each screw is predetermined and inserted so that rotation is impossible to achieve.

Further information can be found in the AC-74 Anomaly Investigation Summary Report.