Analysis and Repair of Cracks in the Space Shuttle Main Propulsion System Propellant Feedlines

Abstract

In June 2002, inspections of the Space Shuttle Atlantis revealed three cracks in a flowliner in one of the Main Propulsion System liquid hydrogen propellant feedlines. Further inspection revealed similar cracks in some of the feedlines from Space Shuttles Endeavour, Discovery, and Columbia, as well as the Main Propulsion Test Article (MPTA), the original Space Shuttle Program's three engine test stand. The cracks raised concerns that debris could break away from the flowliner and be ingested by the Space Shuttle Main Engine (SSME), causing possible engine failure and subsequent loss of the mission or vehicle. These concerns resulted in the grounding of the Space Shuttle fleet until a safe repair could be developed and implemented. The flowliner cracks were found to be the result of low-stress, high-cycle fatigue caused by backflow conditions from the Space Shuttle Main Engine. The design team considered leaving the cracks in place for flight as well as several repair options including stop-drilling the cracks, cutting the crack out, slot enlargement, and weld repairing the cracks. In order to return the flowliner part to near print conditions without inducing further unknown environmental or systems effects, weld repairing of the cracks was selected. Fatigue testing of the weld repair proved that the weld repair technique would retain adequate fatigue life, and flight rationale was achieved using a reverse-fracture analysis technique to predict environmental loads and resultant crack initiation and growth rates. Post-flight non-destructive evaluation inspection requirements for the flowliner have been developed for subsequent flights of the Space Shuttle, which began in October 2002. The path through this investigation showed the ability to solve extremely complex problems quickly while balancing the need for safety of both the crew and vehicle.