NSTS Orbiter Auxiliary Power Unit Turbine Wheel Cracking Risk Assessment

The National Space Transportation System (NSTS) Orbiter has three hydrazine-powered auxiliary power units (APU). These APUs provide power during both ascent and descent phases of the mission for gimballing the Space Shuttle Main Engines (SSME), and for deflecting the flight control surfaces. Each APU drives a hydraulic power source for these control requirements. This paper discusses the concern for flight safety which led to a detailed risk assessment associated with the continued use of turbine wheels, with this known cracking problem. While test data strongly suggested that the turbine blade cracking would not result in the loss of a full turbine airfoil, the probability of such an event had to be estimated. The steps in this study included a determination of the causes of cracking, a determination of the 0.99/95 percent reliability-based life of cracked turbine blade wheels subject to continued use, and finally, the estimation of fleet-wide probability of blade liberation, if the wheels were used beyond the 0.999/95 percent reliability-based life. The determination of the 0.999/95 percent reliability life was based on a Weibull probability interpretation of high-time testing of cracked turbine wheels. The fleet-wide reliability study was undertaken to relieve the potentially substantial cost of turbine wheelmore » inspections that might result from limiting the allowable risk to 0.001 ({equals} 1.0 - 0.999).« less