Availability tradeoffs for today's complex systems (military radar)

For the US Navy's relocatable over-the-horizon radar (ROTHR) project, tradeoffs involve millions of dollars. This work describes the problems solved, work performed, and results achieved for the selection of ROTHR spare parts to optimally support mission availability at least cost. A reliability block diagram (RBD) describing the complete system-to-critical-part relationship was developed first. This RBD described 305 unique removable units in over 2800 part applications, requiring 83 pages of drawings. Parts exhibiting electronic adjacency were modeled with each part as the center of a subsystem which could have itself up or both the adjacent parts up for subsystem success. For parts with preceding power circuitry bringing down groups, sets of binomial probabilities showing all combinations were developed. The optimal selection of critical spare parts resulted in achieving the system operational availability goal of 95% at a saving of approximately $4.0 per ROTHR installation compared to simple protection level sparing methods.<>