Estimating Spares Requirements for Space Station Freedom
Using the M-SPARE Model
Published in: Estimating Spares Requirements for Space Station Freedom: Using the M-SPARE Model (Bethesda, MD : Logistics Management Institute, July 1993)
The Logistics Management Institute developed a methodology that estimates the optimal orbital replaceable unit (ORU) spares inventory for NASA's Space Station Freedom. NASA is using this methodology to select a spares inventory that maximizes station availability, i.e., the probability that no critical system is inoperative for lack of an ORU spare over the resupply cycle. Spares are ranked in order of decreasing benefit per cost (the improvement provided to station availability per dollar) and added, in that order, to the inventory until a target resource expenditure or availability is reached. The methodology also develops optimal spares inventories constrained by the spares weight the shuttle can carry, the spares volume the station can store, or a combination of resources. To implement our methodology, we developed the Multiple Spares Prioritization and Availability to Resource Evaluation (M-SPARE) model that operates on a personal computer. M-SPARE presents the maximum availability for an entire range of resource expenditures. The model also converts annual spares requirements over any period of the station's life into funding estimates for the next 9 years. To address other spares-related costs, the model also estimates repair budgets. In this guide, we describe the M-SPARE methodology, operation, and analytical capabilities.