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)

Posted on on July 01, 1993

by Robert C. Kline, Craig C. Sherbrooke

Read More

Access further information on this document at

This article was published outside of RAND. The full text of the article can be found at the link above.

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.

This report is part of the RAND Corporation External publication series. Many RAND studies are published in peer-reviewed scholarly journals, as chapters in commercial books, or as documents published by other organizations.

Our mission to help improve policy and decisionmaking through research and analysis is enabled through our core values of quality and objectivity and our unwavering commitment to the highest level of integrity and ethical behavior. To help ensure our research and analysis are rigorous, objective, and nonpartisan, we subject our research publications to a robust and exacting quality-assurance process; avoid both the appearance and reality of financial and other conflicts of interest through staff training, project screening, and a policy of mandatory disclosure; and pursue transparency in our research engagements through our commitment to the open publication of our research findings and recommendations, disclosure of the source of funding of published research, and policies to ensure intellectual independence. For more information, visit

The RAND Corporation is a nonprofit institution that helps improve policy and decisionmaking through research and analysis. RAND's publications do not necessarily reflect the opinions of its research clients and sponsors.