Determining the number of spares in an inventory/repair system which supports equipment with scheduled usage

The authors address the problem of finding the minimum number of spares for systems of equipment which are used on a scheduled, periodic basis. When spares are expensive it is important to have as few spares in the system as possible subject to management’s specification of the required likelihood of having equipment available for use when needed. Several varieties of this problem are addressed in the literature, but all of them assume the equipment is needed or must be available for the time rather than according to a schedule. The authors present the base case of a series of stochastic models that can address several versions of this problem and which explicitly address the aspect of scheduled usage. Numerical examples based on NASA’s space shuttle are presented. A comparison is made between the results of the present model and an approach in the literature that is in common use. The comparison shows that the usual approach errors on the side of too many spares in the system.