<p>Additive manufacturing (AM) provides many potential benefits when compared to conventional manufacturing (CM) for critical systems, including shorter lead times. It is important to quantify the value of the possible time savings associated with AM and to consider this benefit along with the additional capital, material, and labor costs associated with AM when supporting a fleet of critical systems. While existing works primarily focus on either the time-savings impact on inventory or the manufacturing costs of AM, this paper combines both effects into a comprehensive analysis. The model developed in this paper focuses on systems that currently use subsystems with CM parts that need to be returned to their original equipment manufacturer (OEM) for repair when they fail; AM allows some failures to be resolved without requiring the return of the subsystem to the OEM and thereby reduces the spare resupply time. The model monetizes the cost avoidance associated with using AM parts by comparing two scenarios, one using AM parts and the other using CM parts, under the constraint that both achieve the same spare subsystem inventory protection level. Protection level is the probability that at least one spare will be available in the inventory all the time. Models for single-echelon and multi-echelon sparing are developed. A case study of an Electronic Support Measures system used in helicopter fleets deployed on aircraft carriers is provided. The results of the case study demonstrate that although manufacturing costs are larger when using AM parts, the life-cycle cost for a fleet of systems can be substantially less due to decreases in failure resolution times enabled by AM part repairs in the field.</p>