HYBRID SPARE-PARTS LOGISTICS IN ENGINEERING SYSTEMS: A 3S MODEL FOR STORE, SOURCE, OR SYNTHESIZE DECISIONS

Spare-parts availability is a critical condition for technical continuity in production, transport and other engineering systems. Traditional inventory strategies provide rapid response but may create high holding costs, obsolescence and tied capital, whereas external sourcing introduces lead-time dependence. Additive manufacturing adds a third option by enabling selected components to be produced on demand from digital data. This conceptual paper develops a hybrid decision framework that combines these alternatives in a 3S model: Store, Source, Synthesize. The model evaluates spare parts according to criticality, supply lead time, downtime cost, demand frequency, additive-manufacturing feasibility, material and quality requirements, legal and warranty constraints, and lifecycle economics. The study synthesizes peer-reviewed research on additive manufacturing in spare-parts supply chains and positions 3D printing as a selective logistics option rather than a universal substitute for physical inventory. The resulting framework is intended for preliminary classification and decision support; it requires organisation-specific validation before quantitative thresholds are introduced.