What are the design considerations for reusable rocket engines?

Reusability requirements affect all engine aspects: Life requirements (100+ flights vs single use), Inspectability (access for between-flight checks, borescope ports), Maintenance-friendly design (modular components, reduced refurbishment), Robust margins (operating well below limits), Material selection for fatigue/creep life, Health monitoring integration (sensors for anomaly detection), Throttling capability for landing, and Multiple restart capability. Design philosophy shifts from minimum weight to life-cycle cost optimization. SpaceX Merlin/Raptor and Blue Origin BE-4 demonstrate reusable designs. Data from each flight feeds reliability growth. Target 10-100x reduction in $/kg to orbit through reuse. Lessons from SSME (designed for 55 starts) inform modern approaches.