Graded-Compliance Metal-Packaged FBG Strain Sensor for Cryogenic Strain Measurement of Reusable Launch Vehicles

This study develops a graded-compliance, metal-packaged fiber Bragg grating (FBG) strain sensor for cryogenic structural health monitoring of metallic propellant tanks and pipelines on reusable launch vehicles (RLVs). The serially connected frame enables customizable sensitivity and expands the effective measurement range via strain desensitization for large-magnitude cryogenic strains. This design reduces interfacial shear stress and improves long-term stability under cyclic cryogenic loads, while glass brazing fixation avoids the failures associated with organic adhesives. The ground characterizations down to 77 K show high linearity (R2 > 0.998) over −4000 to 6000 με. The tensile sensitivity decreases by approximately 4.3% from room temperature to 77 K, with a tension–compression sensitivity discrepancy of 2.4% under cryogenic conditions. The limited sensitivity drift across 77 K–123 K and favorable batch consistency support a unified full-temperature-range sensitivity calculation method, reducing the onboard computational burden. Within the cryogenic operating range of 77 K–123 K, the maximum full-scale error is ±1.01%. Batch-produced sensors have passed environmental qualification tests, and units from different batches were deployed in three independent RLV flight campaigns in 2024, 2025, and 2026. Continuous and stable strain signals were obtained during ground propellant loading and in-flight phases under severe thermo-mechanical loads, consistent with the theoretical predictions.