Effects of Crew Seat Inclination on Multi-Organ Injury Risk in Astronauts During Off-Nominal High-g Landing Impact

During off-nominal landings, spacecraft reentry capsules can experience impact accelerations far exceeding the 8-12 g typical of nominal returns, with peak values surpassing 50 g and rise times under 100 ms. Such loading conditions pose multi-organ injury risks to astronauts. This study systematically investigates how seatback inclination modulates these risks.

Drop tower tests were performed at three severity levels (Low: 18 g, Mid: 32 g, High: 45 g) using a Hybrid III 50th percentile male anthropomorphic test device. The measured velocity pulses then drove THUMS AM50 v7.0 simulations across five seatback inclinations (0°, 10°, 20°, 30°, 40°). Injury risk was evaluated using dynamic injury criteria (HIC₁₅, N_ij, Thoracic Dmax, and Lumbar Fmax) supplemented by tissue-level mechanical parameters (Brain CSDM0.2, Lung MPS and CSDM0.343, Myocardial VMS, Aorta MPS). Segmented linear regression with logit-transformed probabilities quantified the inclination-injury probability relationship. A sensitivity analysis varying the occupant-seat contact condition (friction coefficient ± 50%) tested for cross-over interactions and assess the robustness of the inclination angle rankings.

All dynamic injury criteria worsened monotonically with increasing inclination; the lumbar spine were the most sensitive region. Tissue-level responses revealed conflicting organ-specific demands: Brain CSDM0.2 and Aorta MPS reached minima at 0°, Lung MPS at 40°, while Myocardial VMS displayed a U-shaped minimum near 20°. Lung MPS and Lung CSDM0.343 trended in opposite directions-peak local strain was highest at 0°, whereas the volume fraction exceeding the injury threshold peaked at 40°-indicating a shift in lung deformation mode from localized compression to diffuse stretching. Segmented regression confirmed that injury risk is strongly correlated with seat inclination, with risk ratios per degree ranging from 1.003 (neck, Low impact) to 1.739 (lumbar, beyond 10°). The sensitivity analysis detected no cross-over interactions, confirming that the relative ranking of inclinations was robust to variations in contact conditions.

No single inclination simultaneously minimized injury risk across all anatomical regions. The supine posture (0°) offered the greatest protection for the brain, spine, and aorta, but maximized lung strain. A lower inclination range (0°-20°) provided a more favorable overall balance than substantially reclined postures (30°-40°), though the precise optimum differed by organ system. These findings provide quantitative guidance for adaptive crew restraint system design.