Aerospace

Vertical load fluctuations alter nose landing gear (NLG) system stiffness and complicate shimmy dynamics. Based on the full-scale NLG static stiffness test data, the relationship between shock absorber stroke and system stiffness was fitted, and a nonlinear shimmy model considering time-varying loads was established. The numerical solution was achieved using the established Simscape model. The research results show that, under constant load conditions, considering the nonlinear growth characteristic of NLG system stiffness with shock absorber stroke, the lateral shimmy amplitude of the NLG is significantly reduced, while the rotational shimmy amplitude increases slightly; among these, lateral stiffness plays a dominant role in influencing shimmy stability. In addition, time-varying loads aggravate shimmy through two paths: first, the fluctuation of load amplitude directly changes the force state; second, vertical movement causes changes in the shock absorber stroke, which in turn leads to dynamic adjustment of system stiffness. This is of great help in guiding the stiffness design of the NLG system and accurately evaluating shimmy stability.

Low Earth orbit (LEO) satellites, characterized by low altitudes, high velocities, and strong ground signal reception, have become an essential and dynamic component of modern global navigation satellite systems (GNSS). However, orbit decay induced by atmospheric drag poses persistent challenges to maintaining stable trajectories. Frequent orbit maneuvers, though necessary to sustain nominal orbits, introduce significant difficulties for precise orbit determination (POD) and navigation augmentation, especially under complex operational conditions. Unlike most existing methods that rely on Two-Line Element (TLE) data—often affected by noise and limited accuracy—this study directly utilizes onboard GNSS observations in combination with real-time precise ephemerides. A novel time-series indicator is proposed, defined as the geometric root-mean-square (RMS) distance between reduced-dynamic and kinematic orbit solutions, which is highly responsive to orbit disturbances. To further enhance robustness, a sliding window-based adaptive thresholding mechanism is developed to dynamically adjust detection thresholds, maintaining sensitivity to maneuvers while suppressing false alarms. The proposed method was validated using eight representative maneuver events from the GRACE-FO satellites (May 2018–June 2022), successfully detecting seven of them. One extremely short-duration maneuver was missed due to the limited number of usable GNSS observations after quality-control filtering. To examine altitude-related applicability, two Sentinel-3A maneuvers were also analyzed, both successfully detected, confirming the method’s effectiveness at higher LEO altitudes. Since the thrust magnitudes and durations of the Sentinel-3A maneuvers are not publicly available, these cases primarily serve to verify applicability rather than to quantify sensitivity. Experimental results show that for GRACE-FO maneuvers, the proposed method achieves near-real-time responsiveness under long-duration, high-thrust conditions, with an average detection delay below 90 s. For Sentinel-3A, detections occurred approximately 7 s earlier than the reported maneuver epochs, a discrepancy attributed to the 30 s observation sampling interval rather than methodological bias. Comparative analysis with representative existing methods, presented in the discussion section, further demonstrates the advantages of the proposed approach in terms of sensitivity, timeliness, and adaptability. Overall, this study presents a practical, efficient, and scalable solution for real-time maneuver detection in LEO satellite missions, contributing to improved GNSS augmentation, space situational awareness, and autonomous orbit control.

The adaptive cycle engine (ACE) can modulate thermal cycle characteristics by adjusting variable geometry components, enabling rational distribution of bypass flow rates. As a key component of the ACE, the rear variable area bypass injector (RVABI) significantly influences the engine bypass ratio and consequently alters engine performance. RVABIs are typically categorized into three configurations based on their design: Translation Type, Rotary Type, and Hole Type. Previous studies have not fully elucidated the overall operating characteristics, internal flow mechanisms, and applicable scenarios of these different RVABI configurations. To address this problem, this paper first introduces and validates a three-dimensional (3D) simulation methodology for RVABIs. Subsequently, criteria for reasonably evaluating the operating characteristics of different RVABI configurations are defined. Following this, the differences in operating characteristics and internal flow mechanisms among the three RVABI configurations are systematically compared. Finally, the application scenarios for each configuration are identified. This work provides valuable insights to guide the configuration selection and parameter design of RVABIs in practical engineering applications.