Skip entry strategy for vehicle re-entry

This dissertation presents the development of a guidance algorithm for the re-entry of a low Lift-to-Drag (L/D) ratio vehicle returning from the Moon, employing a skip entry strategy inspired by the Apollo re-entry guidance. The primary goal of the algorithm is to guide the vehicle from the first atmospheric entry interface to the second, while ensuring that structural stress and thermal loads remain within tolerable limits for the vehicle. To achieve this, a Numerical Predictor-Corrector technique was implemented, utilizing bank angle modulation to control the vehicle trajectory. The predictor component numerically integrates the complete, unsimplified equations of motion, ensuring a more accurate representation of the vehicle dynamics compared to simplified models, which may introduce approximation errors affecting guidance precision. The corrector component adjusts the bank angle command at each process cycle and is implemented as a PID (Proportional, Integrative, Derivative) controller, with its gain values optimized using a genetic algorithm. In nominal scenario, the vehicle consistently achieves high-precision targeting. Monte Carlo analyses incorporating various uncertainties, including variations in atmospheric density, aerodynamic coefficients, and initial entry conditions, highlighted that the vehicle very low L/D ratio limits its control authority. This limitation is particularly evident under conditions of low atmospheric density and reduced lift coefficients, resulting in decreased accuracy in reaching the second Entry Interface Point. The reduction in precision is deemed acceptable, as trajectory errors can be effectively corrected during the final phase of re-entry. The methodologies developed in this research offer a robust framework for future lunar return missions, particularly for vehicles with challenging aerodynamic characteristics.