Switching-Curve Guidance Strategies for Two-Stage Discrete-Event Drag-Modulated Aerocapture

This paper presents a two-stage discrete-event drag-modulation guidance method for aerocapture missions at Mars and Titan. In this method, two jettison events define a three-segment ballistic-coefficient profile during a single atmospheric pass. A nominal trajectory is selected through apoapsis matching and a constraint linking the exit flight-path angle to the spacing between the jettison angles. Nominal and dispersed scenarios are then generated using a face-centered central composite design. Based on these scenarios, three alternative switching-curve guidance strategies are developed in the flight-path angle domain, with velocity, specific orbital energy, and cumulative drag loss serving as the switching variables. The guidance method is evaluated for three Mars elliptical-orbit cases and one Titan circular-orbit case through Monte Carlo simulations with dispersions in entry states, ballistic coefficients, and atmospheric density. An ascent/descent density-ratio model is also developed to represent atmospheric uncertainty more realistically. The results demonstrate that the drag-loss switching strategy provides the best overall performance for the Mars cases, whereas the energy-based switching strategy performs slightly better for Titan. Relative to single-stage switching-curve guidance, the proposed two-stage method improves robustness and reduces the postaerocapture orbit-correction cost. It also achieves performance comparable to two-stage numerical predictor/corrector guidance while requiring a simpler onboard implementation.