Generalized Energy Depletion Guidance for Aerobraking at Mars, Venus, Earth, and Titan

Aerobraking drag-passage control has traditionally relied on open-loop periapsis trimming, with few closed-loop concepts capable of responding to real-time density transients. Energy Depletion Guidance is one of very few such algorithms that rotates an orbiter’s solar panels to modulate the ballistic coefficient, allowing the vehicle to track a commanded heat rate and heat load profile while maximizing orbital-energy loss. This paper generalizes Energy Depletion Guidance along three directions. First, the admissible control set is extended to jointly enforce a structural limit in addition to the thermal limits. Second, the Energy Depletion Guidance is reformulated with an explicit targeting mode in which the drag-passage guidance problem is cast as a fixed-time optimal control problem with a terminal constraint on specific orbital energy. Third, the Mars closed-form drag-passage approximation is generalized to Venus, Earth, and Titan. Furthermore, the resulting generalized Energy Depletion Guidance framework is implemented in the high-fidelity SpaceAGORA.jl and exercised for drag passages at all four bodies. Results show that the closed-form solutions for Mars, Venus, Earth, and Titan had a maximum percentage error of 0.8%, 0.1%, 0.06%, and 0.05%, respectively. Single-pass simulations using Energy Depletion Guidance, along with Monte Carlo campaigns, show that heat rate and heat load remain within bounds for all simulations, while the maximum drag is violated on average of 9.6%, 27.2%, 40.4%, and 30.8%, for Mars, Venus, Earth, and Titan, respectively, with periapsis altitude strongly governing energy depletion and drag limit violation. Finally, in targeting mode, Monte Carlo simulations show that single-pass simulations achieve post-pass apoapsis targeting errors on the order of 10–60 m.