Propellant-Optimal Powered Descent Guidance Revisited

San Diego State University, San Diego, CA 92182 This paper presents new and significant advances in propellant-optimal powered descent guidance based on the indirect method. The paper starts by reviewing and improving a recent indirect method relying on analytical equations, and pointing out certain numerical perils of the approach. By taking a fully numerical approach, the developments in this paper take relatively simple and concise software to implement, producing a fully automated algorithm capable of rapidly and reliably generating the complete optimal powered descent trajectory subject to imposed constraints including one on the thrust pointing direction, without the need for any empirical assumptions or hard-to-make user-supplied guesses. Furthermore, a “solution degradation" phenomenon not recognized before is brought to attention in the closed-loop solution of the propellant-optimal powered descent problem with a fully constrained final state that can pose a potential safety risk. This risk is mitigated in the proposed guidance approach where guidance in the later half of the powered descent is by tracking an onboard generated optimal reference trajectory with a globally convergent tracking guidance law.