Indirect Optimal Control Techniques for Multimode Propulsion Mission Design

Multimode spacecraft propulsion has the potential to greatly increase the maneuvering capability of spacecraft in comparison to traditional architectures. This technology combines two or more propulsive modes into a single system with a single propellant. Trajectory design techniques for spacecraft with this capability, however, are limited and typically require manual selection of the burn sequence. In this study, indirect optimal control formulations with automatic mode selection are developed and applied for the first time for multimode spacecraft with two modes of operation. Minimum-fuel transfers are solved in polar coordinates and Mean Equinoctial Elements. Propellant-constrained minimum-time problems are also solved for the first time using a penalty function approach. An interior-point constraint formulation is also provided. The focus of this work is the development of trajectory optimization techniques for multimode-propelled spacecraft. Sample transfers are presented for each coordinate choice and optimization objective and are compared to single mode approaches. The results demonstrate viability of the proposed techniques and show the multimode approach can reduce the time-of-flight in comparison to a low-thrust only trajectory while providing mass savings over high-thrust only solutions.