Optimizing heliocentric transfers of a spacecraft equipped with electric thrusters of different types

The paper addresses the issue of generating the optimal nominal program for controlling the thrust of a cruise propulsion unit (PU) of an interplanetary spacecraft (SC), consisting of several different types of electric thrusters. The optimization criterion consists in minimizing propellant consumption at fixed transfer duration. Determined are: PU thrust direction and the sequence of firing/shutting down individual thrusters. Pontryagin's maximum principle was used to obtain analytical expressions for optimal nominal control programs: determined were dependences of thrust direction and firings/shutdowns of individual thrusters on the dynamic system phase and conjugate coordinates. Used as an example of an application for the optimization procedure was the heliocentric section of the Earth - Mars transfer trajectory. Calculations were performed for a prototype with parameters similar to the SC of the international project DEMOCRITOS, which intends to use a Russian nuclear power system to supply power to multiple electric thrusters of different types. Numerical computations were run to solve boundary problems and simulate SC motion with the obtained optimal nominal control program for the propulsion system for two types of electric thrusters and different duration of transfer. Propellant consumption and firing time vs. required transfer duration were analyzed for both types of thrusters.