Interplanetary trajectory design for a hybrid propulsion system

A fuel-optimal interplanetary trajectory design is investigated for a hybrid propulsion system based on solar sailing and solar electric propulsion. The optimal-control problem is formulated using the calculus of variations and Pontryagin's maximum principle. The solution to the corresponding two-point boundary problem is a bang–bang control for the solar electric propulsion engine that is solved by an indirect method combined with the homotopic technique. Two interplanetary missions, including the Venus and Apophis rendezvous, are used to analyze the fuel consumption for different combinations of the two propulsion systems. The departure time associated with the launch window and the transfer time are two factors that influence the fuel consumption. The optimal departure time can be obtained for a given launch window time range. Additionally, a solar sail used as an auxiliary system to solar electric propulsion can decrease the propellant expense to zero if the transfer time is sufficiently long. Solar electric propulsion used as an auxiliary to a solar sail can significantly shorten the transfer time, while consuming only a small quantity of propellant. Therefore, a hybrid of the two systems can avoid their respective limitations and lead to the development of pure solar sails for space missions.