Seasonal efficiencies of solar sailing in planetary orbit

Increasingly solar sailing is perceived as a practical means of spacecraft propulsion. This paper concentrates on further understanding the orbital dynamics of a solar sail in a planet-centred environment, explaining and investigating the seasonal variations encountered when using a solar sail to perform orbit manoeuvres at Earth and Mercury. Solar sail performance is shown to depend on orbit inclination, with the optimal inclination achieved when placing the orbit within the ecliptic plane. When the orbit plane is not coincident with the ecliptic plane the loss of optimality is shown to generate an out-of-orbit-plane force when attempting to follow a locally optimal energy gain control law. The variation of escape time with initial orbit inclination is calculated and confirmed to decrease as sail acceleration increases. The method of variation of sail performance with orbit inclination explains the increasing loss of optimality between locally optimal and globally optimal trajectories previously found as orbit inclination is increased, it also explains the variation in escape times found between July and December launches, due to the difference in orbit alignment with the ecliptic plane. Mercury escape times are investigated for start epochs throughout the Hermian year and shown to vary significantly. The variation is directly related to the highly eccentric nature of Mercury's orbit, however the optimal launch date at Mercury is not directly related and varies with sail characteristic acceleration. Additionally, the time difference between maximum and minimum escape times was also considered and found to never exceed half a Hermian year, thus showing while the variation in escape time is substantial, the optimal start epoch for an escape trajectory to minimise Julian date at point of escape is always the present point in time