Optimal Solar Sail Interplanetary Trajectories with Constant Cone Angle

A simplified solar sail steering law is discussed in which the sail clock angle is used as the only control variable, while the sail cone angle is maintained constant along the whole spacecraft heliocentric trajectory. Within this control strategy, the steering law that minimizes the flight time is obtained with a closed form solution using a classical variational approach. Sub-optimal performances, that is, optimal constrained solutions, have been analyzed for different mission scenarios assuming three-dimensional (ephemeris free) problems. Numerical simulations show that for suitable combinations of cone angle and spacecraft characteristic acceleration, the percentage increase of flight time of the simplified steering law is moderate when compared to a minimum time solution, in which the sail cone and clock angles are both allowed to vary during the whole transfer phase.