Compound Solar Sail with Optical Properties: Models and Performance

摘要

A compound solar-sail model, in which the optical properties of the collector, reflector, and director have been taken into account, has been investigated. A compact expression for the resultant force acting on the sailcraft has been derived in which two force coefficients depend only on the optical properties of the reflecting material, whereas the third one also depends on the collector geometry. Unlike the ideal case, the sailcraft acceleration is different from zero for any cone angle. This new model allows one to establish a more realistic comparison between the performance attainable with conventional and compound solar sails. To this end, the optimal control law for minimum-time three-dimensional interplanetary trajectories has been solved using an indirect approach. The resulting steering law generalizes a similar result available for an ideal compound sail. Applications to transfers toward Mars and Venus have been discussed. Assuming that the main losses of the compound sail are concentrated in the collector, the minimum transfer times obtainable with a compound sail are shorter than those of a conventional sail with optical properties and are close to those of an ideal flat sail. As there exists a critical cone angle beyond which the sailcraft acceleration of the optical model is greater than that with an ideal model, even better performance of compound sails are expected for those missions requiring high values of cone angles. We finally note that a fully realistic comparison between conventional and compound solar sails needs a more detailed study. In fact, it is likely that the compound sail will have a significant mass penalty compared to a flat sail because of the mass of the additional mirrors and the structure required to join them together. An important issue would be to determine the payload mass delivered for a given launch mass, taking into account both trajectory optimization and the mass budget of the flat and compound sails. This is beyond the scope of the current Note and is left to future research.