OPTIMAL LOW-THRUST INTERPLANETARY TRAJECTORIES BY DIRECT METHOD TECHNIQUES

A direct optimization method has been developed and utilized to comput e a wide range of optimal low-thrust interplanetary trajectories. This method replaces the optimal control problem with a nonlinear programm ing problem which in turn is solved by using sequential quadratic prog ramming. The direct approach is capable of modeling multiple powered a nd coast arcs, planetary gravity assists, and constant-thrust and vari able-thrust electric propulsion systems. The advantages of the direct approach include reduction in the design space, ease of establishing g ood initial guesses for the design parameters, and improved flexibilit y for handling ''mixed'' optimal control problems with continuous cont rol functions and discrete control parameters. Numerical results are p resented for three interplanetary mission examples and the results fro m the direct method show an excellent match with published optimal tra jectories. In addition, optimal trajectories are obtained for a new lo w-thrust interstellar transfer problem.