OPTIMIZATION OF VERY-LOW-THRUST, MANY-REVOLUTION SPACECRAFT TRAJECTORIES

Optimal minimum flight time solutions are obtained for continuous, ver y-low-thrust orbit transfers using a direct-transcription approach to convert the continuous optimal control problem into a nonlinear progra mming problem. The thrust accelerations used are characteristic of sol ar electric and nuclear electric propulsion, resulting in trajectories that require many revolutions of Earth to achieve the desired final o rbits. Among the problems examined are transfers from low Earth orbit to geosynchronous orbit (GEO) and orbit raising from GEO to a specifie d radius. All initial and terminal orbits are circular, with motion co nstrained to the equatorial plane. Motion of the spacecraft is describ ed using the equinoctial orbit elements. The variation of spacecraft m ass and acceleration due to fuel consumption is modeled. The orbit tra nsfers include the effect of Earth's oblateness through first order as well as third-body perturbations from the moon.