GTO to GEO Optimal Trajectory Profiles and Electric Propulsion System Configuration

Sammanfattning

Quick and reliable computational methods for optimized orbital transfers are crucial for projects at preliminary stages. They enable an initial, realistic sizing of the propulsion subsystem, one of the major components of satellite design. This thesis work, conducted at ReOrbit Oy, presents a minimum-time optimal trajectory for the orbit raising of a micro-satellite from GTO to GEO, assuming continuous firing by electric propulsion. The Delta-v requirements resulting from this simulation lead to the selection of an appropriate electric propulsion system, elaborating on the design of its configuration in terms of fuel and thrust requirements. This is done by taking into account, other than the major contribution given by the orbit raising, additions due to in-orbit maneuvers performed twice a day over a 10-year lifetime, like station-keeping corrections and reaction wheels desaturation. The optimization method is a direct-indirect hybrid for low-thrust orbital maneuvers, employing Pontryagin’s Minimum Principle for the transcription into a nonlinear programming problem. The initial guess required to start the optimizer is obtained with Lyapunov control theory. An orbital averaging technique is implemented, enabling fast computation of multiple trajectories during the optimization. Disturbances from the J2 zonal harmonic, solar radiation pressure, third-body effects of the Sun and Moon, and atmospheric drag up to 1500 km of altitude are included in the dynamic model. Eclipse conditions are assessed with a cylindrical shadow model, as the solar electric propulsion experiences a zero thrust period when in Earth’s shadow. The electric propulsion system configuration is determined with trade-off studies and comparisons between different suppliers. The chosen outline includes 4 Xenon thrusters, with complementary power processing units and propellant management systems, resulting in a total transfer time of less than 4 months. The same propulsion system is employed both for the transfer trajectory and the in-orbit maneuvers, by changing the thruster’s configuration once in GEO.