Optimal low Earth orbit transfers with drag sails

Satellite operations in low Earth orbit will be extremely frequent in the near future, and consequently the optimization of trajectories between such orbits is becoming of extreme interest. Low-thrust electric propulsion will arguably be the preferred option for many missions, as it provides benefits in terms of propellant consumption. Low-thrust trajectories in low Earth orbit require several revolutions around the Earth and they are carried out in an environment where the two-body problem approximation is not suited to describe the motion of a satellite. The objective of this thesis is to illustrate a general methodology to take into account the effects of the oblateness of the Earth and aerodynamic drag, and obtain significant solutions to the minimum-time and minimum-propellant problems. An indirect optimization approach is applied to transfers between almost circular low Earth orbits, considering the effects of drag and the asphericity of the Earth. The exploitation of drag sails is investigated in a case study, considering a small 15-kg spacecraft in an initial orbit similar to that of the International Space Station. Depending on the maximum frontal area of the sail, minor to significant improvements can be obtained in terms of minimum-time. The propellant consumption is improved significantly for a negative altitude change and in some cases zero-consumption maneuvers are achieved by extracting the sail at the right time.