Multiple fly-by trajectories
Multiple fly-by trajectories i.e. gravity-assist trajectories for interplanetary missions. When travelling among the planets, one needs to minimize the propellant mass needed by the spacecraft and its launch vehicle. The amount of propellant needed depends largely on the chosen route. Gravity assisted flyby maneuvers can be used to transfer angular momentum from a planet to a spacecraft approaching the planet from behind in it's progress about the sun. Finding the right order and exact nature of such flyby maneuvers introduces a combinatorial component into the overall fuel minimization problem, making it exceedingly difficult to solve.