The design capacity for synergetic gravity assists (powered flyby’s) changes possible types of optimal interplanetary trajectories. The application of Particle Swarm Optimization (PSO) is used to determine optimal mission trajectories from Earth to planets of interest, via synergetic gravity assist maneuvers. In order to verify the design results from PSO, past missions are re-examined from a new design perspective. Test results are obtained for Voyager 1, Voyager 2, and Cassini. The results closely resemble those of actual mission data, providing support for the new design method involving PSO and synergetic gravitational assists. The computation of these solutions offers the significant benefit of costing one to two minutes of wall-clock time with standard desktop or laptop computing systems. In addition to these past missions, the work then extends the design method to a newly proposed multiple gravity-assist (MGA) mission from Earth to Saturn that could take place within the next few years. The best solutions from PSO for the MGA routes are on the order of one half to one third the propellant cost as compared to the direct routes for the launch and arrival dates chosen.