| Double lunar swingby (DLS) escape trajectories employing a multi-month Solar-Perturbed Loop (SPL) between swingbys can be very beneficial for trajectory performance, especially for lower energy objectives, such as near-Earth asteroids and comets. However, trajectory constraints approaching and departing the swingby that terminates the SPL can result in a large propulsive maneuver at that swingby, negating the benefits of this approach compared to direct trajectories.;Introducing a propulsive impulse during the SPL adds degrees of freedom usable to reduce the total propulsive cost. An indirect or "primer vector" approach is used to develop a first-order necessary condition for cost reduction in the time-fixed problem, using the unpowered SPL as the reference trajectory, and to find the gradients of the cost and constraint functions with respect to time and position of the included impulse. The cost function introduced here uses a simplified model of the ;The solution space in which unpowered SPL's exist is characterized, using sun-phase angle at the second swingby (S2), time-of-flight between swingbys, and flight path angle at S2 as key parameters. Regions in this space for which a first-order cost reduction is possible are identified. Full optimization of selected trajectories is performed with view to gaining insight into the effect of the included impulse, and finding conditions that the mission designer should seek or avoid while constructing these trajectories. |
