ABOUT COMBINING TISSERAND GRAPH GRAVITY-ASSIST SEQUENCING WITH LOW-THRUST TRAJECTORY OPTIMIZATION

Gravity-assist maneuvers have the potential to be mission enablers, due to "free energy" they provide. The efficiency of low-thrust propulsion is further one means of improving mission payload mass. Combining both for a given mission possibly improves overall mission performance, which makes it desirable to investigate low-thrust gravity-assist missions. For means of investigating a broad range of mission options, the System Analysis Space Segment department of DLR is working on methods of combining the optimization of low-thrust trajectories and gravity-assist sequences with the help of the Tisserand Criterion and shape-based trajectory models. The hurdles faced by violations of Tisserand Criterion premises are shortly discussed along with their repercussions on planning a gravity-assist sequence for a low-thrust mission. A methodology, based on benchmarking the results with nongravity-assist trajectories is presented in this paper, grounded on solution populations combining the optimization of the trajectory and the selection of the next gravity-assist partner. Furthermore it is shown how the solution space can be reduced with the help of constraints originating in the maximum possible Δv gain and the gravity-assist partner pool.