The trajectory optimization program is based upon the zero sphere of influence patched conic model of the solar system; that is, heliocentric trajectories are computed under the assumption that the sun is the only attracting body. Once the terminal heliocentric velocities are known, the velocities relative to the terminal body are evaluated and equated to planetocentric conditions at infinity. This method is alternatively referred to as "matched asymptotes ". It is generally regarded as sufficiently accurate for purposes of preliminary mission analysis and for propulsion system sizing and selection. This model is virtually identical to that employed in MAnE.
Trajectory optimization is achieved with the indirect techniques of the Calculus of Variations. While the solution to the boundary value problem that results with this approach is considered by many to be overly difficult, the robust iterator that is used in the program, combined with the analytical process expounded in the Mission Analyst’s Guide substantially reduces the severity of the problem. Advantages of indirect techniques are that convergence tends to be quadratic in the neighborhood of the solution and that convergence assures that the solution is locally stationary.
The ultimate boundary value problem to be solved is comprised of an equal number of independent variables and end conditions. The end conditions are, in turn, comprised of the constraints that are specified for the problem and the applicable transversality, or optimality, conditions for the problem posed. The transversality conditions are automatically formulated within HILTOP so that the user need not be concerned with specifying those that apply. This is but one of the many user-friendly features encoded in HILTOP to significantly reduce the learning curve for productive use.