  Space missions that use low-thrust propulsion technology are becoming
increasingly popular since they utilize propellant more efficiently and thus
reduce mission costs. However, optimizing continuous-thrust trajectories is
complex, time-consuming, and extremely sensitive to initial guesses. Hence,
generating approximate trajectories that can be used as reliable initial
guesses in trajectory generators is essential. This paper presents a
semi-analytic approach for designing planar and three-dimensional trajectories
using Hills equations. The spacecraft is assumed to be acted upon by a constant
thrust acceleration magnitude. The proposed equations are employed in a
Nonlinear Programming Problem (NLP) solver to obtain the thrust directions.
Their applicability is tested for various design scenarios like orbit raising,
orbit insertion, and rendezvous. The trajectory solutions are then validated as
initial guesses in high-fidelity optimal control tools. The usefulness of this
method lies in the preliminary stages of low-thrust mission design, where speed
and reliability are key.
