VARITOP is designed to optimize interplanetary low-thrust trajectories where the ratio of thrust acceleration to local gravity is high enough to preclude the use of perturbative techniques but low enough that trajectory arcs cannot be approximated using velocity impulses. This program is similar in concept to an impulsive ballistic patched conic program where the planets are assumed massless in calculating the interplanetary transfer trajectory. Planetary positions and velocities are computed from stored analytic conic elements of the nine major planets. Conic elements are available for both numbered and unnumbered asteroids and for short period comets.

Several types of interplanetary missions can be investigated such as rendezvous, flyby, and an orbiter class mission. A number of trajectory parameters such as launch date, arrival date, and the departure and arrival excess speeds can be optimized together with several propulsion and vehicle parameters, such as initial power, specific impulse,, and initial mass. The conditions required to optimize these parameters are derived from the Transversality Conditions that are associated with the Variational Calculus used in the formulation.

Several propulsion options can be handled by VARITOP including the unconstrained variable thrust model initially used by Irving and Blum, a constant specific impulse model, a constant thrust acceleration model, and a solar sail model. These propulsion options are power-limited in the sense that the effective propulsive power is constrained by the input power available to the system. This input power can be either constant such as from a nuclear reactor or variable as a function of heliocentric distance such as from a solar array. The effective thrust force for a solar sail propulsion option is modeled as a function of the size and orientation of the sail and the inverse square force of the solar pressure impinging on the sail.

VARITOP is a low thrust trajectory optimization program that has been used for low thrust mission studies for the past 30 years. VARITOP has been used by NASA/Glenn personnel since 1987. This program is derived from other low-thrust trajectory optimization codes used in preliminary mission studies at JPL in the early 1960's and has been modified extensively to handle various proposed low-thrust propulsion and mission options.

Adapted from: VARITOP User's Guide, JPL