Swingby

GSFC mission design capabilities have significantly improved over the last decade. The success of GSFC support is based in an accurate numerical computational regime. Before 1990, mainframe computers were the only resource to compute high fidelity trajectories for libration orbits. The software of choice at that time was the Goddard Mission Analysis System (GMAS). This software had complete optimization functionality as well as the capability to model all the required perturbing forces. The software was unique at the time since it allowed object modules to be linked into the run sequence as a way to allow the user access to data for trajectory analysis. During the early 1990’s, the GSFC operational PC program called SWINGBY was developed. SWINGBY was developed as a replacement for GMAS but with an interactive graphical user interface to provide instantaneous feedback of the trajectory design in multiple coordinate systems. It was designed to be a generic tool to support a variety of missions including, lunar, planetary, libration, and deep space and of course gravity assisted trajectory designs. SWINGBY provides complete mission analysis and operations for the WIND, SOHO, ACE, and was used for TRIANA analysis and MAP. Additionally, the lunar orbiter missions of Lunar Prospector and Clementine also used Swingby for mission design and maneuver planning. With the unprecedented success of SWINGBY, GSFC invested in a COTS program called Astrogator, produced by Analytical Graphics Inc. that is based on Swingby design and mathematical specifications.

It is important that trajectories be modeled accurately. The software must integrate spacecraft trajectories very accurately and model both impulsive and finite maneuvers. SWINGBY allows this by incorporating various high order variable or fixed step numerical integrators (Runge Kutta8/9, Cowell 12th order, and Bulirsch-Stoer). Precise force modeling includes up to 100x100 Earth and lunar gravity potentials, solar radiation pressure (also as a Solar Sail capability), multiple 3rd-body perturbation effects and an atmospheric drag model. Varying user-selected parameters to achieve the required goals performs trajectory targeting and optimization. A shooting method using a differential corrector (DC) is widely used to achieve orbit goals in these programs although both provide the user with a limited Quasi-Newton / Steepest Descent method. All three methods use numerical partial derivatives to calculate the direction for convergence. The DC in SWINGBY uses the first derivative information. The partial derivatives are calculated by numerically propagating to the stopping condition, changing the independent variable with a small perturbation and re-propagating. The change in the goals divided by the change in the variables are used to compute the partials. The usual sequence of a forward shooting method is to vary the initial conditions though predefined perturbations. The initial conditions include the orbital initial conditions, an applied delta V, or spacecraft design parameter to meet goals that include orbital parameters such as period, position, velocity, amplitude, etc. SWINGBY includes B-plane and rotating coordinate targets. This software tool is excellent for prelaunch analysis including error analysis, launch window calculations, finite engine modeling, and ephemeris generation.