Gravitational Forces

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FreeFlyer allows the user to include the effects of the built-in or user-defined celestial bodies in the Force Model of a Spacecraft. A simple gravitational potential model (such as a 1x1 or point mass model) will improve performance (i.e., decrease execution time), but will decrease accuracy. For these reasons, simpler models are recommended for long-term analyses or quick lifetime studies. A complex gravitational potential model will increase fidelity, but increase execution time. Complex models are recommended for shorter duration analysis or when execution time is not a primary concern.


FreeFlyer includes a variety of gravitational potential files for the Earth, Moon, Mars, and Venus. For a complete list of the gravity models included with FreeFlyer, see the Gravity Model Files page in the Data Files section of the Appendix.


The total force on the spacecraft can be viewed using the Spacecraft.Acceleration property. Additionally, individual components of the total force on the Spacecraft (drag, Earth gravity, solar radiation pressure, etc) can be viewed separately with the Spacecraft.Accelerations array. In the example below, the individual forces due to Earth gravity, Moon gravity, and Sun gravity are displayed. The total force on the Spacecraft is shown using Spacecraft.Acceleration property, and computed using the individual components of the forces from the Spacecraft.Accelerations property.


For i = 0 to Spacecraft1.Accelerations.Count-1;


      // Report the value of each acceleration component:

      Report Spacecraft1.Accelerations[i].Name, Spacecraft1.Accelerations[i].Acceleration[0].Format("%e"),




      // Manually compute the total acceleration on the Spacecraft:

      totalAcceleration = totalAcceleration + Spacecraft1.Accelerations[i].Acceleration;




// Report the total acceleration:

Report "Spacecraft.Acceleration: ", Spacecraft1.Acceleration;

Report "totalAcceleration: ", totalAcceleration;


The example script above generates the following output when the Force Model only accounts for forces due to the Earth, Sun, and Moon gravity.



To learn more about modeling the gravitational forces of Earth and other celestial bodies, continue to the following pages:

Point Masses

Zonal and Tesseral Potential Terms

Zonal and Tesseral Potential with Solid Tides



See Also

ForceModel Properties and Methods

Solar System settings that affect Spacecraft Propagation