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The propulsion model in FreeFlyer allows for comprehensive thruster and tank simulation. Using a combination of Tank, Thruster, and Valve objects, most chemical and electrical propulsion systems can be modeled. A thrust event object (either FiniteBurn, ImpulsiveBurn, or OutgassingEvent) is applied to a Spacecraft by the Maneuver command.


The Maneuver command must be used with either a FiniteBurn, ImpulsiveBurn, or OutgassingEvent object.


There are a variety of Sample Mission Plans (included with your FreeFlyer installation) that demonstrate various applications of these topics. Continue to the Maneuvering Samples page to view descriptions and images of these examples.



General Procedure for Maneuver Modeling in FreeFlyer

Each of these steps can be performed through either the Object and Command Editors (GUI) or through FreeForm ScriptEditors.


Create and Edit any Tank objects; Attach to Spacecraft

Create and Edit any Thruster objects; Attach to Spacecraft

Attach Tank(s) to Thruster(s), specifying Valves if necessary

Create and Edit ImpulsiveBurn, FiniteBurn, and/or OutgassingEvent object(s)

oImpulsiveBurns and OutgassingEvents do not require hardware to be configured on the Spacecraft

Apply the thrust event(s) using the Maneuver command.

oThe Spacecraft.Maneuver() method can also be used in place of a Maneuver command.



Introduction to Burns

Impulsive Burns

If the Maneuver is performed using an ImpulsiveBurn object, the Spacecraft’s velocity is instantaneously changed by the amount specified by the ImpulsiveBurn. In addition, the mass of the spacecraft is depleted by the amount calculated from the rocket equation based on the user-specified Specific Impulse (Isp).


Finite Burns

If the Maneuver is performed using a FiniteBurn object, the finite thrusts generated by the active Thrusters are included when integrating the Spacecraft’s equations of motion. The mass remaining in the Tanks that feed the active thrusters is calculated by integrating the mass depletion equation. Tank pressure is adjusted as the mass is depleted using the ideal gas law. The Thrust and Specific Impulse (Isp), which are considered to be functions of the TankPressure, change as the pressure changes. A FiniteBurn can only be performed if at least one Tank and one Thruster have been configured to the Spacecraft.


Outgassing Events

If the Maneuver is performed using an OutgassingEvent, accelerations specified by the OutgassingEvent are included when integrating the Spacecraft’s equations of motion. The OutgassingEvent is modeled according to an analytic exponential decay model based on the user-specified time constant and does not deplete mass.



See Also

Attach Command

Maneuver Command