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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.
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.
Introduction to 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).
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.
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.