Finite Maneuver Segment

The Finite Maneuver segment has three tabs that define its parameters: Attitude, Engine, and Propagator.

Attitude

The Attitude Control setting enables you to select the mode in which STK will prescribe the maneuver pointing direction. The following subsections describe each of the available Attitude Control options and their associated parameters for finite maneuvers.

Acceleration History File

Use this option to select an acceleration history file that describes how the maneuver will execute. Astrogator will determine the maneuver's duration from the file and will assume that the first data point is the start of the maneuver. When you select this option, the Engine and Propagator tabs are not available because the file completely describes the maneuver already. Astrogator will determine the attitude of the satellite during the maneuver by pointing the satellite such that the engine is aligned with the acceleration vector specified in the file and aligning it according to the constraint vector you specify in the More Options dialog box.

Parameter	Description
Filename	Click the ellipsis button and then browse to and select the file containing the acceleration history data.

Along Velocity Vector / Anti-Velocity Vector

The satellite object’s attitude is defined as the Delta-V vector aligned with or opposite to the spacecraft's inertial velocity vector. The inertial reference frame depends on the central body of the satellite object. Either the thruster set or an engine model defines the Delta-V in the body frame. You can use More Options to specify the engine direction in the engine model. Finally, the constraint vector in More Options completes the attitude definition.

Parameter	Description
Attitude Update	You can specify how the maneuver pointing and, potentially, the spacecraft's attitude are updated. The options are: Inertial at ignition - The maneuver pointing is defined by the Attitude Control parameters at ignition and remains unchanged throughout the maneuver. The beginning of a segment is always the end of the previous segment. However, the ignition point may be different than the beginning of the Maneuver segment if using burn centering; ignition refers to the time that the engine starts firing. This approach is used for inertially fixed spacecraft and fixes the thrust direction in the inertial direction that is calculated for the beginning of the burn. Inertial at start - The maneuver pointing is defined by the Attitude Control parameters at the beginning of the Maneuver segment and remains the same throughout the maneuver. This approach is used for inertially fixed spacecraft and fixes the thrust direction in the inertial direction that is calculated for the beginning of the burn. Update during burn – The maneuver pointing is updated throughout the maneuver in order to maintain the required thrust direction in the specified direction at every instant throughout the burn. Therefore, the thrust vector either rotates with the specified coordinate system (with the Thrust Vector option) or tracks with the spacecraft's inertial velocity vector (with the Along Velocity or Anti-Velocity Vector option).

Parameter

Description

Attitude Update

You can specify how the maneuver pointing and, potentially, the spacecraft's attitude are updated. The options are:

Inertial at ignition - The maneuver pointing is defined by the Attitude Control parameters at ignition and remains unchanged throughout the maneuver. The beginning of a segment is always the end of the previous segment. However, the ignition point may be different than the beginning of the Maneuver segment if using burn centering; ignition refers to the time that the engine starts firing. This approach is used for inertially fixed spacecraft and fixes the thrust direction in the inertial direction that is calculated for the beginning of the burn.
Inertial at start - The maneuver pointing is defined by the Attitude Control parameters at the beginning of the Maneuver segment and remains the same throughout the maneuver. This approach is used for inertially fixed spacecraft and fixes the thrust direction in the inertial direction that is calculated for the beginning of the burn.
Update during burn – The maneuver pointing is updated throughout the maneuver in order to maintain the required thrust direction in the specified direction at every instant throughout the burn. Therefore, the thrust vector either rotates with the specified coordinate system (with the Thrust Vector option) or tracks with the spacecraft's inertial velocity vector (with the Along Velocity or Anti-Velocity Vector option).

Attitude

Using this option, attitude is the rotation between the body axes and the selected reference axes, specified either as a set of Euler angles or as a quaternion. Astrogator finds the Delta-V direction in the inertial frame using this attitude definition and the thruster direction in the body frame. The thruster direction is determined either by the thruster set used or the engine direction specified in More Options

Parameter	Description
Attitude Update	See the options defined in Along Velocity Vector / Anti-Velocity Vector.
Ref Axes	These are the reference axes used in modeling this maneuver.
Euler Angles	These define a rotation from the reference axes to the body frame; enter values for angles 1, 2, and 3, and select a Sequence option.
Quaternion	The vector components (qx, qy, and qz) and the scalar component (qs) define the quaternion from the reference axes to the body frame.

File

In this option, you provide an imported attitude file. Astrogator finds the Delta-V direction in the inertial frame using this attitude definition and the thruster direction in the body frame. The file is specified by either the thruster set or, if using an engine model, by the engine direction specified in More Options.

Parameter	Description
Filename	Click the ellipsis button and then browse to and select the file containing the attitude data.
File Time Offset	This is a time offset relative to the file time. For example, if the offset value is two seconds, then the attitude for the maneuver at the current simulation time in STK is restricted from the file at the simulation time plus two seconds. An STK time of 12:00:00.000 would use the information in the file for 12:00:02.000. This is the same as increasing the epoch in the file by two seconds. See Attitude File for more information.

Parameter

Description

Filename

Click the ellipsis button and then browse to and select the file containing the attitude data.

File Time Offset

This is a time offset relative to the file time. For example, if the offset value is two seconds, then the attitude for the maneuver at the current simulation time in STK is restricted from the file at the simulation time plus two seconds. An STK time of 12:00:00.000 would use the information in the file for 12:00:02.000. This is the same as increasing the epoch in the file by two seconds. See Attitude File for more information.

Plugin

Attitude is defined by an external plugin. The plugin can specify a set of Euler angles or a quaternion from some reference axes to the body frame. For more information, see the IAgGatorPluginAttCtrl interface page in the STK Programming Help. Astrogator finds the thrust direction in the inertial frame using this attitude definition and the thrust direction in the body frame, specified either by the thruster set or, if using an engine model, by the engine direction specified in More Options.

Parameter	Description
Plugin Name	This is the desired external plugin.
Plugin Table	This is basic information about each attitude controller plugin added to the segment, including Name, Value, and Description.

Thrust Vector

With this attitude control setting, you specify the Delta-V vector in some reference frame, using either Cartesian or spherical components. Astrogator then computes the attitude so that the total thrust vector in the body frame, as specified by the thruster set or engine model, aligns with this vector in the reference axes. Astrogator uses the constraint vector in More Options to complete the attitude definition.

To allow independent changes of a finite maneuver's Cartesian thrust vector components, and to preserve your input, Astrogator does not automatically enforce normalization of the vector, but defers this to later. The vector is normalized before being used internally. To preview this normalization, noneditable, informational user interface fields reflect the unit vector.

Parameter	Description
Attitude Update	See the options defined in Along Velocity Vector / Anti-Velocity Vector.
Thrust Axes	This is the set of axes in which the thrust direction is specified.
Cartesian	These are the components of the selected thrust axes (usually X, Y, and Z) that define the Delta-V vector in the reference axes.
Spherical	These are the azimuth, elevation, and magnitude that define the Delta-V vector in the reference axes. Azimuth is the angle between the X axis and the projection of the vector in theX-Y plane, positive toward Y. Allow Negative Spherical Magnitude – If you select this, the magnitude field of spherical elements is allowed to accept negative values. This option may be useful with targeting problems if you are uncertain of the direction of the maneuver. When using a negative value for magnitude, the Delta-V is in the opposite direction of the specified azimuth and elevation. The actual azimuth will be 180 degrees from the specified azimuth, and the actual elevation is negative of the specified elevation.

Time Varying

With this attitude control setting, the thrust vector is specified in some reference frame using time-varying representations of the spherical azimuth and elevation angles. The independent variable in the associated expressions is the elapsed maneuver time. Astrogator computes the attitude so that the total thrust vector in the body frame, as specified by the thruster set or engine model, is aligned with this vector in the reference axes. Astrogator uses the constraint vector in More Options to complete the attitude definition.

Parameter	Description
Thrust Axes	This is the set of axes in which the thrust direction is specified.
Azimuth and Elevation Coefficients	Az0 / El0 - the constant polynomial term Az1 / El1 - the linear polynomial term Az2 / El2 - the quadratic term Az3 / El3 - the cubic term Az4 / El4 - the quartic term AzA / ElA - the amplitude of the sine term AzF / ElF - the frequency of the sine term AzP / ElP - the phase for the sine term Azimuth is the angle between the X axis and the projection of the vector in the X-Y plane, positive toward Y. Elevation is measured upwards from the X-Y plane.

Parameter

Description

Thrust Axes

This is the set of axes in which the thrust direction is specified.

Azimuth and Elevation Coefficients

Az0 / El0 - the constant polynomial term
Az1 / El1 - the linear polynomial term
Az2 / El2 - the quadratic term
Az3 / El3 - the cubic term
Az4 / El4 - the quartic term
AzA / ElA - the amplitude of the sine term
AzF / ElF - the frequency of the sine term
AzP / ElP - the phase for the sine term

Azimuth is the angle between the X axis and the projection of the vector in the X-Y plane, positive toward Y. Elevation is measured upwards from the X-Y plane.

Engine

The Engine tab defines the magnitude and the nature of the propulsion.

Parameter	Description
Engine Model	Use Engine Model to quickly model the firing of a single engine. For more information, see Engine Model.
Thruster Set	Use Thruster Set to fire multiple engines simultaneously and to simulate off-pulsing or complex engine pointing. For more information, see Thruster Set.
Pressure Mode	Pressure-Regulated indicates that constant pressure is maintained in the fuel tank as the propellant mass decreases. Blow-Down indicates that pressure decreases in accordance with the Ideal Gas Law as propellant is consumed.
Thrust Efficiency	This is the Thrust Efficiency value. Any number above zero is valid, with typical values around 0.98 to 1.02. A value = 1.0 means "perfect" or "nominal" behavior. A value of 0.98 is a "2% cold" burn, while a value of 1.02 would be a "2% hot" burn. A value >1.0 means the engine has performed better than expected.
Thrust	This setting determines whether the thrust efficiency value will affect acceleration calculations only (Affects Accel Only) or will affect both acceleration and mass flow calculations (Affects Accel and Mass Flow). For example, if the thrust efficiency is set to Affects Accel Only, then an efficiency of 0.98 means that only 98% of the allotted fuel is spent, resulting in 98% of the desired thrust. A thrust efficiency affecting acceleration only may represent some problem in the combustion chamber. An efficiency that additionally affects mass flow rate may represent an inefficiency in the propulsion tanks and feed lines.

Propagator

The Propagator tab enables you to define the duration of the maneuver and the method for calculating its ephemeris. Most of its parameters are the same as in the Propagate segment. However, there are two parameters unique to propagation within a finite maneuver: Center Burn and Bias. These parameters are defined in this table.

Parameter	Description
Center Burn	If you select this, and when possible, the maneuver is centered on the maneuver segment start time. Maneuver propagation begins from an interpolated or actual state in the ephemeris occurring prior to the maneuver segment start time. By default (i.e., with no bias), the time of the "ignition" state is the maneuver segment start time minus one-half of the burn centering time. This option is useful for ensuring that a finite maneuver seeded from an impulsive maneuver is accurately modeled. This option is only available for use with a duration stopping condition.
Bias	This value adjusts the center time of the burn. A positive value centers the burn after the previous segment ends by the amount specified in the Burn Center Bias field. The bias ranges from [-1 * Duration, + 1/2 * Duration]. This field is only available if you selected Center Burn. When adjusting the Bias value so that the burn center occurs earlier in time (negative value), it may be possible to place your satellite before previously executed stopping conditions. This could result in segment terminations earlier than expected.

Parameter

Description

Center Burn

If you select this, and when possible, the maneuver is centered on the maneuver segment start time. Maneuver propagation begins from an interpolated or actual state in the ephemeris occurring prior to the maneuver segment start time. By default (i.e., with no bias), the time of the "ignition" state is the maneuver segment start time minus one-half of the burn centering time.

This option is useful for ensuring that a finite maneuver seeded from an impulsive maneuver is accurately modeled. This option is only available for use with a duration stopping condition.

Bias

This value adjusts the center time of the burn. A positive value centers the burn after the previous segment ends by the amount specified in the Burn Center Bias field. The bias ranges from [-1 * Duration, + 1/2 * Duration]. This field is only available if you selected Center Burn.

When adjusting the Bias value so that the burn center occurs earlier in time (negative value), it may be possible to place your satellite before previously executed stopping conditions. This could result in segment terminations earlier than expected.