Maneuver Segment
Impulsive Maneuver | Finite Maneuver
The Maneuver segment can be used to model a spacecraft maneuver. While the parameters for Impulsive and Finite Maneuvers are similar in many respects, there are important differences between them that warrant discussing them on individual terms. There are two controls described immediately below that affect what type of maneuver the segment will model. All other maneuver parameters are described as a function of the maneuver type.
Maneuver Type
Select Impulsive or Finite from the Maneuver Type drop-down menu to define the type of maneuver that the segment will execute. You can define parameters for both types for the same segment, but only the type that is selected as the active type will be used when the MCS is run.
The Impulsive Maneuver calculates a state by adding the defined delta-V vector to the velocity of the final state of the previous segment, adds this new state to the ephemeris, and passes it to the next segment.
Seed Finite From Impulsive
You can convert an Impulsive Maneuver segment to a Finite Maneuver with the same properties by clicking Seed Finite From Impulsive. This button is inactive if the Impulsive Maneuver has not yet been run at least once.
How To...Define a Finite Maneuver from an Impulsive Maneuver with Astrogator
Astrogator will apply the results of the last run of the Impulsive Maneuver - based upon the parameters (including the engine model and thruster set) you specified for it - in each place where there is a corresponding parameter for the Finite Maneuver, overwriting any settings you may have previously made. The estimated burn duration of the Impulsive Maneuver will be used to define a duration stopping condition for the Finite Maneuver.
About the Finite Maneuver Seeding Process and Data 
Impulsive Maneuver
An Impulsive Maneuver is comprised of two tabs that you configure to define the maneuver that you want to model:
Attitude
The Attitude tab defines the attitude and delta-V direction of the spacecraft. The Attitude Control that you select determines whether you will specify the attitude or delta-V direction; Astrogator will calculate the other.
More Options.... Opens the More Attitude Options window, which allows you to specify attitude options, including engine acceleration direction, constraint vector, and attitude before and after the maneuver. These options are used to calculate the attitude of a spacecraft during the maneuver if it has not been fully specified by the attitude control.
Attitude Control. The contents of the Attitude page are defined by the Attitude Control setting. Select an Attitude Control option from the drop-down list. The Attitude Control option will determine the attitude parameters available for you to specify. You can select from among:
- Along Velocity Vector -- Attitude is such that the delta-V vector is aligned with the spacecraft's inertial velocity vector.
- Anti-Velocity Vector -- Attitude is such that the delta-V vector is opposite to the spacecraft's inertial velocity vector.
- Attitude -- Attitude is defined using Euler Angles or a Quaternion.
- File -- Import an attitude file to set the maneuver.
- Thrust Vector -- The total delta-V can be specified in cartesian or spherical form with respect to the thrust axes.
Along Velocity Vector and Anti-Velocity Vector
This attitude control setting puts the delta-V direction in the same or opposite direction as the spacecraft's inertial velocity vector (with respect to the central body of the satellite object). The delta-V in the body frame is defined by the thruster set used, or by the engine direction specified in the More Attitude Options window if an engine model is used. The constraint vector in the More Attitude Options window is used to complete the attitude definition.
| Parameter | Description |
|---|---|
| Delta V Magnitude | Enter a value in the selected distance unit per selected time unit, e.g., m/sec. |
Attitude
With this attitude control setting, the rotation between the body axes and the specified reference axes is defined as a set of Euler angles or as a quaternion. Astrogator will find the delta-V direction in the inertial frame using this attitude definition and the thruster direction in the body frame specified either by the thruster set used or, if an engine model is used, by the engine direction specified in the More Attitude Options window.
| Parameter | Description |
|---|---|
| Delta V Magnitude | Enter a value in the selected distance unit per selected time unit, e.g., m/sec. |
| Ref Axes | Click the  ellipsis button and select the reference axes to be used in modeling this maneuver. |
| Euler Angles | Select to enter values for Euler 1st (Phi), Euler 2nd (Theta), and Euler 3rd (Psi), and select a Sequence option. This defines a rotation from the reference axes to the body frame. |
| Quaternion | Select to enter values for the vector components (qx, qy, and qz) and the scalar component (qs) of the quaternion. This quaternion is from the reference axes to the body frame. |
File
The attitude during the maneuver is specified from an attitude file. Astrogator will find the delta-V direction in the inertial frame using this attitude definition and the thruster direction in the body frame specified either by the thruster set used or, if an engine model is used, by the engine direction specified in the More Attitude Options window.
| Parameter | Description |
|---|---|
| Delta V Magnitude | Enter a value in the selected distance unit per selected time unit, e.g., m/sec. |
| Filename | Click the  ellipsis button and select an attitude file to import. |
| File Time Offset | Specify a time offset in the selected time unit. The time offset can be used to adjust the time stored in the attitude file. In other words, the time in an attitude file is specified as seconds from epoch (see Attitude File for more information). If the attitude of a maneuver is controlled by this file, then the attitude at any instant during the maneuver is the same as that specified in the file. If the offset value is, for instance, two seconds, then the attitude during the maneuver is determined as the attitude specified in the file two seconds after the current time. This is effectively the same as increasing the epoch in the file by two seconds. |
Thrust Vector
With this attitude control setting, the delta-V vector is specified in some reference frame, using either Cartesian or spherical components. Astrogator will compute 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. The constraint vector in the More Attitude Options window is used to complete the attitude definition.
| Parameter | Description |
|---|---|
| Thrust Axes | Click the ellipsis button and select the thrust axes to be used in modeling this maneuver. |
| Cartesian | Select this option to enter values for the components displayed for the selected thrust axes (usually X, Y, and Z). This defines the delta-v vector in the reference axes. |
| Spherical |
Select this option to enter values for the azimuth, elevation and magnitude of the delta-v vector. This defines the delta-v vector in the reference axes. Azimuth is the angle between the x-axis and the projection of the vector in the x-y plane, positive towards y. Select Allow Negative Spherical Magnitude to allow the magnitude field of spherical elements to accept negative values. This option may be useful with targeting problems if you are not certain of the direction of the maneuver. When a negative value is used 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 will the negative of the specified elevation. |
Engine
The Engine tab defines the magnitude and the nature of the propulsion. By default, an Impulsive Maneuver has no magnitude.
Engine Model. Select Engine Model, then click the ellipsis button to select the option to be used in modeling this maneuver. Engine Model can be used to quickly model the firing of a single engine.
Thruster Set. Select Thruster Set, then click the ellipsis button to select the option to be used in modeling this maneuver. Use Thruster Set to fire multiple engines simultaneously, and to simulate off-pulsing or complex engine pointing.
Update Mass Based on Fuel Usage. Select whether to have the mass of the spacecraft updated on the basis of fuel usage. This option has no effect on the delta-V itself; the maneuver will stop decreasing fuel mass at zero, but the maneuver will be performed as specified without regard to available fuel. If selected, the mass of the spacecraft will be updated by an approximated delta-M, using the rocket equation.:
V = Veln(m0/mf)
where Ve = exhaust velocity, m0 = initial mass and mf = final mass, and thrust and Isp are held constant to their values at the beginning of the burn. See the technical notes for the derivation of the equation relating change in mass to V.
Finite Maneuver
A Finite Maneuver is comprised of three tabs that you configure to define the maneuver that you want to model:
Attitude
The Attitude tab defines the attitude and thrust direction of the spacecraft. The Attitude Control that you select determines whether you will specify the attitude or thrust direction; Astrogator will calculate the other.
More Options.... Opens the More Attitude Options window, which allows you to specify attitude options, including engine acceleration direction, constraint vector, and attitude before and after the maneuver. These options are used to calculate the attitude of a spacecraft during the maneuver if it has not been fully specified by the attitude control.
Attitude Control. The contents of the Attitude page are defined by the Attitude Control setting. Select an Attitude Control option from the drop-down list. The Attitude Control option will determine the attitude parameters available for you to specify. You can select from among:
- Along Velocity Vector -- Attitude is such that the total thrust vector is aligned with the spacecraft's inertial velocity vector.
- Anti-Velocity Vector -- Attitude is such that the total thrust vector is opposite to the spacecraft's inertial velocity vector.
- Attitude -- Attitude is defined using Euler Angles or a Quaternion.
- File -- Import an attitude file to set the maneuver.
- Plugin -- Attitude is defined using a COM plugin.
- Thrust Vector -- The total thrust vector can be specified in cartesian or spherical form with respect to the thrust axes.
Along Velocity Vector and Anti-Velocity Vector
This attitude control setting puts the total thrust direction in the same or opposite direction as the spacecraft's inertial velocity vector (with respect to the central body of the satellite object). The thrust direction in the body frame is defined by the thruster set used, or by the engine direction in the More Attitude Options window if an engine model is used. The constraint vector in the More Attitude Options window is used to complete the attitude definition.
| Parameter | Description |
|---|---|
| Attitude Update | Select between:
|
Attitude
With this attitude control setting, the rotation between the body axes and the specified reference axes is defined as a set of Euler angles or as a quaternion. Astrogator will find 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 used or, if an engine model is used, by the engine direction in the More Attitude Options window.
| Parameter | Description |
|---|---|
| Attitude Update | Select between:
|
| Ref Axes | Click the ellipsis button and select the reference axes to be used in modeling this maneuver. |
| Euler Angles | Select to enter values for Euler 1st (Phi), Euler 2nd (Theta), and Euler 3rd (Psi), and select a Sequence option. This defines a rotation from the reference axes to the body frame. |
| Quaternion | Select to enter values for the vector components (qx, qy, and qz) and the scalar component (qs) of the quaternion. This quaternion is from the reference axes to the body frame. |
File
The attitude during the maneuver is specified from an attitude file. Astrogator will find 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 used or, if an engine model is used, by the engine direction in the More Attitude Options window.
| Parameter | Description |
|---|---|
| Filename | Click the ellipsis button and select an attitude file to import. |
| File Time Offset | Specify a time offset in the selected time unit. The time offset can be used to adjust the time stored in the attitude file. In other words, the time in an attitude file is specified as seconds from epoch (see Attitude File for more information). If the attitude of a maneuver is controlled by this file, then the attitude at any instant during the maneuver is the same as that specified in the file. If the offset value is, for instance, two seconds, then the attitude during the maneuver is determined as the attitude specified in the file two seconds after the current time. This is effectively the same as increasing the epoch in the file by two seconds. |
Plugin
A plugin is used to specify the attitude during the maneuver. 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 page on IAgGatorPluginAttCtrl interface in the Integration help system. Astrogator will find 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 used or, if an engine model is used, by the engine direction in the More Attitude Options window.
| Parameter | Description |
|---|---|
| Plugin Name | Click the ellipsis button and select a plugin. |
| Plugin Table |
The plugin table lists basic information about each plugin attitude controller added to the segment. You can also specify the plugin attributes using this table.
|
Thrust Vector
With this attitude control setting, the thrust vector is specified in some reference frame, using either Cartesian or spherical components. Astrogator will compute 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. The constraint vector in the More Attitude Options window is used to complete the attitude definition.
| Parameter | Description |
|---|---|
| Attitude Update | Select between:
|
| Thrust Axes | Click the ellipsis button and select the thrust axes to be used in modeling this maneuver. |
| Cartesian | Select to enter values for the components displayed for the selected thrust axes (usually X, Y, and Z). This defines the thrust vector in the reference axes. |
| Spherical |
Select to enter values for the azimuth and elevation of the velocity vector. This defines the thrust vector in the reference axes. Azimuth is the angle between the x-axis and the projection of the vector in the x-y plane, positive towards y. |
Engine
The Engine tab defines the magnitude and the nature of the propulsion.
Engine Model. Select Engine Model, then click the ellipsis button to select the option to be used in modeling this maneuver. Engine Model can be used to quickly model the firing of a single engine.
Thruster Set. Select Thruster Set, then click the ellipsis button to select the option to be used in modeling this maneuver. Use Thruster Set to fire multiple engines simultaneously, and to simulate off-pulsing or complex engine pointing.
Pressure Mode. Select between:
- Pressure-Regulated -- Constant pressure is maintained in the fuel tank through some pressurization mechanism as the propellant mass decreases.
- Blow-Down -- Pressure decreases as propellant is consumed and the volume occupied by the pressurant gas consequently increases. This is based on the ideal gas law.
Thrust Efficiency. Enter the thrust efficiency value. Any number above zero is valid, with typical values around 0.98 to 1.02. a value of 1.0 means 'perfect' or 'nominal' behavior. a value of 0.98 would be termed a '2% cold' burn, while a value of 1.02 would be a '2% hot' burn. A value greater than 1.0 means the engine has performed better than expected.
Thrust. Indicate whether the thrust efficiency value:
- affects acceleration calculations only
- affects acceleration and mass flow calculations
If the thrust efficiency is set to Affects Acceleration Only, then an efficiency of 0.98 means that only 98% of the fuel will be spent to get 98% thrust. For example, a thrust efficiency affecting acceleration only may represent some problem in the combustion chamber. An efficiency that also affects mass flow rate may represent an inefficiency in the propulsion tanks and feed lines.
Propagator
The Propagator tab defines the duration of the maneuver and the method of calculating its ephemeris.
There are two parameters unique to propagation within a finite maneuver - Center Burn and Bias. These parameters are defined immediately below, while the rest of the parameters on the Propagator tab are the same as the parameters of the Propagate segment.
Center Burn. This option is only available for use with a duration stopping condition. If selected, you are setting the maneuver to start half the time before the previous segment ended. This option is useful for ensuring that a finite maneuver seeded from an impulsive maneuver is accurately modeled.
Bias. This field is only available if Center Burn has been selected. Enter the value by which to adjust the centering of the burn. A positive value will center the burn after the previous segment ends by the amount specified in the Burn Center Bias field. The bias may range from minus the duration value to positive half of the duration value.
More Attitude Options
Clicking More Options... will open More Attitude Options window, which presents some or all of the following parameters to be specified, depending on the selected Attitude Control option. These options are used to calculate the attitude during the maneuver if it hasn't already been fully specified with the Attitude Control selection.
| Parameter | Description |
|---|---|
| Scope of Use | Select between:
If the attitude calculated during the maneuver is to be used for other STK functions, then it will be used for the satellite during the time at the maneuver, and during the specified lead and trail times. This attitude will also show up in reports, graphs, sensor access calculations, and in the 3D Graphics window. If the attitude page definition option button is selected, then the actual attitude during the maneuver is ignored, and the satellite is considered to always be in the attitude specified by the page for all other calculations in STK, including the 3D Graphics window. |
| Direction opposite engine exhaust | If an engine model is being used instead of a thruster set, specify whether the engine acceleration (the direction opposite the engine's exhaust) is aligned with positive or negative X, Y or Z body axis. To specify a direction other than along a body axis, use a thruster set - in which case this option is disabled. |
| Constraint Vector | For the attitude control settings that don't fully define the attitude, the spacecraft attitude is aligned with the thrust direction based on the orientation of the engine(s). Use these fields to specify a constraint vector in spacecraft body coordinates to complete the attitude definition. Specify the Representation for the vector:
|
| Attitude Before and After Maneuver | Specify the Lead Duration and Trail Duration in the selected time unit. These fields are used to specify how long before the maneuver starts the maneuver attitude, and how long to maintain that attitude after the maneuver. Outside this time span, the attitude on the Attitude page is used unless there are multiple maneuvers in the MCS.
The attitude during the lead time is held inertially fixed as the attitude at ignition. Similarly, the attitude at the maneuver's end is used as the inertial attitude after the maneuver segment. This attitude may be different than the attitude used when the engine shuts down in the case where a satellite runs out of fuel during a maneuver. The transition from the attitude specified on the Attitude page to the maneuver attitude is modeled as an instantaneous re-orientation. This is also true at the end of the maneuver attitude when transiting back to the Attitude page definition. |
About Thrust Vectors
The term "thrust vector" is used to describe the direction of acceleration applied to the satellite. This direction is opposite to the exhaust of an engine. For example, for a single chemical rocket engine mounted to a satellite, the thrust vector is opposite to the direction of the flames.
If multiple engines are being used together in a thruster set, the thrust vector is along the direction of the overall effective acceleration. This is determined by calculating the acceleration vector of each individual thruster, with both the direction and magnitude. The thrust vector is then calculated along the direction of the vector to be the sum of all the acting acceleration vectors.
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