Part 16: Design Trajectories with Astrogator

STK Premium (Space) or STK Enterprise
You can obtain the necessary licenses for this tutorial by contacting AGI Support at support@agi.com or 1-800-924-7244.

This lesson requires an STK 12.7 or newer to complete in its entirety.

The results of the tutorial may vary depending on the user settings and data enabled (online operations, terrain server, dynamic Earth data, etc.). It is acceptable to have different results.

Capabilities covered

This lesson covers the following STK Capabilities:

  • STK Pro
  • Astrogator

Problem

Engineers and operators require a quick way to design high-fidelity spacecraft trajectories for mission planning and operations. In this scenario, you will design a launch from a launch pad and successfully inject a satellite into a geosynchronous equatorial orbit (GEO).

Solution

Use STK's Astrogator capability to design:

  • A basic launch phase to place a satellite into a parking orbit (low Earth orbit or LEO)
  • A transfer orbit injection (TOI) maneuver to transfer from LEO to GEO
  • A synchronized orbit injection (SOI) maneuver to circularize the orbit at GEO

What you will learn

Upon completion of this tutorial, you will have a basic understanding of:

  • Astrogator

  • The Launch Vehicle object

Video guidance

Watch the following video. Then follow the steps below, which incorporate the systems and missions you work on (sample inputs provided).

Creating a New Scenario

First, you must create a new STK scenario; then build from there.

  1. Launch STK ().
  2. Click Create a Scenario in the Welcome to STK dialog box.
  3. Enter the following in the New Scenario Wizard:
    4. Option Value
    Name: STK_Astrogator
    Location: Default
    Start: Default
    Stop: + 5 days
  4. Click OK when you finish.
  5. Click Save () after the scenario loads. STK creates a folder with the same name as your scenario for you.
  6. Verify the scenario name and location in the Save As dialog box.
  7. Click Save.

Save ( ) often during this lesson!

Updating the Insert STK Objects tool

Ensure the Launch Vehicle () appears in the Insert STK Objects tool.

  1. Click Edit Preferences... in the Insert STK Objects tool.
  2. Select the Launch Vehicle check box when the Preferences dialog box opens.
  3. Click OK to close the Preferences dialog box.

Creating a Launch Vehicle

Insert a Launch Vehicle () object.

  1. Select Launch Vehicle () in the Insert STK Objects tool.
  2. Select the Insert Default () method.
  3. Click Insert....
  4. Right click on LaunchVehicle1 () in the Object Browser.
  5. Select Rename in the shortcut menu.
  6. Rename LaunchVehicle1 () to LaunchToLEO.

Selecting a Simple Ascent propagator

Use the simple ascent propagator to define the ascent trajectory from a launch point to an orbit insertion point. The simple ascent propagator creates an ascent trajectory based on launch and insertion parameters. The trajectory is a simple curve rising vertically from the launch pad that turns over smoothly to insert the launch vehicle into orbit with a zero flight path angle at the insertion point using the specified velocity.

  1. Open LaunchToLEO's () properties ().
  2. Select the Basic - Trajectory page.
  3. Ensure Propagator is set to SimpleAscent.
  4. Enter 7.3 km/sec in the Burnout Velocity field. This will keep the resulting orbit near circular.
  5. Click Apply to accept your change and to keep the Properties Browser open.
  6. Select the 2D Graphics - Attributes page.
  7. Change the Color: to teal.
  8. Click OK to accept your change and to close the Properties Browser.

Viewing the launch vehicle trajectory

View LaunchToLEO's () launch vehicle trajectory and ground track in the 3D Graphics window.

  1. Bring the 3D Graphics window to the front.
  2. Zoom To LaunchToLEO ().
  3. Use your mouse to zoom out so you can see the launch vehicle trajectory and ground track.

    4. Launch Vehicle Trajectory

  4. Notice that the Launch Vehicle's () default location is Cape Canaveral.
  5. Notice that the trajectory ends at burnout.

Inserting a satellite

Insert a Satellite () object that you will use to create the satellite orbit.

  1. Insert a Satellite () object using the Insert Default () method.
  2. Rename Satellite1 () to GEO_Sat.

Using Astrogator

Set GEO_Sat's () propagator to Astrogator. You will use Astrogator to design your spacecraft trajectory. The STK Astrogator capability contains specialized analysis for interactive orbit maneuver and spacecraft trajectory design.

  1. Open GEO_Sat's () properties ().
  2. Select the Basic - Orbit page.
  3. Open the Propagator: shortcut menu.
  4. Select Astrogator.

Setting up the Mission Control Sequence

The Mission Control Sequence (MCS) is the core of your space mission scenario. The MCS functions as a graphical programming language, in which mission segments dictate how Astrogator calculates the trajectory of the spacecraft based on the general settings that you specify for the MCS itself.

The MCS is defined by selecting and organizing MCS Segments in a manner that produces your desired trajectory. By default, an Astrogator satellite's MCS contains two segments: an Initial State () segment and a Propagate () segment.

Deleting the Initial State segment

Since you are modeling a Launch Vehicle (), remove the Initial State () segment. You will replace it with a Follow () segment .

  1. Select Initial State () in the MCS.
  2. Click Delete Segment () in the MCS toolbar.
  3. Click Yes to confirm deletion.

Adding a Follow segment

Use the Follow () segment to set GEO_Sat () to follow LaunchToLEO (), and then separate from LaunchToLEO () at the end of its ephemeris.

  1. Right click on Propagate () MCS.
  2. Select Insert Before... .
  3. Select Follow () in the Segment Selection dialog box.
  4. Click OK to close the Segment Selection dialog box.
  5. Select Follow () in the MCS.
  6. Select the General tab.
  7. Click the Leader ellipsis ().
  8. Select LaunchToLEO () in the Select Leader dialog box.
  9. Click OK to close the Select Leader dialog box.
  10. Open the Joining: shortcut menu in the Additional Options frame.
  11. Select Join at End of Leader's Ephemeris.

    12. By selecting this joining parameter, GEO_Sat () uses the LaunchToLEO's () final ephemeris point as the initial and final state of the Follow Segment. Also, the separation parameter is automatically set to Separate at End of Leader's Ephemeris.

Setting the Fuel Tank configuration

Set GEO_Sat's () fuel mass and its maximum fuel mass.

  1. Select the Fuel Tank tab.
  2. Set the following in the order shown:
    3. Option Value
    Maximum Fuel Mass 6000 kg
    Fuel Mass 5000 kg
  3. Click Apply to accept your changes and to keep the Properties Browser open.

Running the Mission Control Sequence

Run the Mission Control Sequence to calculate the trajectory of the spacecraft.

  1. Select Propagate () in the MCS.
  2. Note the current stopping condition is Duration, with a Trip value of 43200 sec (0.5 day).
  3. Click Run Entire Mission Control Sequence () in the MCS toolbar.
  4. Bring the 3D Graphics window to the front.
  5. Click Home View () to view GEO_Sat's () trajectory.

Satellite in a Low Earth Orbit

Specifying the satellite engine model

Use a realistic engine model in order to produce accurate results.

  1. Return to GEO_Sat's () propterties ().
  2. Click Component Browser () in the MCS Toolbar.
  3. Open the Show Component Type shortcut menu.
  4. Select Astrogator Components.
  5. Select Engine Models () in the View All Astrogator Components list.
  6. Select Constant Thrust and Isp () in the Engine Models list on the right.
  7. Click Duplicate component () in the Engine Models toolbar.
  8. Type Test Engine in the Name: field when the Field Editor dialog box opens.
  9. Click OK to close the Field Editor dialog box.
  10. Double-click Test Engine () in the Engine Models list.

Modifying Constant Thrust and Isp

Modify the Constant Thrust and Isp engine model to specify the thrust and Isp for your engine.

  1. Double-click Thrust when the Test Engine dialog box opens.
  2. Enter 13500 N in the Real Number: field when the Real Number Field dialog box opens.
  3. Click OK to close the Real Number Field dialog box.
  4. Double-click Isp in the Test Engine dialog box.
  5. Enter 2000 s in the Real Number: field when the Real Number Field dialog box opens.
  6. Click OK to close the Real Number Field dialog box.

    7. This high value will only affect the amount of fuel that the satellite will burn through over the course of the maneuvers. You are doing this so that you do not need to modify the spacecraft mass values several times over the course of the mission.

Adding a component to your collection

You can save a custom-built component and make it available in other scenarios.

  1. Click Add to Collection in the Test Engine dialog box.
  2. Click Close to close the Component Browser.

Designing the Transfer Orbit Injection (TOI)

Use the Propagate segment () to fly to the first maneuver time. The orbit is circular and therefore the burn can take place at any time and result in a similar Delta-V. You require an inclination as close as possible to zero once you enter GEO. To minimize the required delta-v to both circularize and change inclination at GEO, you will combine those maneuvers into one. For that to be successful, the apogee of the transfer orbit will be the ascending or descending node of the orbit. This can be achieved by starting the TOI burn on either the ascending or descending node.

  1. Return to GEO_Sat's () properties ().
  2. Right click on Propagate () in the MCS.
  3. Select Rename in the shortcut menu.
  4. Rename Propagate () to Prop to TOI.

Updating the Stopping Condition

Update Prop to TOI's () Stopping Condition to stop at the ascending node after two full revolutions in the parking orbit.

  1. Click New... () in the Stopping Conditions frame.
  2. Select AscendingNode () in the New Stopping Condition dialog box.
  3. Click OK to close the New Stopping Condition dialog box.
  4. Select Duration in the Stopping Conditions frame.
  5. Click Delete () in the Stopping Conditions toolbar.
  6. Enter 2 in the Repeat Count field. This will end the Propagate Segment on the second ascending node.
  7. Click Apply to accept your changes and to keep the Properties Browser open.
  8. Click Run Entire Mission Control Sequence () in the MCS toolbar.

Defining a Target Sequence - Start Transfer

Insert a Target Sequence (). You will use a Target Sequence () to calculate the Delta-V required to move GEO_Sat () into a transfer orbit.

  1. Return to GEO_Sat's () properties ().
  2. Right-click Prop_To_TOI () in the MCS.
  3. Select Insert After... in the shortcut menu.
  4. Select Target Sequence () in the Segment Selection dialog box.
  5. Click OK to close the Segment Selection dialog box.
  6. Rename Target Sequence () to Start Transfer.

Inserting a Maneuver segment

Insert a Maneuver () segment. This first maneuver Delta-V will be solved to achieve a particular altitude at the end of the transfer ellipse.

  1. Right-click Start Transfer () in the MCS.
  2. Select Insert After... in the shortcut menu.
  3. Select Maneuver () in the Segment Selection dialog box.
  4. Click OK to close the Segment Selection dialog box.

    5. The Maneuver () Segment appears below the Return () Segment.

  5. Drag and drop the Maneuver () Segment below Start Transfer ().
  6. Right click on Maneuver () in the MCS.
  7. Select Properties... in the shortcut menu.
  8. Type TOI in the Name: field when the Edit Segment dialog box opens.
  9. Open the Color: shortcut menu.
  10. Select white.
  11. Click OK to accept your changes and to close the Edit Segment dialog box.

    12. A Maneuver () segment isn't visible in the 2D and 3D Graphics windows. Propagate () segment colors are visible. By making Maneuver () segments white is just a way to focus on Propagate () segment colors.

Insert a Propagate Segment

Insert a Propagate () segment that will be used to determine the stopping condition.

  1. Right click on TOI () in the MCS.
  2. Select Insert After... .
  3. Select Propagate () in the Segment Select dialog box.
  4. Click OK to close the Segment Select dialog box.
  5. Right click on Propagate () in the MCS.
  6. Select Properties... in the shortcut menu.
  7. Enter Transfer in the Name: field when the Edit Segment dialog box opens.
  8. Open the Color: shortcut menu.
  9. Select yellow.
  10. Click OK to close the Edit Segment dialog box.
  11. Click Apply to accept your changes and to keep the Properties Browser open.

Setting the engine model

Use the Test Engine model you updated earlier.

  1. Select TOI () in the MCS.
  2. Select the Engine tab.
  3. Click the Engine Model ellipsis () in the Propulsion Type frame.
  4. Select Test Engine () in the Select EngineModel dialog box.
  5. Click OK to close the Select EngineModel dialog box.

Selecting the control parameter

Select Delta-V Magnitude as the independent variable.

  1. Select the Attitude tab.

    2. The default Attitude Control: is Along Velocity Vector (body X).

  2. Click the Delta-V Magnitude: target ().

    3. Notice the target now has a check mark. Selecting the target makes Delta-V Magnitude an independent variable. This tells Astrogator to determine the Delta-V magnitude based on user determined results. You will set up those results in an upcoming section.

  3. Click Apply to accept your changes and to keep the Properties Browser open.

Propagating to apoapsis

Update Transfer's () Stopping Condition to stop at apoapsis.

  1. Select Transfer () in the MCS.
  2. Click New... () in the Stopping Conditions frame.
  3. Select Apoapsis () in the New Stopping Condition dialog box.
  4. Click OK to close the New Stopping Condition dialog box.
  5. Select Duration in the Stopping Conditions frame.
  6. Click Delete () in the Stopping Conditions toolbar.
  7. Click Apply to accept your changes and to keep the Properties Browser open.

Selecting the results variable

Set the radius of the orbit, R Mag, as the equality constraint. Astrogator will use R Mag to determine the required Delta-V magnitude.

  1. Select Transfer () in the MCS.
  2. Click Results... at the bottom of the MCS.
  3. Expand () Spherical Elems () in the Available Components: list when the User - Selected Results - Transfer dialog box opens.
  4. Select R Mag ().
  5. Move () R Mag () to the selected components list.

    6. This will enable you to set the radius of orbit at the end of the Propagate Segment.

  6. Click OK to close the User - Selected Results - Transfer dialog box.

Setting up the targeter

Set up the differential corrector profile to change the Delta-V Magnitude to achieve a desired radius of orbit.

  1. Select Start_Transfer () in the MCS.
  2. Open the Action: shortcut menu.
  3. Select Run active profiles.

    4. Run active profiles runs the mission control sequence allowing the active profiles to operate.

  4. Select Differential Corrector in the Profiles frame.
  5. Click Properties... ().

Setting the control parameter

Use Delta-V Magnitude as the control parameter (independent variable).

  1. Select the ImpulsiveMnvr.Pointing.Spherical.Magnitude Use check box in the Control Parameters frame when the Differential Corrector dialog box opens.

Setting the equality constraints (Results)

Use R_Mag as the equality constraint (dependent variable), and set the radius of orbit goal to be 42238 km.

  1. Select the R_Mag Use check box in the Equality Constraints (Results) frame.
  2. Click the Desired Value cell.
  3. Enter 42238 km in the Desired Value cell.
  4. Click OK to close the Differential Corrector dialog box.
  5. Save () your scenario.

Running the Entire Mission Control Sequence

  1. Click Run Entire Mission Control Sequence () in the MCS toolbar.
  2. When complete, look at the top of STK.

    3. You will see a message informing you whether or not running the entire MCS converged or didn't converge.

  3. Look at the StartTransfer.Differential Corrector data dialog box.

    4. This shows you data based on running the active profile.

  4. Close the StartTransfer.Differential Corrector data dialog box.
  5. Bring the 3D Graphics window to the front.

    6. TOI Iterations

    You can see the iterations, the last one placing the satellite at the desired location and altitude.

Defining a Target sequence: Finishing the transfer

Now that the transfer has been analyzed, you will follow a similar process to create the SOI maneuver and circularize the orbit at GEO. At the same time, you will bring the inclination to 2 deg. You will target the inclination slightly above the desired inclination, so that it will drift down to the desired inclination (0 to 1 degree) over time.

  1. Return to GEO_Sat's () properties ().
  2. Right-click the bottom Return () Segment in the MCS.
  3. Select Insert Before... in the shortcut menu.
  4. Select Target Sequence () in the Segment Selection dialog box.
  5. Click OK to close the Segment Selection dialog box.
  6. Rename Target Sequence () to Finish Transfer.

Inserting a Maneuver segment

Insert a Maneuver () segment. You will solve for the X (Velocity) & Y (Normal) components and the Delta-V vector in those references axes needed to achieve a circular orbit at GEO.

  1. Right-click Finish_Transfer () in the MCS.
  2. Select Insert After... in the shortcut menu.
  3. Select Maneuver () in the Segment Selection dialog box.
  4. Click OK to close the Segment Selection dialog box.

    5. The Maneuver () Segment appears below the return () Segment.

  5. Drag and drop the Maneuver () Segment below the Finish_Transfer ().
  6. Right click on Maneuver () in the MCS.
  7. Select Properties... in the shortcut menu.
  8. Type SOI in the Name: field when the Edit Segment dialog box opens.
  9. Open the Color shortcut menu.
  10. Select white.
  11. Click OK to close the Edit Segment dialog box.

Inserting a Propagate segment

Insert a Propagate () Segment will be used to determine the stopping condition.

  1. Right click on SOI () in the MCS.
  2. Select Insert After... .
  3. Select Propagate () in the Segment Select dialog box.
  4. Click OK to close the Segment Select dialog box.
  5. Right click on Propagate () in the MCS.
  6. Select Properties... in the shortcut menu.
  7. Type Prop 1 Rev in the Name: field when the Edit Segment dialog box opens.
  8. Open the Color: shortcut menu.
  9. Select blue.
  10. Click OK to close the Edit Segment dialog box.
  11. Click Apply to accept your changes and to keep the Properties Browser open.

Setting the engine model

Use the Test Engine model you updated earlier.

  1. Select SOI () in the MCS
  2. Select the Engine tab.
  3. Click the Engine Model ellipsis () in the Propulsion Type frame.
  4. Select Test Engine () in the Select EngineModel dialog box.
  5. Click OK to close the Select EngineModel dialog box.

Selecting the control parameter

Use Thrust Vector as the attitude control setting. Then select the Delta-V vector's Cartesian X (Velocity) and Y (Normal) as the independent variables.

  1. Select the Attitude tab.
  2. Open Attitude Control: shortcut menu.
  3. Select Thrust Vector.
  4. Click the X (Velocity) target ().
  5. Click the Y (Normal) target ().
  6. Click Apply to accept your changes and to keep the Properties Browser open.

Propagating for one day

Update the Prop 1 Rev () Duration Stopping Condition to stop after one day.

  1. Select Prop 1 Rev () in the MCS.
  2. Enter 86400 sec in the Trip: field in the Stopping Conditions frame.
  3. Click Apply to accept your change and to keep the Properties Browser open.

Selecting the results variable

Set Eccentricity and Inclination as the equality constraints.

  1. Select Prop 1 Rev () in the MCS.
  2. Click Results... at the bottom of the MCS.
  3. Expand () Keplerian Elems in the Available Components: list when the User - Selected Results - Prop 1 Rev dialog box opens.
  4. Move () Eccentricity () to the selected components list.
  5. Move () Inclination () to the selected components list.
  6. Click OK to close the User - Selected Results - Prop 1 Rev dialog box.

Setting up the targeter

Set up the differential corrector profile to change the Delta-V Magnitude to achieve a desired radius of orbit.

  1. Select Finish_Transfer () in the MCS.
  2. Open the Action: shortcut menu.
  3. Select Run active profiles.

Setting the control parameter

Use Delta-V vector's Cartesian X (Velocity) and Y (Normal) as the Control Parameters (independent variables).

  1. Select Differential Corrector in the Profiles frame.
  2. Click Properties... () in the Profiles toolbar.
  3. Select the ImpulsiveMnvr.Pointing.Cartesian.X Use check box in the Control Parameters frame when the Differential Corrector dialog box opens.
  4. Select ImpulsiveMnvr.Pointing.Cartesian.Y Use check box.

Set the equality constraints (Results)

Set Eccentricity and Inclination as the Equality Constraints (dependent variables). Target an eccentricity of 0 within a 0.001 tolerance, and a 2 degree inclination within 0.01 degree of tolerance.

  1. Select the Eccentricity Use check box in the Equality Constraints (Results) frame.
  2. Enter 0.001 in the Tolerance: field.
  3. Select the Inclination Use check box.
  4. Select the Desired Value cell.
  5. Enter 2 deg in the Desired Value cell.
  6. Enter 0.01 deg in the Tolerance: field.
  7. Open the Method: shortcut menu in the Scaling frame.
  8. Select By tolerance.
  9. Click OK to close the Differential Corrector dialog box.
  10. Click Apply to accept your changes and to keep the Properties Browser open.
  11. Save () your scenario.

Running the Entire Mission Control Sequence

  1. Click Run Entire Mission Control Sequence () in the MCS toolbar.
  2. Look at the Finish Transfer.Differential Corrector: Finished data window.
  3. Close both the Start Transfer.Differential Corrector:Finished and Transfer.Differential Corrector: Finished data windows.
  4. Bring the 3D Graphics window to the front.

    5. SOI Iterations

    You can see the iterations, the last one placing the satellite at the desired eccentricity and inclination.

Removing iterations

Remove the visible iterations in the 3D Graphics window.

  1. Return to GEO_Sat's () properties ().
  2. Right-click the Bottom Return () Segment.
  3. Select Insert Before... in the shortcut menu.
  4. Select Propagate () in the Segment Selection dialog box.
  5. Click OK to close the Segment Selection dialog box.
  6. Click Run Entire Mission Control Sequence () in the MCS toolbar.
  7. Close both the Start Transfer.Differential Corrector:Finished and Transfer.Differential Corrector: Finished data windows.
  8. Bring the 3D Graphics window to the front.

Final Orbit

Generating an MCS Segment Summary report

The MCS Segment Summary report provides run summary data for the currently selected segment in the MCS tree. The summary report gives information on maneuver times, expected Delta-V magnitude, estimated burn duration, estimated fuel usage, and other important data.

  1. Return to GEO_Sat's () properties ().
  2. Select Start_Transfer () in the MCS.
  3. Click Summary () in the MCS Toolbar.
  4. Look at the data in the report.
  5. When finished, close () the summary report.
  6. Select Finish_Transfer ().
  7. Click Summary () in the MCS Toolbar.
  8. Look at the data in the report.
  9. When finished looking through the report, close any reports, tools and properties you still have open.
  10. Save () your work.

Summary

This was a basic introduction to Astrogator. In this lesson, you did the following:

  • Used a Launch Vehicle object to launch a vehicle that followed an ascent trajectory from a launch point to an orbit insertion point.
  • Inserted a Satellite object, switched the propagator to Astrogator and used a Follow Segment.
    • This allowed the Satellite to follow the Launch Vehicle object, to separate from the Launch Vehicle at the end of its trajectory, and to place the Satellite object into a LEO.
  • Used a Target sequence, a Maneuver segment, and a Propagate segment to place the Satellite orbit into a TOI.
  • Finalized its orbit by creating another Target Sequence that placed the Satellite into a GEO.
  • Used an MCS Segment Summary Report to determine maneuver times, required Delta-V, estimated burn duration, and estimated fuel usage.

On your own

Throughout the tutorial, hyperlinks were provided that pointed to in-depth information of various tools and functions. Now is a good time to go back through this tutorial and view that information. Further Astrogator tutorials can be found HERE.