Rename Propagate1 to Prop To Periapsis. Now create the commands necessary to perform the Target sequence. This will insert two separate commands: Target1 and EndTarget1.
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Right-click Target1 and click Rename. Type Hohmann Transfer and click OK. Right-click Hohmann Transfer , point to Append , and click Vary. Additional Target Sequence Commands. We know that two maneuvers are required to perform the Hohmann transfer. We also know that for our current mission, the final orbit radius must be 42, km and the final orbital eccentricity must be 0. You use the Target sequence to solve for those precise maneuver values. You must tell GMAT what controls are available in this case, two maneuvers and what conditions must be satisfied in this case, a specific orbital radius and eccentricity.
You accomplish this using the Vary and Achieve commands.
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You use the Achieve command to tell GMAT what conditions the solution must satisfy—in this case, the final orbital conditions. We need a Propagate command after the Target sequence so that we can see our final orbit. A new Propagate3 command will appear. Rename Propagate3 to Prop One Day. Under Condition , replace the value Prop One Day Command Configuration. Now that the structure is created, we need to configure the various parts of the Target sequence to do what we want.
Double-click Vary TOI to edit its properties.
Notice that the variable in the Variable box is TOI. In the Initial Value box, type 1. In the Max Step box, type 0. Double-click Perform TOI to edit its properties. Under Parameter , replace DefaultSC. ElapsedSecs with DefaultSC. Prop to Apoapsis Command Configuration. Notice that Goal is set to DefaultSC. This is what we need, so we make no changes here. In the Value box, type Next to Variable , click the Edit button. See the image below for results. In the MaxStep text box, type 0.
In the Burn list, click GOI. Next to Goal , click the Edit button. In the Value box, type 0. In the Tolerance box, type 0. Before running the mission, click Save and save the mission to a file of your choice. Now click Run. As the mission runs, you will see GMAT solve the targeting problem. Each iteration and perturbation is shown in DefaultOrbitView window in light blue, and the final solution is shown in red.
After the mission completes, the 3D view should appear as in to the image shown below. You may want to run the mission several times to see the targeting in progress. If you were to continue developing this mission, you can store the final solution of the Target sequence as the initial conditions of the TOI and GOI resources themselves, so that if you make small changes, the subsequent runs will take less time.
To do this, follow these steps:.
In the Mission tree, double-click Hohmann Transfer to edit its properties. Now re-run the mission. If you inspect the results in the message window, you will see that the Target sequence converges in one iteration because you stored the solution as the initial condition. One of the most common operational problems in space mission design is the design of a finite burn that achieves a given orbital goal.
A finite burn model, as opposed to the idealized impulsive burn model used for preliminary design, is needed to accurately model actual spacecraft maneuvers. The goal of this finite burn is to achieve a certain desired apogee radius. Since the most efficient orbital location to affect apoapsis is at periapsis, the first step in this tutorial is to propagate the spacecraft to perigee. To calculate the duration of the perigee burn needed to achieve a desired apogee radius of km, we must create the appropriate targeting sequence.
Create and configure the Spacecraft hardware and FiniteBurn resources. Create a Target sequence to achieve a km apogee radius. To model thrust and fuel use associated with a finite burn, we must create a ChemicalThruster and a ChemicalTank and then attach the newly created ChemicalTank to the ChemicalThruster.
In the Resources tree, right-click on the Hardware folder, point to Add , and click ChemicalThruster. A resource named ChemicalThruster1 will be created. A resource named ChemicalTank1 will be created.
Use the drop down menu to the right of the Tank field to select ChemicalTank1 as the fuel source for ChemicalThruster1. ChemicalTank1 Configuration. ChemicalThruster1 Configuration. For a general finite burn, if desired, we can specify how both the thrust and the fuel use depend upon fuel tank pressure. The user does this by inputting coefficients of certain pre-defined polynomials. To view the values for the thrust coefficients, click the Edit Thruster Coef. For this tutorial, we will use the default ISP polynomial coefficient values but we will change the ChemicalThruster1 polynomial coefficients as follows.
In the Resources tree, double-click ChemicalThruster1 to edit its properties. Click the Edit Thruster Coef. Replace the default C1 coefficient value of 10 with ChemicalThruster1 Thrust Coefficients. The exact form of the pre-defined Thrust polynomial, associated with the coefficients above, are given in the ChemicalThruster help. We note that, by default, all of the Thrust coefficients associated with terms that involve tank pressure are zero. We have kept the default zero values for all of these coefficients.
We simply changed the constant term in the Thrust polynomial from 10 to which is much larger than the thrust for a typical chemical thruster. The Thrust and ISP polynomials used in this tutorial are shown below. In the Resources tree, double-click DefaultSC to edit its properties. Select the Tanks tab. In the Available Tanks column, select ChemicalTank1. Then click the right arrow button to add ChemicalTank1 to the SelectedTanks list.
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Click Apply. Select the Actuators tab. In the Available Thrusters column, select ChemicalThruster1. Then click the right arrow button to add ChemicalThruster1 to the SelectedThrusters list. In the Resources tree, right-click the Burns folder and add a FiniteBurn. A resource named FiniteBurn1 will be created.
Use the menu to the right of the Thruster field to select ChemicalThruster1 as the thruster associated with FiniteBurn1. The Target sequence we will later create uses the Vary command to adjust a user defined target control variable in order to achieve the desired orbital goal of raising apogee to km.
We must first create this variable which we will name BurnDuration.