Since last time we talked about how Next Giant Leap was using STK/Astrogator for trajectory analysis for the Google Lunar X PRIZE, I figured it would be a good time to talk about how to use it for launch vehicle trajectories. Now before we go any further, let’s talk about licenses. For this one you will need the Astrogator license. Not sure if you have one? Well let’s check. Open up STK and go to Help -> License Viewer. If your License Viewer looks like this …
… don’t panic, it’ll be ok. Just e-mail support (support@agi.com) and they will set you up with a demo license so you can check this out. Now that we are good with our licenses, let’s get started.
Open up an STK scenario and insert a satellite object. I know we are working on launch vehicles, but Astrogator is a satellite propagator, so for this instance we need to use a satellite object. Now let’s pause here for a moment to talk about what Astrogator is exactly. Astrogator is a specialized analysis module for orbit maneuver and spacecraft trajectory design. It calculates the satellite’s ephemeris by executing a Mission Control Sequence (MCS) that you define according to the requirements of your mission. Now we aren’t going to go into too much depth with Astrogator, just enough to get our launch vehicle launched, but if you want more details on it, check the end of this post, where I’ll link you to some great resources. OK, enough with that; let’s move on to the fun stuff!!
Now that you have a Satellite object in your scenario, double click it to open up the Properties Browser. Start at the Orbit page and change the Propagator from the default of TwoBody to Astrogator. When you do that, the panel that comes up should look like the following:
Click on the Initial State icon to highlight it and then click on the red X in the tool bar above to delete that segment. Do the same for the Propagate segment so we have a nice clean MCS. Alright, click on the Insert Segment button (if you’re not sure where it is, look in the pic above. It’s called out). This brings up the Segment Selection box, which gives you all the segments you can use in an Astrogator MCS. But since we are only worried about modeling the launch, select the Launch icon and hit OK. Now the Launch Segment is used to model a simple spacecraft launch and is similar to the Ascent Propagator on the Launch Vehicle object. This means that the trajectory is just 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. Why is this useful? Well you can use this segment to help vary your launch time to meet other mission requirements that you may have. Alright that’s enough of the details; let’s walk through this panel a bit shall we?
Let’s start at the top of the panel, with the Central Body option. STK gives you the ability to launch from a Central Body of your choosing, which is great for interplanetary missions, you don’t have to launch from just Earth, you can launch from the Moon or Mars if you want. For this example let’s stick to Earth. Step Size allows you to specify the time intervals between calculated ephemeris points; we’ll leave that set at 5 secs. The Use State From Previous Segment check box (this is a new feature in STK 8.1) allows you to define the launch location parameters by using the state of the spacecraft at the end of the previous segment. But since we don’t have a previous segment, we’ll leave that unchecked. The Pre-Launch Time allows you to specify the amount of time the launch vehicle will hold on the launch pad before it will launch, so if you want it to launch a minute after the scenario starts, then just put in 60 secs and the launch vehicle will hold off until a minute into the scenario before it will launch. But we aren’t going to use that today either so leave it set as 0 secs.
Alright let’s jump down to the Launch portion of the panel. So our launch coordinate type can be either Geocentric (measured from the center of mass of the Earth or central body) or Geodetic (measured along the normal to the surface of an ellipsoid defined with reference to the Earth or central body). We are going to stick with Geodetic. There are a couple of ways to set our launch latitude, longitude, and altitude:
1.) By hand – good old fashioned typing in your coordinates
2.) Import Facility Location – This takes you to the facility database that allows you to search for a facility and then use its lat, long, and alt.
3.) Select Facility – Allows you to select a facility from your STK scenario to pull your coordinates from. NOTE: You must have facilities in your scenario before you define your launch segment in order for this option to appear.
Let’s use the Import Facility Location button, check site name, and type in Cape* (remember * is a wild card in search). This returns 17 different sites with the name Cape in it. Select Cape Canaveral ETR USA LC and hit OK, and then hit close on the Facility Database panel. Now our launch latitude, longitude, and altitude are set.
Let’s now look at setting our burnout parameters. We have four different options for how we can define our burnout – Geocentric, Geodetic, Launch Az/Alt, and Launch Az/Radius. We’ll stick to the default Launch Az/Alt for this example. Next, we can specify our Time of Flight, which we’ll leave set at 600 sec. We can set our Azimuth, Downrange Distance, and Altitude at which the burnout will occur, and we’ll leave the defaults there as well. If you click the Burnout Velocity… the custom function panel will come up and allow you to specify Burnout Options. You can Use Fixed Velocity (the inclination of the final state of the launch segment is determined by the arc between the launch and insertion positions, and the horizontal flight path angle is set to zero) or Use Inertial Velocity (the final state of the launch segment is solely and completely determined by the burnout position and velocity) and set the parameters accordingly. The Satellite Properties button brings up the Spacecraft Configuration panel that allows you to specify the physical values of the spacecraft parameters and fuel tank based on the satellite that you’re launching. Finally, we have the Ascent Type, which allows you to specify the order of the spline used to generate the motion along the ellipse of your trajectory. Use Ellipse: Cubic Motion for motion that is computed based on given positions and velocities and Ellipse: Quartic Motion for motion that is based on given positions, velocities, and initial accelerations. We are just going to stick to Elllipse: Cubic Motion for this example. Alright so here is what your MCS should look like now that we have set all of our parameters:
How did I get my Launch segment to be pink you might ask? Well just double click the segment to bring up the Edit Segment panel. Here you can change the name (don’t forget it’s always important to name your segments something specific to keep them organized), add a comment, change the coordinate system, and change the color. Alright so now that we have our properties set, let’s hit the Run Segment button to actually compute the trajectory of our launch. After the run is complete you’ll notice that at the bottom of the panel that Initial is no longer –Not Set- but 1 Jul 2007 12:00:00.000 UTCG and Final is now 1 Jul 2007 12:10:00.000 UTCG, representing the Start and Stop times of our launch segment. Want a summary of that data generated from your launch segment? Just click the Summary button (it’s the white scroll next to the Run Segment button). This brings up an Astrogator MCS Segment Summary, which has the Parameters in Cartesian, Keplerian, Spherical, Other Elliptical Orbit Parameters, Geodetic, and Geocentric.
So there you have it, how to create a launch trajectory using STK/Astrogator. There are some really great Astrogator resources out there. Here are my favorites:
- www.agi.com/astrogator – Complete AGI information on Astrogator including presentations, videos, and VDFs.
- www.astrogatorsguild.com – Astrogator experts Mike Loucks (Space Exploration Engineering Corp) and John Carrico (Applied Defense Solutions) have posted STK scenarios, training, papers, and more. Visit this site, it’s a great resource!
- http://blogs.agi.com/astrogator/ – AGI developers Dr. Matt Berry and Paul Black talk about new features and unique problem solving with Astrogator.
Well I’m pretty sure you guys are sick of hearing from me for the moment, so we’ll wrap this up. Let me know if you have any questions!
Till Next Time,
Rocket Girl (aka Amanda)
NOTE: You need a STK/Attitude license to perform some of the analyses you see in this blog. Don’t have one? No worries, just email the Support Team at support@agi.com and they will set you up with a demo license. So go, go get the license, I’ll wait….
Ok, got the license? Good. So last time we talked about how we model the shuttle trajectory in STK, but what about the attitude? Right after the shuttle clears the tower it goes into the roll program to help with things like structural loading during launch, set orbital inclination, and improved ground-based communications. Well, how do we go about modeling that roll program in STK? It’s a great question and it might get a little tricky, but I think we can figure this out. So let’s get started!
Now that you have a launch vehicle object in STK that has the external trajectory file as its propagator, open up the Properties panel for that launch vehicle and click on Attitude under the Basic heading. At the top of the Attitude panel you’ll see Standard in the drop-down box. Most of the time, the options in the Standard attitude type are good enough, but the attitude we want to model is a bit more custom, so we want to change that in the drop down to Multi Segment. The Multi-Segment option lets us build a sequence of attitude profiles. We are going to be dealing with the Basic Segments section of the panel. There is already a default segment set up, called ECFVelRadial, so we are going to start by editing that segment. Click the Edit Segment… button to open up its properties. Change the Name to Launch, since we are going to be modeling the shuttle attitude at launch. In the Type drop-down menu, select Aligned and Constrained. In this attitude profile the body-fixed axes are oriented using aligned and constrained pairs of vectors. The Aligned Vector Type needs to be set at Cartesian with X = 1.0, Y = 0.0, and Z = 0.0. The Reference Vector need to be set as STS-118 Zenith(Detic) (to set that just hit the Select button which will take you to the Vector Geometry Tool to select the vector). The Constrained Vector Type needs to be Cartesian with X = 0.0, Y=0.0, Z=1.0. The Reference Vector needs to be set as the STS-118 North vector. Before you close out the panel, make sure the Start time is that of your scenario. Now we have the attitude of the shuttle model for launch. Let’s move on to the roll program.
In the Basic Segments section of the Attitude panel click the Add Segment button. Change the name of the Segment to Roll and set the Start time to 10 seconds after the launch time of the shuttle (it takes the shuttle about 10 seconds or so to clear the tower so we want to start the roll program after it makes it past the tower). Change the Type to Fixed Time Slew, this attitude profile performs a shortest distance rotation between the attitude of vehicle at the end of the preceding segment and its attitude at the beginning of the next segment. Then OK out of this panel.
Now it’s time to end this attitude profile. Click the Add Segment button to add the final attitude segment. Change the name to EndRoll and change the Start time to 10 seconds after the beginning of the roll. Set the attitude type to ECF velocity alignment with nadir constraint. With this attitude profile, the vehicle’s X axis is aligned with the Earth-fixed velocity vector direction and the Z axis is constrained in the geocentric nadir direction. But since we need the shuttle to be heads down in this attitude profile, we need to apply a Constraint Offset of 180.00 deg. Get out of that panel and hit apply and there you have a roll program in the attitude of your shuttle! Check out the pictures below of what we just did in 3-D.
OK, the first picture is of the shuttle just clearing the tower after launch. This is T+10 seconds; I got that text annotation in the top left corner by going to the 3D window properties browser >Annotation. Then in the text box I clicked the Add… and then added the T+10 Seconds text, changed the color to green, and changed the Position to Pixel and left the defaults. On the actual launch vehicle, I added the BodyXY plane (which is the gray, translucent plane running through the body of the shuttle), the Roll Axis vector, and the Flight Roll Angle and its magnitude in degrees. These were all in the Launch Vehicle properties browser > 3D Graphics > Vector. The plane, vector, and angle are all predefined in STK’s Vector Geometry Tool. The imagery in the background of the launch pad at Kennedy Space Center is coming from AGI’s Globeserver. The second image is T+15 seconds and the third is T+20 seconds. So this gives you an idea of what the Multi Segmented attitude profile is doing over time in 3D to model the roll program of the shuttle.
See that wasn’t too bad right?! Well, that’s all for now!
Till Next Time,
Rocket Girl (aka Amanda)
Since STS-126 launched last week, I figured it would be a good time to talk about how you can model that in STK. Everybody ready?
The best way to model this is with external files – basically take any trajectory data that you have for the shuttle, put it into a format that STK can ingest, and there you have it. What am I talking about? One of the ways STK allows modeling launch vehicles is through what we call the STKExternal propagator. This allows you to read the ephemeris for a satellite from a file that must be in a *.e format. I know, you are asking what all that means.
An ephemeris file is just an ASCII text file formatted for STK that ends with the .e extension. What data needs to go in this text file? Well you need the vehicle position and velocity data at certain times, which STK uses to support analysis and animation of the vehicle over time. If and when it’s necessary, STK will even interpolate between points to perform the analysis in the same coordinate frame the data is supplied in. There are two main things to keep in mind when creating these ephemeris files. First, note how the data is formatted – you know, Cartesian X, Y, Z or geodetic latitude, longitude, and altitude. Second, determine what coordinate system the data is in – inertial or fixed frame. There are some keywords that need to be in the file, and there are about 11 different data formats. We don’t have time to go through all of that today, but luckily for us STK’s extensive help system has all the details on exactly what you need to have in the files. I’ll give you more details later on in the blog, but I know you are eager to get started now!
As always here is a picture from STK of the shuttle launch using the .e file. Now I promised in the last blog to talk about vectors. So let’s do that real quick.
You’ll notice in the picture that I have a ToLP vector and the FlightRoll angle displayed. These were created in the Vector Geometry Tool (VGT) inside of STK. One of the things that separates STK from all the other tools out there is that it’s a physics-based software suite, meaning we give you the vectors for every object you have in your scenario. So go ahead and launch VGT inside STK (to do that just click the icon on your toolbar that has the two arrows crossed). Find your Launch Vehicle Object in the list of VGT objects and click on the “+” icon to expand it. Here you can see all the predefined vectors, axes, angles, coordinate systems, points, and planes for the launch vehicle (and there are a lot of them). In this case I wanted to display a vector on the shuttle showing me where the launch pad is in relation to the shuttle. So I had to create it.
Next to create, click on the single arrow button to bring up a panel that allows you to create a new vector. I named the vector ToLP and kept the type as the default of displacement. The Origin Point I kept as my
Looking for some good resources on how you can use STK for shuttle analysis? Check out www.agi.com/vdf for some great VDF’s on shuttle launch, docking with the
That’s it for now!
-Rocket Girl (aka Amanda)
Hey y’all. For those of you who are new, I’m Rocket Girl, the AGI UC blogger and an aerospace engineer here at AGI. Now that the UC is over I’m moving on to blogging about tips and tricks on how to perform launch vehicle and missile analysis in STK. Now remember, this is an open forum, so don’t be shy. Make sure you ask those burning questions that you have about launch vehicle and missile analysis in STK. Anything you’ve ever wanted to know, just ask and we’ll talk about it. Alright enough with the intros, let’s move on to the fun stuff!
Did you guys know that there are four different ways to model a launch vehicle trajectory with the launch vehicle object inside of STK? Today we are going to talk about one of those ways, using the Simple Ascent propagator. Now the Simple Ascent propagator creates an ascent trajectory based on launch and insertion parameters. The trajectory is a simple curve that rises vertically from the launch pad and turns over smoothly to insert the launch vehicle into orbit with a zero flight path angle at the insertion point using a the velocity you specify. Once you get your launch vehicle object into your scenario just simply open up the Properties Browser and go to the Trajectory page. The propagator will already be set to Simple Ascent (it’s the default in STK). So go ahead and change your Start Time if you need to – it will automatically match your scenario start time. Now let’s look at some of the other things we can change. Launch Latitude can either be in the Geodetic or Geocentric, which ever prefer. You can either type in the launch latitude and longitude or you can go over to the 2D map, click on the point that you want to launch from and STK will automatically fill in the latitude and longitude data. Next, you can set your launch altitude (or launch radius if you’re using Geocentric). OK great, so we have our launch location entered, let’s look at our burnout velocity and burnout location.
The burnout velocity is the velocity of the launch vehicle at the stop time, or the burnout time. Once you have that specified, you need to got down and set your burnout latitude and longitude (the same way you changed your launch latitude and longitude) and your burnout altitude (or radius). Now that all your parameter are sit, hit OK, which will save all your launch vehicle settings and close the Properties Browser. Jump to your 3D window, and you can watch the animation of your launch vehicle. Below is a picture of what mine looks like.
So I launched from Cape_Canaveral_ETR_USA_LC, which I got from the Facility Database inside of STK. To do this go to Insert Facility from Database, when the Facility Database panel comes up check Site Name, I put in Canaveral* (* after a name is a wildcard character), which brought up all facilities with Canaveral in the name and I chose which one I wanted. Then I went to the 2D map and clicked on the facility icon to get my Launch Latitude and Longitude. I left all the other parameters as the default, and there you have it!
I have some Tick Marks spaced 60 seconds apart shown on the trajectory. So this gives me a visual cue along the trajectory that lets me know where the vehicle is in the path every minute. You can find this in the Launch Vehicle properties browser > 3D Graphics > Trajectory. Finally, I have a dynamic data display in the corner, which is an LLA report. So it gives me dynamically the LLA position of the launch vehicle as it animates. You can add that from the Launch Vehicle properties browser > 3DGraphics >Data Display.
Alright we have our launch vehicle modeled using a Simple Ascent propagator. Whew! I think I need a