[Tutorial] Geostationary Orbit Constellations for Dummies [MechJeb] (2024)

The title says it all. I would put this wee guide on the wiki, but the wiki is rather...restrictive. I also couldn't find any simple guides like this for KSP anywhere after a lot of searching, so here it goes.

First off: No complex math. You don't even need to know how to multiply or divide. The only skill you need is basic addition and subtraction, and an ability to read the data MechJeb gives you (yes, you can fly the thing yourself the entire way, though I can't imagine why anyone would want to do this eight-or-so times without using at least Smart A.S.S., which is what I use for all my orbital maneuvers for this after the ascent. It will get repetitive the more satellites you put up).

Second: I say geostationary because I'm comfortable with it, I'm not interested in arguing semantics, and geo is from the Greek for "earth" though typically in the sense of "land" or "ground", not specifically this planet we meatbags call home.

Third: I assume here that the reader has a firm grasp of orbital maneuvers, including phase changes (adjusting the altitude of an orbit), plane/inclination changes, basic ship piloting, and perhaps even a cursory knowledge of what all the lovely buttons do in MechJeb. There are tutorials already for all of this, so I'm not going to cover any of it.

Finally: The goal of this tutorial is not just to describe how to get a payload into geostationary orbit, but how to get it at more-or-less a precise longitude with a minimum of fuss. That is, one burn from a parking orbit to GEO, or close to GEO, with minimal adjustments if the user wishes.

Step 1: Understand What a Geostationary Orbit Is

...and the difference between "geostationary" and "geosynchronous". Really, I can't do a better job here than Wikipedia, and for the sake of brevity, I won't go into detail. Short version: a geostationary orbit will keep some orbital body above the same longitude at all times. Put more simply, to an observer on Kerbin, a satellite in geostationary orbit will remain stationary in the sky (well, hopefully, or at least with so little deviation it's unnoticeable). A practical example: Geostationary satellites are used to provide streaming data to large numbers of clients, such as in the case of satellite TV providers. The simplicity of using a simple receiver that doesn't need to track the satellite far out-weighs the cost of putting a satellite in GEO.

Now, Kerbin's GEO altitude is 2.870Mm. Get a piece of space junk up at that height, such that its apoapsis and periapsis are both at 2.870Mm, and you have yourself a GEO.

Step 2: Build a Rocket

Anything that can get your desired payload to an altitude of 2.870Mm above Kerbin will do (that's Mega-meters, or 2.870 million meters). Test it out with your payload, see if it can get you there in one shot, with fuel and an engine to spare, preferably, for fine-tuning; I have a soft spot for the NERVA for this task. An easy way to do this is to just set MechJeb's ascent autopilot to 2,870km and let it go. If you get your payload there with fuel to spare, you're in good shape and have enough delta-v to get your payload there. Note that any rocket that has enough delta-v to escape Kerbin (e.g. Mun or Minmus launch vehicles) will likely have enough delta-v to get to a stable GEO (the circulisation burn could mess with this; I haven't done any specific math on this point, nor do I plan to).

Step 3: Launching

I've seen more than a few mentions of GEO constellations requiring precise launch timing, as one would use for a rendezvous with a spacecraft in low-orbit. This simply isn't the case. If we consider the case of a direct ascent to GEO, and a similar ascent profile for each launch, each satellite will end-up in roughly the same position because, relative to the rotating surface of, the final point of the ascent hasn't moved. So, the bottom line? Launch whenever you darn well please, as we need to fix this later anyway.

This first launch is a dry run. We're out to collect data, and little more, you may even wish to do this in a test save if you're as OCD as I am about not littering space more than necessary. Launch your vehicle to a low-altitude of your choosing, just write it down. This is where all subsequent launches will be going. I usually go with 200km (eventhough this has the mild downside of being a little slower because of time compression limits...I don't really mind). Once you are at your parking orbit (you did remember to write that altitude down, right?) and inclination is at 0 (or within a few degrees), open MechJeb's surface information window. At the very bottom you will see Longitude. This the the key piece of data we'll be using.

Step 4: Transfer

Wait until your craft is approaching 0 degrees longitude. Then commence a burn to enter a Hohmann transfer orbit with apoapsis at 2.870Mm. Warp to the AP, then bring your periapsis up to 2.870Mm as well. If you are using a relatively low-thrust engine (or even a high thrust one; you'll need to expend quite a bit of delta-v to circularise this orbit), you may end-up burning for so long that your apoapsis begins to rise again. That's okay, get it close, we've got our data.

Note the current longitude. We'll call this x.

And for the sake of actually giving an example, let's say we arrived at a new longitude of 138 degrees. Ta-da! That 138 degrees is very, very important. I hope there are lightbulbs going off now...

Step 5: Interpreting What the Heck I Just Told You To Do

Here's where the math comes in. The beautifully easy, peasy math. Let's say that we now want to deploy a payload such that when it's in GEO, it's (approximately; done right this method should get one within a few degrees longitude) at 0 degrees longitude. We simply do: 0 - x = longitude of burn. In this case, we get -138 degrees. So, following the same pattern as our test flight, exiting our parking orbit of 200km at -138 degrees longitude (or at least, starting our burn there) will have the satellite arrive in GEO on or close to a longitude of 0 degrees.

To further illustrate, here's a table showing the (exceedingly simple in comparison to most math stuff with orbital mechanics in KSP) math that gives us the longitudes to burn at for all eight orthagonal points, giving a nice, evenly spaced set of 8 satellites (negative values used so they correspond correctly to longitude as displayed by MechJeb):

0 - 138 = -13845 - 138 = -9390 - 138 = -48135 - 138 = -3180 - 138 = 42-135 + 138 = 3-90 + 138 = 48-45 + 138 = 93

And unless I completely messed-up (I'm a little sleep-deprived of this writing) that should make sense. See? I keep my promises. Not so much as a multiplication sign. But wait! There's more!

Step 6: So You Don't Know How to Add

No problem! All you need to know is how many degrees you want to separate your satellites by. Then just initiate your burn from points that are equally spaced by longitude. So if you don't really care where exactly your satellites end-up above Kerbin, you could simply do transfer burns at 0, 45, 90, 135, 180, -135, -90, and -45 degrees longitude. Just make sure you do the same thing with the same rocket every time. Changing the throttle, thrust (i.e. engines), mass, anything, will throw everything off, requiring you to go back to Step 3 for that rocket.

When you're finished, you're free to correct the orbit as you normally would, by reducing periapsis, increasing apoapsis, and returning, burning normal or antinormal to adjust inclination, etc. I've managed to get within a few degrees longitude of my intended destination (about a degree or two off; close enough for me), with eccentricity of 0.0003 and inclination of <0.1 (which is the standard margin of error for inclination of real-world GEO satellites).

I hope that made some modicum of sense, and that it's somewhat helpful. I've found this to be an exceedingly easy way to have a nicely-spaced constellation of comsats around Kerbin. You know, for Kerbin TV...or whatever they do to rot their brains.

Edited June 1, 2016 by phoenix_ca
Fixing typos