[Tutorial] Planetary intercepts - or how to orbit in the right direction

[KSK]

This is probably old news to the seasoned Kerbalnauts out there but it's something that took me a while to figure out. So I thought I'd write a tutorial in case anyone else out there was having the same problem. That problem being - how do I set up an intercept with a planet (or moon) so that I end up in a consistent prograde or retrograde orbit?

The Mun isn't a problem. It's big enough to have a significant effect on your spacecraft's trajectory and I could set up a recognisably similar trajectory for everything I sent to the Mun by eyeballing it. For Minmus though it wasn't so easy. It's a smaller target, it's a lot further away and setting up a consistent trajectory was proving difficult. As a result, my early Minmus missions ended up in prograde or retrograde orbits more or less at random.

This is fine for 'footprint and flag' missions but not so helpful for building space stations or for docking larger craft in orbit. And if Minmus wasn't working, then what chance Duna, or anywhere further afield? It was time to figure this out properly, so without further ado.

A guide to consistent orbiting

1. To figure out your eventual orbit, you need to know your velocity relative to the planet as you approach.

Here's a rough diagram to illustrate the point. In each case we burn retrograde at periapsis (Pe) to drop into orbit around Minmus. If our velocity relative to Minmus is pointing in one direction, we get a clockwise (retrograde) orbit. If it points in the other we get a counterclockwise (prograde) orbit. The trick is working out the relative velocity ahead of time, so lets look at that next.

2. We can do this the complicated way...

Here's a simple example. To a reasonable approximation we're on course to cut straight across Minmus's orbit. We can therefore estimate our relative velocity by considering motion along Minmus's orbit and normal to it. Our velocity along Minmus's orbit is essentially zero, so our relative velocity becomes 0-x where x is whatever Minmus's orbital speed is. The exact numbers don't matter, the point is that relative to Minmus we are travelling back along its orbital path. In contrast, we have a small velocity normal to Minmus's orbital direction and at any given time Minmus has zero velocity in that direction. So our relative velocity becomes y-0 or in other words nothing changes. Add the two up and we get our relative velocity.

This is a whole lot easier to see on the diagram. Red lines are our relative velocities along Minmus's orbit and normal to it. Green line is the resultant velocity.

3. Or we can do this the easy way.

A much easier way of doing this is simply to mouse over the 'Minmus encounter' point on the Map to display the Minmus' location at that encounter point. We can then estimate our direction of travel relative to Minmus by drawing an imaginary line between the encounter point and Minmus. As you'll see, that works out to be fairly similar to the direction we estimated in Step 2 by fiddling around with adding vectors.

4. But does that work in practice?

Lets try. Minmus is way over there at the encounter point, so draw that imaginary line. Looks like our relative velocity is angled back along Minmus's orbit at about 20 degrees to its direction of travel. Remembering back to that first diagram, we can predict that our eventual orbit will be clockwise (or retrograde) around Minmus.

Once we cross into Minmus's sphere of influence, the Map will show the correct relative velocity and we can see if that prediction is correct. A quick burst of timewarp and

Not bad!

5. So lets set up the Minmus insertion burn.

Note that our prograde marker on the maneuver node is pointing in the direction we expect. That's a nice check.

And there we have it. I've added a maneuver node to highlight our orbital direction. As predicted - a nice retrograde orbit.

Hope this all made some sense. Have fun planning those missions!

Edited April 30, 2013 by KSK

[Al-Rabban]

Thanks for the tutorial, I like you have been able to eyeball but this really helps clarify what happens when I enter the sphere of influence.

Cheers!

[Temstar]

Seems needlessly complicated, here's my method:

Assuming a prograde orbit relative to the parent body you're orbiting:

• For going to moon and superior planets, if your apapsis is above the orbital altitude of the body you're going to (the target moves in from underneath you), you will go into a retrograde orbit
  
• For going to inferior planets (or inferior moons for a moon-to-moon transfer), if your periapsis is below the orbital altitude of the body you're going to (you zoom by below the target), you will go into a retrograde orbit
  

An even simpler way is to look at your result orbit after encounter if you don't fire your engines or use aerobrake to insert into orbit:

• If the result orbit has a higher apapsis than the original orbit, you will go into prograde orbit if you capture
  
• If the result orbit has a lower apapsis than the original orbit, you will go into retrograde orbit if you capture
  

[KSK]

Thanks Al-Rabban for the kind words!

Temstar - I'm not going to argue with that ribbon! I'll give your second method a try and see how it works for me. Not that I'm doubting that it works per se, but having just gotten my head around one way of thinking about this it may take some time to adjust Thanks for the reply.

[Seaview123]

Pretty cool tutorial, thanks!

[Pecan]

Pretty cool tutorial, thanks!

And almost two years old!

Once you create your initial manoeuvre node double-click on the target body (eg; Minmus) to focus the view on it and see your orbit within its SOI. Then make minor adjustments to the node to approach the body the way you want to.