How do I intercept my service module on an Apollo style Mun landing?

[sokid2k]

Either using mechjeb 2 or manually doing it. I'm having trouble figuring when to launch to get it just right and save the most fuel.

[iamaphazael]

One way to do it is this:

1: Quicksave

2: Launch your ascent stage into orbit at the height your SM is at and circularize. Time how long it takes from the time you launch to the time you hit circular orbit. Also note how far downrange your ascent stage ends up once the circularization burn is complete (that is, hoiw far around the planet are you from where you started.

3: Quickload.

4: Watch your SM orbit a few times and time how long it takes to complete one orbit. Once you have that number, wait until the SM gets to that point where your ascent stage is going to end up and note what time it is.

5: Subtract the time it takes to get your ascent stage up from the time it takes to complete one orbit. Add the result to the current time. That's when you should launch.

6: When the time comes, launch your ascent stage and try to fly as close to the same profile as you did the first time. Don't worry at this point about rendezvousing. Just get your craft into a circular orbit at the right altitude. If you've done your math right, you should find that you end up in a similar orbit to your SM, reasonably close to it. Now, you can handle this just like any other rendezvous and make slight tweaks to your orbit to catch up or hang back.

7: ?

8: Profit

[rryy]

It all depends on the altitude and inclination of the service module's orbit. Assuming an equatorial orbit, set the service module as target and when the pink target marker is at around 70º, launch into orbit. Get your apoapsis to cross the orbit of the service module and set up a maneuver node at apoapsis to burn prograde so that the encounter distance is as close as possible. Do NOT circularize, or else you will waste delta-v burning retrograde again to catch up to the service module. Once you get close, cancel out your relative velocity.

Edited June 16, 2013 by rryy

[Margaul]

Manually, you can try to figure out when to launch...usually when the command/service module is maybe 1-2 minutes from passing directly overhead. However, it's not a big deal fuel wise if you miss. The Mun's gravity is very weak, so you can theoretically adjust an orbit by using RCS once you have very similar orbits. If I were you, I would get into the same orbit of the command module. Then if you are behind the command module, make one side of your orbit just a little lower in altitude. This will allow you to catch up to the command module (It might take a few orbits, but such is the life a space traveler) Conversely, if you are in front of the command module, raise your altitude a little bit so the command module can catch up to you.

It's of course nice if you can launch and get your orbit in such a way that you will be right near your command module as soon as your orbit is fixed up, but it's hard. It's the equivalent of trying to throw a rock at something that is sailing through the air and actually hitting it.