Problem getting back to Kerbin from Mun

[hhatch]

I managed to build a rocket that got me a Mun landing. I thought getting back to Kerbin would be the easy part. As you can see my orbit is significantly inclined. I tried taking off in many different directions but I always get orbits that are way out of line with Kerbin. What am I doing wrong ?

 

[Gordon Fecyk]

Where was your original landing location on the Mun? Was it on or near its equator?

If you're just trying to get back to Kerbin from an inclined orbit and you don't care about where you land, your inclination shouldn't matter. The pictured orbit will get you home on a single aerobrake pass quite safely.

If you want to fix your inclination and you don't mind staying in space for another couple of Kerbin days, you could raise periapsis to above 70 km and just coast back up to apoapsis, to fix your inclination there for the least amount of fuel.

Otherwise you should try to launch from the Mun's surface to as close to an equatorial orbit as you can get, and adjust your inclination there before burning back toward Kerbin.

[Rocket In My Pocket]

What he said^

If it's just a Command Pod re-entry; that's fine. I wouldn't bother changing it myself.

[hhatch]

I figured out the orbit inclination problem. I simply was not following the 90 degree vector. However, this all of a sudden showed up. How did I get these numbers for apoapsis and periapsis ?

 

[Cpt Kerbalkrunch]

13 minutes ago, hhatch said:

I figured out the orbit inclination problem. I simply was not following the 90 degree vector. However, this all of a sudden showed up. How did I get these numbers for apoapsis and periapsis ?

 

No two burns are exactly alike. By changing the inclination of your last burn, you ended up in a slightly higher orbit. You just needed to burn retrograde a bit more to get your Pe back around 35km again.

When you make an escape burn, look at your projected orbit. You want your orbital path to be a straight line following Mun's orbital path to the retrograde side (or prograde if you wanted to raise your orbit). The same applies to interplanetary transfers as well. The closer you are to a straight line on your escape, the more efficient your burn is.

[Geonovast]

@Snark has a really good tutorial for Mun returns.. that I can't seem to find.

If he links to it here, I promise to bookmark it this time.

[Snark]

3 minutes ago, Geonovast said:

@Snark has a really good tutorial for Mun returns.. that I can't seem to find.

I do?  Wow... if anyone finds it, link it to me? 

Pretty sure I've offered advice on a lot of topics, but I don't recall doing a tutorial per se.

[Geonovast]

Just now, Snark said:

I do?  Wow... if anyone finds it, link it to me? 

Pretty sure I've offered advice on a lot of topics, but I don't recall doing a tutorial per se.

Well... you've posted some really helpful pictures anyway.  I guess I just assumed it was from a tutorial you'd done.

[Zhetaan]

22 hours ago, hhatch said:

I figured out the orbit inclination problem. I simply was not following the 90 degree vector. However, this all of a sudden showed up. How did I get these numbers for apoapsis and periapsis ?

Without seeing the burn itself, I'll say that you most likely burned at the wrong time, so part of your prograde with respect to the Mun became radial-out with respect to Kerbin.  It's not a major problem; you have enough delta-V to correct easily your periapsis.

The conventional wisdom that tells you to use equatorial orbits for Mun landings and such exists because it simplifies returns:  if you're in an equatorial Mun orbit, then burning prograde at the 'right time' ends up moving you retrograde with respect to Kerbin, and so you can return with only one burn.  When you are in an inclined orbit, the situation gets trickier, because the 'right time' doesn't occur every orbit of your spacecraft, but rather occurs only twice per Mun orbit of Kerbin at the points where your axis of ascending/descending nodes lines up with the Mun's prograde/retrograde.  If you can't wait for that, then you can burn to zero inclination and then leave at your pleasure, but it costs fuel.

On the other hand, if you're only slightly inclined, then you can burn for return as you would from an equatorial orbit without caring for the misalignment, but your slight misalignment will add up to a slight deviation from retrograde when you enter Kerbin's sphere of influence.

Edited December 15, 2017 by Zhetaan

[Geschosskopf]

On 12/14/2017 at 8:43 AM, hhatch said:

I managed to build a rocket that got me a Mun landing. I thought getting back to Kerbin would be the easy part. As you can see my orbit is significantly inclined. I tried taking off in many different directions but I always get orbits that are way out of line with Kerbin. What am I doing wrong ?

As @Gordon Fecyk says, inclination doesn't matter.   What matters is getting your Pe close to Kerbin.  Even if you're trying to land as close to KSC as possible, you can do that after a few inclined orbits as Kerbin rotates under you.  If you have the fuel for it, put your Kerbin Pe at about 100km and then circularize.  Then wait for KSC to rotate close to your orbit and de-orbit so you land near it.  If you don't have the fuel for that, take what you get with a direct re-entry from Mun or aerobrake 1 or more times.

When you leave Mun, you will always be in an inclined Kerbin orbit UNLESS you were landed on Mun's equator AND took off heading due E or W.  In every other situation, you will always have at least a few degrees of inclination.  Given that the rocket equation dictates that you're only returning a tiny fraction of your starting Mun rocket anyway, don't worry about the recovery funds for landing the tiny remaining bit at the antipode of KSC.  What matters is coming home at all, which is what gives you the rep and science.  The funds are already long gone.