Most efficient way to turn back

[laythe depression]

I’m at the beginning of a new career. I have passed around Duna and rather than slowing down and getting to Duna orbit I intersected with Ike. Now I am in a very eccentric orbit with 2000Dv to turn back to Kerbin. According to Alexmoon Launch Window Chart I need around 1600Dv to turn back to Kerbin with insertion burn.

Should I change my inclination first then get into Duna or Kerbol orbit than intercept with Kerbin or get into Duna SOI and then Intercept Kerbin? What is the most efficient way to return? I know I have sufficient delta v, I can also  aerobrake but I could have easily left with less in Ike orbit.

https://drive.google.com/open?id=12KCDbTFH5GjdpveQ5222lt455O23JrsW

https://drive.google.com/open?id=1XEfxn4-YIsebzZKhDK6Ncp_k1r1CeF8D

[Zhetaan]

@laythe depression:

To start, welcome to the forum!

I think that the thing to remember is that Ike orbits Duna with a sphere of influence of only about one million metres.  Duna's is nearly fifty times larger.  Your Ike-centric inclination is extreme, but your Duna-centric inclination is going to be much less so.  Add to that the fact that though Ike has somewhat low gravity, its small sphere of influence means that your orbit is always going to be very close and thus inclination changes will be expensive.  There is probably a point where it's cheaper to make the change at Ike and then go to Duna's sphere, but if your intention is to make an interplanetary trip, then you'll probably want to transfer out to Duna high orbit and return from there, with or without a dive for Oberth effect assistance as you see fit.

However, if you want the most efficient return, then that would entail using both your orbit around Ike and Ike's position to effect an Oberth dive nearly to Duna's atmosphere at the right time to get a Kerbin transfer.  However, without a fuel depot to offset the cost of setting up the encounter in the first place, the savings probably aren't worth the trouble.  Duna and Ike just don't have enough gravity.

[Aeroboi]

First of all a unintended Ike encounter from Duna low orbit is quite probable. Ike has a good chunk of gravity relative to that of Duna so it's spere of influence extends far and it's orbit is slightly eccentric so there's a high chance you come close to Ike on your departure from Duna. Next time make sure Ike is nowhere near like on the other side of where you'd be going so you don't run into this problem next time.

What you want to do is wait until Ike's orbit is exactly between Duna and that of the Sun. According to the first picture this shows me you want to timewarp until Ike has travelled a little over one quarter orbit around Duna. Then you want to do your burn retrograde in relation to the Sun so that you fall towards it when kerbin is at the right location. This burn is done either at peri or apo around Ike and depends which direciton your orbiting.
According to the first picture you seem to be orbiting clockwise from the orientation of the picture.

So what you do is wait until Ike has traveled just over one quarter orbit, then make a maneuver node at Ike Periapsis to meet Kerbin.

Because your orbit is highly eccentric you should already have a lot of energy so you will need a lot less then 1600m/s

Edited September 14, 2018 by Aeroboi

[Kryxal]

The VERY first thing to do is alter your orbit so you don't plow into Ike the next time around ... a small burn at AP should do that, or you can break orbit to head for Duna.  You're probably going to be best off thinking through how you want to do things, because getting to either a high Duna orbit or a low eccentric one shouldn't be too difficult, but they can interact badly with return windows if you're not careful.

[bewing]

You realize that you've got a couple years to wait for an efficient launch window back to Kerbin?

[Plusck]

If you are already in a low Ike orbit, it is cheaper to burn straight back to Kerbin from there than to return to Duna orbit first. The interplanetary transfer calculators can't show this because they'd need to add another stage in the calculations (escape velocity from Ike translating into an escape velocity from Duna) to work it out.

I don't think it matters much whether it's equatorial or not: if it's inclined, the main effect will to make your Kerbin intercept inclined and therefore a bit trickier to hit without needing a correction burn on the way.

And if it's eccentric, that's fine (and is in fact an advantage) as long as you need to burn at Pe.

And that last point is where you may have a problem. Your Pe is entirely on the wrong side of Ike to burn back to Kerbin at this point in Duna's orbit. It's quite possible that you're perfectly set up to go to Dres with a minimal burn, now, if that interests you, but you'll only barely be able to capture once you got there, with no way of going home...
However, when Duna is on the other side of the sun, that Pe would be in the right place. If you have a transfer window at that time (approx. 400 days), then you can burn home straight from Ike Pe. This is a distinct possibility, so you're probably best doing nothing at all until you have a better idea about that.

So as others have said:

• you'll probably need to return to Duna orbit before burning for Kerbin, unless your transfer window is in 400 days or so;
• your next transfer window to Kerbin is a long way in the future, anyway, so make the most of your low passes around Ike to get EVA and gravioli science from multiple biomes if possible (and you may want to lower Pe to about 15km for this purpose);
• when you are ready to burn back to Kerbin, you have two conflicting things you want to do:
  • raise your Ike orbit to send you into a dive down to Duna so that your Duna Pe is as low as possible on the prograde (with respect to the sun) side of Duna, to minimize your Duna-Kerbin transfer burn; and
  • raise your Ike orbit to send you into a Duna orbit that has its AN/DN roughly aligned with prograde/retrograde, so that the burn back to Kerbin needs little or no correction.

Frankly, I doubt you can do both of those last two things at the same time. To get the lowest Duna Pe, then you should escape Ike when it is between Duna and the sun, and you'll end up in an inclined eccentric orbit with Pe on the dark side of Duna (as we see it now) which is maybe the right way to be facing when the next transfer window comes up, but only if that is in about 600 days.
Therefore the simplest option would be to minimize the inclination of your resulting Duna orbit, which means escaping Ike with minimal excess velocity when your Ike Ap (as you see it now) is as far away from Duna as possible (and which is currently prograde with respect to the sun). The burn back to Kerbin is going to be expensive, but doable if you're very careful not to waste fuel setting it up.