You should've stopped for directions Jeb...Or, why is the moon going the wrong way?

[Zorgoth1]

Short version:

I'm in low orbit around Duna, rotating in the opposite direction to Ike (see below). I want to reverse my orbit to make a landing on Ike before returning to Kerbal. I have about 5k delta-V. Is this possible? What is the most efficient way to do this?

Longer version:

I decided to RP and do a "realistic" mission to Duna. I built a large crew ship and a separate lander + transport vessel - Apollo style. While I was waiting for the transfer window, I build 2 lander probes and sent them on a fast burn to scout a nice area on Duna and Ike. The Kraken ate one but I found some nice undulating hills on Duna with the other. I decided to up my game and launch a rover with sky crane (except mine ends up on the ground, so it's a ground crane - or just crane) to accompany my 2 planned big ships to Duna.

I launched my 3 ship flotilla out of Kerbal SOI and felt very pleased with myself as they arrived in the appropriate order and I even did some nice aerobraking. That's when I started having some problems. I realized that it is much harder to place a heavy lander (my rover-crane) on a target when there is an atmosphere. Fine. Jeb can walk. Then transferred Jeb and Bob into the lander can and realized I needed a lot of fuel to slow them down enough to survive landing on Duna (far, far away from their Rover and my lander). They planted the flag, got up into LDO with just enough fuel and monoprop to execute some docking, crew/science transfers, and fuel balancing. Rover aside, mission objectives still look good. Then I started to plan a transfer to Ike orbit. Only to realize I was rotating in the opposite direction of Ike. I'd areobreaked in the wrong direction.

I tried ditching my transfer craft and docking the lander to the crew ship and burning straight normal to my Duna orbit, hoping to flip over, but this is turning out to be a mess, especially since my crew ship is unbalance with the lander on it. Maybe I need a super elliptical orbit and then burn retrograde when my speed is the lowest?

Any ideas on how to best reverse orbit with ~5k of Delta-V, a need to land on Ike and of course return safely to Kerbal?

And no, I don't have a save to go back to pre-aerobraking.

Edited June 25, 2014 by Zorgoth1

[FenrirWolf]

5k delta-v gives you room to do pretty much whatever you want.

I'd just transfer directly to Ike like normal.

[problemecium]

It's very possible to plan a maneuver node that transfers you to Ike such that your orbit when you get there goes the other way. Do that.

[LethalDose]

Yeah, what FenrirWolf said: You've got loads of dV. You can do pretty much whatever. Don't feel bad, I did the same thing on my first Duna mission, too, just with lots less dV.

Reversing your orbital direction is basically making a 180 degree inclincation change, and The "best" way to make an inclination change is at low orbital speeds. So, if you're looking to save dV on reversing your orbital direction, I'd recommend that you push your Ap as high as you can/want/feel like, then burn retrograde until it's uh, prograde. Then make sure your Pe is high enough to not slam into Duna. You can aerobrake again to drop your Ap and save a lil more dV.

I think in my situation, I just burned to Ike directly, where I had a hard (meaning high throttle, not difficult) braking maneuver to get into orbit. Worked great, still had plenty of dV to get back to Kerbin. That was with a mostly full rockomax 32 tank, a Poodle engine, and a Mk1-2 pod with all the other bits (RCS, antennae, etc.). I don't know how much dV that works out to, but it's way less than 5k. you'll be fine.

Edited June 24, 2014 by LethalDose

[cantab]

Most efficient is lithobraking on Ike. But your ship probably isn't built for it.

Next best is as LethalDose says, raise apopasis until it's nearly escape, then reverse your orbit at apoapsis, then aerobrake to put your apoapsis level with Ike's orbit.

[Pecan]

Nooo, cantab, no plane-change! Go to Ike from whichever orbit around Duna you have; adjust your intercept/orbit around Ike by fine-tuning - that's all it should take.

The most obvious similarity is with "I'm in a West-equatorial Kerbin orbit - how do I get to Mun?" questions and the answer is, "The same way you get to Mun from East-equatorial orbits".

[BudgetHedgehog]

Nooo, cantab, no plane-change! Go to Ike from whichever orbit around Duna you have; adjust your intercept/orbit around Ike by fine-tuning - that's all it should take.

What he said. As long as your Duna AP is just inside Ikes SoI, you'll have very little velocity with regards to Ike so you can either burn radially to push your P to the "correct" side or you can just land as normal - Ike is pretty similar to Mun and has only a slightly faster sidereal rotational period so you won't be losing much if you land "retrograde", possibly a couple hundred m/s or more, but seeing as you have 5k to begin with, I'd say you're safe.

If you really want to do things properly and land "prograde", then yes, a bi-elliptic inclination change transfer is your best bet - burn until your Duna Ap is almost escaping and once you get to it, burn retrograde until your PE is at the desired height then re-circularise at the desired altitude. This could require more fuel than just transferring to Ike directly though. Alt+F5 before you do anything so you can go back to this spot before you try, just to be sure.

[allmhuran]

Nooo, cantab, no plane-change! Go to Ike from whichever orbit around Duna you have; adjust your intercept/orbit around Ike by fine-tuning - that's all it should take.

The most obvious similarity is with "I'm in a West-equatorial Kerbin orbit - how do I get to Mun?" questions and the answer is, "The same way you get to Mun from East-equatorial orbits".

The problem with just going directly to the target is that the relative velocity between you and it will be vastly higher. In other words, it's an extremely expensive capture. It's true that with 5k delta-v this isn't an issue. But it's probably helpful to know what you could do instead in case a similar situation comes up again in another mission without so much fuel to spare.

Cantab's suggestion is sound, but there's a better one, it just requires a bit of patience. Use Ike to turn you around.

Step 1: You're going to need some amount of inclination relative to Ike. If you're coming in perfectly equatorially this won't work. But only a fraction of dv is required to fix this.

Step 2: Get an Ike intercept and adjust your trajectory to move your periapsis towards the north or south pole. The closer the better, but the further you try to move it the more it will cost. If you want to be really stingy, just move it a little bit and repeat this procedure over and over, with an adjustment of just a meter per second or two each time.

Step 3: Fly past Ike. Assuming you flew past the south pole, your orbit around Duna will now be much more inclined.

Step 4: Repeat steps 2 and 3, moving your Ike periapsis a little more each time, eventually past the pole and back up the other side of the moon.

Congratulations, you have now slingshotted your way into an opposite orbit for very little delta v!

Edited June 25, 2014 by allmhuran

[Tortoise]

EDIT: Nevermind, I don't know.

[cantab]

Well, let's guesstimate delta-V for Ike capture from an Ike Transfer Orbit, for the prograde and retrograde cases.

Ike's orbital speed: ~300-320 m/s (as per Wiki)

Orbital speed at apopasis of an Ike Transfer Orbit, with Pe = 370 km (Duna's radius included) and Ap = 3200 km, can be found by the equation here http://en.wikipedia.org/wiki/Elliptic_orbit#Velocity and is 140 m/s

So if you're in a retrograde transfer orbit you'll have around 280 m/s extra speed relative to Ike on entering its SOI. How exactly that affects your capture burn I'm less sure on, but a "penalty" of 280 m/s is the obvious guess.

Now compare this to my previously suggested approach.

Extra delta-V to escape Duna compared to establishing an Ike transfer: 60 m/s as per the Reddit delta-V charts.

Orbital speed at apoapsis of a near-Duna-escape orbit with Pe = 370 km and Ap = 47000 km: 10 m/s

The extra cost to reverse your orbit is then just 80 m/s if you aerobrake back down, or 140 m/s is you don't.

Either way, to be honest it's pretty small. Duna's low mass, compared to say Jool or Eve, helps us by making all our orbital speeds in its SOI lower.

[Zorgoth1]

Thanks all for the responses. I have had 100% success utilizing lithobraking. Jeb tested, Jeb approved.

And the pushing the Ap out to the edge of the SOI and then reversing works pretty well too.