Any tips on returning to Kerbin immediately after Duna transfer?

[jkopp415]

Probably an awful question but I was wondering if it was possible to return to Kerbin fairly soon (a few days in game) after an interplanetary transfer to Duna instead of waiting for the next transfer window. I am also trying to keep the delta-v as low as possible.

[bewing]

Basically no. The transfer windows don't line up that way if you are going for minimum deltaV transfers.

 

[OHara]

As you may well know, the goals of immediate return and low-delta-v are in conflict.  The usual low-delta-V Hohmann transfer has you en-route when Kerbin and Duna are closest to each other, ('conjunction') with Kerbin passing on the inside.  Then you wait until half a journey-time before the next conjunction to return, and the conjunctions are a couple years apart.
You could find a low-delta-V solution that has a long transit time, sitting in orbit around the sun until the next alignment rather than waiting on Duna, but that would seem to miss the point. 

You could look into leaving a couple months early so that you arrive at Duna during Kerbin-Duna conjunction, and the leave essentially immediately.   The transfer-planners are not set up for exactly this, but you can use them to find the least-expensive trip with a fixed arrival date (the conjunction).  It would cost more delta-V but maybe not too much more.

Edited July 3, 2018 by OHara

[blakemw]

It is possible but it's hard.

You have to use some combination of leaving early in the transfer window to Duna and a fast transfer (low trip time), when using Transfer Window Planner it's arrival date you care about, you want it to be as soon as possible. This will result in you arriving late in the Duna->Kerbin transfer window, allowing you to (nearly) immediately transfer back. The deltaV requirements are not absurd, I can't remember exactly what, but budget maybe 2000m/s extra than for perfect Hohmann transfers.

Edited July 3, 2018 by blakemw

[Kesa]

You can't optimize time and Dv simultaneously, and if you want to go back home, there is no point in minimizing Dv. The more Dv you spend, the sooner you'll arrive, and spared Dv is useless if your destination is home.

The question you should ask yourself is "how soon can I go home using no more than X Dv, and how?" where X is your total Dv available. And likely, you'll burn all your remaining fuel minus margin.

Maybe try using this tool https://alexmoon.github.io/ksp/ to figure out when and how to leave (I would try with manoeuver nodes alone first though).

 

Using this tool and assuming some informations, I found that if you have about 2000 m/s (instead of the minimal 650) you can leave right away and save 170 days. If you have about 1000 m/s Dv, you can wait about (a bit more) the same time you would have waited for an ideal transfer, but take a route 60 days shorter.

Edited July 3, 2018 by Kesa

[Zhetaan]

It's certainly possible.  You can always fly outside the transfer window; just don't be surprised if you run into the wall of too little delta-V.

As others have said, you have to trade delta-V for time.  Actually, you have to trade delta-V for time twice, because there are no brakes in space.  Burning for extra speed to get home will have you hitting Kerbin's atmosphere at faster-than-nominal interplanetary speeds, and may require a braking burn to ensure you survive your return.

There is an easy way to ensure that you get an encounter on return for a near-minimum of delta-V (near-minimum is subject to interpretation, but let's go with the terminology), but it requires two complete orbits, and that adds to the transit time.  What you do is burn away from Duna to rendezvous with Kerbin's orbit and pay no attention to where Kerbin actually is:  you aren't looking for an encounter on the first orbit, you just want a periapsis that is on the Kerbin orbital line.  Once you are on that line, you burn to raise or lower your apoapsis (possibly lowering it so much that it becomes the new periapsis) so that you get a Kerbin encounter on the next orbit.  If this strikes you as similar to orbital rendezvous with a space station, then you have much to go on:  burning for capture is essentially the same as rendezvousing and matching orbital velocities with the target planet.

Ensuring a near-minimum of time is another matter entirely; keep in mind that most orbital manoeuvres are normally performed under constraints of fuel rather than time.  Perhaps that changes when you add life support (there are arguments for using less-efficient windows if the extra fuel needed for the fast route masses less than the extra food needed for the slow one), but without knowing your rocket's capabilities, I cannot advise you on the suitability of any particular route.  Eventually, given infinite (and massless) fuel, the most time-efficient transfer involves constant thrust with a midpoint turnover, but even that requires that you consider your rocket carefully:  constant thrust with an Ant versus a Rhino makes a difference, after all.

[jkopp415]

Thank you all for your great answers, I knew I had to leave Kerbin orbit eventually!

[Kryxal]

Unfortunately for you, the transfer windows line up poorly for this.  The return window for Duna is actually earlier than the outbound window, and the next window isn't for a while.  Anyway, you should be able to manage on about 7 km/s for to-orbit and back from LKO.  Look for an arrival about day 90/360, depending on your day length, from the 1/1 start.

[OHara]

In the context of Mars missions, the short-stay trip is  called an 'opposition-class' mission.

I'll put what I think is the most satisfactory short-stay mission-plan to Duna in the spoiler.

Spoiler

The sun, Kerbin and Duna are in a line on year 1 day 330, so that will be the date of the visit on Duna.  
Leave Kerbin on day 10, 320 days en route, arrive Duna day 330.
Leave Duna day 330,  325 days en route, arrive Kerbin day 655 (a.k.a Year 1, Day 229)
Total delta-V is 3000m/s each way (of which aero-braking could save nearly half) not including any landings.

I plotted a free-return trajectory, passing by Duna without stopping and returning to Kerbin,  in the image.  By leaving early enough to take 3/4 of an elliptical orbit to Duna, that orbit can dip closer to the sun and help us catch up to Duna without as much fuel.  Shorter trips cost much more fuel.

For comparison, the minimum-fuel trip needs 1650m/s each way with a schedule:
Leave Kerbin on day 231, 275 days en route, arrive Duna day 506, and stay for 862 days
Leave Duna on day 1093, 267 days en route, arrive Kerbin day 1361 (a.k.a Year 4, Day 83)