Need help planning my Duna Flyby/return rocket.

[DunaManiac]

Just asking, can I go around Duna and come back to Kerbin by a gravity assist? Or should I boost up onto a higher, but prograde orbit?

[Spaceman17]

yes you can but you have to achieve orbit around duna by ike graviti assist then turn your engines when duna is in the correct position respect kerbin and thencorrect your orbit at half way to ensure reentry

the best way i seen it works is launching a orion capsule with sls block 2 in a supossed configuration of crew. but kerbin and duna has to be in a launch window

[Cpt Kerbalkrunch]

You really are a @DunaManiac. 

From what I understand of your question, you actually don't need to do anything at all. A simple flyby is just going to leave you in a solar orbit with Ap at Duna and Pe at Kerbin. You'll eventually encounter Kerbin again without even trying. I don't know the resonance of the two, so I don't know how many orbits it'll take, but you can always place a maneuver node to see where your going to intersect again and the best way to shorten the timeframe.

[Kryxal]

Probably the easiest way to do a straight flyby would be to plan an orbit that would take 2 years and do a long-range fly-by of Duna, probably on the way back down from apoapsis.  It would require a correction burn, but if you're only in Duna's SoI for an hour or two, it should be fairly minor.

[Zhetaan]

Simply entering and leaving Duna's orbit makes a gravity assist; you get gravitational influence on your flight path every time you pass by any celestial body that is not the destination of the flight.  Whether that influence counts as an assist depends on whether you consider it assistance.  Maybe it's really gravitational hinderance (I'm looking at you, Ike).

Since you ask about boosting to a prograde orbit, I wonder whether you mean something along the lines of a free-return trajectory such as many of the Apollo missions used.  If that is the case, then no; the gravitational relationship is not the same, so the concept does not apply.  Free-return (with the retrograde flyby) is only possible when the vessel goes from a primary to a body orbiting that primary and then returns to the primary.  Duna does not orbit Kerbin, so this is not possible.  If you are already in a retrograde orbit around Duna, then you can return from that orbit without needing to switch directions; just reverse the rules about when to leave.  The object is to return-burn so that your exit vector overlays Duna's retrograde orbital vector, and you can do that no matter whether you orbit prograde or retrograde to Duna itself.  You'll only be burning at local night rather than local day.

If you were asking about honest-to-goodness gravity assists, the problems with gravity assists at Duna are that Duna is relatively low-mass, so there isn't much assistance it can give you, and that it has an atmosphere, which puts a hard limit on how much of the possible assistance you can actually use.  That's not to say that it can't help you; I'm only warning you not to expect dramatic results such as you can get from Eve and Jool.

There is a Kerbin-Duna-Kerbin flight path that begins at Kerbin on day 239, costs 1303 delta-V, encounters Duna on day 372, flies by at an altitude of 65.7 km over Duna's surface, and returns to Kerbin on day 1087.  The total flight lasts for 848 days and costs 2611 m/s.  I include that because I do not know the final flyby altitude and do not wish to obligate you to an aerocapture that it may turn out you can't actually do.  I found some earlier and shorter flights, but the only cheaper ones I found fly by Duna at much higher altitudes.

[Foxster]

I suggest building some spare dV into your craft (say about 8.5kdV to allow plenty of wiggle room) and aim to achieve a low flyby of both Duna (<140km) and Ike (<50km), as well as high passes, doing all the science you can in all four locations.  

This will likely mess up a simple low-dV slingshot but there's a pretty good chance of an Ike encounter anyway so why not just build in some extra dV and go for it? You'll collect a great deal of science this way. 

If you can get the dV up around 10kdV then you'll be able to fit in an Ike landing too without too many problems. 

[Spricigo]

1 hour ago, Zhetaan said:

There is a Kerbin-Duna-Kerbin flight path that begins at Kerbin on day 239, costs 1303 delta-V, encounters Duna on day 372, flies by at an altitude of 65.7 km over Duna's surface, and returns to Kerbin on day 1087.  The total flight lasts for 848 days and costs 2611 m/s.  I include that because I do not know the final flyby altitude and do not wish to obligate you to an aerocapture that it may turn out you can't actually do.  I found some earlier and shorter flights, but the only cheaper ones I found fly by Duna at much higher altitudes.

May you explain how did you found that path? 

 

[Zhetaan]

I used Flyby Finder, by @PLAD.  It takes a bit of guesswork to figure out a set of initial conditions that will yield results, but it works.

[PLAD]

I got slightly different results from Zhetaan on the lowest dV K-D-K flight in the first window and the lowest flyby altitude flight. You can see the Flyby Finder readouts along with images of what the flights look like in the game. I'm using the latest KSP and v0.85 of Flyby Finder, maybe that was different? His start date and flyby date are right on.

 

 

 

Both flights leave on the same day and fly by Duna on practically the same day (at vastly different altitudes), but the arrival times back at Kerbin are different. Check out the last panel though, there are many different windows for this flight path. Caution: It is risky to go for the absolute lowest flyby altitude possible, there are always little inaccuracies in the departure burn magnitude and time that make the needed flyby altitude change a bit, and if you are skimming the atmosphere then you can't correct with a lower flyby. That's why I went for a 70-km flyby in my example.

Edited July 31, 2017 by PLAD