• 0
The Flying Kerbal

How To Do A Duna Flyby & Kerbin Return?

Question

OK guys... after playing KSP for three years, and while by no means a Scott Manley, I do feel I've made real progress in this game.  Initially I'd run here as soon as I hit a problem looking for help, but gradually this became less and less, until now!  However after four days of frustration, cursing, swearing, and almost putting something solid through the monitor of the computer, I have to admit on this one I'm beat. ;.;

I'm playing a career in which I'm trying to stick only to "World First" type contracts, keeping private contracts to an absolute minimum.  And things were going great, right up until I got a contract when I didn't expect it, requiring a science gathering flyby of Duna and return to Kerbin.

For a number of reasons I don't want to enter Duna orbit, not least because I haven't unlocked powerful enough antenna to make communications to Duna possible.  Also, if I were to enter orbit of Duna, I would then not be offered the contract to do so at a later date, loosing money in the process.

So what I'm trying to do is a two year orbit which would see my vessel just skimming through the Duna SOI for a very short time, maybe an hour or two, before continuing it's Kerbol orbit, ending back encountering Kerbin two years after setting out.

I know it's possible, I've seen screenshots of it on various forums including - I'm almost certain - this one.  However I can't find any with a guide explaining how to perform this manoeuvre, such as timings, angles, etc.

So can anyone talk me through how to do this?  Keep it as jargon free as possible as there is so much of it and as I've already said, I'm no Scott Manley.

Thanks all. 

Share this post


Link to post
Share on other sites

8 answers to this question

Recommended Posts

  • 1

A Kerbin sky circle around the sun takes1 year by definition.  Duna's bigger sky circle takes longer.  Ship's sky circle from Kerbin, out to Duna and back also takes longer than Kerbin but not as long as Duna.  When you come back to Kerbin it will not be back where it started. but into its next sky cricle/year.  If you want to set up a sky circle that takes exactly 2 Kerbin years it will need to go out a lot further than Duna, only incidentally passing that as you go (or return) and wasting the rest of the time in deep space.

You can try to set this up by initially creating a time to drive the engines so that the ship's sky circle takes exactly two years but doesn't meet anything else.  Then adjust when the time to drive the engines is placed until the sky circle just passes by Duna.

Share this post


Link to post
Share on other sites
  • 0

After leaving Duna's SOI, you're in an orbit around Sun, and you need to do a Hohmann Transfer to Kerbin. In the end, it is essentially like going from Kerbin to Mun or Minmus.

Provided you have all required data, you could calculate the Hohmann Transfer, that is however a not-so-easy task, to say it mildly. So, apart from doing the math by yourself, you have two options then:

Either use a tool/mod, that does the calculation for you, that tool would be MechJeb.

Or you do it by try and error, fiddling around with maneuver nodes until you find an acceptable encounter.

Edited by VoidSquid

Share this post


Link to post
Share on other sites
  • 0

I would like to suggest more options, in addition to what @VoidSquid mentioned:

- there is a mod, which allows to visualize the locations of ships and planetary bodies at given time in the future: [ SlingShotter ]
- and there is a mod, which visualizes graph for ideal transfer windows between given planetary bodies: [ Transfer Window Planner ]

Share this post


Link to post
Share on other sites
  • 0

Transfer Window Planner though "only" makes porkchop calculations for transfer between CBs orbiting a same central CB (e.g. Mun to Minmus, or Kerbin to Duna), it does not calculate Hohmann Transfers.

Share this post


Link to post
Share on other sites
  • 0
16 hours ago, The Flying Kerbal said:

So what I'm trying to do is a two year orbit which would see my vessel just skimming through the Duna SOI for a very short time, maybe an hour or two, before continuing it's Kerbol orbit, ending back encountering Kerbin two years after setting out.

This won't happen. Do it step by step, pass by Duna first, THEN try and approach Kerbin. Maybe there's just the right maneuver you can make to pull this off, but you will be 10 hours into the planning when you could be 10 hours into the playing.

Share this post


Link to post
Share on other sites
  • 0

I'd figure it to be a matter of calculating the transfer (periapsis at Kerbin, period 2 Kerbin years, apoapsis as needed to make it happen) then finding an intercept with Duna (and pass in FRONT so as to bring your orbit around Kerbol down) then fine-tuning.

Share this post


Link to post
Share on other sites
  • 0

OK everyone, first I want to thank you all for taking the time to respond to my initial request for help, all messages, comments and suggestions were very much appreciated.

I have say this has been one of the most frustrating and time consuming missions I've ever attempted in KSP, when Fierce Wolf said I'd waste ten hours trying to st this up, he wasn't exaggerating!  I'm playing a career game using the Rusty Star Rockets mod.  I have imposed a few rules with keeping the use of non-RSR parts to an absolute minimum and only accepting contracts offered by the Kerbin World-Firsts Record-Keeping Society and RSR, being the two that have been the most challenging up to now.

This wasn't helped by being offered this contract immediately after fulfilling the World's First contract to plant a flag on the Mun; I haven't even been to Minmus yet!  This has meant there isn't much of the tech tree unlocked, so technology I need to control vessels on interplanetary missions is still unavailable.  Therefore to pull this off, I had to insure the encounters at Duna and Kerbin (return) were achieved by the initial transfer burn out of LKO, with a mid-course correction very shortly after entering Kerbol orbit if necessary.

So finally after much cursing and swearing, I hit upon the manoeuvre that would achieve this, and the Duna probe finally blasted out of LKO tonight!  I haven't a clue how I found this, even with all the help from you guys, I'd have to say it was down more to luck than judgement!  There will be a slight correction needed, but for now everything's going splendidly.  After so much time and effort, I decided that after completing the transfer burn, I'd complete a few simple RSR contracts that were outstanding, and then call it a night - completing the Duna flight tomorrow.

If all goes well I might post a few images showing how it all played out.

Anyway, thanks once again everyone, and "Fly Safe" as a certain Scotsman would say!

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Answer this question...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.