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Ncog Nito

Interplanetary transfers and windows of oppurtunity...

Question

Good evening to you all.  I'm sure the answers to this question can be found somewhere on the forum, but apparently I'm not entering the right search criteria.  I am running an unmodded 1.2.2 version of KSP and want to know how to accomplish an interplanetary transfer using launch windows and gravity assists.  Let's just say my target is Duna.  How do I accomplish setting a flight plan from Kerbin SOI to the orbit of Duna?

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11 minutes ago, Ncog Nito said:

So, @bewing, what your saying is, don't try and set the entire flight plan from Kerbin?  Let's say, plan Kerbin to the Mun, with a low Pe.  Then from the Mun Pe, set a new maneuver, on top of the theoretical maneuver, to adjust course to your next target, like Duna, and so forth?  This is just an example, and, sorry, just trying to get mechanics down right.

Yup. Or, there's the entire concept of multiple maneuver nodes that hasn't really been discussed. You can do a lot of "theoretical" orbital maneuvering by putting a maneuver node on an orbit ... and then a maneuver node on the resutling orbit ... and then a maneuver node on the result of that one ... ad nauseum. So you can either know in advance that it's going to work -- in which case you don't need to do much planning-ahead. Or you can plan ahead with a sequence of maneuver node burns.

 

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Ok, I understand the maneuver nodes and have been using them frequently, adjusting the different icons (prograde, retrograde, radial, etc...), and swinging the node around an orbit to see what would be the best path, lowest dV, or shortest time to target, but say I am trying to get to Duna, the maneuver paths I have set only show "Kerbin Escape" times.  I have yet to see one that shows a theoretical path other than a solar orbit.  What am I missing or doing wrong?

25 minutes ago, bewing said:

Maneuver nodes. In stock it's all done by creating a theoretical orbit where you expect your ship to be, and then creating a maneuver node on that orbit, and then swinging the maneuver node around on the orbit and seeing how far and in which direction a particular dV burn will get you.

 

 

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When you say "using launch windows" -- are these ones you are looking up on the internet? Or are you asking how to find when they occur using only the stock game?

 

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9 hours ago, bewing said:

When you say "using launch windows" -- are these ones you are looking up on the internet? Or are you asking how to find when they occur using only the stock game?

 

I am asking how to find when they occur using the stock game, as well as how to chart a flight plan, and using gravity assists..

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7 minutes ago, Ncog Nito said:

I am asking how to find when they occur using the stock game, as well as how to chart a flight plan, and using gravity assists..

Maneuver nodes. In stock it's all done by creating a theoretical orbit where you expect your ship to be, and then creating a maneuver node on that orbit, and then swinging the maneuver node around on the orbit and seeing how far and in which direction a particular dV burn will get you.

 

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Zoom the map out until you can see the orbit of Duna.  The markers around Kerbin won't show you anything beyond the kerbin escape.  You'll want to pull the node out until the trajectory crosses Duna's orbit, and then play with the node until you get either a closest approach marker or an encounter somewhere along Duna's orbital path.  For best results, wait until Duna is about 45 degrees ahead of Kerbin in it's orbit, that's your transfer window.  There are also a few java applets and ingame mods that can calculate the optimum window down to the second.

Edited by Capt. Hunt

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1 minute ago, Ncog Nito said:

  I have yet to see one that shows a theoretical path other than a solar orbit.  What am I missing or doing wrong?

Maybe you just need to see how its done.

This one makes look easy: https://www.youtube.com/watch?v=TuDbhHcuZik

This one is more detailed https://www.youtube.com/watch?v=TyRfAVwDnLs

Another possibility its a setting issue: draw path conics limit too low.

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1 hour ago, Spricigo said:

Maybe you just need to see how its done.

This one makes look easy: https://www.youtube.com/watch?v=TuDbhHcuZik

This one is more detailed https://www.youtube.com/watch?v=TyRfAVwDnLs

Another possibility its a setting issue: draw path conics limit too low.

Ok, the first link was helpful, the second has an error and will not run.  Thanks for the help @Spricigo, with this and the tidbits from @bewing and @Capt. Hunt, I should be able to figure it out from here, eventually.

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How do you incorporate gravity assists into this?  Say, doing a flyby past the Mun, and then to Duna?

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18 minutes ago, Ncog Nito said:

How do you incorporate gravity assists into this?  Say, doing a flyby past the Mun, and then to Duna?

Usually by not doing gravity assists. :confused: Seriously, gravity assists add more complexity than is reasonable for the average game/player.

The idea is that instead of plotting a direct trajectory to your destination you instead plot a trajectory that will collide* with a different celestial body and then follow to the destination. It makes you flight time longer and requires better precision. In practice you are planing a 2nd (or 3rd or Nth) maneuver along the the way. which would be non-sense if you actually needed to expend fuel for it, but happens to coincide in time to a moment a celestial body will be there to perform the maneuver for you.

Don't worry, its not collide like in "hit the surface".

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2 minutes ago, Spricigo said:

 

Usually by not doing gravity assists. :confused: Seriously, gravity assists add more complexity than is reasonable for the average game/player.

The idea is that instead of plotting a direct trajectory to your destination you instead plot a trajectory that will collide* with a different celestial body and then follow to the destination. It makes you flight time longer and requires better precision. In practice you are planing a 2nd (or 3rd or Nth) maneuver along the the way. which would be non-sense if you actually needed to expend fuel for it, but happens to coincide in time to a moment a celestial body will be there to perform the maneuver for you.

Don't worry, its not collide like in "hit the surface".

I know what gravity assists are and the fundamentals that make it feasible.  Such as the Voyager probes, they used gravity assists to sling shot themselves passed each planet in order to get to the next with very low, if any, consumption of fuel.  I'm just trying to figure out how to plot the trajectories with the game basics.

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6 minutes ago, Ncog Nito said:

I'm just trying to figure out how to plot the trajectories with the game basics.

If you already know there's a gravity assist coming up that you want to use, you can plan it with maneuver nodes like anything else. You would adjust the trajectory of your ship within the assisting body's sphere of influence to produce the desired orbit after exiting that SOI.

If you want to find a gravity assist to use for a particular purpose, nothing in the stock game will help with that. Your options are:

  1. An external calculator program like Flyby Finder
  2. Use a complete route precalculated by someone else
  3. Experiment with gravity assists until you understand them well enough to look for them yourself, but this still requires a lot of time and trial and error

(You may notice that both of those links lead to posts by @PLAD, this forum's resident expert on gravity assists. You can learn a lot about them by checking out his post history.)

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1 hour ago, Ncog Nito said:

How do you incorporate gravity assists into this?  Say, doing a flyby past the Mun, and then to Duna?

With great difficulty. I think there was a tool called KSP-TOT or something like that for detailed mission planning. It's a very non-trivial problem. For the case of a Mun flyby on the way to Duna, if you don't mind a distant flyby, you could delay your transfer for a few orbits until it grazes the Mun's SOI, though the closer the Mun is, the harder it'll be to fine tune the maneuver to stay on course to Duna.

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1 minute ago, Ncog Nito said:

 I'm just trying to figure out how to plot the trajectories with the game basics.

 

5 minutes ago, Spricigo said:

 In practice you are planing a 2nd (or 3rd or Nth) maneuver along the the way  to coincide in time to a moment a celestial body will be there to perform the maneuver for you.

Well, lets use Voyage as an example than. Someone at NASA noticed that was about to open good transfer windows from Earth to Jupiter, than from Jupiter to Saturn, than from Saturn to Uranus and finally from Uranus to Neptune. So they planed the flight path so each planet bended the trajectory toward the next one.

How much the trajectory will be changed depends on (1) how close you get to the planet (2) how long you stay in the planet's gravity well. The direction of the change is opposite to the side you pass.

In KSP we have some advantages over real life (like the fact there is no equipment wearing out or kerbal running out of snacks), but the basic idea remain the same.

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Gravity assists: When you do a gravity assist at planet X, you end up going in the same direction as Planet X with a lot more speed. So you aim. You say, I want to be going in the same direction as planet X when I get right here at point Y on the orbit of Planet X. And then you plan it with maneuver nodes like any other interplanetary trip -- to hit Planet X at Point Y. You want to hit it at a nice low altitude, and then you make adjustments at the Pe. Note that the adjustments are suprising -- to give yourself a stronger gravity assist to go faster to your destination, you burn retrograde at the Pe, for example.

 

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For me, the simplest way to visualize a Hohmann transfer (minimum energy transfer) is that it will leave the original orbit on the opposite side of the primary from where the target body will be when it gets there.  And that's the tricky part.  You can get the orbital period, roughly, by picking any osculating orbit (one that just touches both origin and destination orbits); then you need a way to see where your target will be in half that time (because you're only going to ride the osculating orbit from one kiss to the other, not all the way around).  You can visualize this, nearly enough, by knowing the period of the target body as well as that of the osculating orbit.  This lets you determine how much of an orbit the target will traverse while you're coasting up or down the gravity well.

Then you find your launch windows by looking for times when the target is the right fraction of an orbit ahead of or behind the original body.  For instance, if you're transferring from LKO to Mun, you want to make your burn close to orbital Munrise -- that will extend your orbit approximately at right angles to the Mun's current direction, but the time you take to get out to Mun's orbit is close to a quarter of Mun's period.

Doing this between planets is a little trickier, because neither the origin nor the target orbit can be treated as almost zero time (as can a thirty-ish minute orbit compared to Mun's 36 hours or so) -- but the idea of burning on the opposite side of the orbit from where the target will be when you get there always holds.

I don't think there are tools in the stock game that can let you set up gravity assists other than by trial and error (and you may need hyperedit, or even to start a new save and warp to a particular range of dates, to even attempt to set up a maneuver node). 

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36 minutes ago, bewing said:

Gravity assists: When you do a gravity assist at planet X, you end up going in the same direction as Planet X with a lot more speed. So you aim. You say, I want to be going in the same direction as planet X when I get right here at point Y on the orbit of Planet X. And then you plan it with maneuver nodes like any other interplanetary trip -- to hit Planet X at Point Y. You want to hit it at a nice low altitude, and then you make adjustments at the Pe. Note that the adjustments are suprising -- to give yourself a stronger gravity assist to go faster to your destination, you burn retrograde at the Pe, for example.

 

So, @bewing, what your saying is, don't try and set the entire flight plan from Kerbin?  Let's say, plan Kerbin to the Mun, with a low Pe.  Then from the Mun Pe, set a new maneuver, on top of the theoretical maneuver, to adjust course to your next target, like Duna, and so forth?  This is just an example, and, sorry, just trying to get mechanics down right.

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2 minutes ago, bewing said:

Yup. Or, there's the entire concept of multiple maneuver nodes that hasn't really been discussed. You can do a lot of "theoretical" orbital maneuvering by putting a maneuver node on an orbit ... and then a maneuver node on the resutling orbit ... and then a maneuver node on the result of that one ... ad nauseum. So you can either know in advance that it's going to work -- in which case you don't need to do much planning-ahead. Or you can plan ahead with a sequence of maneuver node burns.

 

Thank you @bewing.  That's what I have been trying to figure out!  I.ve been under the assumption that you had to plan this all out from one maneuver from Kerbin.  Now it makes sense.  Plan several out along the flight path to your destination.  Also, thanks to the rest for the valuable input.  I will take a look at @PLAD post history as @HebaruSan suggested as well.  It was all much appreciated.

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3 hours ago, Ncog Nito said:

I know what gravity assists are and the fundamentals that make it feasible.  Such as the Voyager probes, they used gravity assists to sling shot themselves passed each planet in order to get to the next with very low, if any, consumption of fuel.  I'm just trying to figure out how to plot the trajectories with the game basics.

The key to the Voyager missions was that they were planned to take advantage of a rare celestial event where all of the planets were aligned perfectly for the grand tour.  Allegedly in KSP, this occurs at game start, but since I generally play career, I've never been able to take advantage of this.

 

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1 hour ago, Ncog Nito said:

 I.ve been under the assumption that you had to plan this all out from one maneuver from Kerbin.

Important to notice that you "carry"  the results of previous maneuvers.  

Let's say you planned: departure from Kerb,  gravity assist at Eve,  another gravity assist at Eve,  gravity assist at Kerbin and finally arrival at Jool.  

But  when you departed from Kerbin you weren't precise so,  while you still meet Eve,  not as close as you planned.  You need a correction in the trajectory or you will miss the 2nd encounter with Eve. And by consequence, not reaching Jool on the planned date. 

Eventually a new encounter with Eve (or another place net)  can put back on track.  But that may be too late if some contract  expires in the mean time. 

Edited by Spricigo

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