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Interplanetary Transfers


I'm going to try asking this one last time.  I will ask that, no matter who responds, please understand that I'm frustrated and honestly need a direct answer.  Don't beat around the bush, don't give me multiple explanations, don't point me to things I've already pointed out that I've seen.  Please and thank you in advance.

With that said, I need help on interplanetary transfers.  I've seen the spreadsheet to calculate transfer windows.  I've seen the stuff about calculating dV.  I've seen the tutorials on transferring from Kerbin to the Mun and/or Minmus.  I can get from Kerbin to Minmus and back with zero issue.  I can craft rockets.  I can fly them all over Kerbin's SOI.  What I cannot do is transfer outside of Kerbin's SOI to Duna.  Or even inward towards Sol to Eve.  I cannot do it, no matter how hard I try.

This morning, I literally transferred from Kerbin to the Mun, and then from Mun to Minmus.  Why?  Because I was under the impression that this was good practice (which it is), and that to go from Kerbin to Duna (as an example) you simply go to one of Kerbin's floating rocks, create a node, zoom out to the solar system view, and do the same thing.  Well, that didn't work.  I'm orbiting Minmus, trying to create a node, and I can't even get an intercept to Duna's orbit, let alone an encounter.  All that happens is that the Ap and Pe continue to shift and move as if I'm doing something, and then the orbit actually flips around to a different direction, and then...nothing.  I even tried doing a blind Minmus escape, just throttling up until the Ap on the other side of the rock went into an escape trajectory, at which point I just found myself in orbit around Sol.  So I guess I can leave the SOI, I just can't get to anybody else's.

What I need is a simple, easy-to-follow, step-by-step explanation of how to transfer to another planet.  I don't want a video where someone has built a rocket in warp speed and doesn't explain things.  I don't want someone to say "Well, just create a node".  I don't want some convoluted answer about transfer windows or dV calculation because, once I know how to do it, I can calculate those on my own with the tools that are out there.  I don't want someone to say "Let MechJeb do it for you" because that doesn't teach me how to do it on my own.

I cannot ask this more plainly and more simply.  I'm stuck, and I need help.  If I cannot get the help I need, this game ends up going to the wayside in my "Unable to Play" pile of games.  Yeah, there's lots to do in Kerbin's SOI.  But that's not even scratching the surface of this game.  Please, someone, help.  Just give me a simple set of instructions to follow.  "Do this, then do this, then do this".  With images, if at all possible.

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@Popestar, as others have said, this is tough stuff to understand.  There is little in your life's experiences that can be adapted to Astronautics.  Everything you will have to learn using a lot of the 1st Principles of Physics as they apply to the Solar System, be it in KSP or in real life.  This is tough, but with care and taking your time to skim lightly and come back often to this tricky topic, can be mastered.

The key is to take this on in little bites that you can accumulate.  Try to imagine you're in Map Mode and picture the orbits involved, zooming in to see the parking orbits, zooming out to see the orbits around Kerbol.  Imagine in a general overview the thrust of the burns and how they will affect velocities with respect to the bodies around Kerbol and Kerbol itself.

Although I've referenced a source filled with formula, don't bother with those formulae and the actual numbers.  Getting the mental images at various scales and the ideas in mind is the key to understanding.  The rough size and direction of the numbers involved is good enough.  The formulae and detailed numbers are layered onto that understanding later if necessary.  Read the text and get the feel for the nature of things in Interplanetary Transfer.

Because Interplanetary Transfer is usually done parking orbit to parking orbit, it comes down to 2 timings, Time #1 and Time #2.  Each of these Times is also associated with an Angle to help determine it.

Below I'll express this as travelling from Kerbin to Duna to shorten wording.  It can be adapted to any pair of a current body orbiting Kerbol and a target body orbiting Kerbol.  Or any current and target bodies orbiting any central massive object.  The big difference is the changes in determining Times #1 and #2 between going out (say from Kerbin to Duna) and going in (say from Kerbin to Eve).

On the chart @Snark referenced (copied below), the Planetary phase angle is determined on the left pane and gives Time #1, while the Ejection Angle is determined on the right pane and gives Time #2.  Given the two bodies as current and target, these two Angles can be predetermined (and have been by others), which then can be used to determine the two Times.

  1. Time #1: When in the orbit of Kerbin the delta-V cost to Duna is near minimum.  Anything well away from this time is a direct hyperbolic trajectory and needs a LOT more delta-V.
    1. The relative positions of Kerbin and Duna in their orbits around Kerbol determines the Planetary phase angle.  This is used to determine Time #1.
    2. How much delta-V over the minimum the spacecraft has available determines how big the Launch window is around this Time #1.
    3. This produces a transfer orbit from Kerbin to Duna.  This is usually a half of an elliptical orbit, called a Hohmann transfer orbit.
    4. The speed of Kerbin orbiting Kerbol is how fast the spacecraft is moving with respect to Kerbol.  To get into the transfer orbit, with respect to Kerbol, the spacecraft needs to either speed up to go out (say from Kerbin to Duna) or slow down to go in (says from Kerbin to Eve), in both cases matching the speed in the transfer orbit at the start near the current body orbiting Kerbol.
    5. Besides that speed change in #1.4, the spacecraft needs to escape from Kerbin.  This then leads to....
  2. Time #2: Near Time #1 within the Launch Window, when in the parking orbit around Kerbin to start the Ejection burn to get into the right hyperbolic escape orbit leaving Kerbin with the needed Hyperbolic excess velocity (the remaining relative velocity of the spacecraft after it has effectively escaped Kerbin) either Prograde for out (say from Kerbin to Duna) or Retrograde for in (say from Kerbin to Eve) that matches the speed change in #1.4.  The whole burn delta-V will be larger because the spacecraft needs to escape from Kerbin as well.
    1. While the spacecraft is under thrust, this is called the Power track and is an accelerated trajectory.  How much angle around the current body this power track does, along with the angle the hyperbolic escape orbit does before leaving the SOI of Kerbin, gives the Ejection angle.  This is used to determine Time #2.
    2. The hyperbolic excess velocity is either Prograde with respect to Kerbin's velocity around Kerbol and thus added to the spacecraft's velocity with respect to Kerbol to go out (say from Kerbin to Duna); or Retrograde with respect to Kerbin's velocity around Kerbol to go in (say from Kerbin to Eve).
    3. Given a standard Prograde parking orbit, when going out (say from Kerbin to Duna), the burn will be around the dark side of Kerbin.
    4. Similarly, when going in (say from Kerbin to Eve), the burn will be around the lighted side of Kerbin.

Again, on the chart @Snark referenced, the Planetary phase angle is determined on the left pane and gives Time #1, while the Ejection Angle is determined on the right pane and gives Time #2.


On 2/14/2021 at 5:42 PM, Snark said:

Does this help?



Further information can be gleaned from the master document I learned from many decades ago.  This one slim volume, already decades old when I read it, has compressed information in spades about all parts of Astronautics, from getting into orbit to going from the Earth to Mars.  I had to seriously up my game to understand many parts of it, including learning a new Calculus tool, Lagrangian mechanics, to plot the path of the Power Track.

That level of study isn't needed here.  I'd suggest just skimming over the calculations just to get a better feel for all the other details.

The Mars Project (1953) by Wernher von Braun

Archive.org has it online now.


Here's where it begins on Interplanetary Transfer and covers Time #1 and the Planetary phase angle.


Here's where it starts talking about the Time #2, the start of the ejection burn and the Ejection angle.


This took me a lot of effort to understand.  But I got there.  Now I get to apply that knowledge to KSP.

Once you start to get an idea about Interplanetary Transfer, @HebaruSan's Planning Node is a new fantastic tool to plot Interplanetary Trajectories.   Its two parts are exactly equivalent to determining my Times #1 and #2 above.  Because it's working in the game, the determination also automatically handles the level of details like the current and target bodies being in elliptical orbits which are tilted with respect to one another.

While it's possible to manually plot, or use the tools in good mods like MechJeb, this sort of exacting tool will be a KSP gamechanger. :)



Edited by Jacke
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Hey man, I think most of us really are trying to help, it's just we're a bit confused too – specifically, about what it is, is confusing you.

So I went back to your original question, and here's my best effort at a simple, 1-2-3 explanation of something that really isn't simple at all... not until you've done it a hundred times, anyway: how to execute a basic Hohmann transfer from one planet to another. 

First, determine your launch window. For example, go to http://alexmoon.github.io/ksp/ and enter the date, desired starting orbit, and destination. Note that while the utility gives you an extremely precise answer, in practice there's a fair bit of slop there, it doesn't matter much if you burn some hours or even days before or after. So let's go to Duna: I entered year 1, day 1, starting orbit 150 km, target: Duna. I got an ejection burn of 1018 m/s on Year 1, day 286.

  1. Build a craft capable of executing the burn. It needs to have the dV obviously, but until you're more comfortable with this stuff, give it sufficient TWR to execute it in one go – since we're practicing, make it 0.5 or more (Kerbin gravity).
  2. Fly your craft to your starting orbit – in this case, 150 km – and then time warp to the transfer window.
  3. Set a manoeuvre node anywhere on your orbit, and extend the prograde to match the computation from the transfer window planner, more or less. Let's say 1020 m/s. 
  4. Move the manoeuvre node so that you eject from the Kerbin system prograde. Meaning, in a tangent, in the same direction Kerbin is orbiting the Sun.
  5. Observe your planned orbit. It should intersect Duna's orbit, or touch it (if you're really lucky with your transfer window). Set Duna as your target, and observe the encounter arrowheads.
  6. Move the manoeuvre node ever slightly forward and back on your 150 km Kerbin orbit, and watch the encounter markers close in. Once they're pretty close, carefully adjust prograde/retrograde on your manoeuvre node. Boom, you've got your encounter.
  7. Since you have pretty good TWR, you can execute the burn quite "mechanically" -- just point your craft at the marker on the navball and burn when indicated. You're on your way!

OK, at this point you have an encounter with Duna, but it's probably not a really good one, so you'll want to refine it. So plan a mid-course trajectory correction.

  1. Put a manoeuvre node about halfway between Kerbin and Duna.
  2. Focus on Duna.
  3. Adjust the burn until your trajectory is as close to equatorial to Duna as possible, and pretty low, say 60 km. Most of this will be normal/anti-normal, I suspect, with a little bit of prograde/retrograde. If Ike is in the way, add a whiff of radial in/out to delay to get there at a more suitable time.
  4. Time-warp to it, and burn. Set the thrust limiter to 1% or use RCS for fine-tuning.

And that's it really.

Once you've got this technique down, you can start worrying about more efficient ways to get there: 

  • Start from Munar orbit – trickier, because you'll have to burn down your Kerbin Pe to 100 km or so, in such a way that your orbit points in the direction you want to eject, and you're at Pe within your transfer window; the ejection burn at Kerbin Pe will be the same as with the basic version (Minmus isn't worth it IMO, timing it is much harder because of the longer orbital period and also there's some inclination to deal with)
  • Do this with a more efficient, lower TWR craft, splitting the burn into two: first one raises your Ap to 800 km or so, second one ejects
  • Read up on and experiment with other types of transfers
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Before I ask my question, I want to stress that I'm not pointing my finger at anyone, it's a genuine question, not trying to mock or anything.

Why is an interplanetary transfer so different from a Kerbin - Mun transfer? For me, they're kind of the same, the only difference is with a Mun transfer you don't have to wait for a window. You target your location, you put a maneuver node on a specific point in your orbit (that a mod like transfer window planner can show you, or a website like https://ksp.olex.biz/) and match its time of execution with the transfer window, you extend prograde until you intersect the orbit of your target, you finetune the maneuver node until you get the encounter you want. If necessary you do a mid-course correction.

For me, that's it really...
With Moho and Dress it's a little more complex due to their inclination, but for me that's just like a minmus transfer on a bigger scale.


Edited by modus
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With Kerbin --> Mun, in a general sense its parent-->child. You never actually "fully leave" the SOI of Kerbin, yes you enter the SOI of Mun but that's enclosed within Kerbin's SOI. So its one 'step'.

With Kerbin-->Duna you have to go via a solar orbit, so its child-->parent-->child. 

One analogy I like to use is snooker. Kerbin--> Mun is equivalent to hitting the cue ball precisely, so it hits another and pots it. Kerbin-->Duna would be a "plant shot" ie hitting the cue ball to hit an intermediate, to hit another into the pocket.

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34 minutes ago, modus said:

Why is an interplanetary transfer so different from a Kerbin - Mun transfer?

You instinct is right, they are kind of the same. I'm just speculating there but I think what is most often the cause of confusion is that the craft depart from "an orbit withing an orbit". It's not so obvious that the maneuver is intended to change the current orbit the craft sares with Kerbin to an orbit that will intercept Duna (or whatever other pair of celestial bodies) and even then it add anothe leyers of complexity.  Does it makes sense to you?


edit:because  @paul_c  just beat me with a cleaner explanation.

Edited by Spricigo
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2 minutes ago, modus said:

Why is an interplanetary transfer so different from a Kerbin - Mun transfer?

Imagine your spacecraft is somehow circling around a space station in orbit around Kerbin and you get the analogy to an Interplanetary Transfer.  You have to align in two ways:

  1. Where in the orbit around Kerbin.
  2. Where in the circling around the space station.

And those #1 and #2 align with the Times #1 and #2 in my explanation above.  In reality, the spacecraft isn't circling around a space station, so Time #2 isn't a factor.

The other difference is in delta-V magnitude.  Once you get enough delta-V to do a Hohmann Transfer from Kerbin parking orbit to Mun or Minmus, it's not much more to do a Free-Return Trajectory to Mun or a Fast Transfer (long Elliptical) to Minmus, which is how I do them respectively.   The Free-Return Trajectory is great for Mun flybys that return to the vicinity of Kerbin without another major thrust maneuver.  By the time I fly to Minmus, it's going to be with an orbiter / lander and often there's resources on the spacecraft that are better to minimize by shortening the flight time.  In both cases the spacecraft reserve delta-V is about the same size as the difference between a Hohmann Transfer and the faster trajectories, so it's not hard to tweak the design for a bit more delta-V.

Once it's Interplanetary, the minimum delta-V values for a Hohmann Transfer are much larger and those for other transfers even moreso.  There's no equivalent of the Free-Return Trajectory because Kerbin won't be in the right place upon return; to do a Duna flyby and return to Kerbin needs a special orbit that goes beyond Duna and needs to allow for the effect of the Duna flyby and is a bit tricky.   And to do a Fast Transfer sees the delta-V requirements quickly become too much.  Extra and reserve delta-V are for widening the Launch Window.

Going after a difficult target like Moho, on a significantly elliptical orbit with significant inclination, make the nuances worse.  Finding the minimal delta-V in a combination of maneuvers is hard.  Slight changes in a burn or what sequences of burns are done can greatly increase or decrease the delta-V required.  The usual approach here is a lot of reserve delta-V and aim to do each maneuver as best as possible but having the delta-V to take what can be achieved.

Often KSP is played without any life-support or other flight-time constraints, so using multiple orbits and passes to minimize delta-V is common.  Once there's any factor which makes time matter, like life-support, radiation, etc., things are rather different.  My use of Fast Transfers to Minmus comes from my years of playing Better Than Starting Manned, where the Minmus missions were done before any solar panels were unlocked and the lifetime of the craft was down to how long the batteries could run a probe core.  However, a Minmus Fast Transfer isn't that much more delta-V than a Hohmann Transfer (about an added 500m/s).

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3 minutes ago, paul_c said:

With Kerbin --> Mun, in a general sense its parent-->child. You never actually "fully leave" the SOI of Kerbin, yes you enter the SOI of Mun but that's enclosed within Kerbin's SOI. So its one 'step'.

With Kerbin-->Duna you have to go via a solar orbit, so its child-->parent-->child.

Yeah I get that, but in the end, what you do (plan node, extend node, wait, burn, ...) is the same, no?

6 minutes ago, paul_c said:

One analogy I like to use is snooker. Kerbin--> Mun is equivalent to hitting the cue ball precisely, so it hits another and pots it. Kerbin-->Duna would be a "plant shot" ie hitting the cue ball to hit an intermediate, to hit another into the pocket.

This I don't really agree with, unless you use a gravity assist.

Anyway, I don't want to steal the thread. My transfers work out just fine, it was just something I wondered, reading all these good answers:wink:

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One of the items I didn't see anyone talk about here (maybe I missed it) was the in-game node editor that's available for precise changes to a node:


Once I discovered this I had a much easier time creating nodes and getting to where I wanted to go... I rely on MJ2 and Astrogator for the more tedious routine work, but for my first asteroid capture mission, as well as setting up multiple mid-course corrections for my first Eve and Gilly adventure, being able to edit plane and pro/retro-grade burn values (as well as adjusting the thrust limits of the engine on the ship) made all the difference compared to relying on fine motor skills with the mouse.  

I think I've seen a mod or two that offers additional features for editing nodes, I just haven't tried them.  

Edited by maddog59
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