3 STEPS to Learning Interplanetary Transfers

[lemon cup]

lemon cup's Illustrated Guide to
Interplanetary Transfers
IN THREE  EASY STEPS

 

PREFACE
Are you new to KSP and eager to visit other planets? Are you an accomplished pilot, but just never quite had the time or attention to devote to learning interplanetary transfers? A seasoned veteran returning from hiatus only to discover unfamiliar changes to gameplay? Or do you mostly have the hang of it but still run into a few bumps in the road from time to time?

You may be surprised to know that many KSP players have never visited another planet other than Kerbin. There are probably several reasons, not the least of which is the lack of an in-game tutorial for doing so,  as well as the drastically different process and added level of difficulty compared to playing around in Kerbin orbit - which already has a steep learning curve. 

There is in fact nothing wrong with that of course. Despite what anyone says, there is no wrong way to play KSP, and a player can reach a sense of just as much fun and accomplishment having only ever visited Kerbin orbit,  as someone who has visited all of the planets and their moons. But if you are looking to learn a new skill or refine your current techniques, please enjoy this guide. My goal here is to strike a balance between SHORT AND SIMPLE, while also going into as much DETAIL as possible.

THE 3 KEY CONCEPTS 
Okay, so the title was a lie! The "steps" that I mentioned are actually ideas, or concepts. You need know only these 3 things in order to successfully transfer to another planet. In summary they are:
      1) WHEN to go (Transfer Window)
      2) HOW MUCH rocket power you need to get there (DeltaV)
      3) HOW TO use a maneuver node to get an encounter (Plotting the Course)

Before we go on I just want to point out: A simple, step by step list of "do exactly this" instructions is all that you need to successful go to another planet. But that certainly would not teach you WHY you did those things and HOW you did them.  Learning and coming to understand these ideas in full, you will notice an immense feeling of excitement at how simple interplanetary transfers are. Don't overthink it! If you run into issues along the way, come back to these 3 concepts, start to figure out which one you're hanging up on, and reread that section below. YOU WILL GET THIS!

PREREQUISITES
Now, like anything in this game we have a few caveats. If you are setting out to attempt transferring to another planet, one should assume a good understanding of the basics of (Kerbal) spaceflight. Before reading this guide, you should know the following:
   -how to get to orbit
   -how to design a craft up to the task
   -what DeltaV is, and how to get more (or less) of it
   -basic orbital mechanics and terminology (highly recommend all the in-game tutorials )
   -orbital directions (prograde, normal, radial, etc.) what they mean, and what happens when you go that way!
   -a decent understanding of how to plot nodes, and the patience to fine tune them!

What you DON'T need to know:
   -math (besides addition and subtraction)
   -astronomy
   -advanced physics
   -anything you can't learn from a quick Google or Youtube search!

Again: Don't overthink this! I will break down and take you through everything you need to know to make this happen.
 

STEP 1: The Transfer Window

The first big hurdle of understanding to get over. It is the first thing you must correctly understand before anything else can take place, but it can seem abstract and confusing at first. Let's take this one apart and talk about what a transfer window is. If you already fully grasp this concept, you may go to the next step. If you don't, YOU MUST READ THIS STEP FIRST.

What the heck is a transfer window?
A Transfer Window is the point at which TWO OBJECTS orbiting the same PARENT BODY are going to ALIGN to enable a perfect Hohmann transfer. A minimum energy Hohmann transfer will be used because it saves the most fuel, and once you understand how to do that, you can apply that knowledge to do other, less efficient transfers if you want. But for now, we are going for efficiency! For the sake of this tutorial, we will use Duna because it is the easiest of Kerbin's neighbors to get to. 

Take a look at this picture from the Tracking Station. It shows the Sun in the middle, with Kerbin straight to the right and Duna up there minding it's own business. I've drawn some white lines so that we can take a look at the angle. For Transfer Windows, angle is everything.

Cool, nice lines. So what do they mean?
Well, imagine that you are Kerbin. That's right, you are the little blue planet. You have a giant rocket strapped to one side of you and you want to go visit your neighbor, Duna for a cup of tea. Well, you know you'll use a Hohmann transfer, since that is the most efficient after all. And you know what that Hohmann transfer will look like - you'll burn straight Prograde somewhere, then, exactly 180 degrees away - on the other side of the orbit - your apoapsis(AP) will get higher and higher. It will be just high enough to touch Duna's orbit. The big question is: when do you (Kerbin) make that burn, so that when you reach AP, Duna will be there to greet you?

Of course, you'll wait for the two planets to reach a specific alignment, and make the burn then. This is the Transfer Window, and it is just like waiting for the right time to rendezvous with another spacecraft in Kerbin orbit, only around the Sun instead! Like the picture above shows, you will wait until Kerbin and Duna form a 44 degree angle with the Sun (they don't ever change orbits so you'll always be using that angle).  As you can see, Duna is a bit too far ahead still. Kerbin is coming up behind it and causing the angle to grow smaller, so the correct course of action is to WAIT (looks like about 2 weeks maybe?) until the alignment is just right. 

How the heck did you get that?
How did I know that the Transfer Window to Duna looks like that, and that it should be roughly 44 degrees? Because (and here's the first excellent news) SOMEONE HAS ALREADY DONE ALL THE WORK FOR US! You have 2 methods to determine Transfer Angle:
   -Eye ball it (just like we're doing here) by using the wonderful online sources available to us such as This Picture, or This One, or many others like them.
   -Use a mod! MechJeb and KerbalEngineerRedux both have functions to tell you when (how many days) you'll wait to transfer to another planet. Also there is Transfer Window Planner, which is dedicated solely to helping you find the perfect time to set your maneuver and be on your way. 

Okay, so how do I take that knowledge and apply it?
      1. Go to the Tracking Station
      2. Zoom out until you can see Duna's and Kerbin's entire orbits.
      3. Orient the camera to be looking directly down, like you see above, with Kerbin exactly to the right.
      4. Time warp until Kerbin catches up to Duna and forms that magical 44 degree angle, or until just before your mod of choice tells you to.
           (Remember that as the planets move, the camera will too, so keep readjusting to get the best view)
      5. Remember to give yourself enough time to get your rockets in orbit first!

Wait you forgot one thing - I'm not Kerbin!
Yep that's right, you are a spacecraft orbiting Kerbin. But as far as the Transfer Window goes, that doesn't matter! We will figure out how to contend with that little tidbit later. For now all that matters is getting Kerbin and Duna in the right spot.

STEP 2: The DeltaV

Okay, you've set up your Transfer Window to the best of your ability; you eyeballed it as hard as you could, and the planets look to be aligned with each other.

Now, get your craft into orbit. For this, you must aim for a nearly perfect circular orbit, with a nice 0-degree inclination around Kerbin. There are a few considerations you should take into account when launching your craft but none are as important as your DeltaV. 

I know what DeltaV is, and I know how to get more of it. But how much do I need?
For any craft to make a transfer from a circular 80km Kerbin orbit to Duna (not counting the amount you'll need for when you actually get there) you need roughly 1080m/s of DeltaV.

You said there was no math... I feel like you used math to get that...
Well, technically I didn't use math (and here's the second bit of excellent news): SOMEONE HAS ALREADY DONE ALL THE WORK FOR US! Behold:

Here we have the Community DeltaV Map for stock KSP. It is the go-to tool used to find out how much rocket power you need to get places ASSUMING your Transfer Window is correct. The numbers you see represent the amount of DeltaV you need to perform various orbital maneuvers in regards to each planetary body. 

Here we go with the math again... look at all those numbers!
No no, don't worry about all those numbers. One step at time here, I will go over how we got 1080m/s in a nut shell, but I don't want to spend too much time on this. Remember, this tutorial is only showing you how to transfer to another planet, not anything past that. Let's focus on that for now.

At the bottom of the map you see Kerbin. The number 3400 above it means that you need about that much DeltaV to get to an 80km orbit. The number 950 above that: how much you need to thrust Prograde(from the 80km orbit) to escape Kerbin orbit and break out into Solar orbit. Still with me?

Now you'll see Duna represented by the red line. The first number, from the bottom, is 130. This means that, from your new Solar orbit, you will thrust an additional 130m/s Prograde to meet Duna's orbit. Add that to 950, you get 1080m/s.

But that isn't how we're going to work this one, not quite anyway. We are going to go straight from Low Kerbin Orbit, and try to get an encounter with Duna in one shot.

Okay, I'm ready. Let's do this!

STEP 3: Plotting the Course

Believe it or not, if you are now reading this and feel good about the above 2 concepts, you are in a very good spot. You've already managed to get through the two hardest-to-grasp ideas when it comes to interplanetary transfers. This next part is going to be the most familiar to you as you've already played a fair amount of KSP and have a good understanding of basic spaceflight and plotting maneuvers. The tricky part is putting it all together.

Before we go on, it's checklist time! We are about to plot a single maneuver node to go from LKO to Duna. Make sure you have accomplished the following:
      1. Time-warped to the Transfer Window, to the best of your ability. (44 degrees)
      2. Achieved a circular 80km orbit around Kerbin in the craft of your choice.
      3. Designed the craft with at least 1080m/s of DeltaV (plus 100-200 for safety)
      4. Said goodbye to whatever or whoever is onboard. Chances are (unless you've planned very far ahead) it ain't coming back. 
      5. Switched to the Map Screen, zoomed out, and select Duna as your target (Don't forget this one!)

With that out of the way, we are now ready to assume control of our spacecraft and fly to Duna! Again, this part can be tricky because it requires fine tuning and a discerning eye. But, Don't overthink this! I will tell you what to look for and how to get the right results. I'll break this part down into sections for ease of learning.

PLACE THE NODE
Here is the easiest part, all you have to do is place a node and type a number into a box. Consider the following:

You can see my craft orbiting Kerbin at about 80 km. All I have done is clicked any random spot along my orbit and placed a maneuver node. When you do, that fancy box at the bottom left will pop up - that is the Maneuver Node Editor. In the Prograde block, type the number 1080 and then press enter.  (NOTE: you can see I've actually typed 1050, that is because I was going off a different DeltaV chart at the time, and it is normal to have a deviation anywhere between 10-100m/s from what any given source will tell you. We are going for rough values here! The intent is to get us close enough, and fine tune as needed.)

So, here is the breakdown:
      1. Warp a short time so that your craft ends up on the light side of Kerbin (you'll find out why soon enough).
      2. Pick any random spot on your orbit path (spoilers: we'll be moving it soon)
      3. Click to place an empty Maneuver Node.
      4. Enter "1080" into the Prograde block and press "Enter"

Very good, as you can see you now have a burn planned that will take you from a circular orbit around Kerbin, to escaping it's Sphere Of Influence (SOI). Let's proceed.

MOVE THE NODE
Now we will pick up the maneuver node and start dragging it around our craft's orbit, the spot we want to get it is on the dark side of the planet, about 20-30 degrees after "Sun Down".  We are going to do this with the explicit purpose of obtaining one thing: the correct EJECTION ANGLE.

I have no idea what that is, maybe we should just build another space station in LKO...
Allow me to demonstrate what an Ejection Angle is. 

You can see that we've chosen to thrust Prograde in the direction of the green arrow, but we won't be going in that direction for long. We will leave Kerbin orbit, but not before it's gravity pulls us into a new direction, represented by the red arrow. That new direction is the Ejection Angle, and it is basically the spot at which you will exit Kerbin's SOI.

So how do we know where to put the node to get the right ejection angle?
We will zoom out on the Map Screen and use some of the clues that we can see.  Examine this:

Now you can see a very important piece of information. We can now see Kerbin Prograde illustrated by the blue arrow. This is the direction Kerbin is currently travelling around the Sun. Remember, we are shooting for a perfect Hohmann transfer out to Duna. That means that we should LINE UP our Ejection Angle with Kerbin Prograde...! Keep in mind Kerbin is travelling roughly 10,000m/s around the sun, and we don't have the rocket power to fight that! We need to go with the flow, and use Kerbin's velocity to our advantage. 

Like I said, let's try to drag that node around the spot on the dark side of Kerbin. You'll see that doing that causes your Ejection Angle to point pretty close to Kerbin Prograde. Zoom in and out to get the best views, and take your time.

How do I know when it's close enough?
Check it out:

Dragging the maneuver node around can be touchy and hard to do with a high degree of accuracy. But the goal is to get The Sun PE pretty close to Kerbin right now. You can see that here, I've got it sitting actually inside Kerbin's SOI, which is very good but you don't need it quite that close. Zoom out and you can see that as long as you are pretty close, you should be okay. Remember, we are dragging the node around so that our Ejection Angle is lined up with Kerbin Prograde, and The Sun PE ends up pretty close to us. 

Here's the breakdown, it's simple now that we've gone over the basic concept:
      1. Drag the node to roughly 20 degrees past "Sun Down" on the dark side of Kerbin. 
      2. Take time to ensure your ejection angle lines up with Kerbin Prograde, as close as you can get it.

Now let's take a deep breath, we're almost there! We are one step away from getting an encounter with Duna.

FINE TUNE THE NODE
If you are lucky, you might already have an encounter at this point! If you are unlucky however, you could be pretty far off. Let's check, still using the Map Screen. Remember this part WILL NOT work unless you have selected Duna as your target. (NOTE: If you are way off, head down to the TROUBLESHOOTING section below, then meet me back up here when you've got it sussed out.)

Take note of a few things in the picture above. I've zoomed out and placed the camera in a good spot to see what's going on. On the other side of the Kerbolar system, you can see those light blue Closest Approach markers. I've right clicked them to pin the values shown underneath and keep them highlighted while I work on getting them closer to each other. 

I've got an approach coming in at 88,000km. Anything inside 100,000km is very close and you only need a bit of fiddling to get an encounter. Let's start by messing with the node and see what we can get.

I'll begin by selecting the Gizmo Tab on the Maneuver Node editor and adding in different directions to see if the Closest Separation gets bigger or smaller. This is usually always necessary. Since Duna is slightly inclined, you may need to add some Normal and Antinormal here to get a good encounter. Other than that, you'll need to balance each Prograde and Retrograde input you make with Radial In and Radial Out inputs. When in doubt, just click the directions that make the distance to Closest Approach go down!

 

When you get it close enough, those Closest Approach markers will disappear and turn into the tell-tale Encounter indicators! But we're not done, let's go in for an up-close inspection of Duna's SOI and see what our approach with the planet looks like. 

Small adjustments are crucial here. With the Gizmo tab selected, grab your Input Scale Slider and drag it down to the bottom few settings to get much smaller direction inputs for fine tuning. See below.

Just like we did in the step above, click or drag various maneuver directions until your approach to the planet is as close as you want it.

Phew! Okay, now what...?
Actually, that's it! You have just successfully plotted a maneuver to another planet! And before we look back on our work, keep in mind that your first Interplanetary Transfer may not be pretty, efficient, or even survivable! The important part is that you managed to get that node put down, and understand why you took the steps you did.

In summary, we did the following in this step:
      1.  Use the Gizmo Tab (or the tool of your choice) to add different direction values to the maneuver node.
      2. Watch the Closest Approach markers to see what directions result in decreasing the distance.
      3. Switched to viewing Duna's SOI and fine tuning our approach with the planet.

   All that is left is to execute the node at the right time, and prepare for a long journey. With SAS on, click the "Node" button just to the left of the Nav Ball so that SAS stays on target for you. 

After you execute the node, depending on how closely you followed it and how well your craft performed, you may find that upon looking at your map screen again, you are no longer set for an encounter. Don't panic! My best advice is  to go ahead and time-warp out until you exit Kerbin's SOI, then plot a Mid-Course Correction Burn. These are usually pretty cheap, and for highly inclined planets like Moho and Eeloo, it is almost always necessary to do these at the Ascending or Descending Nodes. 

Now before we wrap this up and break down everything we learned, let's try to problem solve some issues you may be having with interplanetary transfers.

 

TROUBLESHOOTING THIS STEP

Don't worry if you are reading this right now scratching your head. When trying to set up the maneuver node, several problems can arise and prevent you from getting a good encounter. Here are the most common issues.

I've set up my ejection angle perfectly but my Closest Approach is way off.
This is usually because you have not lined up the Transfer Window correctly, you could be a little off. Some steps to getting past this:
-Zoom out of the map screen and assess your orbit. Does your projected orbit touch Duna's? If it is coming up short for example, add some Prograde to your maneuver, then drag the node a bit to readjust your Ejection Angle (since it will have deviated slightly after adding more). 

It's still way off.
-Start dragging the maneuver node around a little bit and changing your Ejection Angle. This will alter your Radial In and Radial Out values IN RELATION TO THE SUN, therefore causing your intercept of Duna to shift around and possibly get close enough for you to go back to the editor and fiddle with directions, until you get an encounter. The downside to this is that you will use more DeltaV to make this happen.

No good. I still can't get it, or I can but it wastes too much fuel. 
Reset your maneuver node to our original value of 1080m/s Prograde. Your Transfer Window is too far off. To figure out by how much, switch to the Gizmo tab and begin clicking the small Plus symbol at the bottom to skip orbits further and further into the future. When you do this, keep an eye on the Closest Approach indicators. Is the value getting smaller?
-if yes, you are a bit too early on your transfer window. Time warp out a few days to a week, and try again.
-if the value gets larger, that means you have missed your Transfer Window by too much. Load a previous save and try to line up your Window a bit better, or time warp to next year and try again! And consider a mod that makes lining up transfer windows easier, sometimes they can be a real pain.

I can get an encounter but it costs more DeltaV than the map says it should.
Depending on how the planets are aligned, you may be trying to catch Duna at a bad time, when it is at Apoapsis AND is off inclination with Kerbin. The worst offender is Moho - due to its high inclination, some transfers can be over twice as expensive! As far as Duna goes, it will never be prohibitively misaligned as to prevent you from getting a good encounter, you may just have to live with an extra 100m/s of DeltaV cost.

I think that about covers it. 

THE BOTTOM LINE

 

Now that we have tackled the painful learning process, I think the best way to round out this guide is to take what we know, and review the entire process. Use this checklist whenever you want to practice transfers and continue to improve your newfound skill. And yes there are more than 3 steps here, sue me! Your training on the 3 core concepts took place way up there - everything down here is bonus material at this point 

1. Find the proper Transfer Window for the planet you want to go to, and line the angles up accordingly.
2. Use a DeltaV chart to get the right values to plug into your Maneuver Editor.
3. Get into a circular low Kerbin orbit (80-100km is best)
4. Set the maneuver node down somewhere on your orbit and insert your DeltaV value into the Prograde block.
5. Drag the node around your orbit until you line up your Ejection Angle with Kerbin Prograde (or Retrograde for Eve and Moho)
6. Target the planet of choice and watch your Closest Approach indicators as you make small direction inputs with the Maneuver Editor.
7.  Achieve an encounter, then switch to the planet's view and make very small adjusts to get even closer. 
8. Select "Node" on your SAS wheel and make the burn when it's time!

And that concludes our training for today! Still foggy? That is perfectly okay, it may take several tries and rereading this guide or other materials before it finally clicks. It took me many hundreds of hours before really getting these concepts down, but hopefully I've given you the tools you need to get out there and start crashing Kerbals into far away worlds. So with that I'll say, "Fly Safe."

 

Edited March 3, 2021 by lemon cup

[Leocrabe225]

Hey! First, thank you for this, I managed to make an encounter with Eve before I read that, but the extra info and details are really appreciated. 

Then comes my question, once you reach another planet, how do you get back with the least deltaV ? Should I re orbit and wait for the transfer window, and then do the maneuver all over again? (Which makes me wait a year 'cause you miss it by the time you reach Eve if I understood correctly) Or is there an even better method?

Final question, When I managed to meet with Eve, I had an orbit with a TERRIBLE angle. Is there anything I can do without needing 4000m/s of deltaV ? Or should I avoid having such a terrible meeting angle (how?)

I hope you'll see this, thanks again for this amazing tutorial!

 

Leocrabe225

 

[lemon cup]

12 hours ago, Leocrabe225 said:

Hey! First, thank you for this, I managed to make an encounter with Eve before I read that, but the extra info and details are really appreciated. 

Then comes my question, once you reach another planet, how do you get back with the least deltaV ? Should I re orbit and wait for the transfer window, and then do the maneuver all over again? (Which makes me wait a year 'cause you miss it by the time you reach Eve if I understood correctly) Or is there an even better method?

Final question, When I managed to meet with Eve, I had an orbit with a TERRIBLE angle. Is there anything I can do without needing 4000m/s of deltaV ? Or should I avoid having such a terrible meeting angle (how?)

I hope you'll see this, thanks again for this amazing tutorial!

 

Leocrabe225

 

Glad it helped!

I am out of practice in stock KSP and never really got the hang of Eve so I will have to defer you to youtube videos on that one.

I can advise you on arriving though, which is nearly as in depth but can be summarized nicely.

Your initial ejection from Kerbin matters a lot on how much energy you have on arrival. The more accurate your Kerbin ejection is, the less energy you will have to put into injecting at Eve. But equally important to the nature of your arrival orbit is your Correction Burns and Pre-Orbit burn that you need to preform in the coast phase.

Even the most accurate transfer will probably need a small correction burn, the best place to do your first is at the AN (ascending node) or DN(descending node) of your target body, this is your opportunity to match planes and get a nice equatorial trajectory for when you arrive. A good rule is to pack a good RCS array that can fire in all directions. Once you execute the node, you can correct for error using small bursts of RCS.

When nearly done with the coast phase, I like to plan a Pre-Orbit burn before entering the planet’s sphere of influence. This is a good spot to burn Radial In or Radial Out, trying to get your periapsis as close as possible to the minimal safe height, for max gravity breaking. Even better, try to enter on the advancing side of the body, the side that is facing “forward” as it orbits the sun.

You can also do an aerobrake here, but be extremely careful with Eve! I recommend a powered capture into a highly eccentric orbit first, then whittle it down with careful aerobraking passes. 

So in a nut shell: plant a maneuver node somewhere close to AN/DN during coast. Then zoom into the target planet and adjust the node using the maneuver node tab on the lower left of the HUD. Plan more nodes closer the the planet to get as close an incoming periapsis as possible. 
 

There is more but I risk this becoming it’s own guide haha. I know there is lots more info on youtube, here are some keywords to get you started.:

interplanetary match planes
correction burn
gravity break