In this chapter I show you how to plan an interplanetary travel in Kerbal Space Program. The lander is described in the previous chapter, and the second part is here.

Chapter 6. Interplanetary Travel on Budget, part 1


Before you start (or if you want to skip reading and just fly), here are the two interplanetary calculators for KSP:

  • Launch Window Planner. It provides you abundant information on many ways to reach target planet, and I use it exclusively.

  • Interplanetary guide and calculator. Shows you when to do the Hohmann transfer, you can do this almost visually, without any special tools. But it does not guide you for more expensive but faster transfers.

  • Aerobraking calculator. Instead of guessing, saving and reloading, just calculate and do aerobraking in one pass.

You may just calculate the requirements, and then build a rocket and fly it according to the optimal transfer figures. Otherwise, here’s a bit of theory.

Transfer orbit

If there’s no atmosphere at destination planet (Moho, Eloo), you perform a typical Hohmann transfer. If there’s an atmosphere on destination planet (Eve, Kerbin, Duna, Jool, Laythe), you don’t need the insertion burn. Travel becomes a lot cheaper.

The Hohmann transfer orbit is tangent to both orbits, and it requires the least acceleration on both ends (there’s a bit more optimal version, but it saves just a couple percent of fuel). It requires precise timing, which the calculators show you. But it’s not the only way. With a steeper, crossing transfer orbit you can reach the destination faster (and aerobraking can cancel the excessive speed if you need), or slower (if you have to wait for the target to catch up).


Hohmann transfer (image from Wikipedia). 1) original orbit 2) transfer orbit 3) destination orbit.


Crossing orbit transfer.

A steeper transfer orbit lets you meet the target earlier. Or you can cross the orbit, ascent higher, let the planet catch up and encounter while descending.

You also can escape at some angle. See the three transfer options. If we use aerobraking, the steepness of arrival does not matter much: we can cancel out any extra velocity. What matters is the departure, and here are the three basic choices.


Tangential escape is the cheapest one, and the crossing escape (in the middle) is the most expensive. 10-20 days before the launch window, when you’re behind the Sun, you need to dive deeply to catch up with Duna. You’ll have to distort the orbit so much that it may take up to 5000 m/s of 𝛥v.

When you do an escape burn, what escape it will be depends on timing.


By the way, I said that a crossing escape is the most expensive, but the extra cost varies. If you escape from Jool to lower planets, it’s still quite affordable. Why? Here’s the answer.


Hope this all explains why launch windows put such hard limits, and several days earlier or later make such big difference.

The Rocket and The Trip

Last moment advice: write down the day of departure from the calculator and don’t launch in advance, because warping some days in low orbit is impossible. Warp on any landed object or a flag till that day and then launch the ship.

I built the Duna expedition rocket with a bit of extra fuel, and also made the interplanetary stage circularize the ship around Kerbin, so that ascent stages just fall down, leaving no space debris.


Last ascending stage is about to fall on Kerbin.

When you have the plan and 𝛥v for away and return burns, build a ship for them. Away and return stages usually weigh very differently: the away one is much heavier and needs more thrust.


Interplanetary stage uses Poodle engine


The rest of the ship needs LV-909 engine only.

Setting up Departure Node

Once you have an estimation of the burn from the calculator, just set the node in a reasonable position (or look it up here, at the correct angle to planet prograde). And add as much 𝛥v prograde as the calculator tells.

Set the destination planet as target and you’ll see the encounter markers if you cross it’s orbit. If not, either adjust the radius by adding more speed, or adjust the plane (on equatorial orbit it’s just the normal marker, on inclined orbits it’s more tricky).

Now you see the encounter markers. See the target position at encounter. It will be ahead or behind your position. Don’t try shooting into that marker. If you try hitting the marker behind, surprisingly, it will move even further back. The marker is not target, it just means you come too early (or too late if it’s ahead). You should adjust orbit to come later by... adding more prograde speed (yes, orbits are counterintuitive), so that you’ll move slower in the upper end.


Another option is to use only prograde marker and move the maneuver node along the orbit to change the departure angle and the crossing angle. Just pull and push back and forth and you’ll finally meet the sweet spot.


Departure node. The day before I did this flight, KSP 0.22 was released, and I had no plug-ins at all. Turned out it’s doable without them.

As soon as there’s encounter in the orbital map, it’s almost enough. KSP calculates positions with floating point values which have low resolution on big distances, and positions drift chaotically as you travel further, so it’s useless to adjust a precise flyby. So, if you were lucky to have a precise enough maneuver, just burn and adjust closer to the encounter.

This is it for now. The next chapter, the last on Duna, will have no theory and will just show how this ship actually flies. Stay tuned!

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See also

One Giant Leap
a photo gallery of Apollo program (with comments in Russian)