# 90m/s enough for Eve encounter in KSP? (reading delta-v maps)

I'm trying to understand this cool delta-v map, in particular I don't get how 90m/s delta-v should be enough to go from a transfer orbit to an Eve encounter:

In my attempt, I go to a transfer orbit by burning straight up against Kerbin's orbit until I get an escape:

This escape burn already gives us about half the required orbital transfer to Eve:

But it's not even close to the 90m/s that is allegedly sufficient for an encounter. For that, I need about 400m/s:

So what's going on?

• you need to look at the second number. Which is 430 in this case, and seems to match up with your value quite well Commented Jun 7, 2022 at 14:34
• @Topcode The second number is "maximum plane change delta-v" (from the map legend). The second number says "10" for Duna, so that can't be correct. Given that the number indeed matches well, maybe the numbers are erroneously swapped for Eve.
– John
Commented Jun 7, 2022 at 14:53
• @Topcode Just checked what delta-v the inclination change would take for Eve and I get something above 300, so I'm not sure about the number swap hypothesis.
– John
Commented Jun 7, 2022 at 15:00
• Is this a launch window thing? Commented Jun 7, 2022 at 16:13
• @John I’m not too sure I don’t use these maps often, but I think that the might be 90 only for the most efficient burn, which you are of course not doing. Commented Jun 7, 2022 at 19:32

The starting point is "elliptical orbit around Kerbin with Pe of 10 km above atmosphere (80km ASL) and Ap at the edge of SOI", not solar orbit. If you try to do the burn while in solar orbit, it's much less efficient (because of Oberth effect).

In theory this means you should be in a (correctly aligned) elliptical orbit around Kerbin, and do the burn of 90m/s at the periapsis.

In practice such an orbit is a pain to align, and it's easier to read as 930+90=1020m/s burn from LKO to Eve intercept.

Experimentally, I see a burn of ~1030m/s from LKO to Eve intercept, which seems in line with the map. (Mechjeb was used to plan this one specifically)

• Note also the "430" above the "90": depending on the relative positions of Kerbin and Eve, you may need up to 430 m/s delta-V to deal with the inclination change.
– Mark
Commented Jun 8, 2022 at 3:36
• Ah, of course! I have since used those more efficient burns (but only for the convenience reasons you also stated) and been wondering why I required so little delta-v in total. For some reason it didn't click. Thanks!
– John
Commented Jun 8, 2022 at 12:46

As stated previously by Gedas:

If you try to do the burn while in solar orbit, it's much less efficient (because of Oberth effect).

Other than this you also need to take transfer windows into account. This means that the minimum delta-V required changes with time. Calculating it is not easy, but in the case of Eve and Duna it shouldn't be that hard, you can find many guides (such as this one):

However in the case of planets with high inclinations (they don't orbit in the same plane) and highly elliptical orbits - mainly Moho and Eeloo - this method becomes less accurate.

If you have the time and patience to maximize the efficiency of your transfer, I would like to recommend mods, such as the Transfer Window Planner.