Launch Window Planner curiosity, anyone know what's going on here?

[rousseau]

I've found this launch window planner very helpful- much thanks to the creator http://alexmoon.github.io/ksp/

Something strange caught my eye though, when looking at expected dv to reach the interior planets. Moho and Eve both have these discontinuities at specific times. I can only zoom in so far, but this pattern appears to continue to a point. What this means practically, is that launching before this moment, has high energy costs for getting there slightly faster than ideal (and less substantial costs for getting there slower); while after this moment, has high energy costs for getting there slightly slower (and less substantial costs for getting there faster).

So- is this an error in the utility, an error in ksp, or a real phenomenon? and if so- what is going on at that point?

[Red Iron Crown]

Can you share the input parameters you used to get this result? I'd like to poke around the porkchop plot.

[GoSlash27]

I don't really know, but I suspect it's the insertion burn. Those would represent times when you arrive carrying excessive DV that has to be burned off in order to establish orbit. Just a guess.

Best,

-Slashy

[Jouni]

Patterns like that are quite common, even if you plan only for a flyby/aerocapture. My guess is that the optimal transfer (for a given departure time and a time of flight) puts the periapsis slightly below the target. Fast transfers below the red line result in an encounter before reaching the periapsis, while slow transfers above the line have an encounter after the periapsis. To get an encounter at the periapsis, you have to make the transfer burn to a less than optimal direction, unless the timing is exactly right.

[cantab]

I believe these only occur when you limit it to "ballistic" trajectories, and arise because of difficulties with the inclination. They go away if you choose to search for "optimal" trajectories that can include a mid-course correction to sort the inclinations out.

[allmhuran]

Yep, can confirm, at least empirically, that this is a result of inclination.

The longer duration trip for higher energy is a result of burning to put your intersection point at a spot not 180 degrees from your Kerbin departure, but actually a little further around your path in order to ascend/descend through the target's orbit. The reason for the higher dV in such cases is that to do this you must drop your periapsis down inside the target's orbit so that you get the crossing point with its orbit, otherwise you'd be gaining altitude from your periapsis and moving further away from the target orbit.

One of the early ejections to Eve, for example, will give you an intercept about 230ish degrees around from your ejection from Kerbin, and a periapsis quite a long way below Eve's orbit. You'll cross Eve's orbit twice, with the second crossing at the ascending node.

[rousseau]

Ah, cantab you are correct, these points go away when set to optimal.

Red Iron Crown these are the exact parameters i put in for the graph in the first post:

Wonky orbital paths due to inclination seems reasonable. I'm tempted to open a new sandbox save and try out a launch at these points just to see.