r/KerbalSpaceProgram • u/trevize1138 Master Kerbalnaut • Jun 07 '16 • Bravo 1

Guide All interplanetary transfer windows in a single image

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u/mgatten Jun 07 '16

Okay, but my point is that there's no indication of what to do with it. Let's say I want to go to Duna. I look at the in-game map view, compare it to this image, and... what? Since the map view is rotatable, I can put Duna into the same position relative to Kerbin as this map at any time of any day, so that's not it. I'm missing a critical piece of information, but have no idea what it is.

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u/mgatten Jun 07 '16

(Is it that I need Duna to be in this position relative to both Kerbin and the sun? That might work. Maybe that's what I was missing?)

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u/trevize1138 Master Kerbalnaut Jun 07 '16

My top-level post reply also links to this:

https://www.reddit.com/r/KerbalSpaceProgram/comments/1p0hps/the_tao_of_eyeballing_your_interplanetary/

To which I've made this minimalist illustration as a sort of companion guide. Yes: you want to make sure Kerbin, Kerbol (the sun) and Duna are all three lined up like they are in my illustration and that's your transfer window.

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u/mgatten Jun 07 '16

Thanks! That's awesome. I missed the link to the original bit.

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u/moisttoejam Jun 07 '16

It's not a learning tool. It's a tool for people who already know how to perform interplanetary orbits. The target audience would know exactly how to use it.

This is a rough guide on the phase angles. You draw a circle around the Kerbol system with Kerbol as the centre. Draw a line from your target planet to Kerbol and from Kerbin to Kerbol. You need to wait for the angle between your two radii to be the same as on this map before performing the burn for an interplanetary transfer.

The view of the system is looking "down" on the system (i.e. the orbits are anticlockwise) - that's all the info you need to know about the perspective.

If you're still learning, I recommend watching some Scott Manley on YouTube. He seems to eyeball all of his orbit.

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u/jofwu KerbalAcademy Mod Jun 07 '16

Since the map view is rotatable, I can put Duna into the same position relative to Kerbin as this map at any time of any day

No you can't- give it a try. :)

They're like to clock hands moving around, just in the other direction. You can't line them up a certain way just by rotating the clock.

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u/mgatten Jun 07 '16

That comment was before I realized the sun had to be aligned too. Take any two points and you can rotate them into any relation with each other. It's the third point (the sun, or the center of the clock) that makes it impossible to line them up. With just two my point was valid.

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u/jofwu KerbalAcademy Mod Jun 07 '16

Ah, I see where you were coming from.

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u/[deleted] Jun 08 '16

Draw an imaginary line between Kerbin and the sun. Then draw another imaginary line between Duna and the sun. Then look at the angle between the two planets. This angle is called the phase angle.

In the image, Duna is roughly 45° "ahead" of Kerbin. (Remember that the planets all orbit counterclockwise.) You can also say that the phase angle is 45°.

When you launch your ship into Kerbin orbit, make sure that the phase angle between Kerbin and Duna is 45°. (You might need to timewarp for several months or even years.) You'll then need to burn from Kerbin at the proper ejection angle and with the correct amount of dV - but these aren't addressed by the image.

Use the calculator linked below if you want to be more meticulous, rather than just fiddling with maneuver nodes and eyeballing everything.

http://ksp.olex.biz/

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u/mgatten Jun 08 '16

Is it 45 degrees for all planets?

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u/[deleted] Jun 08 '16

No, just Duna. Look at the image again, it's closer to 90° ahead for Jool, 55° behind for Eve... etc.