Transfer Window Planner & Precise Node/Precise Maneuver editor

[Seeker89]

Hey everyone. 

I have been looking at this for sometime and through many play through, and I just don't get it. 

Spoiler

Kerbin (@48000km) -> Duna (@100km)
Depart at:      Year 1, Day 20, 0:00:00
       UT:      410400
   Travel:      323 Days, 1:38:58
       UT:      6982738
Arrive at:      Year 1, Day 343, 1:38:58
       UT:      7393138
Phase Angle:    127.17°
Ejection Angle: 0.09° to prograde
Ejection Inc.:  0.09°
Ejection Δv:    2671 m/s
Prograde Δv:    2670.7 m/s
Normal Δv:      4.7 m/s
Heading:        89.90°
Insertion Inc.: -0.28°
Insertion Δv:   1077 m/s
Total Δv:       3747 m/s

What does all this mean? and how can I make a node with one of the node editors?

[Streetwind]

A lot of this information is supplemental. Only four figures are relevant for making a node: the departure time, the ejection angle, the ejection dV, and the normal dV.

Out of these four, you can more or less forget the ejection angle again. Stock KSP has no way to determine it, and eyeballing is often not precise enough. A mod like PreciseNode may allow you to set it - I wouldn't know, I've never used it. And if you have Transfer Window Planner installed as an ingame mod, you can make it visually display the ejection angle so you know where to click to create the node. But technically it is not even necessary. I will explain why later. For now, just put the node anywhere you want. If you are going to the inner planets, put it on Kerbin's day side; if you're going to the outer planets, put it on Kerbin's night side.

Then, the departure date. You should try to make your node so that the burn occurs reasonably close to this number. For Moho, be as precise as you possibly can; you want less than an hour of deviation. For Eve, less than maybe 4 hours. For all other planets, precision is not so important, but it certainly helps - and in low Kerbin orbit, there's no reason why you shouldn't easily be able to get within the hour. When setting up your node, you can use the "orbit forward" function to place it as far into the future as you need.

 

That leaves just two relevant values: the dV figures. But, you might ask, why ignore the prograde dV cost? And, you will notice that prograde dV plus normal dV adds up to more than the ejection dV. How can that possibly be? The answer to both is: Pythagoras. Because you are combining a prograde vector and a normal vector in the same burn, you don't need to pay the sum of both - you only need to pay the square root of the sum of their squares. A² + B² = C², ---> Prograde² + Normal² = Ejection². You can save a lot of dV this way if the normal component is large, which is why the transfer planning tool pretty much always recommends you such a combined ejection burn. Also, this fact makes it very easy to plan your node.

Start by applying the normal dV component. In your example, you would add 4.7 m/s worth of dV in the normal direction to your new node.

As the second step, start pulling the prograde marker to add dV, and keep adding that until your node reads the same number as the total ejection dV. You don't need to count how much you add. Because of the pythagoran theorem, all that matters is that the final total matches the ejection dV. This is why you can ignore the prograde dV reading.

 

You now have a node that is set for roughly the right time, and it has precisely the required dV in precisely the right directions. Now we return to the ejection angle, which in stock KSP, you cannot determine in any way.

But what you can do: you can pick up the node and drag and drop it along your orbit around Kerbin (or use the maneuver tool to move it in steps). Assuming that orbit is equatorial and at the right altitude (so that it matches the input you used in the transfer planner), and the node is set to the correct departure time and ejection dV (which you already did in the steps above), that means that there exists a position somewhere along your orbit where this node automagically results in an encounter with your destination. Target your destination, this will help you find this spot more easily by dragging the node around, because it will show you how the close approach markers move.

Now that you have your encounter, check one last time if your departure timing is still acceptable. And voila, you have successfully planned your node! And you didn't even need any mods for it. Of course, if you have mods that help, those might make it even easier... but the above approach always* works.

 

* Unless the planner gave you a trajectory that involves a mid-course correction. In which case you have to plan that second node, too, before you can start dragging the first node around to find the encounter. Or you could just tell the planner that you want a ballistic trajectory, which never involves a mid-course correction.

 

Edited February 2, 2023 by Streetwind

[monstah]

Question: how do I get to make TWP show me the ejection angle visually? I never knew it had that feature.

[Streetwind]

3 hours ago, monstah said:

Question: how do I get to make TWP show me the ejection angle visually? I never knew it had that feature.

This crudely annotated screenshot should show it off sufficiently

(Pay no mind to the weird planet names, I'm testing planet packs in that game instance.)

[monstah]

30 minutes ago, Streetwind said:

This crudely annotated screenshot should show it off sufficiently

(Pay no mind to the weird planet names, I'm testing planet packs in that game instance.)

O_O!

I've mostly used TWP from the VAB, but I never once noticed this button!

[Streetwind]

Yeah, you need to be in space for this to work.

[JadeOfMaar]

On 10/28/2016 at 11:21 AM, Streetwind said:

This crudely annotated screenshot should show it off sufficiently

(Pay no mind to the weird planet names, I'm testing planet packs in that game instance.)

Those are not weird planet names. And ooooooh, Pythagoras.