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FloppyRocket

How to convert an alex moon launch window into a maneuver node?

Question

Sorry for posting this but I haven't been able to find an answer to exactly this question. Plenty of discussion about the Launch Window Planner mod, but I'm not using mods.

Situation:

I'm orbiting Dres and need to get back to Kerbin. I've got 1,700 m/s of delta-v to work with. Yes, I realize I might have to abandon the mission and go build a bigger rocket. But I'd like to evaluate how far off my rocket design is.

Part 1:

Without using any mods , how do I convert this transfer from the alex moon Launch Window Planner web page into a Maneuver Node?

k5HvSxQ.jpg

I've tried going to the correct UT time mark for the departure time, entering the two components of the Ejection burn (the 'i' info button says it's prograde 1263 m/s and normal 143 m/s), and from that I get basically into a large solar orbit that goes nowhere near Kerbin. So I'm doing something wrong - no idea what.

Update: Upon further review, I realize the alex moon site doesn't know anything about my orbit around Dres, so just using the UT departure time isn't sufficient. So likely that's where all those angle values come into play, but there no way to enter those angles into a maneuver node. So perhaps a mod is required, if I want to avoid just endlessly tweaking on the maneuver node and dragging it around hoping to find a Kerbin encounter.

Update to the update: (see strikeout above)

Part 2:

If I was to use a mod, which one that has been updated to work with KSP version 1.6.1?

 

 

Edited by FloppyRocket
long live the typos

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I think the ejection burn looks like it's a pure prograde burn. So the fact that you had a normal component is what messed you up.

Edited by bewing

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88 degrees retrograde means you'll want to eject almost directly retrograde, with (I believe) a slight "upward" component, so your apoapsis comes a bit after ejection.  Probably the easiest way to do this is, match the magnitude of the ejection burn, then slide your node around first to eject retrograde, then to adjust the apoapsis and periapsis as called for.  Add a second node for fine-tuning halfway there before executing the first burn, and if you're not seeing a fairly close approach, play with it a bit to see what's going wrong.

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Frankly, I use the calculator (actually the Transfer Window Planner mod which gives you the calculator in the game) to get a date and time. Then I make a node at that date and time, drag it around to the correct location, and pull it prograde enough to get the encounter (or close enough). I don't bother with any numbers other than the time and ejection dV.

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4 hours ago, bewing said:

I think the ejection burn looks like it's a pure prograde burn. So the fact that you had a normal component is what messed you up.

When i clicked on the "i" info button, the text box showed the prograde and normal parts of the burn.

2 hours ago, 5thHorseman said:

Frankly, I use the calculator (actually the Transfer Window Planner mod which gives you the calculator in the game) to get a date and time. Then I make a node at that date and time, drag it around to the correct location, and pull it prograde enough to get the encounter (or close enough). I don't bother with any numbers other than the time and ejection dV.

I tried that method, but it seems like when I set the burn dv at the date and time from the planner, it doesn't cause an intercept. I have to slide the node around as much as 30 days one way or the other, and and adjust the burn. That seems wrong, since all of the trajectory physics are on rails and the practice should match the predictions. So I'm likely doing something wrong.

4 hours ago, Kryxal said:

88 degrees retrograde means you'll want to eject almost directly retrograde, with (I believe) a slight "upward" component, so your apoapsis comes a bit after ejection.  Probably the easiest way to do this is, match the magnitude of the ejection burn, then slide your node around first to eject retrograde, then to adjust the apoapsis and periapsis as called for.  Add a second node for fine-tuning halfway there before executing the first burn, and if you're not seeing a fairly close approach, play with it a bit to see what's going wrong.

Is that 'retrograde" with respect to Dres' orbital direction, or with respect to my orbit around Dres? My retrograde direction changes pretty quickly when I'm in orbit, and the window planner can't know what my orbital phase is, it only asks for my orbital altitude.

So probably it must be retrograde with respect to Dres' orbital motion, minus a couple a degrees one way or the other (whatever "88 degrees to retrograde" means). I've seen the reference figures for the angles, and I don't understand what "degrees to" means.

Edited by FloppyRocket
adding more replies

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It's with respect to the orbit of Dres.

Transfer Window Planner (online version): https://alexmoon.github.io/ksp/

Olex's interactive interplanetary guide and calculator: http://ksp.olex.biz/

For your purposes, you want to look at Olex's page, and the ejection angle.  I think I was actually wrong above, and the calculator is specifying that you burn 88 degrees before reaching the "retrograde" position relative to the Kerbol-Dres pair, from a prograde orbit.  Then again, in this instance that's not going to be all that far off a retrograde burn.  Just plug in Dres and Kerbin on that page and it will give you a (rough) solution that will help you visualize things.

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6 hours ago, FloppyRocket said:

So probably it must be retrograde with respect to Dres' orbital motion, minus a couple a degrees one way or the other (whatever "88 degrees to retrograde" means). I've seen the reference figures for the angles, and I don't understand what "degrees to" means.

The angle is in reference to the direction Dres is travelling and will assume a prograde orbit. In the case of 88° the manoeuvre node will be on the Sun facing side of the orbit almost directly perpendicular to Dres' direction of travel and the burn will be retrograde relative to Dres' direction of travel.

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I'm playing unmodded right now as well. What I usually do is make a maneuver node with the right amounts of dV at the departure time and just drag it around the orbit. If you zoom out far enough, you should see it moving around the system and get it to where the apoapsis is just touching the orbit of Kerbin. At the correct time, that should get you the intercept. How big is your orbit around Dres? If the period is just a few hours or less, than anything pointed in the right direction should be close enough (i.e., +/- a couple of hours shouldn't change the dV requirements more than maybe a couple of m/s). If you have a highly-eliptical and/or long-period orbit, then you might not be going the right direction at the right time. In that case, you would have to do some sort of 'pre-burn' to hit the right spot in the orbit at the right time.

FWIW, I've reached the conclusion that setting up the stock maneuver nodes is a frustrating and unnecessarily time-consuming process, and I'm planning to add a mod to make them easier as well.

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I've done it that way, but MechJeb has "Advanced Interplanetary Planner" which has the same porkchop display. You click somewhere, and it creates your node. 

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10 hours ago, FloppyRocket said:

When i clicked on the "i" info button, the text box showed the prograde and normal parts of the burn.

I tried that method, but it seems like when I set the burn dv at the date and time from the planner, it doesn't cause an intercept. I have to slide the node around as much as 30 days one way or the other, and and adjust the burn. That seems wrong, since all of the trajectory physics are on rails and the practice should match the predictions. So I'm likely doing something wrong.

Is that 'retrograde" with respect to Dres' orbital direction, or with respect to my orbit around Dres? My retrograde direction changes pretty quickly when I'm in orbit, and the window planner can't know what my orbital phase is, it only asks for my orbital altitude.

So probably it must be retrograde with respect to Dres' orbital motion, minus a couple a degrees one way or the other (whatever "88 degrees to retrograde" means). I've seen the reference figures for the angles, and I don't understand what "degrees to" means.

That Alex Moon calculator really does work exactly as advertised. You just have to fiddle a bit.

This is a sandbox attempt using roughly your date. I get an intercept with a single 1400 m/s burn...

Spoiler

EJzlJ4a.png

Basically, the manouvre node is at about 90°, heading retrograde. You do need a slight normal addition (upwards) because otherwise you cross Kerbin's orbit at the wrong time. That normal addition means that you dip down a bit further than Kerbin's orbit and then meet Kerbin as you are heading back up. This is a bit clearer here:

Spoiler

c8tFrMD.png

So yes, you can definitely get home on less than 1700 m/s. :D

This is from a 40 km orbit, but with Dres's weak gravity, a higher or lower orbit won't make all that much of a difference.

edit:

After more fiddling, I got a better intercept with lower dV cost, meeting Kerbin exactly at the right place (at solar Pe), with about 1380 m/s total. Still not perfect compared to the Alex Moon calculator, but close.

That "88° to retrograde" angle you are supposed to use is a bit confusing. In any event, as @Reactordrone said, it necessarily has to be almost directly between Dres and the sun since you need to exit almost directly retrograde. As I understand it, "88° to retrograde" means you start at retrograde (with reference to Dres's solar orbit) and count back 88°... And so your exit from Dres's SOI will be heading slightly out and away from the sun. It didn't quite work for me because the closest approach I managed to get was when it was more like 95° to retrograde.

Dres is hard.

 

 

Edited by Plusck

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