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5thHorseman

The Equatorial Minmus Challenge

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Posted (edited)

A thread in Gameplay Questions got me thinking that it'd be fun to see who can actually do this the best. I am by no means sure of the best strategy though I've thought of a couple.

The Equatorial Minmus Challenge

The Goal

Transfer to Minmus from LKO in such a way that you enter Minmus' orbit as equatorially as possible.

The Setup

Use Alt-F12 to put a craft into equatoral Minmus orbit, so you can check your score. Launch or Alt-F12 your actual ship into LKO, less than 100km orbit. Your Kerbin Orbiter can be in any orbit of Kerbin (tilted from 0 to 90, retrograde, eccentric), so long as the ap is under 100km.

The Rules

You may do 2 "burns" once in LKO (Defined as an orbit with less than 100km Ap): One to eject from LKO, and one to slow down at Minmus.

  1. The LKO burn can actually be any number of burns (so you can tweak it to be exact) but needs to be represented by a single maneuver node. NO BURNING ONCE YOUR SHIP IS OVER 100KM.
  2. The Minmus burn must be in Minmus' SOI, at Pe, and may only be retrograde.

The Proof

2 images: One of your maneuver node from LKO to Minmus, and another in orbit of Minmus showing your An/Dn variance.

The Score

The score is simple: Your An/Dn variance, either from the game or from KER, between your craft and the one you hacked into Minmus orbit. Lowest score wins.

Unless someone points out how a mod could help you, I'm willing to allow ALL MODS for this. Even MechJeb. As this is more of a planning than an execution challenge.

The Duration

This challenge is open indefinitely, but will close if someone gets 0.00 inclination as nobody can do better than that.

The Leaderboard

  1. 0.12943 - @Pand5461 (Reported by Kerbal Engineer Redux)
  2. 2.36 - @OHara (Reported by Precise Maneuver)
Edited by 5thHorseman

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Something tells me that the key to this is timing your departure so that you arrive right as Minmus crosses the ecliptic plane...

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5 minutes ago, MaverickSawyer said:

Something tells me that the key to this is timing your departure so that you arrive right as Minmus crosses the ecliptic plane...

That is one of the considerations for sure.

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After trying this for a good half hour or so, I conclude that wow it's hard.

I was able to get two maneuvers set up with a 3.6 degree final Minmus inclination, starting at a 60 degree Kerbin orbit. That took quite a while and a lot of fiddling.

I was never able to execute that maneuver, though, with anything resembling enough accuracy in the time I'd allowed (under 100km orbit) to get anywhere NEAR that. Frequently, I couldn't even get my encounter set up before getting over 100km. I don't think I got anything close to even 15 degrees even allowing for burning above 100km.

One thing I did determine, though, was that your lowest inclination was when you Pe was matched with the target equatoral orbit. At least, with my "approach from near polar" strategy.

As this thread hasn't gotten any replies from interested parties and it really does seem ridiculously difficult to the point that I sure don't want to try it anymore, I'm going to close it unless someone replies within the next few days with a submission.

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8 hours ago, 5thHorseman said:

I'm going to close it unless someone replies within the next few days with a submission.

Ah, don't close it, this will be one of those challenges that get an entry every few months.   

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3 hours ago, Gargamel said:

Ah, don't close it, this will be one of those challenges that get an entry every few months.   

And then 3 months later they'll ping me because I've not replied because I'm not checking the thread. :D

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Don't you have to match planes with Minmus anyway to comfortably be able to transfer to it?

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1 hour ago, RedPandaz said:

Don't you have to match planes with Minmus anyway to comfortably be able to transfer to it?

Nope. For years now I just transfer to the next An/Dn point and burn higher than Minmus' orbit so I meet it on the way out or the way back.

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12 minutes ago, 5thHorseman said:

Nope. For years now I just transfer to the next An/Dn point and burn higher than Minmus' orbit so I meet it on the way out or the way back.

but stll, that's not the elegant way XD

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2 minutes ago, RedPandaz said:

but stll, that's not the elegant way XD

I find that people tend to define "elegant" as "the way I like to do it."

I personally find it inelegant to do an unnecessary and expensive (relatively) plane change, and I also find it inelegant to time warp to the correct time on the ground to launch into the proper orbit. Maybe if the game had tools to tell me when that time was, it'd be a little more elegant.

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1 minute ago, 5thHorseman said:

I find that people tend to define "elegant" as "the way I like to do it."

I personally find it inelegant to do an unnecessary and expensive (relatively) plane change, and I also find it inelegant to time warp to the correct time on the ground to launch into the proper orbit. Maybe if the game had tools to tell me when that time was, it'd be a little more elegant.

I don't time warp on the ground, I just get into an orbit, change my plane, and then find a nice transfer window

 

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2 minutes ago, RedPandaz said:

I don't time warp on the ground, I just get into an orbit, change my plane, and then find a nice transfer window

 

Unnecessarily and expensively :wink:

But in any case, that is immaterial here. The purpose of the challenge isn't to be elegant, it's to find a way to do something difficult yet simple to define.

(Also, matching planes with Minmus first is totally allowed in this challenge. And it won't get you an equatorial orbit)

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7 hours ago, 5thHorseman said:

(Also, matching planes with Minmus first is totally allowed in this challenge. And it won't get you an equatorial orbit)

Does Minmus have an axial tilt? If not, then if you set up your transfer burn so that you launch into Minmus' orbital plane, i.e. so that your craft is launched from the celestial longitude of one nodal point, and then time it so that you encounter Minmus at the other nodal point, you ought to end up in an equatorial orbit. If there is an axial tilt OTOH, then I can see how it would become pretty darn hard...

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There's no tilt, or if it is, it's so small as to be unnoticeable.

 

Speaking of which... do ANY of the stock celestial bodies have axial tilt?

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59 minutes ago, MaverickSawyer said:

do ANY of the stock celestial bodies have axial tilt?

IIRC no due to limitations in unity.

 

As for the way I normally get to minmus - its usually a burn at the ascending/descending node of whatever orbit I'm in plus some waiting till when it actually matches up nicely.

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I think I am gonna have to show you all how this is done. 

When I launch to minimus in early career mode I launch in a window where if you track a heading offset from 90° you arrive into LKO already aligned with minimus' orbit. It saves some DV which is important early on in career mode on higher difficulties. 

Hold my beer. (Actually this will probably take a fair few attempts to get it perfect).

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2 hours ago, herbal space program said:

Does Minmus have an axial tilt? If not, then if you set up your transfer burn so that you launch into Minmus' orbital plane, i.e. so that your craft is launched from the celestial longitude of one nodal point, and then time it so that you encounter Minmus at the other nodal point, you ought to end up in an equatorial orbit. If there is an axial tilt OTOH, then I can see how it would become pretty darn hard...

Minmus' tilt is not in line with its orbit. It's in line with Kerbin's axial tilt. So ejecting to Minmus from Kerbin will cause you to arrive at Minmus with a tilt relative to Minmus' equator.

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46 minutes ago, 5thHorseman said:

Minmus' tilt is not in line with its orbit. It's in line with Kerbin's axial tilt. So ejecting to Minmus from Kerbin will cause you to arrive at Minmus with a tilt relative to Minmus' equator.

Seems like what you need to do in that case is jus eject equatorially so that you encounter Minmus right at the nodal point. I'll give that a try right now...

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Posted (edited)

Well, Minmus is descending through the nodal point at its orbital velocity times sin 6°, about 30m/s, so to enter its SOI while staying above its equator you need to be moving down also at 30m/s. minmus.jpg 

Transferring to Minmus' orbit, 24'000km from Kerbin, results in a rather slow velocity there, about 3000m/s × 700km /24000km = 85m/s since we have the same angular momentum as we did moving 3000m/s at 700km above Kerbin.   So we need about 20° inclination in the orbit that leaves Kerbin so that 85m/s × sin 20° = 30m/s

The transfer burn doesn't give much freedom to adjust the arrival orbit, so I adjusted the inclination before the transfer burn to tune the angle at which I arrived at Minmus.

Leaving the final orbit quite loose around Minmus makes aiming easier, but I couldn't get lower than 2.4° before getting frustrated.

I found the mods Precise Manuever and Better Burn-Time to be quite helpful here.

 

Edited by OHara
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Posted (edited)
18 hours ago, 5thHorseman said:

And then 3 months later they'll ping me because I've not replied because I'm not checking the thread. :D

I'd think this challenge would be a self serve scoring system...  Lowest Inclination wins. 

 

And see, look what I started.....

Edited by Gargamel
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7 hours ago, OHara said:

Leaving the final orbit quite loose around Minmus makes aiming easier, but I couldn't get lower than 2.4° before getting frustrated.

Well analyzed! I'm willing to assume you could perform that last burn, as it's not the hard part of the challenge. Welcome to the top (and only slot) on the leaderboard!

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8 hours ago, OHara said:

Well, Minmus is descending through the nodal point at its orbital velocity times sin 6°, about 30m/s, so to enter its SOI while staying above its equator you need to be moving down also at 30m/s.  

Transferring to Minmus' orbit, 24'000km from Kerbin, results in a rather slow velocity there, about 3000m/s × 700km /24000km = 85m/s since we have the same angular momentum as we did moving 3000m/s at 700km above Kerbin.   So we need about 20° inclination in the orbit that leaves Kerbin so that 85m/s × sin 20° = 30m

 

Yes, that's the ticket. That part occurred to me after I posted last night. I have no time to try this for a couple of days, but I will as soon as I do. Without resorting to Precise Node, I think a high TWR transfer stage augmented by an extra-beefy RCS system will do a lot to increase accuracy. If you can just get to an encounter with the short initial burn, you can then focus the view on Minmus and use RCS to tweak you approach while you watch the results. If you start right at 70km circular, there should be a fair amount of time for that before hitting the 100km limit. Anyway, I'll try with this approach later in the week.

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The opposite approach would probably be more rewarding. 

Reach Minmus at its northernmost or southernmost points in its orbit, 90° from either nodal point.  Then Minmus is moving in its equatorial plane relative to Kerbin, the craft is moving slowly relative to Kerbin, let the craft reach its apoapsis and wait for Minmus to come by.   The biggest velocity involved is Minmus's 270m/s orbital velocity, which is the desired plane.

 

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Peace of cake.

A careful choice of the initial orbit, maneuver timing and direction, and the end result:

kVIfoIp.png

Full video (not disclosing the initial orbital plane):

 

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