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Descending Node or Ascending Node?


I likeOxidizerrfuel
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3 hours ago, I likeOxidizerrfuel said:

Hello dear reader

Which is better and more fuel effecient? I remember hearing about the Dn and An but I don't know which one is more effecient. 

Hello dear forumgoer

If you are making a plane change maneuver (as I assume you are if you are interested in the An and Dn) then you want to perform the maneuver at whichever node is farther from the body you are orbiting.
The highest of the two nodes will allow the cheaper maneuver which will achieve the same desired plane change.


Happy landings!

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What's a Minmus maneuver?  

You mean changing inclination to match the Minmus orbit? 

...or waiting for Minmus to arrive at it's target ascending or descending node so you can travel straight to the moon from Kerbin equatorial orbit?

In either case...  I don't think it matters which side you choose.

The only Minmus maneuver that I'm aware of involves waiting a few days for that moon to reach one of those two transfer windows so I don't have to waste time/fuel matching its inclined orbit.

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4 hours ago, XLjedi said:

The only Minmus maneuver that I'm aware of involves waiting a few days for that moon to reach one of those two transfer windows so I don't have to waste time/fuel matching its inclined orbit.

Am I missing something here?

Assuming you are launching from the default pad, all you need to do is wait a maximum of 3 hours (half a day) until your position on Kerbin matches the crossover point of Minmus's orbit then launch at either +6 or -6 degrees inclination.  Even by eye I can get within about 0.5 degree of Minmus's orbit.  After that you can go TMI any time you like.

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17 minutes ago, I likeOxidizerrfuel said:

I don't want a complex answer. Please, make it simple and easy to understand.

23 hours ago, I likeOxidizerrfuel said:

I remember hearing about the Dn and An but I don't know which one is more effecient. 

Neither!:cool:

If you want an answer that is more helpful than that, then please let us know what you actually want to do. Without knowing that we cannot tell you how to efficiently get it done.

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23 hours ago, I likeOxidizerrfuel said:

Which is better and more fuel effecient? I remember hearing about the Dn and An but I don't know which one is more effecient.

More efficient at what?  They're essentially the same thing, they're just opposite orientations.   Asking "which is better, An or Dn" is like asking "which is better, left or right".

Can you describe what problem you're trying to solve?  i.e. why you're asking the question?

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@I likeOxidizerrfuel Don't worry if the answers you're getting seem a little unhelpful. It's just their way of saying that your question is incomplete.

If you're in a circular orbit and you just want to change the inclination of that orbit then it doesn't matter if you change your inclination at the ascending or the descending node. In a circular orbit there is no difference between the amount of fuel used to burn at either one.

HOWEVER - if your orbit is highly elliptical (a stretched out oval); or if you plan to also burn into a transfer trajectory during the inclination change; or if your trajectory is hyperbolic, crossing between different spheres of influence - then it does matter at which node you make your inclination change.

It just doesn't really matter if it's called the ascending node or descending node because those names are fairly arbitrary. If you simply flipped the map upside down then you'd just reverse the names. Those are just the places where the plane of your orbit crosses the plane of another orbit or the equator of a celestial body. It doesn't matter what you call those points; it just matters where along your orbit they are.

Edited by HvP
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On 2/24/2020 at 6:16 PM, I likeOxidizerrfuel said:

Which is better and more fuel effecient? I remember hearing about the Dn and An but I don't know which one is more effecient. 

 

17 hours ago, I likeOxidizerrfuel said:

I don't want a complex answer. Please, make it simple and easy to understand.

I assume that you're talking about a burn to change your inclination to match Minmus's orbital plane.  In that case, whichever one is closer to the apoapsis will be the more efficient node.

The simple explanation is that inclination burns are dependent on your orbital speed:  the faster you are going, the more burn is necessary to deflect your path.  Anything in an elliptical orbit moves the fastest at its periapsis and slowest at its apoapsis, so making the inclination change as close to the apoapsis as possible is the best way to increase the efficiency of the burn.

Note that this is not always possible:  a circular orbit, for example, has the same velocity throughout and so there is no most efficient choice.  A burn with a deadline, such as a transfer window, may require you to change inclination closer to the periapsis in order to hit the window at the right time (though this can usually be avoided by launching and making the inclination change ahead of time).  An escape orbit or flyby has no apoapsis, but you can still choose the node that is farther from the periapsis.

There are other times when you can get more efficiency in other ways.  For example, choosing the right time allows you to build an inclination change into your launch, and this is efficient because your launch speed is low (it's not zero, though; you have Kerbin's rotation to consider).

The reason that the efficiency is not tied to the DN or AN is because the DN and AN are reference markers for where your orbit crosses the equator.  AN, or ascending node, is where you cross going in the direction of local 'up' (towards the north pole), and DN, or descending node, is where you cross going local 'down' (or towards the south pole).  These depend on the orientation of the planet, but have nothing to do with the size of your orbit, your orbital speed, or the locations of the apoapsis and periapsis, so there cannot be a definite answer as to which is more efficient.

Edited by Zhetaan
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