Minmus Approach - How do I get into an orbit here?

[Vaireon]

I'm very new to this game, only been playing a few days. My proudest achievement so far is landing an unmanned probe on the Mun and returning it to Kerbin.

On this mission, I am attempting to land a manned probe on Minmus, I escaped Kerbin with no problems and fixed my ascension/descension nodes on the way there. However, I overshot one of my maneuvers by a tiny bit and ended up coming up to Minmus on a very strange angle.

Whenever I try and create a new Maneuver to get into orbit around Minmus, my new orbit always appears out of place or not around Minmus. And at one point if I try dragging the retrograde vector any further the maneuver node starts glitching and jumping rapidly. I'd like to ask how you would recommend I get into orbit around Minmus in this situation.

For imgur images -- The first Image shows my approach towards Minmus and my fuel resources. I'm using a Terrier engine with 3 small fuel tanks. The second image shows what happens whenever I try to make a maneuver into orbit. This happens all along my flight path, even at the pariapsis and around Minmus. Any help is much appreciated.

Thanks!

 

[Foxster]

I'm unsure of the problem. You do look to be on course for encountering Minmus, so as you enter it's SOI (orbit path turns blue) you can tweak the Pe to something like 50km, then just before Pe burn retrograde until you have an orbit. No?

[Vaireon]

6 minutes ago, Foxster said:

I'm unsure of the problem. You do look to be on course for encountering Minmus, so as you enter it's SOI (orbit path turns blue) you can tweak the Pe to something like 50km, then just before Pe burn retrograde until you have an orbit. No?

My Pe is currently at around 27km, do you suggest I raise it when in the SOI, then burn retrograde? I hadnt thought of that, thank you

[Plusck]

Hi and welcome to the game

I really don't see any problem with this approach. It seems to be pretty close and you should have no difficulty circularising at Pe there. Your orbit is going to end up clockwise rather than anticlockwise, but that is no real problem for Minmus, just a slight added cost on landing. I always seem to end up going the wrong way around Minmus myself...

Maneuver nodes can be problematic with SOI changes. Hopefully that will improve with the next release, but in the meantime we just have to make do. However, what you describe sounds more like an actual bug: when my nodes glitch they are a pain but the resulting orbit (when it gives one) is still correct, but in your case it sounds like the whole thing is off. You could maybe try looking through the Technical Support (unmodded) forum for map glitches to see what solutions people have proposed. Or wait for somebody more knowledgable to give some pointers.

Finally, what happens after you change SOI? Since you got down to the Mun ok, I'd guess that the situation corrects itself once you're inside the SOI. "Ride it out" isn't a great answer, I know. In the meantime, you can also read up on the patched conics configuration, which might help you (for example http://forum.kerbalspaceprogram.com/index.php?/topic/11297-different-conic-drawing-modes/ but I think the default value is now mode 3, not 1).

[Streetwind]

The reason your orbit may appear screwy is because you are changing frames of reference midway through your course plotting. In other words, you are looking at the trajectory from a different point of view before the maneuver node than after. The best idea here is to wait until you have changed frames of reference (in other words, entered Minmus' sphere of influence) and then start planning your next node.

You may ask: "how do I know that I can safely wait?"

The answer to that is simple, because there are only two things you need to look at. The first thing is whether or not you have an encounter at all - whether your trajectory is passing through the target's SoI. Seems unnecessary to point out? Well, wait until you get that one edge case where the patched conics algorithm glitches out and only draws a flickering image of an encounter, which disappears during timewarp even though you are going straight through the SoI...   Hopefully that'll be fixed in 1.1. Until then, make sure you are 100% confident you are actually getting an encounter, even if the game may sometimes appear to "forget" it.

The second thing you need to look at is whether or not you have a periapsis during your encounter. If you do not have a periapsis displayed anywhere inside the target SoI, that means you are on a collision course with said body and you might want to plan a course correction at some point (you still have plenty of time to do this after arriving in the target SoI, by the way). If you do have a periapsis - congratulations, everything is just fine! There's nothing more you need to check. You can, if you want, try to raise or lower said periapsis, but as long as you have one at all, you can calmly and safely cruise to your encounter.

This is true for any encounter with any celestial body in the game.

The "getting into orbit" part is too, by the way. Capturing into orbit around a celestial object simply involves a retrograde burn at your encounter's periapsis. (Now you know another reason why having one is important!) This is another reason why your planned orbit may have looked screwy: you did not plan your maneuver node at periapsis. While it is possible to capture into orbit at any other point, too, fuel usage increases drastically because you need to include a significant radial component into your burn - and it makes planning the burn complicated as well. It'll be like coming to a full stop and then re-accelerating sideways to orbital velocity. Very inefficient. No, you want to do it at periapsis, where your burn is automatically perfectly aligned with the orbit you intend to reach (zero radial component, pure retrograde burn).

As a rule of the thumb, rockets burn more efficiently deep inside gravity wells ("Oberth Effect"), which means that the lower your periapsis is, the less fuel you will need to capture into orbit. That is especially true for missions designed to land on the target body, which will eventually want to nullify the entire orbit under engine power. You will often see recommendations to choose a very low encounter periapsis for this reason.

 

Edited February 9, 2016 by Streetwind

[Warzouz]

I don't understang what this node is, just delete it a redo it. If you want to have a simplier view or your Minmus orbit "Focus" on Minmus, you should get a simple hyperbole and you'll be able to set your node easily.

It seems you're targeting a retrograde orbit. If you're new to KSP, try to get approach on the best side (prograde), even if, with Minmus, dV retrograde is not much. Let it become a habit to circularize prograde so you don't even need to think about it.

You should slightly burn radial now to get your PE on the right side of minmus (prograde), then set your retrograde burn at PE.

On the bright side : you already did the hardest. Circularizing and landing should be easy. Good job.

 

[Vaireon]

3 minutes ago, Plusck said:

Hi and welcome to the game

I really don't see any problem with this approach. It seems to be pretty close and you should have no difficulty circularising at Pe there. Your orbit is going to end up clockwise rather than anticlockwise, but that is no real problem for Minmus, just a slight added cost on landing. I always seem to end up going the wrong way around Minmus myself...

Maneuver nodes can be problematic with SOI changes. Hopefully that will improve with the next release, but in the meantime we just have to make do. However, what you describe sounds more like an actual bug: when my nodes glitch they are a pain but the resulting orbit (when it gives one) is still correct, but in your case it sounds like the whole thing is off. You could maybe try looking through the Technical Support (unmodded) forum for map glitches to see what solutions people have proposed. Or wait for somebody more knowledgable to give some pointers.

Finally, what happens after you change SOI? Since you got down to the Mun ok, I'd guess that the situation corrects itself once you're inside the SOI. "Ride it out" isn't a great answer, I know. In the meantime, you can also read up on the patched conics configuration, which might help you (for example http://forum.kerbalspaceprogram.com/index.php?/topic/11297-different-conic-drawing-modes/ but I think the default value is now mode 3, not 1).

Hi! thanks for the pointers! It did seem to be a bug with the SOI, as I entered it my maneuver nodes worked perfectly again, allowing to enter a stable orbit around Minmus.

Thank you very much!!

[Vaireon]

56 minutes ago, Warzouz said:

I don't understang what this node is, just delete it a redo it. If you want to have a simplier view or your Minmus orbit "Focus" on Minmus, you should get a simple hyperbole and you'll be able to set your node easily.

It seems you're targeting a retrograde orbit. If you're new to KSP, try to get approach on the best side (prograde), even if, with Minmus, dV retrograde is not much. Let it become a habit to circularize prograde so you don't even need to think about it.

You should slightly burn radial now to get your PE on the right side of minmus (prograde), then set your retrograde burn at PE.

On the bright side : you already did the hardest. Circularizing and landing should be easy. Good job.

 

I had no idea I could focus target on Minmus! That makes things a whole lot easier, thank you! 

[Plusck]

2 hours ago, Vaireon said:

My Pe is currently at around 27km, do you suggest I raise it when in the SOI, then burn retrograde? I hadnt thought of that, thank you

I didn't see this earlier.

The answer is no!

Since the screenshot doesn't show your Pe, I'm certain that Foxster was simply assuming it was higher than 50km. If you already have a 27 km Pe then that is excellent - simply burn retrograde when you get there. Lower is always better for burns due to the Oberth effect (i.e. you make use of the higher kinetic energy and lower potential energy of your propellant when you are lower in the gravity well of a body), as long as you don't need to expend significant amounts of energy to get there.

So no, don't raise your Pe. You could lower it a touch if it is cheap to do so (i.e. while still outside Minmus's SOI). Once you reach Pe, you burn retrograde to circularise and don't really need a maneuver node to do so.

Edited February 9, 2016 by Plusck

[Warzouz]

Don't bother about Oberth effect

- It's about optimization/efficiency

- It's not significant on light bodies such as Minmus

I suggest you focus on few maneuvers. Don't forget that if your PE is not satisfactory, you'll have to waste fuel to fix it then do a burn. It's sometime much more costly to change trajectory to a more efficient one than just burn on the trajectory you're on even if it's not optimised.

Remember the old TCP/IP moto : "simple is beautiful"

PS : KSP moto would be : "simple is beautiful but lighter is even better"

[Snark]

3 hours ago, Warzouz said:

Don't bother about Oberth effect

- It's about optimization/efficiency

- It's not significant on light bodies such as Minmus

Agreed that it doesn't make much difference at Minmus, so not really worth worrying about in this particular case.

However, in general, for most places (i.e. bigger than Minmus), Oberth effect can be significant, and it's a good idea to get into the habit.  Just like trying for a prograde orbit, as somebody wisely pointed out: 

8 hours ago, Warzouz said:

It seems you're targeting a retrograde orbit. If you're new to KSP, try to get approach on the best side (prograde), even if, with Minmus, dV retrograde is not much.

To @Vaireon:  the optimal approach in general (though as Warzouz points out, doesn't make much difference at Minmus) is to approach the planet or moon with the lowest Pe you can get away with, and do your circularization burn at really really low altitude.

Even if what you want is a slightly higher circular orbit, you should still do your capture burn down low.  For example, suppose you're approaching a vacuum planet and want to end up in a 100 km circular orbit.  You set up your approach so that it has a really really low Pe (like 10 km, or however low you can get without smacking into terrain), do a retro-burn to capture and lower your Ap to 100 km, then coast up to Ap and circularize there.

On the other hand, if your goal is landing, what you would do would be to aim for the really really low Pe, then burn to circularize there, and then go down to the surface when you get to a location that suits you.

[Vaireon]

Thanks for all the help everyone! I have successfully landed on Minmus and have just recently landed back at Kerbin. 

Love you all