Problem with a maneuver node

[KrisKelvin]

Ok, so I am in low Munar orbit and want to get back to Kerbin. So I tried to make a maneuver node on the far side of Mun and pulled the prograde handle to elongate the orbit towards Kerbin. The brown dotted orbit is forming nicely then suddenly it breaks into a purple dotted line which is very confusing! Why will it not just let me plan an orbit to go back to Kerbin?

Here's a video of what I mean

https://drive.google.com/file/d/1qeiiKctzlwCPXdJzkDeVeRYwy2B8nKXr/view?usp=sharing

Edited August 19, 2020 by KrisKelvin

[Serenity]

38 minutes ago, KrisKelvin said:

Ok, so I am in low Munar orbit and want to get back to Kerbin. So I tried to make a maneuver node on the far side of Mun and pulled the prograde handle to elongate the orbit towards Kerbin. The brown dotted orbit is forming nicely then suddenly it breaks into a purple dotted line which is very confusing! Why will it not just let me plan an orbit to go back to Kerbin?

Here's a video of what I mean

https://drive.google.com/file/d/1qeiiKctzlwCPXdJzkDeVeRYwy2B8nKXr/view?usp=sharing

The purple is the future orbit you gonna have after you insert to Kerbin and leave Mun's influence, is that what you mean?

Edited August 19, 2020 by Boyster

[Reactordrone]

You're burning on the wrong side of the Mun but the new orbit is the one in Kerbin's sphere of influence rather than the Mun and you do want to see where the orbit will be relative to Kerbin.

[KrisKelvin]

23 hours ago, Boyster said:

The purple is the future orbit you gonna have after you insert to Kerbin and leave Mun's influence, is that what you mean?

But it just confuses the picture! I want to see the elliptical orbit that would go around Kerbin.

11 hours ago, Reactordrone said:

You're burning on the wrong side of the Mun but the new orbit is the one in Kerbin's sphere of influence rather than the Mun and you do want to see where the orbit will be relative to Kerbin.

No. I am burning on the correct side of mun (the side opposite Kerbin). A prograde burn there will extend my apoapsis out to Kerbin (which is what I want). But I just want to be able to see the damn orbit I am creating! Not have it suddenly break and change colour. I can't see what I'm doing!

[Spricigo]

3 hours ago, KrisKelvin said:

But it just confuses the picture! I want to see the elliptical orbit that would go around Kerbin.

Point is, that elliptical orbit is an orbit around the Mun.   But for reference, your orbit around Kerbin would look something like this:

 

That assuming it's a pretty circular orbit, it can be a lot messier with high elliptical ones. Still, the important part  is: it will be close enough to Mun's own orbit while the craft stay withing Mun's SoI.

The only trajectory that matter is around the Celestial Body you are within, the only exception is when crossing a SoI boundary and then only for the purprose of crossing the boundary.

 

So we don't see where the orbit goes around kerbin, because for all the game is concerned we go nowhere but Mun's SoI.

 

 

3 hours ago, KrisKelvin said:

No. I am burning on the correct side of mun (the side opposite Kerbin).

 

It's pretty obvious you are burning the wrong side. Just look how the orbit around Kerbin is raising as you increase the burn. 

 

 

[Superfluous J]

Let's say you're driving a car at 50mph, and are just passing a building that you want to visit. So you step out of your car and, because you also are going 50 mph you just fly past.

That's what's happening here. You're leaving the Mun (which is flying around Kerbin) in the direction of Kerbin. However, you still have all that motion from the Mun so you are going that direction.

You need to jump backwards from the car 50mph to stop your velocity, and THEN worry about heading toward the building.

In orbital mechanics, you want to leave the Mun BACKWARDS, not DOWNWARDS. Luckily for your ship, Kerbin has gravity so once you do that, "walking toward the building" will happen automatically: You'll fall toward the planet.

[Spricigo]

8 minutes ago, Superfluous J said:

Let's say you're driving a car at 50mph, and are just passing a building that you want to visit. So you step out of your car and...

Do you own a Chevy Sonic?

 

[Reactordrone]

6 hours ago, KrisKelvin said:

No. I am burning on the correct side of mun (the side opposite Kerbin). A prograde burn there will extend my apoapsis out to Kerbin (which is what I want). But I just want to be able to see the damn orbit I am creating! Not have it suddenly break and change colour. I can't see what I'm doing!

You can effectively ignore the Mun to determine where you need to burn to reduce the altitude of the other side of your orbit. If the Mun wasn't there you would burn retrograde to lower the other side of your orbit so with the Mun in the picture you want the escape from the Mun to be tangential to the Mun's orbit and retrograde, i.e about 90° further around than you're doing it.

https://imgur.com/W8XnKRb

[HebaruSan]

7 hours ago, KrisKelvin said:

A prograde burn there will extend my apoapsis out to Kerbin (which is what I want).

Oh, I see. You're kind of thinking of this as analogous to getting to the Mun, as if Kerbin orbited the Mun rather than the other way around. In that scenario, the proper thing to do is indeed to raise your apoapsis towards the body you want to reach (or, where it will be by the time you reach it).

But it's different when you're already orbiting the satellite body. You can't just raise your apoapsis around the Mun up to Kerbin, because at some point before that time, you will no longer be orbiting the Mun; you'll escape the Mun and be in orbit around Kerbin instead. That's what the game is trying to tell you in the video you captured.

[king of nowhere]

7 hours ago, KrisKelvin said:

But it just confuses the picture! I want to see the elliptical orbit that would go around Kerbin.

 

that's exactly what you are seeing. that's the new orbit. and it's not elliptical because you are going the wrong way.

look, it's the same that happens when you make a manuever towards the mun. you have your ellipse, then suddenly the line breaks and you see a dotted line close to mun. except kerbin is much bigger than mun, so it looks different.

 

Quote

No. I am burning on the correct side of mun (the side opposite Kerbin). A prograde burn there will extend my apoapsis out to Kerbin (which is what I want). But I just want to be able to see the damn orbit I am creating! Not have it suddenly break and change colour. I can't see what I'm doing!

alas, that's not how orbital mechanics work. when you are on mun you want to fall down on kerbin, and to fall down from orbit you need to burn retrograde to slow down. which, in this case, will mean your ellipse must point behind the mun, not towards kerbin.

if you don't believe it, try it. look how much deltaV it would take to get back to kerbin with your manuever. and then look how much it takes the correct way  (should be 250 to 300 if done correctly).

if orbital mechanics weren't quirky, they wouldn't use rocket science as synonim for something difficult. getting to and from mun is the easier part. i'm currently trying to start from gilly, get a gravity assist on eve and intercept moho on an orbital node, i know what i'm doing and yet i spent two hours trying different trajectories without getting a really good one.

[StrandedonEarth]

To put it another way: as well as your orbit velocity around the mun, you also have the mum’s orbital velocity around Kerbin. By burning in the same direction as the Mun is orbiting, you’re increasing your orbital velocity around Kerbin even more, raising your Kerbin Ap orbit on the other side of Kerbin.  

So you need to burn prograde (relative to  the Mun) on the near side of the Mun, backwards to the direction the Mun is orbiting, to decrease your orbital velocity around Kerbin, which will lower your Pe over Kerbin, on the other side of Kerbin.