Can someone tell me whats going on in here?

[Aiden.J]

So I took off from the Mun, and as i was making a maneuver node to escape and go back to Kerbin, i saw something weird. As i created the node, at around 211 m/s, my orbit (which escaped from the Mun), would eventually come back in it's SOI and orbit the Mun, again, with out me burning retrograde. I think my orbital speed relative to the Mun was less than it's escape velocity, but i was still in the Mun SOI when i created the node, so my velocity was higher than Mun's escape velocity (unless, when i came back, my velocity had decreased). Ill check if anybody responds, and i have a screenshot with KER on, but i don't know how to paste it onto this question.

 

Edited May 21, 2017 by Aiden.J

[Red Iron Crown]

Welcome aboard! I'd suggest hosting your screenshot on imgur.com or another image host, then we can see what's going on.

[Cpt Kerbalkrunch]

This might be a stupid question, but are you sure this wasn't a second encounter with Mun on your projected orbital path? Mun is very easy to get an encounter with. In fact, sometimes it's difficult to avoid.

As far as posting a screenshot, any hosting site will do (imgur, dropbox). I've been using postimage. No accounts or downloads necessary. Just go on the site and hit upload image (or choose image, I think). It'll open your files so you can pick an image. Go in your KSP folder. Inside it is a folder labeled "Screenshots". Any time you hit F1 in-game, a screenshot is taken and stored here. Choose the one you want. Once it's uploaded it will have links and addresses to access it. Highlight the one labeled "Direct Link". Copy it with Control C. Then, in your post here on the forum, just hit Control V and the image will pop up in your post. It's pretty easy. You'll pick it up quickly.

[FullMetalMachinist]

18 hours ago, Aiden.J said:

As i created the node, at around 211 m/s, my orbit (which escaped from the Mun), would eventually come back in it's SOI and orbit the Mun, again, with out me burning retrograde.

It sounds like you stop adding dV to the node when you see it re-encounter Mun's SoI. If it is what I'm thinking, then it shouldn't be a problem, and it's actually pretty common. 

What can happen is this: if you burn prograde (compared to Mun's orbit around Kerbin) and just barely escape it's SoI, what happens? You leave the Mun ahead of it and with roughly the same orbit as the Mun, just with a slightly higher AP due to leaving prograde. This puts you in a higher orbit, which means your orbit is slower. Which means that before long the Mun will catch up with you and you'll get pulled into it's SoI again, without having to do any additional burns. 

Same thing happens if you escape retrograde. You'll exit behind the Mun, but will have a slightly lower and therefore faster orbit and will catch up to it and encounter it's SoI again. 

These 're-encounters' should go away once you put enough dV into the burn so that your orbit after leaving Mun's SoI is different enough that it won't happen. 

[Aiden.J]

On 5/13/2017 at 5:06 PM, Red Iron Crown said:

Welcome aboard! I'd suggest hosting your screenshot on imgur.com or another image host, then we can see what's going on.

OK ill try that! Thanks

[Aiden.J]

On 5/14/2017 at 9:23 AM, FullMetalMachinist said:

It sounds like you stop adding dV to the node when you see it re-encounter Mun's SoI. If it is what I'm thinking, then it shouldn't be a problem, and it's actually pretty common. 

What can happen is this: if you burn prograde (compared to Mun's orbit around Kerbin) and just barely escape it's SoI, what happens? You leave the Mun ahead of it and with roughly the same orbit as the Mun, just with a slightly higher AP due to leaving prograde. This puts you in a higher orbit, which means your orbit is slower. Which means that before long the Mun will catch up with you and you'll get pulled into it's SoI again, without having to do any additional burns. 

Same thing happens if you escape retrograde. You'll exit behind the Mun, but will have a slightly lower and therefore faster orbit and will catch up to it and encounter it's SoI again. 

These 're-encounters' should go away once you put enough dV into the burn so that your orbit after leaving Mun's SoI is different enough that it won't happen. 

No, what's happening is what you said, except when my new orbit re-encounters the Mun, it creates an orbit around the Mun. I uploaded a screenshot through imgur, which should help.

[Ohm is Futile]

1 hour ago, Aiden.J said:

No, what's happening is what you said, except when my new orbit re-encounters the Mun, it creates an orbit around the Mun. I uploaded a screenshot through imgur, which should help.

This can also happen since your orbit is not different enough from the Mun's. It's basically the concept of gravity assists. By re-entering the Mun's SoI, you end up in orbit around it because its gravity is strong enough to modify your trajectory. Again, if you burn more, this should no longer happen.

[Aiden.J]

6 hours ago, Ohm is Futile said:

This can also happen since your orbit is not different enough from the Mun's. It's basically the concept of gravity assists. By re-entering the Mun's SoI, you end up in orbit around it because its gravity is strong enough to modify your trajectory. Again, if you burn more, this should no longer happen.

Ohh, so whats happening is as i orbit Kerbin, my relative Position to the Mun is what my orbit is, as explained in this bad drawing (and bad explaination).

Where i cross the Mun's orbital plane is my ascending node (relative to the Mun), and my orbital period matches that of the Mun (relative to Kerbin), and, in the Mun's SOI, my orbital period (relative to it) also matches the Mun's orbital period around Kerbin, so when i orbit once around the mun, the Mun orbits once around Kerbin (and so do i). Its the Mun's gravity is what makes my orbital period perfect, correct? Another way i see it is that, somehow, my rendezvous with the Mun slows my trajectory enough that i orbit it, but how can that happen? Because when a spacecraft does a "gravity assist", it gains orbital momentum (idk if that's a thing) by swinging around the object and taking a minuscule amount of the object's momentum, which would only speed the spacecraft up (again, idk if that's right), which would hold the opposite effect than what happened.

I honestly am really bad at this rocket science stuff, so im pretty sure im wrog and you're right, but im just trying to understand this. Thanks and sorry.

Edit: No, my orbital period to Kerbin is half that of the Mun, so when my orbit intersects the Mun is when im going fastest (im at periapsis to the Mun) and when im farthest away (at my apoapsis and periapsis relative to Kerbin) im going slowest to the Mun (so at apoapsis to Mun); think of it on rails.

Edited May 22, 2017 by Aiden.J

[Alshain]

When you can't quite figure out what is going on with a maneuver like that, switch your focus back to the planet.  The maneuver is viewed relative to the gravity of the body you are focused on.  If you view it from Kerbin's perspective, it all becomes clear.