I need help. Why on earth does this happen?

[BrandonM]

I'm fairly new to orbiting, so if anyone can tell me why this happens, it would be great. 

 

I am following these instructions:

Yes, they are outdated, but nearly the same.

 

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On SOI transit my orbit decides to do this.

 

http://steamcommunity.com/sharedfiles/filedetails/?id=778002232

 

If someone could tell me WHY, that would be wonderful.

 

Thanks.

Edited October 9, 2016 by BrandonM

[Aegolius13]

Does your path show a good altitude over the Mun before the SOI transfer?  Are you warping through the transfer?  That's known to lead to bigger errors when the same recalculates the new SOI.  

In any event, you can fix that orbit pretty cheaply by doing a "radial out" burn until your periapsis reaches the desired altitude.  It will be cheapest if you do it as soon as you change SOI.

[BrandonM]

2 minutes ago, Aegolius13 said:

Does your path show a good altitude over the Mun before the SOI transfer?  Are you warping through the transfer?  That's known to lead to bigger errors when the same recalculates the new SOI.  

In any event, you can fix that orbit pretty cheaply by doing a "radial out" burn until your periapsis reaches the desired altitude.  It will be cheapest if you do it as soon as you change SOI.

Alright thanks. I'll post back here if i'm still having problems.

 

[Capt. Hunt]

when you set up your maneuver node to make your Trans-Mun-Injection burn, it will tell you what your resulting munar orbit will be, you can adjust that trajectory by tweaking the maneuver.  It probably just needed a little more or less prograde.

Hint: the less you pull out the axis marker on the maneuver node, the finer the adjustment will be.

Edited October 9, 2016 by Capt. Hunt

[Plusck]

30 minutes ago, BrandonM said:

I'm fairly new to orbiting, so if anyone can tell me why this happens, it would be great. 

 

I am following these instructions:

Yes, they are outdated, but nearly the same.

 

-----------------------------------------------------------------------------

On SOI transit my orbit decides to do this.

 

http://steamcommunity.com/sharedfiles/filedetails/?id=778002232

 

If someone could tell me WHY, that would be wonderful.

 

Thanks.

Well, that video isn't really giving you an "Apollo-style" mission since it isn't a "free return" transit (for a free return, you go the wrong way round the moon and are sent back to a low-Pe orbit around the starting planet).

So... you're either burning too much from LKO, or not enough, because instead of being caught by the Mun's gravity and whipped around behind it (standard practice, what the video shows, NOT what Apollo did), you are actually impacting it. If you had burned a touch more from LKO, you would have gone around the far side.

In any event, it's easy enough to correct - it just gets increasingly more expensive to correct as you approach the Mun. Since it is cheaper (for landing purposes) to get into a normal eastwards orbit, you should have just slowed down a bit while some distance away from the Mun.
Where you are now, however, it will be cheaper to get into a retrograde orbit - for that need to burn radially out (probably about two-thirds of the way towards radial out from prograde would be best). If you want to go prograde, you'll need to burn radially-in (holding the position for the burn since the markers will whip around rapidly, so you finish the burn heading radial-out... if you see what I mean).

 

[BrandonM]

Thank you guys, will get back to you later

[DerekL1963]

28 minutes ago, Plusck said:

Well, that video isn't really giving you an "Apollo-style" mission since it isn't a "free return" transit (for a free return, you go the wrong way round the moon and are sent back to a low-Pe orbit around the starting planet).

Not all Apollo missions used a free return trajectory - less than half of them did, through Apollo 11.  They stopped using free return from 12 onwards because it sharply limited landing site availability.

[Plusck]

29 minutes ago, DerekL1963 said:

Not all Apollo missions used a free return trajectory - less than half of them did, through Apollo 11.  They stopped using free return from 12 onwards because it sharply limited landing site availability.

Really? From everything I have seen, they always seemed to use a retrograde orbit which was either a pure free-return orbit or very close to it (so if they had to abort for any reason, only a very slight correction would be needed to get back on a free return. Apollo 13, for example, did so with an 11 m/s burn...).

[FullMetalMachinist]

1 hour ago, Aegolius13 said:

Are you warping through the transfer?  That's known to lead to bigger errors when the same recalculates the new SOI.  

Minor (slightly off topic) correction here. You actually do want to warp across SoI transitions, just do it at a slow warp (5x-50x). The reason is that while on rails the orbit code uses more precise numbers than it does when out of warp (normal time). See here for more info:

 

[DerekL1963]

54 minutes ago, Plusck said:

Really? From everything I have seen, they always seemed to use a retrograde orbit which was either a pure free-return orbit or very close to it (so if they had to abort for any reason, only a very slight correction would be needed to get back on a free return. Apollo 13, for example, did so with an 11 m/s burn...).

"Very close" != "Free return", either you are or you aren't - if 13 had been on a free return trajectory, they wouldn't have had to make a burn to get onto a free return trajectory.  (Close only counts in horseshoes, hand grenades, and thermonuclear weapons.  Not so much in orbital mechanics.)  IIRC, their periapsis w/o getting back on free return was a couple of thousand miles up.

Some of the confusion may come from the hybrid nature of the actual trajectory...  At S-IVB burnout, they were on free-return but after T&D and extracting the LM they'd make a burn with the SPS and depart from free return.  Apollo 13 made that burn at about 30:40 MET.

http://history.nasa.gov/ap13fj/05day2-mcc2-tv.htm

Edited October 9, 2016 by DerekL1963
Clarified comment on periapsis

[Plusck]

33 minutes ago, DerekL1963 said:

"Very close" != "Free return", either you are or you aren't - if 13 had been on a free return trajectory, they wouldn't have had to make a burn to get onto a free return trajectory.  (Close only counts in horseshoes, hand grenades, and thermonuclear weapons.  Not so much in orbital mechanics.)  IIRC, their periapsis w/o getting back on free return was a couple of thousand miles up.

Some of the confusion may come from the hybrid nature of the actual trajectory...  At S-IVB burnout, they were on free-return but after T&D and extracting the LM they'd make a burn with the SPS and depart from free return.  Apollo 13 made that burn at about 30:40 MET.

http://history.nasa.gov/ap13fj/05day2-mcc2-tv.htm

Yes, I understand.

Still, if you aim to insert into a prograde orbit but fail, you're going to be missing the atmosphere by several million miles. "Almost" free return is therefore useful and quite meaningful, if you have a variety of means at your disposal to make minor changes to your trajectory (even if your main engine fails).