Abrupt direction change when in new SOI and grav. assists?

[ROXunreal]

Greetings. I'm three days new to the game but have a basic understanding of most of the astronomical concepts involved.

One thing I don't understand is that (as far as I know) because of the game mechanics, upon entering a new body's SOI the direction of your craft changes abruptly and unpredictably. I don't understand how I'm supposed to "aim" for third bodies during gravity assists in these circumstances, and I can't seem to find any relevant info on this in internet searches.

This is what I mean:

This is the plan.

Looking good.

Suddenly upon reaching the "Mun encounter" node, THIS happens. Everything is changed. I'm heading in a completely different direction that was not indicated in the flight path prediction?????

I also don't get the broken flight path lines?

Edited December 24, 2014 by ROXunreal

[tetryds]

What is happening here is very simple:

When you have an orbit around Kerbin, your conics will display the orbit relative to Kerbin.

Which means that one orbit around Mün, relative to kerbin, will look nowhere near an elipse.

But after you enter Mün's SOI your orbit will be relative to the Mün.

That means that both orbits are pretty much the same, you just change your reference.

So, if you timewarp you will see that the Mün will move and your ship will move with it, and you will escape exactly like it's shown on the first orbit.

You can change the modes, and make it show like on the last picture all the times, you just need to go on settings.cfg and change:

CONIC_PATCH_DRAW_MODE = 0

So it will display like this:

Right now you have:

CONIC_PATCH_DRAW_MODE = 3

Which looks like this:

Both orbital paths are exactly the same.

Modes 1 and 2 shift the SOI entrance orbit to match with the SOI entrance and exit, respectivelly.

Edit: If you focus on Kerbin and timewarp you will see that even though the orbit drawing changes, the relative ship motion is exactly the one shown on the first image you posted.

The "broken path lines" are because on Mode 3, it always show the orbits relative to the lower SOI on the chain, so the same way your orbit will exit Mün further forward to start being relative to Kerbin again, it will also exit Kerbin a bit further forward and start being relative to the Sun.

This is just a matter of how these orbits are displayed

Edited December 23, 2014 by tetryds

[Ziff]

This is a matter of perception. You are still moving on the lines indicated in the first picture. The first picture shows your actual path through space before, during, and after intercepts with the moon. What you aren't taking into account is the fact that the moon is moving. In the final picture it's showing your path relative to the moon. You have to remember that the moon is also moving. So, by the time you orbit around the moon it will have moved forward and the blue line showing your "Exit" path will reconnect with the other orbit and send you on your way. Also, recognize that the yellow line in the third picture is the same as the purple line in the second picture. There is now a new purple line in the third picture showing your exit from Kerbin's SOI.

[Edit] In other words, just zoom the map out and watch as your time warp. You'll see the ship get pulled in and it's orbit follow along where it should have gone.

Edited December 23, 2014 by Ziff

[eddiew]

What is happening here is very simple:

I wouldn't say it's "simple" but good explanation

Important thing for the OP is that if you just keep time warping and get out of Mun's SOI, the broken lines will merge at the point of escape, you'll be back on the original orbit you expected to be on. In fact, if you'd focussed Mun in the middle screenshot, it should have showed you what the orbit would look like once you were in it's SOI. Your velocity isn't changing, but the way the game is displaying your movement is.

[tetryds]

I wouldn't say it's "simple" but good explanation

Haha, maybe it isn't, but you get used to it.

Thanks.

I tend to use Mode 0, often Mode 3 can become a mess and hard to fine tune maneuvers.

But Mode 3 makes it simpler to plan free return trajectories.

[Anquietas314]

As has been explained this is just the patched conics system in a somewhat counter-intuitive configuration. However, it's worth adding that if you pass through an SoI switch while timewarping above say, 10x or 50x, quite often the orbit around say, Mun won't be what was predicted, sometimes with quite a large difference between them (e.g. periapsis can jump from 5km to 500km or inside the Mun at very high timewarp).

You can also see your encounter prediction relative to the Mun if you focus view on it, as opposed to the encounter relative to Kerbin.

[ROXunreal]

Thanks for the fabulous answers. I tested and did notice that in the end if I followed through regardless of the apparent orbit change I get the same result. It was just rather confusing at first. Well, main thing is that it works.

[DrD]

This confused me also but once I understood it actually helped me tremendously to understand orbital mechanics in general. So your orbit looks one way to someone standing on the Mun but another to someone standing on Kerbin and yet another to someone standing on the sun (Kerbol. I prefer to say sun to avoid confusing typos.))

For example when you thrust away from Kerbin to go to Duna it looks like you're blasting away from Kerbin to someone at the KSC but once you switch to sun SOI it's clear that Kerbin is orbiting away from you and you've in fact slowed down relative to sun.

You'll also notice your speed changes when you enter a new SOI, it goes up quite a lot when entering sun SOI since you now have Kerbins' orbital velocity counted as well.

Neat huh?

And BTW if you don't know (I didn't for a long time) you can click on a body, say the Mun, and select focus, and then you right away see your pending orbit to that body like in Tetryds's picture but solid lines not dashed. To return focus to ship press backspace. Not sure if this is changed in 0.90, I'm still early in my career in this latest version.

[Anquietas314]

the sun (Kerbol. I prefer to say sun to avoid confusing typos.)

Strictly speaking, Squad never actually named it Kerbol - the fans did. I believe the official name is just "the sun".

For example when you thrust away from Kerbin to go to Duna it looks like you're blasting away from Kerbin to someone at the KSC but once you switch to sun SOI it's clear that Kerbin is orbiting away from you and you've in fact slowed down relative to sun.

I think you've confused Duna with Eve here: you speed up relative to Kerbin's orbit to go to Duna, not slow down.

Not sure if this is changed in 0.90, I'm still early in my career in this latest version.

Nah it's still the same

[tetryds]

I think you've confused Duna with Eve here: you speed up relative to Kerbin's orbit to go to Duna, not slow down.

Haha, that caught you too.

When you accelerate and make your AP higher you get slower as you orbit towards it.

But it's true that you only "slow down" as you gain altitude, not at the exact escape moment.

[Specialist290]

While we're on the subject, you can check this handy tutorial if you want more in-depth information on what each conic patching mode draws in reference to. It's an older one, but the information in there should still be valid.

[LethalDose]

Well, I would argue that your premise:

... upon entering a new body's SOI the direction of your craft changes abruptly and unpredictably

is flawed since the change isn't "abrupt": The trajectory is gently curved throughout the predicted track. The changes are more dramatic when the vessel is closer to the new body, but if

I [...]have a basic understanding of most of the astronomical concepts involved.

is true, then you'll understand why (in a word, "gravitation").

As for:

I don't understand how I'm supposed to "aim" for third bodies during gravity assists in these circumstances

Well, gravity assists are a major pain to set up. Not sure how else to say it.

Many experienced players (myself included) believe that there are very few conditions where gravity assists are worth the hassle. Examples include slingshots off Eve to kill velocity for Moho approaches and Jool captures/escapes off Tylo. Other experienced players love them for all kinds of maneuvers.

If you're really set on using assists, then my best advice is the following three points:

• Zero inclination approaches: Make sure you're approaching the body you want to slingshot off at almost a perfect equitorial orbit. Doing so should largely eliminate that sigmoid shape you're seeing, or at least remove the inclination change as a reason you're seeing it. This should make your outcomes more
  
• Node planning: Set your original burn node to get you an AP/PE near your target slingshot body using the pro-/retrograde vectors, then rotate that node around your origin orbit until you get the slingshot result you like.
  
• "Brute force" energy changes: Use gravity assists only when you're trying to make big changes to your orbital energy where the exact AP or PE doesn't matter that much. Jool capture off Tylo would be a prime example of this. In other cases, you're sacrificing too much precision to make your slingshot worth it.
  

Really, if you're 3 days new to the game, don't worry about assists yet. Even if you're an astrophysicist, learn the basics for now. There's some practical skills for this game that you can only really get by playing it. When you can shoot yourself out to Jool or Moho with no problems, then start mucking around with the assists. You'll learn where to use them pretty quickly if you understand the orbital math.

Anyway, hope that all helps. Clear skies and welcome to game.

EDIT:

Two more points that I think may be salient to the discussion:

• You may also want to increase the number of SOI changes you're predicting into the future. That purple trajectory in your last image is your Kerbolar/Solar trajectory. it was there the whole time, it just wasn't getting rendered.
  
• Take your SoI transitions at low timewarp speed (10x or lower). When you transition SoI's/patches, there's invariably some floating point and rounding errors. Thing is, those errors are dependent on the pre- and post- transition frames, which are, in turn, dependent on your time warp. Basically, high time-warp SoI changes can seriously ruin your day.
  

Edited December 24, 2014 by LethalDose
Addenda

[LethalDose]

While we're on the subject, you can check this handy tutorial if you want more in-depth information on what each conic patching mode draws in reference to. It's an older one, but the information in there should still be valid.

Awesome resource.

So if this is > 2 years old, why in the frak is no way to adjust this ingame yet!?