How is velocity translated/preserved during SOI changes? (specific examples inside)

[mekasha]

When changing SOI, does your old heading even matter, or is speed the only constant? I've finally had some luck capturing asteroids doing something a little counterintuitive; exiting Kerbin's SOI at roughly the same time and spot as a target asteroid passing through, and grabbing it in Kerbol's SOI. To manage that, I had to use kind of a crazy trajectory to exit at around the same time in the same place. Once I changed SOI's I was basically on what appeared to be a stable orbit almost directly next to my target asteroid (though still had to burn a bit to match speeds).

Just for a clearer example, if I had two objects exiting an SOI at the same place, at the same time, (at the same speed?) but object A was going "straight" at the point, and object B was approaching that point at a 45* angle, what would happen when they exit? Will they end up in precisely the same orbit? Is the same true if their speeds were different?

Edited April 13, 2015 by mekasha

[Superfluous J]

Your heading and speed both matter when changing SOIs. You can see this very clearly when you eject from Kerbin going backwards in its orbit compared to going forwards. Backwards, it slows you down. Forwards, it speeds you up (relative to Kerbin around Sun)

In your second example, they would exit at the same point, giong different directions. However because Kerbin's velocity is a lot larger than the difference between Kerbin's Velocity and your ships' velocities, it won't be super noticeable in map mode.

[Warzouz]

Sometimes, when quitting SOI, there is a "temporary" trajectory which seems to be wrong. This artifact appears at any warp speed (even x1). The trajectory seems to return to the body we quit and give a new SOI (usually very far). The "out of SOI" trajectory is also wrong.

Then at the predicted time, the correct trajectory come back. The false trajectory lasts only less than 2 minutes.

I think it's nearly always when you quit a smaller SOI to its parent time. I don't recall to have seen this bug (?) when entering smaller SOI.

I use MJ, KER, AlarmClock mods.

What is it ?

[Red Iron Crown]

When entering the SoI of a body, that body's orbital velocity is subtracted from the vessel's velocity. When leaving the body's SoI, that body's orbital velocity is added to the vessel's velocity. Note that this is not straight addition and subtraction, but instead is vector addition; direction very much matters.

Edited April 12, 2015 by Red Iron Crown
Fixed a typo.

[Superfluous J]

Sometimes, when quitting SOI, there is a "temporary" trajectory which seems to be wrong. This artifact appears at any warp speed (even x1). The trajectory seems to return to the body we quit and give a new SOI (usually very far). The "out of SOI" trajectory is also wrong.

Then at the predicted time, the correct trajectory come back. The false trajectory lasts only less than 2 minutes.

I think it's nearly always when you quit a smaller SOI to its parent time. I don't recall to have seen this bug (?) when entering smaller SOI.

I use MJ, KER, AlarmClock mods.

What is it ?

I don't know, but I've seen it too even on stock. It happen all the time. I just ignore it.

It's not "real" in any sense, and not related in any way to how switching SOIs works. It's just a bug and apparently just a visual one.

[Eric S]

I think it's nearly always when you quit a smaller SOI to its parent time. I don't recall to have seen this bug (?) when entering smaller SOI.

I use MJ, KER, AlarmClock mods.

What is it ?

I believe that this bug is related to the bug where warping through an SoI transition can throw off your trajectory. This latter bug was fixed in 1.0 during development, so this bug may be fixed as well.

As near as I can tell, this bug is caused by some kind of confusion as to which SoI you are in. You'll see a point at which you should transfer to the parent body's SoI, but when you reach that point, you get some nonsense trajectory projection that still lists an escape in the future from the lesser body's SoI.

The bug that was fixed had to do with the fact that spacecraft movement and the patched conics predictions were using different code to determine SoI transitions. The fix was to make everything use the patched conics SoI predictions, so hopefully there won't be any more room for craft to be confused about what SoI they're in.

[Reactordrone]

You have to consider the geometry of the different frames of reference. Ejection angle changes how your speed changes relative to the parent body, e.g adding or subtracting velocity from Kerbin's orbital velocity. A large plane change in a small diameter orbit around Kerbin will translate to a small plane change when you get into the much larger diameter of a solar orbit. That method of intercepting asteroids is what I use, i.e matching time and location of entry and exit then waiting for both objects to be in the same SOI. Targeting mode on the navball doesn't really work for objects that are are in different spheres of influence.

[Chaos_Klaus]

Yes, it is all about reference frames. When you change the SoI your velocity stays the same. You still move into the same direction.

Your orbit on the other hand looks totaly different. It seams to have a knee. That is because the referenceframe changed to the new parenting body and that body's own velocity is substracted from yours.

[impyre]

Velocities are preserved, it just doesn't seem obvious due to change in reference. The cause for the "glitch" regarding erroneous paths is due to the fact that the projection is based on forces which have changed. Though your velocity remains unchanged when you change SoI's, your acceleration doesn't (this is because KSP uses two-body calculations for the sake of playability and projections). What this means is that at some point, the game must detect the change of SoI, remove the force due to gravity from the old parent and apply the force due to gravity from the new parent. This can take some time (more or less depending on warp, vehicle part count, the numbers involved, cpu use, and physics deltatime settings). This means there can be a gap when you are in a new SoI but the changes haven't yet been applied... (in extreme cases, the changes may not be applied at all, as you will enter and leave the SoI in the same physics time tick) This leads to things like skipping through atmospheres, missing SoI's, etc.