Relative Velocity Question

[Chads]

Hello!

Question on using the prograde/retrograde markers while in Target mode on the nav ball.

If I get my velocity to 0.0m/s, am I effectively in the same orbit as the target?  Or, over time, will the velocity match begin to degrade?

I'm not exactly sure what I was seeing last night, with respect to velocities and distances, and so I'm trying to get in my head how that would work.  What I noticed is that, despite holding a 5m/s relative speed, I would move toward, then away from, the target (distance would decrease, then increase).  I can visualize that--I'm moving through the plane of it's movement, and at some point, I'm moving further away as opposed to closer.  However, I couldn't find the right spot to keep it steady.  But...if I zeroed out relative velocity, it seemed that I'd sit at the same distance for extended periods of time (I did not play with it with time warp, though--I was low on fuel, and didn't want to completely screw my mission up).

Thanks!

[bewing]

Not quite the same orbit, no. Relative velocity = 0 means, "moving in the identical direction at the exact same speed" is all. But any difference in position means that they are on different orbits. So yes, they will definitely diverge over time.

The closer you are to the target ship, the less the two ships will diverge. Imagine that you have two ships, one at 70km altitude and another at 100km altitude. You match the speed of the 100km ship to the 70km ship. What happens? Well the 100km ship is moving much too fast to stay at 100km now, and it will have a very high Ap.

 

[WanderingKid]

49 minutes ago, Chads said:

If I get my velocity to 0.0m/s, am I effectively in the same orbit as the target?  Or, over time, will the velocity match begin to degrade?

Yes, it will, because you are very close to the same orbit, but not exactly on the same one.  There is no way, really, to be 'exactly' on the same orbit in KSP.  You can get incredibly close if you can get directly behind (or ahead) of the object on its orbit and you're killing relative speed directly on the prograde/retrograde orbit markers as well, but in reality it's almost impossible to do.  I say almost, because someone is going to come in here and show that it *can be done*!  For the rest of us mortals, however...

You can kill relative velocity being four kilometers away from the target, or even on the other side of the planet (though prepare for a mess that way).  All relative 0.0 means is "For now, at this VERY moment, our vector velocity matches".  That's not orbital velocity, that's vector velocity.  So if you think of two ships, one on a lower orbit and one on a slightly higher one, and you're directly underneath the higher one (from the planet's view) and match vector velocity, your orbits will cross up ahead and behind.  Eventually, you create an X from that vector matching, because you're starting from a different orbital position.  That eventual X creates a different vector velocity difference.

This is why typically, when docking, you do three or four burns to get yourself much more aligned with the target orbit.  The first one is a gross change, to get to a reasonable difference and then closer to the target... and its orbit.  The next few are adjustments where if you had a NASA team behind you telling you exactly what marks to hit you wouldn't need, but we approximate by simply burning at the target then correcting a few times by killing relative again at closest point.  Finally, your last docking approaches and adjustments match your orbits to near exactness.  Note, I say near.  If you don't dock you will always find you've had some drift after a while when you come back.

[Spricigo]

1 hour ago, Chads said:

If I get my velocity to 0.0m/s, am I effectively in the same orbit as the target?  Or, over time, will the velocity match begin to degrade?

 

oddly enough: yes AND yes. Displayed 0.0m/s is not fundamental and absolute constant 0 so, given enough time, the crafts will drift away

Quote

What I noticed is that, despite holding a 5m/s relative speed, I would move toward, then away from, the target

What you need to keep in mind is that you need to match velocity and position, not speed and distance. The difference is that the first pair are vectorial physics quantities, with magnitude and direction. What you are describing is just passing by the target. I would make a length description of how to make velocity and position match, but that was already been very well done, so I prefer to just link it:

TL:DR version make coincide with and with while maintaining [speed]=k.[distance]  

In other words: as x_you approaches x_target gradually make  v_you matchv_target

 

Edited June 22, 2017 by Spricigo
edit: also, what the ninjas said

[Spricigo]

11 minutes ago, WanderingKid said:

... NASA...

...Finally, your last docking approaches and adjustments match your orbits to near exactness... 

which, by NASA standards, is more like grossly putting yourself in collision trajectory before ramming your target 

[WanderingKid]

4 minutes ago, Spricigo said:

which, by NASA standards, is more like grossly putting yourself in collision trajectory before ramming your target 

Don't hit the solar panel, Jeb... JEB!  THE SOLAR... *smash*... dammit.