Relative speed fluctuations during docking

[Zruty]

Hi everyone!

When I'm trying to dock the DeltaGlider to the ISS I experience this phenomenom. I eliminate my relative speed (make it zero) using AttitudeMFD. Then the speed slowly begins to increase again.

I'm not asking 'how to get rid of this?' Actually, it's more fun to dock this way. But why does it happen?

Thanks in advance.

 

[Master of Blades]

Is it under Time acceleration?

 

[Jebusy]

same with me.....hmm maybe its because even though the pea is the same as the iss when you want to dock the rest of the orbit altitude is different...so its slowly gets farther all the time...no wait actually that dosnt mean the rvel should increase...meh..i dunno

 

[TSPenguin]

Unless your orbit is exactly the same (PeA,Apa,Plane) you will have drifts.
Besides that you have relative rotation of the ISS and you. And then there is the uneven gravity field of earth.
But all that should occur slowly.

 

[ar81]

See this tutorial
[ame="http://www.orbithangar.com/searchid.php?ID=3074"]Tutorial: DG to ISS[/ame]
It will help you to understand what happens.
Essentially you trade altitude and velocity.
If you bring relative speed to zero, the slightest difference in altitude will make Kepler laws to make your vessel to start moving.
As you go around in orbit, the same relative position means different altitudes and therefore changes in relative velocity.

 

[Zruty]

Thanks ar81 for clarification. I know that you can't have the same speed and different velocity with a target while orbiting. I just though that when the distance is about 100 metres it becomes insignificant: definitely not 0.01 m/s^2 speed change.

I also think I should download the RendezvousMFD for the initial approach procedure handling. I knew I was doing it too Star Wars-like I just hope there is a manual or something for it.

 

[Zruty]

1. My difficulties were probably caused by my desire to do everything at once. I was aligning the spacecraft to the dock port and simultaneously approaching the station. When I do these things separately the fluctuations are much less and are much easier to handle. Sorry for misleading you.

2. Regarding the Rendezvous MFD - there is a restriction on the target's orbit Ecc. Is it because the calculations are actually done with a circular orbit? I guess the elliptic orbit case yields more complicated equations?

 

[ar81]

Rendezvous MFD calculates relative vertical and horizontal distance. If you are too far or orbits are too eccentric, calculations do not work properly.
Your target will be at the origin and you will see your craft moving along a certain path.
Use it when you are very close to your target.

Orbital operations are not too intuitive at the beginning.
But if you thinks about two orbits (your vessel and your target orbits) you may understand what happens with relative velocity. See my tutorial so you understand Kepler's law.

 

[V8Li]

Well, here's an example: ISS at 350Km at 7300 m/s velocity in an orbit of 340/360km. You at 350Km at 7300 m/s (relative velocity is 0). You could just as well be in a 100/580Km orbit. Consider the different vertical speeds alone and you will understand why you will move in a different way. Add to that the different vertical accelerations given by the different orbits and you'll get your fluctuations.

So, if you set a rendezvous from a close matching orbit (let's say 320/360Km) the fluctuations will be smaller and over longer periods of time and you could only use the RCS to dock. Also, if you are 5 meters from the dock at a relative speed of 1m/s and you don't need to move the ship up/down and/or left/right to keep it on course, that means you're in a same exact orbit as the target's with only a few meters differences between Aps or Pes. Got the point?

Note: assuming the RInc is 0.

 

[Zruty]

I used a term 'relative velocity' to describe the speed vector, not the vector's length. So if relative velocity is zero the vertical (tangential, whatever) speeds coincide.