Flight Question Rendezvous problem

[kirog]

Heya! :tiphat:

I have a question regarding rendezvous operations, which has been bugging me for quite some time now...

ISS rendezvous, the hard way: Raising my perigee bit by bit, keeping the ISS in front of me (in SyncOrbit MFD around DTmin 20sec) until I reach my last orbit. Then I do a prograde burn at Apogee to raise my perigee well above 300km and fine tune it so that DTmin is 2.0 to 2.10 seconds, with the ISS still in front of me (ToR-TgT is lower than ToR-ShP). The approach itself is not the problem, but this phenomenon:

I want to be approximately 14-16km behind the ISS at rendezvous point, so far so good. But during the last phase, still quite some time before rendezvous point, my distance to ISS begins to decrease significantly below 14km, then it rises again until it reaches 14-16km at rendezvous point, the desired result. This is a problem though, because I want to do an approach that ends 14km behind the station, which should be the minimum distance. I don't want to come closer before rendezvous point / time and therefore too low, because I am still racing towards my apogee). This is obviously something I don't understand at all....
Has it something to do with RInc not being zero, but 0.01-0.03 degrees?

I can't figure it out.. please help! :facepalm:

 

[Tommy]

Since you are coming "up" from below the ISS, your velocity will be lower than the ISS's when you rendezvous. If your periapsis is low enough, the difference in velocity is enough so that the ISS is actually behind you for a little bit during that last half orbit, and you pass "beneath" it at a distance of only a few km. Then, by the time you've reached the rendezvous, the ISS has pulled into the "lead" again because it's traveling faster.

It doesn't really matter that you are getting closer than 14k, but if you want to avoid that you will need to raise your periapsis. That way you won't be traveling as much slower, and you need to get "ahead" of the ISS in order to have the 15k distance at rendezvous.

Not sure how easy it is to understand that explanation, but hopefully it helps a bit.