what you are experiencing is the phenomena called 'Nodal' Regression'. It is caused by the perturbative torque of the earth's equatorial bulge. In other words...
The Earth like many large bodies in the solar system are not perfectly sphere. The effect is that there is more mass around the equator than other parts of of the planet. What your station is doing like mine is gravity is grabbing it and dragging it a tiny bit causing its orbit to drift west if you launched into the direction of the Earth's spin. If you launched west, the drag would occur and cause the drift to occur to the east. Nodal regression is predictable, is based on two things. First your orbital inclination. The closer to a equatorial orbit, the less this variable will affect your regression. The second factor is altitude. The lower your altitude is the more drift your have, higher altitudes, will slow the drift.
The good news?
Of course I am going to give you good news.... The drift is like I said predictable so that means that after determining your drift in a 24h day, you can then go from there on how much time needs to pass before your station drifts back into the lunar plane by doing some simple math. For my station it is ~2.62 deg drift out of the lunar ground track per 24hour period.
There are 360 degrees in a full circle so by dividing 360 by 2.62 I get 137.40458015 blah blah blah... If I am launching from a ground base that is below latitude of 30 deg, I now know that I have a launch window about every 138 days if I want to use my station and not deal with a plane change. This is really your only option if you want to go for realism,
More Good News?
I love alternatives and I have a few for you.
1) If your not worried about virtual costs of your missions, the Energia HLLV is more than capable of launching a DGIV, XR1 or XR2 into LEO. The only fuel you have to worry about burning at that point is at seperation from the launch vehicle to round out your orbit. If you never used the Energia it ALMOST puts your payload into a circular orbit. The idea is that with a low perigee, you say no to spent orbital debris and drag burns it up in the atmosphere. I usually use this feature if I am going to be using an external tank connected to the docking port for going to Venus or Mars in a hurry. 20 tons of fuel from the Space Station Building Blocks fuel tanks are really a fuel source. I can launch the DGIV and the tank at the same time and dock with it in a few minutes and leave Earth behind.
2) Another option I use cause I love Dan's DGIV when I wanna get into orbit quick and use just the crafts engines is the Velcro Rockets. Sputnik created these really neat drop tanks for the DG, and they are compatable with the DGIV as well. They work with the XR1, but you need to change a setting in the cfg file that allows ground based VTOL pad refueling. It is the same option you would change to allow the scn editor to refuel the craft or FuelMFD for example. With the DGIV, you can launch off plane and still have enough fuel aboard for getting to the moon in 40 hours or less. There is also a SRB booster for the XR1 that states that 50% fuel saving for the vertical launches.
If your looking for another source of smart talk, Dr Martin himself has included a PDF speaking of it with those big fancy math symbol thingies in \Doc\Technotes\Gravity.pdf
