All your suggestions brought me to pay attention to more things and finally to get it up there. Straight up till 25.000m, then full thrust and turning. Thanks a lot for your help. I'm going to check out the aerodynamics parts. @Snark : I only read your reply after succeding, but tried them just for practice. I think filling up the top tank did it, no flipping at 10000m, BUT the whole construction was very wobbly (which might be due to the docking port between the station and the bottom tanks). I suppose it works like a shuttlecock, the mass first against the wind. - - - Updated - - - I thought that it would be better to reduce the weight, since it was my first time. Apparently I was wrong.