The staging and the fuel lines are correct, but what changed them is that weak point (the decoupler above the orange tank) that my rocket has. Sometimes the gray tanks on the boosters just fall off, but it doesn't happen that frequently. Yesterday I tried to this same craft into a 120km orbit around Kerbin, but the weak point and the gray tanks didn't allowed me to do so. I haven't had any malfunctions (except for the one in the video) after about 5 tries, strangely enough. Anyway, now I'm on my way to Gilly toghether with my lander. Not that I need a lander for Gilly Thank you all for your help !