I gave a help (Blazer or BenjaSFS feel free to tell me if the translation is inacurrate, I'll fix it).
That being said,
BenjaSFS☆☆☆ please write in english so that everybody can understand you. If you use an automatic translator and it bugs sometimes, please tell us, we will take it into account.
To me, the main problem is broken drag here. If you play with real drag, the separators will make it less aerodynamic than a brick (they are bugged, each separator is considered fully exposed to the air flow, no matter its placement).
Apart from that, the launcher itself is pretty realistic, the number of stages is correct, the use of Hawk/Titan is a good choice too (the first 3 stages run on solid fuel). The biggest problem to me is how the first stage is splitted. I guess you needed 2 engines there, but due to how their fuel capacity is balanced, the engine below will run out of fuel very soon.