ur turbojet fuel tanks are too big so u need to replace them or u remove 1/2 of the fuel and all of the oxidizer or change tanks that should help cos ur engines are too weak and ur plane is too heavy and also never use the glider like wings its the worst wing to use its only for gliding rather use something else. U can send the craft to meh and il fix it rlly i can FIX or idk but when ur at 0.1 IA (intake Air) Turn the engines off and close the intakes cos intakes produce drag and drag is bad but also add more engines and also lower ur thrust at 40,000 M or Ft and ur thrust should be at 2/3 and at 60,000 ft trust must be 3/3 or full PM me for anything else also bring up ur gear that produces drag