Entering the final door is easy, you just have to understand the mechanics which make it possible. As soon as you land on the ground where you rocket jump, turn your back to the force field and aim at the ground at a 45 degree angle. Then, begin to fast run back(I do not know the technical name of it), but it's where you run in a direction and jump, then jump again once you hit the ground. If you don't do this, then you won't have enough momentum from your rocket shots. When you fast run backwards, right before you enter the force field, jump and shoot at the ground, once you hit the ground jump and shoot again. You should be able to enter the door in time if you follow this right.