The idea is worth something but so is the implementation because ideas are generic and the more specific you make them, the more you actually define an implementation, in the physical world at least.
If your rocket does work, then yes being able to lock everyone else out of it for decades is the entire point.
But once those decades are over, anyone else can copy your idea, which has been documented for all of eternity. There's an almost endless list of inventions before patents that were never made public, and nobody knows how they worked.
Unfortunately, that's the idea for how patents are supposed to work. In the real world what we have is patent trolls.
Exactly, a patent can't be just for "an idea" which is intrinsically generic but rather for the actual implementation with details. Patenting ideas would probably be the very definition of an overly broad patent.
No, because the disclosure has to enable a person of ordinary skill in the art to practice the full scope of the invention. This is called the “enablement” requirement.