Why doesn our space program (NASA) focus more on propulsion systems?
I agree that propulsion is a major stumbling block for NASA, or at least it “looks that way”. There have been major speculations about several nations, not just U.S. possibly having various other more advanced systems, but seems limited to speculation, unless more evidence is provided. One possible plan involves creating a Gravitational Tunnel that is merely taking advantage of certain constants using Gravitational sling effect throughout the solar system, in essence a round trip excursion for exploration and development. This however sounds like a very slow pathway, little better than ion propulsion. Good for regular deliveries of supplies sometime in the future perhaps though. I am rather intrigued by the concept of Gravitational Propulsion, that is; a warping of space (as in gravity) to interact with space or warped space in order to propel, but that needs more work. There of course are many other possible answers; electromagnetic launchers, nuclear as you mentioned tho as the other
