Nuclear-powered aircraft.
While the original "only" flew 47 times, it wasn't very feasible in terms of economics, dealing with the radiation effects on the airframe, etc.
Since then, however, Popular Mechanics wrote a piece on a design that's by and large free from the issues of the one's
flown 60 years ago by the USAF.
Emphasis added to demonstrate that this isn't exactly a new idea - the actual flights preceed my birth by a decade and a half!
By the way, the Popular Mechanics article talked about the late 1990's discovery by researchers at the University of Texas in Dallas, where when hafnium was bombarded with x-rays, it releases gamma rays 60 times more powerful than the energy of the x-ray input.
This article examines this phenomenon, but it apparently splashes cold water on it's liklihood.
Still, Popular Mechanics isn't the National Enquirer, so one has to ask why the splashy article in PM, and why the cold shoulder from the gubbahmint?
Yet
this phenomenon is mentioned in Wikipedia.
More on the controversy,
here.
This raises some questions in my mind, such as, did Carl Collins at UT really goof? Or is this being kabashed? If it's a viable alternative to Jet-A, why not?
Furthermore,
"halfnium-178" and "atomic weight" yields no Google hits. That's rare.