Nuclear powered aircraft
In the 1950/s the United States Airforce and Navy considered nuclear powered aircraft. Instead of burning fuel to heat air and provide thrust, a nuclear-powered jet engine would heat the air from the heat generated by a nuclear reactor. If it had worked, the aircraft could have remained airborne for months at a time, limited only by the endurance of the flight crew.
Here’s a rather good 46-minute-long documentary on the United States’ program to develop nuclear powered flight.
Here’s an article on nuclear powered aircraft.
And here’s an article on a proposed civil airliner, that would be powered by a nuclear fusion reactor—even though nobody knows how to make a fusion reactor yet.