Nuclear propulsion is the system Rickover championed for the Navy. He led the Navy’s nuclear propulsion program, guiding the development of reactors and the overall approach that made the first nuclear-powered submarine possible. This culminated in the Nautilus, launched in the mid-1950s, proving that a submarine could operate for extended periods underwater without refueling. The core advantage is endurance and stealth. A nuclear reactor provides continuous power and energy, so a submarine can stay submerged for months, travel long distances, and avoid frequent surfaced refueling or recharging. That capability transformed strategic and tactical naval thinking, enabling new kinds of undersea operations and deterrence. Diesel-electric propulsion, by contrast, requires frequent access to air to recharge batteries, which limits submerged endurance and makes long-range patrolling more difficult. Gas turbines and steam propulsion exist in other contexts, but the pioneering shift Rickover led was the adoption and safe, reliable use of nuclear propulsion for submarines and other Navy vessels.