The Pratt & Whitney J57 is a turbojet engine that was widely used in military and civilian aircraft from the 1950s to the 1970s. It was known for its reliability and performance, powering notable aircraft such as the Boeing B-17 Flying Fortress and the Lockheed F-104 Starfighter. This engine features a two-spool design, which allows for efficient operation at various speeds and altitudes. The J57 was one of the first engines to incorporate a variable-area nozzle, enhancing its thrust capabilities and fuel efficiency, making it a significant advancement in jet propulsion technology.