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Sir Frank Whittle is renowned for developing the turbojet engine in the 1930s, which is commonly referred to as a jet engine. This innovation revolutionized aviation by enabling faster and higher flight capabilities compared to traditional piston engines. The turbojet operates by compressing air, mixing it with fuel, and igniting the mixture to produce thrust, thus allowing for greater efficiency and performance in aircraft.

This advancement marked a significant milestone in engineering and military aviation, leading to the development of modern jet aircraft. While the term "jet engine" is often used broadly, it specifically refers to any engine that produces thrust from jet propulsion, with the turbojet being a critical type of jet engine developed by Whittle.

The other options, such as rocket engines and piston engines, refer to different methods of propulsion and are not directly associated with Whittle’s notable achievement during this time. Thus, identifying the turbojet engine as the significant invention highlights Whittle's impact on aviation technology and history.