Sir Frank Whittle, a British Royal Air Force engineer, is credited with inventing the turbojet engine in the 1930s. He independently developed the jet engine concept around the same time as German engineer Hans von Ohain. Whittle's design used a gas turbine to produce thrust, which became the foundation for modern jet propulsion. Gottlieb Daimler is associated with internal combustion engines and automobiles, while Roger Bacon was a medieval philosopher, and Lewis Waterman invented the fountain pen.