Whittle had begun thinking about jet engines in 1928, when he wrote a college thesis on future developments in aviation. He was prescient, considering that this was the heyday of the biplane and Lindbergh had only recently crossed the Atlantic. "It seems," he wrote, "that, as the turbine is the most efficient prime mover known, it is possible that it will be developed for aircraft, especially if some means of driving a turbine by petrol could be devised." At first he was thinking of a turboprop, but by 1930 he had realized that an airplane could be propelled by the fast-moving gases of the turbine exhaust alone, and that thrust obtained in that way had advantages, for high speed flight, over thrust obtained by means of a propeller. In that year Whittle took out a patent on an engine design consisting of an air compressor driven by a gas turbine, with a gas-generating burner between them. This was, in general outline, a complete jet engine. Mechanically, it had much in common with the turbosupercharger, which had been developed for aircraft use during World War I, a principal difference being that the heat source for the turbosupercharger - what we now call a turbocharger - is external.