THE EMERGENCE OF THE TURBOFAN ENGINE

If you have looked out the window of an airplane lately, you may have noticed that jet engines are gradually getting shorter and fatter. You will see 737s, the most common airliner in service, with two types of engines of distinctly different shapes. The older models have long, stovepipe-shaped engines under the wings, where the newer ones (or older ones which have been retrofitted with new engines) have rounder, shorter powerplants, with a large shell or nacelle around the outside and a smaller cylinder protruding from the rear. Boeing’s latest, the 777, has relatively short but immense engines – each with diameter equivalent to the fuselage of the 737. This change represents the maturing of the turbofan engine, which in the early 1960s superseded the older turbojet engine. Strictly speaking, for the past thirty-five years we have been living in the fan age more than the jet age.

Turbofans have a number of advantages over turbojets, particularly lower noise and higher efficiency – both key factors in making commercial jet air travel socially acceptable and economically feasible. Yet they appeared relatively late: no aircraft was powered by a turbofan engine until after 1960. Flight Magazine, in its 1957 prediction of aero engines ten years in the future, did not even mention the fan engine.1 As late as 1959, after airlines had begun to contract for turbofan engines, at least one expert was still expressing skepticism about their practicality.2 Once they appeared, however, turbofans almost immediately became the dominant engine for high-subsonic flight – the regime in which commercial airliners fly. In 1960, Flight Magazine declared that engineers had agreed that all high-subsonic engines would be fans.3

Today, turbofans power virtually all large commercial transports, as well as most large military transports and many business jets, and afterburning turbofans power most military supersonic aircraft. While the technology has certainly evolved in the last thirty-five years, the original turbofan configuration nevertheless stabilized quite quickly. Pratt and Whitney introduced the JT8D in 1963, and it remains the single most common jet engine in commercial service – with more than 13,000 sold.

The rapidity, scope, and permanence of the turbofan’s proliferation suggests a new technology with such obvious advantages that it met no resistance and spread rapidly – a veritable “turbofan revolution,” to modify Edward Constant’s phrase.4 But the obviousness argument, that hallmark of corporate histories and trope of technological progress, breaks down upon closer analysis. For the advantages of the turbofan engine, or more generically of the bypass engine, were recognized almost

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P Galison and A. Roland (eds.), Atmospheric Flight in the Twentieth Century, 107—155 © 2000 Kluwer Academic Publishers.

as early as those of the turbojet itself – Frank Whittle patented the idea in 1936, and a number of bypass engines were designed in the mid 1940s. Thus, nearly a quarter of a century elapsed between when fan engines were considered a good idea and when they actually became good enough to put them into service on airplanes. This paper explores this odd historical trajectory by asking two questions. First, if they took over so quickly, why did it take so long for turbofan engines to enter flight? And second, why did turbofan engines emerge when they did?

The answers to these questions include new engineering techniques, government – funded research, military requirements, and corporate competition. The story has a broad historical significance because the turbofan depended on and contributed to a stable configuration for commercial jet air travel at high-subsonic speeds5 – a major feature of today’s technological life. We are also interested, however, in questions of engineering epistemology – i. e. what knowledge do engineers use in design? how is this knowlege developed? and how precisely is it utilized? Walter Vincenti began to address these questions with his series of case studies in aeronautics, and we build on his work, especially regarding the role of uncertainty in design.6

Examining epistemological issues in the design of turbofans sheds light on other questions as well. For example, why, in 1997, might you be likely to fly on an aircraft with engines designed more than thirty years ago? Why do technologies experience periods of rapid change, followed by long periods of stability and incremental change? What follows, we argue, is fundamentally a story of radical and incremental change, but one that ends in a counterintuitive way. Rather than a radical innovation winning out over incremental improvements, we find a radical design that spurred incremental innovation in a competitor. The latter succeeded commercially and established the turbofan as an accepted technology.