WHY THE TURBOFAN EMERGED WHEN IT DID

Let us return to our initial questions. First, given that the turbofan engine was long recognized as promising better propulsion efficiency in high-subsonic flight, and given that the original patent was in 1936, why did turbofans enter flight-service only in the early 1960s? A simple technical answer, recognized to at least some extent from the 1940s on, is that no turbofan was going to offer markedly superior performance until (1) gas generators – i. e., turbojets – had reached a reasonably high level of performance, especially in specific-power; and (2) compressor and fan aerodynamic design had reached a point where a sufficient pressure-ratio could be achieved in the bypass stream for efficient high-subsonic flight without excessive weight. Until these advances in technology had been achieved, turboprops like the Lockheed Electra, with flight speeds around 400 miles per hour, made a great deal more economic sense for most commercial flight. This simple technical explanation, however, masks an underlying complexity. For, the two requisite advances in jet engine technology would not have been sufficient for the turbofan to have emerged until the problem to which it was an answer had been identified as important.

As a first step toward unraveling this complexity, we can identify the several local factors that lay behind General Electric’s developing their first turbofan, the CJ805- 23, when they did: (1) persistent advocates of fan engines within GE, especially Peter Kappus; (2) an established gas generator with sufficient specific-power to drive the fan; (3) the aft fan concept, which allowed the turbofan engine to be developed at remarkably little cost; (4) the realization, which emerged in the last years of the NACA supersonic compressor research program, that comparatively high Mach number transonic stages could be designed without first having to learn how to control shocks; (5) the shift of key figures in this research program from NACA to GE, especially Lin Wright; (6) the advent of the computer, allowing the introduction of streamline-curvature methods for analyzing radial equilibrium effects in compressors; (7) the idea of adapting streamline-curvature methods to provide a through-blade analysis that could define a blade contour precisely tailored for the significant radial redistribution of the flow that occurs within a high pressure-ratio transonic blade row. Three other factors may have been important in

GE’s decision to commit money to developing the CJ805-23: (1) Rolls-Royce’s Conway engine, perceived perhaps by some as heralding the advent of bypass engines; (2) Pratt & Whitney’s overwhelmingly dominant position in high-subsonic flight, achieved initially through their J-57 on the B-52 and then in the process of being repeated by the commercial version of the J-57, the JT3C, on the Boeing 707 and Douglas DC-8; and (3) Wislicenus’s talk at the SAE Golden Anniversary Aeronautical Meeting, promoting the concept of an aft fan engine.

In contrast to the dismissive stance they had adopted in response to the Conway, Pratt & Whitney responded to GE’s engine by designing a competing fan engine of their own. While GE turned to the radical design of a high-Mach-number single­stage aft fan to achieve the requisite pressure-ratio in the bypass stream, P&W relied on a more incremental design, a two-stage forward fan, to achieve this pressure – ratio, compensating for the added weight by employing titanium. In effect, the competitive pressure of GE’s engine forced P&W to leapfrog over the Conway. Although the tip Mach number of P&W’s fan was significantly lower than GE’s, it was still far enough above Mach 1.0 to preclude the use of standard blading of the general type Rolls-Royce had used in the six bypass stages of the Conway. P&W instead had to use transonic blading of the type NACA-Lewis had proven shortly before on their 5-stage and 8-stage demonstrator compressors.

Pratt & Whitney’s leapfrogging over the Conway exemplifies the phenomenon, often noted but rarely analyzed, that just knowing something has been done makes it much easier to match. Other examples abound in twentieth-century history, but no one has yet collected them and systematically studied the phenomenon. Such a study would likely examine the role of uncertainty in technical developments. Once a certainty of outcome is assured – in this case, that a fan engine can supply a quantum jump in performance – engineering fits itself into the space between the boundaries of possibility.

The GE and P&W turbofan engines, taken together with the Conway, raise a historical issue of perhaps less interest to historians of technology than of interest for them. Within the field, the question of “firsts” does not frequently arise in discussion as a historiographic problem – most historians agree it is not the most productive focus of inquiry. Broader audiences, however, particularly engineers, often assume that the business of historians does involve establishing priority and allocating credit. Therefore, narratives which illustrate that technical firsts are not the keys to understanding a complex history can clarify the work of historians of technology for technical audiences. The turbofan case serves this purpose well, because the radical GE design, which was arguably more notable from a technical point of view, did not end up as the commercially successful innovation. Rather, the more incremental design of P&W, spurred by GE’s advances, established the still prevailing configuration for low-bypass engines. Here, as everywhere, the question of firsts becomes a problem of definition: Was the CJ805-23 the first turbofan? What about the Whittle proposals? Or the Metro-Vick engine of the 1940s? Or the Rolls-Royce Conway? Answering these questions requires examining the ontologies embedded in the notions of turbofan and bypass engine – topics, we contend, more worthy of historical attention than questions about firsts. The question of firsts then becomes: how did a particular machine, or individual, or group, stabilize a dynamic category, such as bypass engine, or airplane, or commercial jet air travel? Or destabilize existing categories?