Category Archimedes

We traced the evolution of flight and ground test instrumentation and data from Stage 1 to Stage 3. With the XB-70 project the transition to automated flight test instrumentation, recording, and data analysis essentially was complete. Although there is much overlap between airframe and engine flight testing, engine test cells, and wind tunnels, each test medium imposes its own peculiar instrumentation demands; thus there are variations among them in instrumentation and recording devices utilized at a given stage.

Instrumentation advances between 1940-1969, including computerization, generally increased the quantity of data – the number of variables, parameters, and readings – collected and analyzed but did not increase the accuracy of measurements (see Table 2). Numbers of measurements grew roughly at the rate of increased computational capability.

Primary motivations for undertaking huge costs of automated data collection and processing were

• the need to monitor more channels of data as aircraft themselves became more complex and computer-controlled;

• advantages of being able to monitor data in real time and use it to modify test protocols mid-flight.

The latter can lead to enormous, cost-saving, and ultimately is the economic justification for costly automated data systems with telemetry. For example, when Grumman used a computerized telemetry system backed by a CDC 6400 in development of the F-14 Tomcat they experienced a time saving of 67% compared to prior Grumman flight test programs and performed 47% fewer test flights.59 In less demanding test situations where such cost-saving is not expected, oscillographs and other Stage 2 techniques continue to be used even today.

The First and Second Laws of Scientific Data continue to govern aircraft and engine testing. Thousands of data channels did not obviate the need for modeling data via structurally escalating assumptions. By adding model structures to the data we come to see clearly what is the actual performance of our aircraft and engines. The intelligibility of experimental data largely depends upon correcting for systematic errors, deriving the measures you really want via modeling, and separating real effects from artifacts. We see what otherwise would be lost in the noise of instrumentation and raw data.

Experimental data are not some epistemic “given.” Flight test, test cells, and wind tunnels are quintessentially experimental yet rarely involve hypothesis testing or theory confirmation. Thus they provide marvelous insight into the heart of experiment undistorted by standard yet questionable philosophical views about testing, confirmation, or observation.

ACKNOWLEDGMENTS

Precursor portions were presented in a Year of Data talk, University of Maryland, College Park, September 1992, and to Andrew Pickering’s University of Illinois Sociology of Science lecture series, spring 1995. The assistance of Dr. Jewel Barlow, Director of the Glenn L. Martin Wind Tunnel at University of Maryland is much appreciated. Comments on the draft by Dibner workshop participants, a UMCP audience, and especially Peter Galison were quite helpful.

The following people assisted in the collection of photographs and information: Cheryl A. Gumm, Don Thompson, Jim Young, USAF Flight Test Center, Edwards AFB; Don Haley, NASA Ames Dryden Flight Research Facility; Tom Crouch and Brian Nicklas, National Air and Space Museum; Richard P. Hallion, Office of the

Frequency Modulation 3-10%

Pulse-Duration Modulation 1 -2%

Pulse Coded Modulation __________________________ 1%___________________________________

NOTES:

1 Most data from Kerr 1961.

2 Varies with type of data analysis. See Bethwaite 1963, p. 237, for estimates.

3 Accuracy varies with the quantities measured: Noise and vibrations: 5-10%

Most flight test channels 1-2%

A few selected channels, achievable only by using digital recording: 0.1-0.5%

It is very difficult to achieve 0.1% accuracy in flight test. Ground facilities such as wind tunnels may achieve accuracies of 0.001% at perhaps 8 measurements per second, during this period.

4 Higher frequency measures (e. g., vibrations) tend to have higher errors and require FM recording. For most other signals, PCM is more accurate.

Chief Historian, USAF. Part of the research presented here was supported by an NSF SSTS Award.

Sources for previously unpublished pictures are identified as follows:

Suppe Collection: photographs in my personal collection.

Young Collection: photographs in Jim Young’s collection, USAF Flight

Test Center.

GLMWT: photographs from the Archives of the Glenn L. Martin

Wind Tunnel, University of Maryland; used with the kind permission of Dr. Jewel Barlow, Director.

NASA/NACA pictures, which are in the public domain are identified by NASA photo number or source. Other pictures are reprinted from identified published sources and are used with permission of the publishers.

P&W’s solution to the design problem they found themselves in involved three elements. First, they had to reduce the Mach number at the leading edge of the fan rotor. They were already employing inlet guide vanes ahead of the J-57 and JT3C-6 low-pressure compressor. Inlet guide vanes are used to turn the flow ahead of the rotor, giving it a tangential or circumferential component. This lowers the relative velocity of the flow incident on the rotor blades. The inlet guide vanes for the fan had to provide additional turning of the flow toward the tip, but this was feasible. Thus, in spite of its high tip-speed, the outer portion of the fan blades would not have to be designed for Mach numbers far above P&W’s range of experience.75 The tip Mach number would still have to push beyond anything P&W had done before, but only incrementally beyond.

Second, P&W employed a two-stage fan, replacing the first three stages of the JT3C-6 low-compressor. The design pressure-ratio of the fan was 1.66, or an average of almost 1.29 per stage. While this was well above anything P&W had put in flight before, and hence demanded a significant reach, it was still modest compared with GE’s 1.655 pressure-ratio in its single stage fan, or even the 1.35 average stage pressure-ratio achieved in the NACA 5-stage transonic fan. The stage pressure-ratio, too, required only an incremental step, and not a quantum jump, beyond P&W’s existing technology. The inner portion of the two stages of the fan had to do the work that was originally done by three stages of the low-pressure compressor, requiring higher work-per-stage airfoils. But the 1.66 pressure-ratio of the two stages corresponded to the pressure-ratio across the first three stages of the original compressor. The fan design problem was further simplified by having the fan stream discharge immediately behind the second stage stator vanes, rather than ducting the flow all the way to the rear to join the core engine discharge, as in the Conway. This saved weight by eliminating a long, large radius duct, and it eliminated any need to match the fan discharge velocity with that of the core engine. The long, slender fan blades required part-span shrouds to prevent blade flutter, but P&W already knew how to do this from their experience with long, short-chord blades in their nuclear engine. Thus, while the two stage fan posed a challenge to P&W’s compressor designers, it did not require anything revolutionary in its design.

Third, P&W had to do something about weight. The JT3C-6 turbojet was excep­tionally heavy to begin with, weighing in at more than 4200 pounds, with a thrust-to-weight ratio of only 3.03.76 Although GE’s CJ805 turbojet produced only 11,000 pounds of take-off thrust, compared with the 13,000 pounds of the JT3C-6, it weighed but 2800 pounds, and the CJ805-23 turbofan engine weighed in at only 3800 pounds. Because the two stages of P&W’s fan were replacing three stages in the low-pressure compressor, the difference in weight at the front end of the engine was not so great, provided the fan was designed for low weight. But the added work being done in the bypass stream demanded that a fourth stage be added to the three stage low-pressure turbine. This threatened to push the weight of P&W’s turbofan engine to a point where it would have trouble competing with the CJ805- 23. P&W took several actions in response to this problem. They re-rated the low-pressure and high-pressure spools to operate at slightly different speeds, the low-pressure spool at 6560 RPM and the high-pressure spool at 9800 RPM.77 Instead of simply adding a stage to the low-pressure turbine, they replaced the existing third stage with two new stages, reducing some of the excessive mechanical safety margin in order to save weight.

The most important action P&W took to keep the weight of their turbofan engine down was to switch to titanium in the low-pressure compressor. They had already introduced titanium rotor blades and disks in advanced military versions of the subsonic J-57 – i. e. the J-57 without afterburner. Partly in response to complaints about the weight of the initial version of the JT3C-6, they were in the process of flight qualifying an advanced version, the JT3C-7, using titanium blades and disks in the low-pressure compressor to replace the steel blades and disks of the JT3C-6, reducing the weight by roughly 700 pounds. Because of the high tip-speed and the absence of an established track record in using titanium in commercial engines, conservatism appropriate to a commercial design dictated that the fan blades and disks be made of steel. By shifting to titanium elsewhere in the low-pressure compressor, however, the weight of P&W’s turbofan engine would be no greater than the weight of the JT3C-6.78 The conversion of the JT3C to a fan engine could thus be achieved with no penalty in weight at all.

The Same Point by a Different Route – the JT3D P&W designated their new turbofan engine the JT3D (see Figure 17). Its bypass ratio was 1.4, a little less than GE’s 1.56 owing to the somewhat larger size of the JT3C gas generator, compared with the CJ805; the total air flow of the engine was 450 lbs/sec, 30 lbs/sec more than the CJ805-23, yielding a take-off thrust of 17,000 pounds, compared with 16,100 pounds for GE’s engine. More important, its overall performance parameters were entirely competitive with those of GE’s engine: a specific fuel consumption around 0.55 and a thrust-to-weight ratio a little over 4.2. The virtues of the turbofan are most apparent when the JT3D is compared with the JT3C-6: a 4000 pound thrust increase, consuming as much as 500 pounds less fuel per hour in an engine of essentially the same weight.79 As Table 2 makes clear, the JT3D was no less a quantum jump over the JT3C than the GE CJ805-23 was over the CJ805-3.

P&W designed and flight qualified both a military and a commercial turbofan engine in a remarkably short time. The military version, designated the TF-33, was to replace the non-afterburning J-57 in the B-52 and the КС-135. The commercial version was to replace the JT3C on the Boeing 707 and the Douglas DC-8. Because the engine weight did not increase, the new engines could replace the old without any significant modification of the airframe. Most striking of all, these engine replacements did not necessitate scrapping of the original engines. A JT3C-7 could be converted into a JT3D in the overhaul shop by substituting the two-stage fan and its casing for the first three low-compressor stages, substituting a new third stage and adding a fourth in the low-pressure turbine, and a few other minor changes.80

From the user’s point of view, it seemed as if the JT3C had evolved into the JT3D, in the process yielding a quantum jump in performance.

Nevertheless, while the JT3D was a breakthrough in overall engine performance, it did not require any revolutionary breakthrough in component aerodynamic design. In this respect it was markedly different from the CJ805-23. The fan design required an advance in stage pressure-ratio and tip Mach number beyond P&W’s existing compressor design technology, but only an incremental advance, not a jump to an entirely new design regime. The use of titanium in a conservative commercial engine was new, but it was well on the way to occurring independently of the turbofan, and titanium had already been in use in military engines. Other improvements in performance in the JT3D gas generator were already on the way, motivated by the very high conservatism P&W had exercised in the design of their first generation commercial turbojet.

Precisely because P&W was already employing two-spool engines, they had been in a position to consider a bypass engine along the lines of the Conway as early as 1953 or 1954, either as a possible advance on the military J-57 or as an economically superior first generation commercial engine. The steps from bypassing part of the flow from forward stages in the low-pressure compressor to the fan design of the JT3D were merely incremental. Equally, P&W was in a position to develop the JT3D directly, without inducement from GE, in 1956, when GE was just starting the design of its aft fan. Undoubtedly, a fan version of the JT3 designed at that time would have had a smaller initial advance in performance, but it could easily have matured into the JT3D just from normal incremental advances in design technology within P&W. GE’s J-79 could not have evolved into the CJ805-23, but P&W’s J-57 could have evolved into the JT3D if P&W had been looking toward bypass engines.

Why then was P&W not the first to come up with a superior turbofan engine? Perhaps P&W had no influential in-house proponent of turbofan engines, comparable to Peter Kappus at GE. But this can at most be part of the answer, for the potential of bypass engines to realize high propulsion efficiency in the high subsonic flight speed range had been known for years, and Wislicenus had called attention to it prominently once again in 1955. So, the answer must also include aspects of P&W’s engineering style and orientation.

After initially developing the J-57 in the late 1940s, P&W had maintained a dis­tinctly conservative design approach, deriving its other principal engines from this one and upgrading them more through advances in materials, including alloys that allowed increases in turbine inlet temperature, rather than through advances in compressor aerodynamic design. This conservatism notwithstanding, in the early 1950s they had achieved total dominance in the high subsonic flight regime in military aviation, where GE was offering no competition at all, and from this they had taken a huge lead in the first generation of commercial transports that were under development in the U. S. Largely because of the extraordinary success of the J-57 two-spool compressor, they had become wedded to the comparatively un­sophisticated design methods used for it, choosing not to switch to more advanced methods when they began using digital computers. Given their analytical tools and their approach to advancing compressor design, P&W probably had difficulty envisaging how much of a jump in performance could be achieved in a front fan version of the JT3. An incremental step in tip-speed and pressure-ratio would permit a turbofan engine with a low bypass ratio like the Conway’s, but the gain from this was not dramatic. From the point of view of their compressor designers, a bypass ratio that might offer clear advantages would require a sequence of incremental advances in stage design – a sequence which looks much more tractable when necessitated by competition. Finally, P&W may have been thinking that the real future of commercial aviation lay not in the high subsonic flight regime, but in the supersonic regime. If they thought that the first generation commercial transports were merely stepping stones to supersonic transports, they had little reason to invest in the pursuit of more economically attractive engines for high subsonic flight.81

Whatever the reasons for P&W’s prior lack of interest in turbofans, and however much less of a design breakthrough the JT3D fan was than that of the CJ805-23, the rapidity with which they managed to come up with a folly competitive alternative to GE’s engine was an extraordinary feat unto itself. The first flight test of the engine took place in July 1959, seven months before the first flight test of GE’s CJ805-23 (delayed eight months by engine installation problems).

Without diminishing the original contribution of many figures who were bom and trained in America, the pervasive influence of international factors in the evolution of American aviation has been significant. Prior to World War I, European experience often provided the starting points for successful aeronautical investigations and served as the model for research institutions like the National Advisory Committee for Aeronautics. During and after the war, a considerable number of European emigres brought knowledge and entrepreneurial skills, providing a distinct legacy in both theoretical and applied aeronautics. There were degree programs at a handful of universities, but hardly a nucleus large enough to train hundreds of aero engineers needed to sustain a major aviation industry. Despite production of the DH-4 and biplane trainers during the war, there was still no comprehensive infrastructure to serve the requirements of aeronautics. During the 1920s and 1930s the Europeans helped fill these gaps. They were the theoreticians for the NACA; educators in universities; organizers of professional societies; leaders in industry.

During the decades between World War I and World War II, it might have been possible for Americans themselves to fill in the gaps in the aeronautical infrastructure. But it would have required many additional years, and America may not have been prepared for World War II. America’s postwar success in jet engines and high-speed flight technology likewise received invaluable momentum from foreign legacies. It might have been possible for the United States to develop large rockets for space exploration without the contributions of the von Braun team, but the lunar landing would probably have occurred in the 1970s, not the 1960s. Through professional literature, individuals, and hardware, the European influence on American aviation and aerospace history has been profound. Minus that influence, the record of American achievements in flight would have been dramatically diminished.

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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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.

Although GE’s CJ805-23 was the first flight-qualified turbofan engine, it was not the first to enter commercial service. Because it weighed 1000 pounds more than the CJ805 turbojet, it could not be installed on the Convair 880. It did fit both the 707 and the DC-8, but P&W’s rapid response pre-empted any chance for its replacing the JT3C on either of these aircraft. The CJ805-23 thus had to await the development of a new aircraft, the Convair 990, to enter service. First flight was scheduled for Fall of 1960, with production deliveries scheduled for March, 1961. Aerodynamic performance problems with the aircraft ended up moving the latter date back to September, 1962. Ultimately only 37 Convair 990s were sold. GE attempted to have the CJ805-23 introduced on the Caravelle, replacing the Rolls – Royce Avon, but this too fell through. The breakthrough turbofan engine ended up without an aircraft to fly on.82

The CJ805-23 had some problems in the field. Leakage from the hot turbine stream to the cold fan stream proved more of a problem on production engines than it had on the prototype, necessitating some minor redesign. More seriously, the turbofan bluckets began suffering thermal fatigue cracks, owing to the combination of transient thermal stresses (during start-up and shutdown) and the opposite camber of the fan and turbine blading. For a while the blucket thermal fatigue problem looked like it might be a fundamental fact of bluckets and hence not solvable at all, threatening to create a small financial disaster for GE.83 The problem was solved, but it surely did not help GE convince anyone to consider the engine on other aircraft. The last CJ805-23 was shipped in 1962. Its great engineering achievement notwithstanding, it was by all standards a commercial failure. The contrast between this outcome and the commercial success of P&W’s JT3D led Jack Parker, the head of GE Aerospace and Defense, to remark, “We converted the heathen but the competitor sold the bibles.”84 The fan design of the CJ805-23, however, had a more illustrious history. A scaled-down version of it was installed behind GE’s small J-85 engine to form the CF-700, a 4000 pound thrust engine. This engine flew on business jets into the 1990s, most notably the Falcon 20F and the Sabre 75A. The commercial failure of the CJ805-23 was not the fault of the fan design.

P&W’s JT3D entered service on the Boeing 707 in July, 1960, more than two years before the CJ805-23. Shortly thereafter it began powering Boeing 720B’s and DC-8’s, and the TF-33 entered service on the KC-135 and the eight-engine B-52H bomber, of which the military had ordered 102 in September 1959, and a few years later on the Lockheed C-141. JT3D-powered 707s were still in service into the 1990s, and the TF-33-powered B-52 served in the Persian Gulf War. P&W had delivered 8550 JT3D’s, including JT3C conversions, by 1983. Its success was outdone only by P&W’s JT8D, designed in 1959 on largely the same basis as the JT3D, with more than 13,000 delivered.

INTRODUCTION

The wind tunnel has been an essential instrument for the development of the airplane. From the time of the Wright brothers to the present, it has served aeronautical investigators as an indispensable tool for the improvement of aerodynamic performance. With the emergence of practical aviation on the eve of World War I, European and American countries set up their research programs and built laboratories with wind tunnels to conduct their investigations.

The wind tunnel is a relatively simple instrument, making air flow in a tunnel and measuring the force or moment exerted by wind on a body placed in it. As the theoretical treatment of aerodynamic flow is so difficult and complex, the wind tunnel serves as a useful device to gather empirical data in realms not predicted by theory. And yet, the measured data does not necessarily represent the aerodynamic performance of a real airplane in the sky. The theory of fluid dynamics tells us that the difference between the dimensions of the model and those of a full-scale aircraft would cause scale effect, a phenomenon measured by the dimensionless Reynolds number. Besides scale effect, wind tunnel data could be compromised by errors inherent in experimental procedures and wind tunnel structures such as the aerodynamic effect from the walls of a closed tunnel.

This chapter explores the early use of the wind tunnel by British aeronautical researchers and the controversy over the validity of its use. The main character is Leonard Bairstow, an aerodynamic experimenter who worked on the stability of the airplane through wind tunnel experiments, and who argued for the usefulness of such model experiments. Bairstow and his colleagues at the National Physical Laboratory (NPL) conducted aerodynamic experiments beginning in 1904. While their research produced useful data for airplane designers, investigators became increasingly aware of the discrepancies between the data from model experiments and those from full-scale experiments, as well as discrepancies between the data from different wind tunnels. Those discrepancies form one major thread in this story.

This paper also compares the activities inside and outside the laboratory setting, and the interrelations between these two realms. In his Science in Action, Bruno Latour presented a model to explain the process by which research results are generated from the inside of a laboratory and applied to the outside world, making the laboratory in the end an Archimedean point to move the world.1 The Aeronautics

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Division of the NPL can be considered as such a laboratory. Its history reveals it to be a typical case of Latour’s laboratory, though its story differed from that of the ideal laboratory recounted in Science in Action.

In what follows, I will first briefly explain Bairstow’s stability research at the NPL, and the worldwide appreciation of its aeronautical significance. I will then present two episodes in which Bairstow rather coercively argued for the validity of model experiments and the postwar continuation of stability research. After describing how Bairstow became an influential leader of the British aeronautical community, I will explain how he came to be criticized for his insistent stand. The controversy illuminates not only the strengths and limitations of wind tunnel research but also differing perceptions of research inside and outside the laboratory.

An aircraft gas turbine engine takes in air through an inlet, increases its pressure in a compressor, adds fuel to the high-pressure air and bums the mixture in a combustion chamber, and then exhausts the heated air and combustion products, expanding first through a turbine, where energy is extracted from it to drive the compressor, and finally through a nozzle. Schematics of the three principal types of aircraft gas turbines are shown in Figure 1. In a turboprop engine, the turbine also drives a propeller, connected to the rotor shaft through gears, and it supplies the thrust required by the aircraft; the gas turbine in this case is just an alternative to a piston engine, converting chemical energy into mechanical energy. In a turbojet, by contrast, the thrust comes from the energized flow exiting the nozzle, literally the “jet” of exhaust. In bypass engines a significant portion of the thrust comes from exhaust air that bypasses the combustor and turbine. The bypass air must receive energy from one source or another in order to supply thrust. In the case of a turbo­fan engine the bypass air is pressurized by a fan. The ratio of the bypass air to the air that passes through the combustor and turbine is called the bypass ratio. Bypass engines are typically classified as low or high bypass in accord with this ratio. One

of the two engines on the 737 mentioned above, for example, is an older low-bypass engine, Pratt & Whitney’s JT8D, with a bypass ratio of 1.1 to 1; the other is a more recent high-bypass engine, General Electric’s CF6, with a bypass ratio of 5 to 1 – i. e. less than 17 percent of the total airflow goes through the combustor and turbine. (Hence the shorter, fatter appearance of the larger fan.) Figure 2 displays cutaways of these two engines.

In both turbojet and turbofan or bypass engines, indeed in aircraft engines generally, the magnitude of the thrust is the product of the exhaust mass-flow rate and the difference between the exhaust velocity and the flight speed. Turbojets typically achieve their thrust from a comparatively small mass-flow exiting at a comparatively high velocity. Bypass engines can achieve the same thrust from more mass-flow exiting at a lower velocity. One advantage this gives them is lower

Figure 3. Typical propulsion efficiency ranges for turboprop, turbofan, and turbojet engines. [L. C. Wright and R. A. Novak, “ Aerodynamic Design and Development of the General Electric CJ805-23 Aft Fan Component,” ASME Paper 60-WA-270, 1060.]

exhaust noise, for exhaust noise is a function of exhaust velocity to the 7th power. Their more important advantage, however, is that they offer higher propulsion efficiency in the range from around 450 to 750 miles per hour. By definition, propulsion efficiency is the fraction of the mechanical energy of the exhaust flow that is realized in propulsion of the vehicle. After a little algebraic manipulation, it turns out that:

2 Vflight

propulsion efficiency = ~——— ———–

‘exhaust ‘flight

Thus, the nearer the exhaust velocity is to the flight velocity, the higher the propulsion efficiency. So long as the components of the engine themselves perform at high thermodynamic efficiency, high propulsion efficiency can be turned into fuel savings.

Figure 3, taken from the technical paper describing the design of General Electric’s first successful turbofan engine7, indicates how propulsion efficiency varies with flight speed for turboprops, turbofans, and turbojets. The propulsion efficiency of turboprops drops rapidly above Mach 0.5, roughly 300 miles per hour, because of increasingly severe aerodynamic losses at the tips of propellers of that era.8 Because of their high exhaust velocities, turbojets do not match the maximum propulsion efficiency of turboprops until they reach flight speeds above Mach 1.

Turbofans are, in effect, hybrids, filling the propulsion efficiency gap between turboprops and turbojets. Just as in a turboprop, the flow leaving the combustor is used in part to drive a fan that supplies thrust from a high mass-flow air stream; the ducting leading into the fan controls the air flow entering it, enabling much higher speeds without the tip losses experienced in propellers. Just as in a turbojet, the thrust is coming from ducted flow exiting a nozzle; but the exhaust velocities are comparatively low in the colder fan stream, resulting in higher propulsion efficiency and hence more thrust for the same fuel consumption.

The emergence of the turbofan engine involved four partly overlapping steps: (1) advances in the turbojet by the engine companies in the late 1940s and early 1950s, leading to a new generation of military jet engines with increased power that later provided core gas turbines for bypass engines (including Rolls-Royce’s Conway, the earliest bypass engine to enter flight service); (2) major breakthroughs in axial compressor aerodynamic design by the NACA in the early 1950s which, though intended primarily for supersonic flight, ended up providing the separate technological bases for the contrasting fan designs of General Electric’s and Pratt & Whitney’s first turbofan engines; (3) GE covertly developing a turbofan engine in 1957 that achieved a quantum jump in flight performance by employing an aerody­namically very advanced single-stage fan, located aft of the core engine; and (4) P&W, in response to GE, rapidly developing in 1958 what proved to be the commercially more successful turbofan engine, with performance comparable to GE’s even though it employed less advanced aerodynamics in a two-stage fan at the front of the core engine.

The fact that these four steps do not form a single, simple evolutionary pathway demonstrates, among other things, the futility of attempting to tell the story of the “first” turbofan. Debates over firsts usually degenerate into questions of definition, and here such an approach would miss much of what is instructive in the episode. The critical historical and epistemological points surface from a number of separate threads of development (several of them not specifically aimed at turbofans), as well as from the interaction of several development projects, particularly those at GE and P&W. In place of the notion of “first,” we deploy and expand ideas of “normal” and “radical” design, which Vincenti proposes based on a schema set forth by Edward Constant.9 Vincenti closely examines normal design, where engineers work to improve performance of technologies whose fundamental layout and principles are established. He has little to say, however, about radical design, in which a new technology’s basic arrangement and function are yet to be determined (he believes, perhaps correctly, radical design to have received undue attention from historians). In the following history of the turbofan, however, we show that normal and radical design can interact, even when producing a final result that is in many respects incremental. The normal versus radical distinction remains clear, but less clear is whether the two must, or can, exist as separate trajectories. To trace their interactions, we shall describe the four steps listed above after briefly reviewing early efforts on turbofan engines and the reasons they did not displace the then existing turbojets.

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?

“No Longer an Island” was the phrase that characterized the attitude of British citizens after the Wrights’ European demonstration in 1908 and Louis Bleriot’s successful flight across the English Channel in the following year.2 Immediately responding to this rapid technological development, the British government set up an Advisory Committee for Aeronautics (АСА) consisting of representatives from universities, industry, and the military. The committee’s function was defined as “the superintendence of the investigations at the National Physical Laboratory and… general advice on the scientific problems arising in connection with the work of the Admiralty and the War Office in aerial construction and navigation.”3 Accordingly, an Aeronautics Division was founded in the Engineering Department of the NPL in Teddington, and Department Superintendent Thomas Stanton and his assistant Leonard Bairstow started to measure aerodynamic forces in a new wind tunnel.4

In formulating their research program from autumn of 1911, Stanton and Bairstow were influenced by George Bryan’s new book, Stability in Aviation. Bryan, an applied mathematician, proposed a general theory of stability, suggesting it as a basis for NPL wind tunnel experiments.5 To utilize Bryan’s theory, Stanton and Bairstow had to measure not only lift and resistance but also rotative moments of an airplane model caused by wind from all directions. Bairstow devised original instruments different from those suggested by Bryan and had them constructed by instrument makers at the NPL and the Cambridge Scientific Instrument Company.

Experimental data were produced by the spring of 1913. When the data were plugged into Bryan’s theoretical equation, they produced a measure of the model’s stability. From these calculations, Bairstow offered practical suggestions to airplane designers on the position and size of tail planes to maintain stable flight. Bairstow’s experimental results were summarized in several technical reports and used by Edward Busk, an aircraft designer at the Royal Aircraft Factory in Famborough. Based on the data and the suggestions from the NPL, Busk succeeded in designing a very stable biplane, the B. E.2c, which was mass-produced during World War I. In this intermediary role between Bryan the theoretician and Busk the practitioner,

Bairstow served as a “translator” between scientists and practical engineers, a role described by historian Hugh Aitken and others.6 Bairstow was aptly called “an aeronautical form of the ‘scientific middleman.’”7

Bairstow’s stability research was taken very seriously by aeronautical engineers in Britain and abroad. On the eruption of the First World War in 1914, the British Advisory Committee for Aeronautics decided to classify all technical reports. Neutral Americans lost access to on-going aeronautical research in England. Edwin Wilson at the Massachusetts Institute of Technology became reluctant to continue his stability research for fear of duplication. When the United States entered the war in 1917, the National Advisory Committee for Aeronautics (NACA) in the United States officially requested the АСА to permit access to technical reports. The АСА discussed the matter at its main meeting and decided to open its technical results except for one subject – stability. The stability research of Bairstow and other workers was regarded too important to share even with the Americans.

Bairstow had thus achieved a remarkable prominence for a young man. Bom in 1880, he had studied at the Royal College of Science and entered the Engineering Department of the NPL in 1904. In 1917, he was elected a Fellow of the Royal Society. In the same year, Richard Glazebrook, NPL Director and АСА Chairman, asked him to assume the new post of Superintendent of the NPL’s Aerodynamics Department, which had evolved from the former Aeronautics Division. Despite this favorable offer, however, Bairstow decided to work instead for the Air Board as a scientific researcher and consultant.8 It was in this role that he would become a controversial advocate for a certain kind of aeronautical research.