Front Fan Design – P&W’s Solution

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).