PRATT & WHITNEY RESPONDS: THE JT3D FRONT FAN ENGINE71

Word leaked to P&W in early 1958 that GE was well along the way toward flight – qualifying an aft fan engine.72 Their initial public response took the same dismissive stance that they had taken in response to the Conway, namely that properly designed turbojets could do anything that bypass engines could do. Privately, they adopted a three-pronged approach. One task force was charged with putting together a case that there was no real future in bypass engines, that at best these engines were just a temporary digression that would disappear with continuing advances in turbojet technology. A second task force developed a paper design of an aft fan engine, as competitive with what was known of GE’s engine as they could. A third task force aimed to build, using mostly existing parts, a front fan engine and to put it into operation on a test stand as quickly as possible.

P&W had shown little prior interest in turbofans. But they were developing a large diameter axial compressor for use in a nuclear powered engine.73 The idea was to use blades from the first couple of stages of this compressor in place of the first two or three stages of the low-pressure compressor of a J-57 or JT3C-6, with the flow in the outer portion of the large blades bypassing the gas generator. Even with restaggering of the large blades, there was little hope of these front stages matching the remaining stages of the low-pressure compressor. Hence no one was thinking that this cobbled together front fan engine would achieve any sort of reasonable performance. But P&W could point to it, saying that they too were developing a fan engine; it would buy them time.

Remarkably, in a mere matter of weeks P&W succeeded in having a self­sustaining front fan engine in operation on a test stand. While this engine itself was otherwise unimpressive, the work that went into it showed P&W how to go about designing a front fan engine, largely within the confines of their already existing technology, that could compete with GE’s aft fan engine. P&W had one crucial advantage: they were already employing two-spool engines. Tip-speed restrictions dictate that a bypass fan operate at a comparatively lower RPM. Thus, in the CJ805- 23, for example, the gas generator rotor operated at 7684 RPM at take-off, while the fan operated at 5727 RPM. In the advanced versions of the J-57 and JT3C that P&W had under development, the high-pressure compressor operated at 9500 RPM, and the low-pressure compressor, at 6500 RPM.74 So, the question became whether P&W could design a fan to replace the front part of the low-pressure compressor that would match the performance of GE’s aft fan. The only alternatives were to come up with an aft fan behind the J-57 or to develop a new gas generator. Both of these alternatives entailed considerable cost and a long delay before they could meet GE’s challenge.