THE TURBOFAN EMERGES: GE’S CJ805-23 AFT FAN ENGINE

General Electric’s interest in the turbofan concept goes back to the mid-1940s, when they obtained a preliminary design from Frank Whittle.48 GE had undertaken a substantial effort to develop a fan engine in the early 1950s. The combination of the fan’s larger diameter and tip Mach number restrictions requires its rotational speed to be much lower than the rotational speeds of single spool compressors. To meet this requirement, GE adopted an approach that was standard in turboprop engines, namely using a speed reduction gear to drive the fan off the core engine rotor. Where Whittle had sought to “gear down the jet” aerodynamically, GE did it literally. The first step in the development of such a geared turbofan was to develop an efficient, high specific-power core engine that could drive a highly loaded fan through gears. GE designated the core engine they designed for this purpose, using internal funding, the D-2. On test, it proved to be worse than disappointing. In pursuing exceptionally high efficiency at design speed, GE had compromised off-design operation to such a degree that the overall engine was not self-sustaining until it had nearly reached full RPM. Correcting this fault was going to require an extensive redesign of the gas generator. Instead, GE abandoned the D-2 project.49

Peter Kappus, the principal advocate of the turbofan engine within GE, then began pushing the concept of an aft fan. The idea was to install an independently rotating fan rotor behind the gas generator. The exhaust from the gas generator would drive turbine blades mounted on this rotor, and fan blades would extend from the tips of the turbine blades. This idea dates at least as far back as a Whittle patent50 and the Metro-Vick counter-rotating fan discussed above. One of the leading academic experts on turbomachinery, G. F. Wislicenus of Penn State University, had promoted its advantages in a talk entitled “Principles and Applications of Bypass Engines” presented at the Society of Automotive Engineers Golden Anniversary Aeronautical Meeting in April 1955.51 The most obvious advantage of an aft fan from GE’s point of view was that a new core engine would not have to be developed. GE could use the J-79, or what amounted to almost the same thing, its commercial counterpart, the CJ805. This engine had the specific-power required for a viable turbofan engine. Because the fan component was to be aft of the core engine and not mechanically connected to it, the performance of the core engine could be taken as a given. Only the turbofan component would require development funding.

GE committed funds for the development of an experimental aft fan engine in 1956.52 The responsibility for designing the fan component was assigned to the Flight Propulsion Laboratory. John Blanton (see Figure 9) had responsibility for the overall performance of this component. Blanton, a graduate of Purdue, had joined GE in 1956 after a distinguished career at Bell Aeronautical, where he had risen to Assistant Chief Design Engineer. The detailed aerodynamic design of the fan itself fell under Dick Novak’s compressor aerodynamic design group. Novak, a graduate of MIT, had been with GE since the mid-1940s, starting as a field test engineer in the Mohave Desert, but subsequently coming to focus on the aerodynamics of axial compressors, placing great emphasis on analytical design. Novak assigned the aero­dynamic design of the fan to Lin Wright. A graduate of Wayne State University, Wright had joined GE in mid-1956 after a ten year career as one of the central figures in the NACA supersonic compressor research program, starting at Langley and then moving to Lewis. His last project at NACA-Lewis had been the design and test of a highly loaded 1260 ft/sec tip-speed transonic rotor intended to be the first stage of a two-stage counter-rotating compressor.53