jets fans

The development of the fuel-efficient turbofan engine was foretold 30 years before it became a reality.

By Michael S. Coalson

The turbofan is the logical progression—some would say perfection—of the turbojet. Today, about 48,000 turbofan engines power Western military and ci- vilian aircraft into subsonic and supersonic flight. Almost every aircraft engine produced in the last 40 to 50 years has been a turbofan. Why? Because it is quieter and more fuel efficient than a turbojet and provides greater thrust.

Frank Whittle, one of two independent inventors of the turbojet, recognized early in his theoretical studies that the turbojet's jet velocity was too high. "I had always realized it was desirable to gear down the jet," he said.

In 1936, a year before his turbojet ran successfully, Whittle applied for a patent for a turbofan, or bypass, engine, as he called it. Twenty years would elapse before that engine flew.

A turbojet compressor increases the pressure of incoming air by 10 to 30 times. The compressed air passes through a combustor where, mixed with fuel, it burns at temperatures as high as 2,000°F. After combusting, the hot gas flows to the turbine, which extracts from it the energy needed to drive the compressor. The hot, high-pressure air exits through the nozzle at a speed near 3,000 ft./sec.

In contrast, a typical turbofan engine uses a fan to compress most of the incoming air by only a small amount, about twice the inlet pressure. Much of this fan air then exits through a fan nozzle at relatively low speed, around 1,500 ft./sec., and bypasses the compressor.

The remaining air continues into the compressor of the turbofan engine, where its pressure is increased to a high value near that of the turbojet. As in the turbojet, this air moves into a combustor, where fuel is added and the mixture burns. The hot gas flows through two turbines on its way out, one that drives the compressor and one that drives the fan.

The velocity of the hot gas leaving the core nozzle of the turbofan is lower than the velocity of the hot air leaving the nozzle of the turbojet. Typically, its velocity will be about the same as the velocity of the air exiting through the fan nozzle. The velocity of both the bypass stream and the hot stream are significantly lower than the jet velocity in the turbojet, but a great deal more air has been accelerated to the exit velocity.

The turbofan engine's acceleration of a larger airflow to a smaller jet velocity than the turbojet produces more thrust for the same amount of fuel burned.

Jet Age Goes Commercial

Military jets flew during World War II. Propeller aircraft, however, dominated commercial air transport well into the 1950s, at least in the U.S. But jets were on the way.

Two years before the British de Havilland Comet turbojet airliner entered commercial service in 1952, a Pan American World Airways executive sent Christmas greetings to his associates along with a prescient warning. The card showed three lamps hanging in Boston's North Church tower—not the one or two that Paul Revere was looking for in his midnight ride. The card cautioned that this time the British would be coming by air. A series of crashes, unrelated to its engines, ultimately led to this first commercial jetliner's demise.

In the United States, Boeing had built America's first jet bomber, the B-47, and had gone on to win a competition to build the B-52 in 1948. The two bombers were jet-powered, long-range, high-altitude, high-speed transports. If they could carry bombs long distances quickly under jet power, the reasoning went, then a similar design might be made to carry people. In the spring of 1952, Boeing began developing what would become the 707. But, Douglas and Convair weren't far behind.

Engine manufacturers began noticing this three-way competition. Both Rolls-Royce, with its Conway turbofan engine, and General Electric, with its CJ805-23 fan version of the J79 military turbojet, were eager for the commercial airliner business.

Heavy turbofan marketing to the aircraft makers by GE and Rolls-Royce put Pratt & Whitney, which led the turbojet market with its JT3 and JT4 engines, in a bind. P&W had only recently started production of the J57 military engine (virtually the same as the JT3 commercial engine) and had little interest in changing these engines to turbofans for which it had no design.

Frank Whittle stands next to his turbojet (top). As much as that engine (middle) would change the game, turbofan engines, such as the Rolls-Royce Trent 900 (bottom), would change it even more.

But, a scant three years later, the P&W turbofan was a reality. Based upon the JT3C turbojet, P&W's first production turbofan—the JT3D—had a two-spool compressor. It became a turbofan by changing the two front stages on the front compressor and adding an additional turbine stage (needed to drive the much larger new front spool). So clever was the derivative that Pratt & Whitney offered a kit to convert the JT3C turbojets to turbofans. Several airlines took up the company on its offer.

Cliff Simpson, chief of the Air Force Aeropropulsion Laboratory's turbine engine division, described the result: "The control and the fuel system in both [the JT3D and the JT3C] are the same, and at similar conditions, the fuel flow of both engines is the same, yet the C is rated at 11,750 lbs. of thrust and the D at 18,000 lbs. of thrust. The 707 went from a nerve-wracking 45-second takeoff to a nice, comfortable 25- to 30-second takeoff, and the noise went so low that the ears ceased to complain of the pain."

The first production JT3Ds went into the Boeing 707 and Douglas DC-8. Subsequently, the military version of the JT3D, the TF33,
was installed in the Air Force's B-52Hs, C-135Bs, and C-141s. Some of the commercial aircraft models and all of the military ones are still operating more than 40 years later.

GE made more headway with Convair than it did with Boeing or Douglas. Convair had managed to get orders for its airplanes from Delta and TWA, but was late entering service. A substantial amount of this delay was attributed to Howard Hughes, who was in control of TWA at the time.

Even Higher

In the early 1960s, the Air Force reviewed its development priorities for the next decade. From this review came a requirement for "a large military transport with high bypass engines." It was unusual, even irregular, to have a system requirement and a subsystem solution—the high bypass engine—both specified as a requirement.

How did the specification of the type of propulsion engine get into a requirement for the new larger military transport? Proponents of high bypass engines within the Air Force, led by Cliff Simpson, got it there.

Simpson recalled that in the early 1960s all engine contractors in this country and abroad said the highest bypass engine they knew how to build was 3.5. By 1964, the one exception—General Electric—had built a demonstrator engine with a bypass ratio of 8:1. The JT3D, which P&W designed earlier, had a bypass ratio of 1.37. The high in high bypass was high, indeed.

Up to this point, the design of the nacelle enclosing an engine had been the purview of the airframe contractors. In designing the high bypass turbofan, GE made the nacelle that housed the engine part of the engine design. Early arguments against the high bypass turbofan said that the external losses and drag of the nacelle were proportionately greater due to large flow of relatively low-velocity air from the fan.

Accounting for these calculated installation losses greatly diminished the calculated improvements in fuel efficiency of a high bypass turbofan. Thus, the work of designing and testing the nacelle for the engine was critical to realizing the potential advantages of the high bypass turbofan.

Still, the high bypass turbofan engine was not simply an incremental step beyond the turbofans that were already flying. Attaining the desired fuel efficiency for the high bypass turbofan engine meant doubling the compressor pressure ratio from 12 to 24. While P&W had led the way in the early 1950s by doubling the compressor ratio for turbojets, GE was now raising the ante in the turbofan game.

Cycle analysis said that the higher the compressor pressure ratio, the more efficient the turbofan—provided the turbine inlet temperature could be optimized for the pressure ratio. Doubling the pressure ratio in the turbofans of the day would have required inlet temperatures that would have melted the best materials being used in the best blades of the era.

Gerhard Neumann, the leader of GE's flight propulsion division, attributed the success of the company's high bypass turbofan to "internal air cooling of the white-hot turbine blades." The company predicted a 25 percent fuel savings with the new engines.

P&W was also in the early competition to provide an engine for the large military transport specified in the Air Force's priorities review. P&W's initial prototype engine—the STF200—had a bypass ratio of only 2. Later versions increased the ratio to 3.5 with a lower turbine inlet temperature than GE's engine, the TF39. The STF200 had about 10 percent higher fuel flow than the TF39. The GE TF39, with its 8:1 bypass ratio, won the competition to power the large military transport, the C-5A, in August 1965.

While Pratt & Whitney certainly competed for the engine to power this transport, it never seemed to take the high bypass engine requirement seriously as evinced by its first demonstrator's bypass ratio being little more than that of the then-flying JT3D.

Perhaps corporate interest was absorbed in other efforts of commercial turbofan development, afterburning turbofan development (for the F-111), and a Mach 3+ turbojet engine development (for the SR-71). A few months after GE was selected to build the engine for the C-5A, Lockheed was selected over Boeing to build the aircraft.

While Lockheed and GE were consumed with making the C-5A work, Boeing and P&W teamed to build what became the 747 series of airliners. General Electric had an opportunity to bid a version of the TF39 for the 747, but GE management concluded that major modifications to the TF39 to meet the 747 requirement was too big a challenge while the engine itself was still in development.

It is interesting, though, to compare the P&W JT9D engine for the 747 to the one it offered for the C-5A. It had a bypass ratio of 5 instead of 3.5; not as high as GE's 8, but then the aircraft requirements laid down by Boeing dictated a lower bypass ratio than did the C-5A requirement. The JT9D had a turbine inlet temperature of 2,100°F, compared to 2,366°F for the TF39. The cycle pressure ratio was 24:1 compared to the TF39's 22:1. Not surprisingly, the engines have very similar characteristics.

Thus, by the late 1960s, the high bypass ratio engine was the engine of choice for aircraft flying long distances at high subsonic speeds (0.7 to 0.9 Mach). Aircraft of almost every kind have been powered by turbofan engines. High bypass turbofans have been built with many thrust levels—from less than 500 lbs. to more than 100,000 lbs. We can't seem to get enough of 'em.

Michael S. Coalson, who retired after 38 years as an engineer and program manager in the Aeronautical Systems Center at Wright-Patterson Air Force Base, now serves as senior program manager for Universal Technology Corp. in Dayton, Ohio. In addition to working on engines for the F-111, F-14, F-15, and F-16 aircraft and the air-launched and Tomahawk Cruise Missiles—to name a few—he's taught propulsion and compressible flow at the University of Dayton.

Return to Index