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.
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| 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.