by Orr Kelly
“The damage was similar to what you might get from AAA [Anti-Aircraft Artillery] in actual combat,” Chapin says. “If you can keep the engines running, keep the airspeed on, it will fly.”
CHAPTER FIVE
“Excess Energy” to Fly and Fight
Development of a new jet engine can take a decade or more, longer by far than the airplane itself or most of the other components of which it is made. By its very nature, the jet engine business forces otherwise staid, conservative businessmen to become high-rolling gamblers.
The first F/A-18 didn’t fly until November 1978, but General Electric began spending money on the engines that took it into the air at least thirteen years earlier, in the mid-1960s—when hardly anyone in the navy was giving much thought to the need for a new plane, and certainly before anyone knew what such a plane might look like.
Predicting the time when a new plane will be needed does not take much skill—just looking at the calendar. Combat planes have a first-line lifetime of thirteen to fifteen years. They may continue in service for many years beyond that time but not at the cutting edge of technology and combat power. As the General Electric executives looked at the calendar, it was obvious that the F-4 Phantom, powered by GE engines, would have to be replaced sometime in the latter half of the 1970s, or certainly in the early 1980s. If GE was to stay in the fighter plane engine business, it would have to gamble a good deal of money in its effort to have the right engine ready to go at the right time.
William (“Bill”) Rodenbaugh, the unofficial historian of the GE engine plant in Lynn, Massachusetts, describes the dilemma this way: “If you don’t get ahead of the power curve, by the time you wake up to the fact there is a competition for a new engine, you’re going to lose. You have to anticipate the systems. You have to know what they want. You have to know what the technology’s going to be. You have to know how to translate that into a piece of equipment, and you have to go out and sell the bejesus out of it. You do all of that stuff before it ever happens.”
General Electric had learned that lesson the hard way. The company had gained a huge head start in the jet engine business when it was asked, in 1941, to build America’s first jet engine in its turbine factory at Lynn. It then went on to provide engines for the P-80 Shooting Star, the B-47 bomber, the F-86 Sabre jet, the B-58 bomber, the F-104 Starfighter, and the F-4 Phantom. But, despite its dominance of the market for military jet engines, GE didn’t have a winning engine available in the late 1950s, when the airlines switched from propeller-driven planes to jets. For a full decade, the company was entirely out of the commercial-aircraft engine market and did not fight its way back in until the late 1960s.
It was with that unhappy experience in mind that GE began in the mid-1960s to position itself to capture its share—or, preferably, more than its share—of the future market for military jet engines. This was not a matter of trying to design a specific engine to be ready at a specific time in the early 1970s. In this business, the future is too unpredictable for that. Instead, GE tries to keep a family of about fifteen engines in various stages of development and production and to keep pushing technology so that the company will be ready no matter what happens.
The lineage of the F-404 engine eventually chosen to power the F/A-18 can be clearly identified in engines under development in the mid-1960s, and its ancestry can be traced clear back to the first jet engine more than two decades earlier. If the line is followed forward, it is obvious that the F/A-18’s engine is a sibling of GE power plants in the B-l bomber, the F-16, the F-14, the F-117 stealth fighter, and the B-2 stealth bomber—and a cousin of engines in commercial aircraft, helicopters, ships, and electric generators.
As the people at Lynn looked toward the future and tried to adapt one of their big “family” of engines to use in a new fighter plane, one need was increasingly obvious. The fighter planes then in use by the U.S. military were too big and relatively underpowered. They were, as Rodenbaugh puts it, “refined trucks … that couldn’t get out of their own way.”
The air force, for example, was using the F-4 Phantom, originally developed by the navy as an air superiority fighter, as both a fighter and bomber. Designed to be a nimble dogfighter at 35,000 pounds, the Phantom was being sent into combat at 54,000 pounds, carrying a bigger bomb load than a World War II B-l7 bomber.
The conventional wisdom was that America’s fighters did not have to be nimble if their radar could pick up an enemy before he came in sight of the human eye, and if they carried a missile that could hit the other guy before he even knew he had been spotted. The trick, it was argued, was to put the agility in a small, powerful missile, so you wouldn’t have to put it in the big, relatively awkward, airplane. This was the same thinking that sent the early Phantoms into combat without a gun.
By the mid-1960s, a small group of air force officers and civilians, working out of an office in the basement of the Pentagon, were preaching a new doctrine. They were the evangelists of the lightweight, “wrap around” fighter concept that was to capture the fancy of important members of Congress a few years later. They argued that there would unavoidably be many times when pilots would find themselves in close-in dogfights where the ability of the plane would make the crucial difference. In those situations, they argued, the thing that counted was “excess energy”—the power to climb faster and turn sharper than the enemy or, if need be, to bug out and go home.
They quietly conducted some tests in which first-line fighters were put up against the F-5, the little Northrop fighter being sold to America’s allies who wanted something smaller and cheaper than the big planes in the U.S. inventory. In the tests, the little F-5 blew the big brutes out of the sky. The proponents of the new doctrine were quick to note that the features that made the F-5 a winner—its small size and its agility—were exactly the features of the MiG fighters in service in the Soviet Union and in the air forces of many of its allies.
The test results were confirmed when the big American fighters, despite all their advanced technology, barely held their own against the little North Vietnamese MiGs in the early days of the war in Vietnam.
As word of this new thinking filtered back to Lynn, it was obvious that the engines for the new generation of fighters would have to be designed to provide that “excess energy” needed to win a dogfight. Working with Northrop, GE began trying to build that kind of performance into the engines for the Cobra.
The first chance GE had to design and sell an engine based on the demands of the new doctrine came in 1969, when the air force staged a competition for the engine for its F-15. The F-15 was by no means a lightweight fighter. But, as far as the engine makers were concerned, the demand was the same. The F-15 was to be so powerful that it would be able to accelerate straight up, and it would have enough “excess energy” to make it a winner in a dogfight.
In the competition for the F-15’s engine, GE came head-to-head with Pratt & Whitney, then threatening to move into a commanding position in the military jet engine business—and lost. Was this to be a repeat of the experience of the 1950s, when GE had been driven out of the commercial jet business?
That might have been the case if the air force had not come along with its lightweight fighter competition, pitting the General Dynamics YF-16 against the Northrop YF-17. For GE, it was a new lease on life, but there was no question it would be an uphill fight. The YF-16 would be powered by the same Pratt & Whitney engine the air force had chosen for the F-15. It was in production and flying and had had thousands of hours of tests. GE, on the other hand, had little more than a paper engine and only three years to refine the design, produce five prototypes, and conduct about a thousand hours of tests—only a third of the test time normally devoted to a new engine.
GE set up a small, very competent team of engineers—what is known in the aircraft business as a “skunk works”—that operated seven days a week, around the clock. The air force let them alone, doing without the layers of reports and reviews that encumber most
development projects. Work moved very rapidly.
When the YF-17 took to the air, pilots were dazzled by the performance of its new GE engines. Glowing reports on the engines continued to pour in as the YF-17 was put through its paces in competition with the YF-16. They continued to pour in right up to the point where the air force chose the YF-16, with its Pratt & Whitney engine, as its new lightweight fighter.
GE had had two chances to break Pratt & Whitney’s growing dominance of the fighter plane business and had lost both times.
Life seldom offers a second chance, practically never a third. In this case, GE got that third chance when the navy chose to develop the YF-17 as a strike-fighter.
This was a great opportunity but, in many ways, a daunting challenge. To understand that challenge, it helps to understand how a jet engine works.
As Bill Rodenbaugh likes to explain to visitors, a jet engine is nothing more than a device that takes cold air and turns it into hot air.
But what a device!
In one-thirtieth of a second, the air entering the engine of an F/A-18 is raised from fifty degrees Fahrenheit to 2,500 degrees—hot enough to melt the metal of which the engine is made, if it weren’t for a thin layer of cooler air blowing across the metal.
In one-fiftieth of a second, the air is squeezed from the normal atmospheric pressure of 14.7 pounds per square inch to 370 pounds, enough pressure to rupture any boiler made by man.
The engine’s turbine, spinning at nearly 300 revolutions each second, sucks energy out of the hot air. Each of the sixty-four two-inch-long blades ringing the turbine produces horsepower equal to that delivered by the engine in a large truck.
A moment after the air begins its wild ride, it whistles through a nozzle at the back of the engine at 1,650 miles an hour. It has given up energy and more than a thousand degrees of temperature, but it is still a sizzling 1,400 degrees.
The difference in the pressure at the front of the engine and that at the rear—a distance of only a little over thirteen feet—is what sends a fighter plane streaking through the air.
When a mechanic removes the engine from an F/A-18 and takes it apart, this is what he sees:
At the very front is a large fan, taking up most of the thirty-nine-inch diameter of the engine. If it were not encased in the engine housing, it would look much like the propeller of a conventional engine, although with more blades. The fan divides the air into two streams. One stream flows between the outer casing and the engine itself. The larger stream is squeezed slightly and goes into a compressor where the big squeeze takes place.
From the compressor, the air goes into a combustion chamber—a round tube like a big oil drum. Fuel is injected into the compressed air and burned, raising its temperature.
The hot, dense air then goes through two turbines, causing them to spin. A shaft connected to the first turbine turns the compressor. The second turbine turns the fan up front.
The air, having given up some of its energy, spews out through a nozzle at the back of the engine. On the F/A-18 and other jet fighters, there is one more big chamber at the back end called the afterburner. When the pilot kicks in his afterburner, more fuel is injected into the hot air coming from the engine, and it is burned once more, increasing the speed of the air from 1,650 to 2,600 miles an hour. Pilots use the afterburner sparingly because it burns prodigious amounts of fuel and is grossly inefficient. But it gives a sudden jolt of speed that could be the difference between life and death.
When the designers were working on the engine for the lightweight fighter competition, the problem was fairly straightforward: to provide the plane with as much “excess energy” as possible so it would be a superior dogfighter. They thought first of making the engine without a fan up front. That would be a turbojet. They finally settled on a turbofan engine, but with a fan so small that, while it provided some thrust by pushing a small amount of air around the engine, it sent most of the air through the compressor and turbines. So little air went around the engine, in fact, that some called the engine a “leaky turbojet.”
This was a deliberate compromise between fuel efficiency and excess energy. The more air that is forced through the engine, the less efficient it is at subsonic speeds. The ratio of the percentage of air that goes around the engine compared to the amount that goes through it is called the “bypass ratio.” In a commercial transport engine, where the goal is fuel economy, most of the air bypasses the engine, and the bypass ratio is about six—six parts of air going around the engine to every one that goes through. In the B-l bomber, which may need sudden bursts of power on takeoff and in the target area, but also needs to fly long distances, the bypass ratio is two. In the YF-17, the bypass ratio was about .25: almost all the air was forced through the compressor and turbines.
But that wouldn’t do for a strike-fighter, which not only needed high performance but also required relatively long range. A compromise was reached in which the size of the fan on the original engine was increased, raising the bypass ratio to about .4. In the process, the thrust—or power—of the engine was increased by thirteen percent.
Finding just the right balance between the magic excess energy and the fuel economy needed to reach distant targets was a daunting challenge, but it was not the only one. The navy told GE it wanted something quite different in this engine than it had ever demanded before.
The navy had traditionally ranked the characteristics of an engine in this order of importance: (1) performance; (2) weight; (3) cost; (4) reliability and maintainability; and (5) operability.
In this case, the navy turned the requirements upside down and put them in this order: (1) operability; (2) reliability and maintainability; (3) cost; (4) performance; and (5) weight.
In one sense, this was a psychological trick played on the engineers. As Corky Lenox, who was then program manager, put it: “If you ride herd on the designer for cost and reliability, you don’t have to worry about performance. That’s in his blood.”
As Lenox and Will Willoughby analyzed the situation, the navy’s past emphasis on performance had often been a costly mistake. The result of pushing too hard for performance had been high costs, weight growth, poor reliability and, too often, poor performance.
Willoughby noticed this problem in connection with jet engines almost as soon as he arrived at his new job with the navy.
“When I looked at aircraft on carriers, I immediately came to the conclusion the engines were over-stressed,” Willoughby says. “What tears an engine up is stress. Stress is moving the throttle back and forth, like bending a coat hanger.”
Willoughby tried to convince the navy to put a plug or stop on the throttles of the planes in the fleet to keep the pilots from going to the very edges of the engine’s limits. He calculated that would double, perhaps even triple, the lifetime of the engines. But the fliers in the fleet wouldn’t listen to such a restriction on performance.
Willoughby focused on making the next engine so the pilot would not be able to press the engine to its limits. This meant looking at the way the engine would be used in the fleet, and then designing it so it would provide the desired performance without being over-stressed. But when Willoughby took a good look at the “throttle profiles”—the statistics that supposedly told how the pilots manipulated their throttles—he found they bore little relationship to what actually took place in the air.
“The throttle profiles were all fouled up,” Willoughby says. “Holy cow, what a screw up!”
When realistic tests were run, it was found that a pilot in a dogfight works his throttle back and forth faster and much more often than anyone had imagined.
“The first time I saw one of those throttle plots it blew my mind,” recalls Frank E. Pickering, general manager of the GE aircraft engine engineering division, who was involved in the early development of the F/A-18’s engine.
General Electric later used these realistic profiles to test its new engine. To their surprise, they found the throttle movements were s
o fast and frequent that a human operator couldn’t duplicate them. They had to program a computer to mimic the throttle movements of a pilot in combat.
Designing an engine so the pilot could move his throttle freely without putting undo stress on the engine and causing it to require many hours of maintenance for each hour in the air was one part of the goal labeled “operability.” But the engine also had to be designed so it would operate flawlessly no matter what the pilot did with the plane.
To understand the problem faced by the designers, imagine the wildly distorted air flowing into the engine in the thirty to sixty seconds that a dogfight might last. The pilot would certainly turn his afterburners on and off several times; bank as tightly as possible, one way and then the other, pulling seven and a half Gs; fly straight down and straight up; fly supersonic at one moment and then fly less than a hundred miles an hour with his nose pointing skyward. Finally, he might well want to kick in his afterburners, accelerate as rapidly as possible and head for home, probably trying to outrun a missile as he goes. For all this he needs an engine that will go from idle to full power in three seconds.
If the air coming into the engine becomes too turbulent, a number of things may happen—all of them unpleasant from the pilot’s point of view. His compressor may stall, interrupting the flow of high-pressure air to the combustor and turbines. The engine stops. Or the afterburner may belch, sending a sheet of flame into the rear of the engine, again causing it to stall. Air bypassing the engine may become so turbulent that the engine loses power or a fan blade breaks, destroying the engine.
Such problems were not uncommon. In fact they were a way of life in every combat aircraft. Pilots were warned that, if they performed certain maneuvers, they would suffer an engine failure. Such failures are so common and so predictable that one of the tactics used by a skilled pilot in a dogfight is to force his adversary into maneuvers that will cause an engine stall or loss of power.
