by Tom Clancy
The newest revolution—also American in origin—is stealth. When researching Red Storm Rising, I traveled to what was then the headquarters of the Tactical Air Command at Langley Air Force Base in the Virginia Tidewater. There, a serious and laconic lieutenant colonel from Texas looked me straight in the eye and announced, “Son, you may safely assume that an invisible aircraft is tactically useful.”
“Well, gee, sir,” I replied, “I kinda figured that out for myself.”
Seemingly a violation of the laws of physics, stealth is really a mere perversion of them. The technology began with a theoretical paper written around 1962 by a Russian radar engineer on the diffraction properties of microwave radiation. About ten years later an engineer at Lockheed read the paper and thought, “We can make an invisible airplane.” Less than ten years after that, such an airplane was flying over a highly instrumented test range and driving radar technicians to despair. Meanwhile men in blue suits slowly discarded their disbelief, saw the future, and pronounced it good. Very good. Several years later over Baghdad on the night of January 17th, 1991, F-117A Black Jets of the 37th Tactical Fighter Wing proved beyond question that stealth really works.
The stealth revolution is simple to express: An aircraft can now go literally anywhere (depending only on its fuel capacity) and deliver bombs with a very high probability of killing the target (about 85% to 90% for a single weapon, about 98% for two), and in the process it will give no more warning than the flash and noise of the detonation. Meaning: The national command authorities (an American euphemism for the president, premier, or dictator) of any country are now vulnerable to direct attack. And for those who believe that the USAF was not trying to kill Saddam Hussein, be advised that maybe his death was not the objective. Maybe we were just trying to turn off the radio (i.e., command-and-control system) he was holding. A narrow legal point, but even the Pentagon has lawyers. However one might wish to put it, we were trying, and Hussein was a lucky man indeed to avoid the skillful attempts to flip off that particular switch. Whoever next offends the United States of America might wish to consider that. Because we’ll try harder next time, and all you have to know is where that offending radio transmitter is.
AS in Submarine and Armored Cav, I’ll be taking you on a guided tour of one of America’s premier fighting units and its equipment. In this case, the unit is the 366th Wing based out of Mountain Home AFB, Idaho. As organized today, the 366th is the Air Force’s equivalent of the Army’s 82nd Airborne or 101st Air Assault Division—a rapid-deployment force that can be sent to any trouble spot in the world on a moment’s notice. The 366th’s job is to delay an aggressor until the main force of USAF assets arrive in-theater, ready to go on the offensive. But before we visit these daring men and women in their amazing flying machines, let’s take a look at the technologies that enable an aircraft to move, see, and fight.
Airpower 101
WE’VE all seen TV cartoons that show some clever character fashioning a set of wings and then trying to fly like a bird (with thanks to Warner Bros., Chuck Jones, and Wile E. Coyote). Usually, the sequence ends with the character in a bruised and battered jumble at the bottom of some horrendous precipice, pleading for help. Fitting wings to your arms and flapping them like a bird and leaping off cliffs looks silly, and so we laugh; yet that’s just how humans tried for several hundred years to achieve flight. Needless to say, it didn’t work. It can’t. The approach has to fail because it does not take into account the basic forces that affect flight.
Essentially, two forces help you get into the air and stay there. These forces are called thrust and lift. Working against them are another pair of forces that try to keep you grounded. These forces are called weight (mass and gravity) and drag; and their practical application to fly an aircraft safely from point A to point B constitutes the engineering discipline of aerodynamics.
For an engineer designing a combat aircraft, ignoring those forces seems as absurd as traveling backward in time. At the same time, he or she must press the limits imposed by those forces as far as possible. You want a combat aircraft to fly as close to the “edge” as you can make it. By definition. Putting this another way: To really understand the edge, you have to understand the basic forces. And so, before we look at how well various combat aircraft succeed in approaching the edge, let’s spend a little time going over the four forces—thrust, lift, weight, and drag.
THRUST
This is the force that causes an aircraft to move through the air. It is provided by an aircraft’s engines, and has the same effect on the aircraft whether it is pulled through the air with a propeller or pushed with a jet engine. Thrust is conventionally measured in pounds or newtons. The more thrust an aircraft’s engines can generate, the faster the aircraft will travel, and the more lift the wings will provide. Similarly, when you step on your car’s accelerator, the engine produces more power, the wheels spin faster, and the car moves along the road at a higher speed. This action also causes the air to move past the car at a higher speed.
An illustration of the four primary forces on a powered aircraft: thrust, drag, lift, and weight. Jack Ryan Enterprises, Ltd., by Laura Alpher
In the world of combat aircraft design, the engine’s raw propulsion power is expressed as its thrust-to-weight ratio. This ratio compares the amount of thrust that the engines can produce to the weight of the aircraft. The higher the ratio, the more powerful the aircraft. For most combat aircraft, this ratio is around 0.7 to 0.9. However, really high-performance models, like the F-15 and -16, have thrust-to-weight ratios greater than 1.0 and can accelerate while going straight up!
LIFT
Lift is the force that pushes an object up due to the unbalanced movement of air past it. In an aircraft, the unbalance comes from the different curvature of the upper and lower surfaces of the wings (the upper surface has more curve than the lower), and the movement of air is provided as a consequence of the engine’s thrust. When the moving air comes in contact with the leading edge of the wing, the air separates. Part of the flow passes over the top of the wing, and the remainder below. Given the shape of an aircraft’s wing, the air stream on top has to travel a greater distance than the stream below. If both air streams are to arrive at the trailing edge at the same time, then the air stream above the wing must have a higher speed.
In aerodynamics, there is a simple, but neat, relationship between the speed of a gas and its pressure: The faster a gas travels, the lower its pressure and vice versa. This principle is called Bernoulli’s Law, in honor of the 18th-century Italian scientist who first investigated it experimentally. So if the air stream above the wing is moving faster than the air stream below the wing, air pressure above the wing will be lower than below the wing. This difference causes the air below to push upward and “lift” the wing up. As the speed of an aircraft increases, the pressure difference grows and produces more lift. This wing’s angle, called the angle of attack (AOA) of the aircraft, can have a significant effect on lift.
Initially, lift increases as AOA increases, but only up to a certain point. Beyond this point, the AOA is too large and the air flow over the wing stops. Without the air flow, there is no pressure difference and the wing no longer produces lift. When this situation occurs, the wing (and the aircraft) is said to have stalled. Now, a high AOA isn’t the only thing that will cause an aircraft to stall. If an aircraft’s speed gets too low, the air no longer moves fast enough over the wings to generate adequate lift, and again the aircraft will stall—and any pilot will tell you that stalls can be really bad for your health.
DRAG
Drag is the force that wants to slow the aircraft down. In essence, drag is friction; it resists the movement of the aircraft. This is a tough concept to grasp, because we can’t see air. But while air may be invisible, it still has weight and inertia. We’ve all taken a walk on a windy day and felt the air pushing against us. That is drag. As an aircraft moves through the air, it pushes the air out of its way, and the air pushes bac
k. At supersonic speeds, this air resistance can be very significant, as a huge amount of air is rapidly pushed out of the way and the friction generated can rapidly heat the aircraft’s body to temperatures over 500° F/260° C.
There are two types of drag, parasitic and induced. Parasitic drag is wind resistance associated with the various bumps, lumps, and other structures on an aircraft. Anything that makes the aircraft’s surface rough or uneven, like bombs, rivet heads, drop tanks, radio antennae, paint, and control surfaces (rudder, canards), increases the aircraft’s wind resistance. Induced drag is more difficult to understand because it is directly linked to lift. In other words, if lift is being generated by the wings, so too is induced drag. Since drag is unavoidable, the best that can be done is to minimize it and understand the limits it places on the aircraft’s performance. And the limits are significant. Drag degrades the aircraft’s ability to accelerate and maneuver and increases fuel consumption, which affects combat range/radius. Therefore, a good understanding of drag is needed not only by aircraft designers, but by aviators as well.
WEIGHT
Weight is the result of gravitational attraction of the Earth, which pulls the mass of the aircraft toward the Earth’s center. As such it is in direct opposition to lift. Of all the forces involved with flying, gravity is the most persistent. To some extent, we can control the other three. But gravity is beyond our control. In the end, it always wins (unless you’re riding a spacecraft fast enough to escape the Earth’s gravity entirely—about 25,000mph [40,000 kph]!). Thrust, lift, and drag are all accounted for in the design process of the aircraft. But when thrust or lift become insufficient to maintain the aircraft aloft, gravity will bring the plane down.
ENGINES
Once you understand the physics of flight, and you can build a sufficiently lightweight power plant, getting an aircraft into the air is a relatively simple matter. But operating high-performance aircraft in the hostile environment faced by today’s military aircraft is quite another thing. These machines are anything but simple.
With complexity comes problems. The heart of a good aircraft is a good engine—the thing that makes it go! More fighter programs have been plagued by engine troubles than by any other source of grief. So, what’s the big deal in making a good jet engine, you might ask? Well, try and imagine building a 3,000-to-4,000 lb./1,363.6-to-1,818 kg. machine that produces over seven times its own weight in thrust and is made with tolerances tighter than the finest Swiss watch. It has to operate reliably for years, even when pilots under the stress of combat or the spur of competition push it beyond its design limits.
To give you a better picture of how exact these engines are made, look at a human hair. While it may look pretty thin to you, it would barely fit between many of the moving parts in a jet engine. That’s what I mean by tight tolerances! Now, let’s spin some of those parts at thousands of revolutions per minute and expose a few of them to temperatures so high that most metal alloys would melt instantly. One can now begin to appreciate the mechanical and thermal stresses that a jet engine must be designed to handle every time it runs. Should even one of the rapidly rotating compressor or turbine wheels fail under these stresses and come into contact with the stationary casing, the resulting fragments would shred the aircraft just as effectively as missile or cannon fire.
Since a combat aircraft’s performance is so closely tied to its propulsion plant, the limits of engine technology are constantly being pushed by designers and manufacturers. Their goal is to design an engine that is lighter than its predecessors and competitors, but produces more thrust. To accomplish this, an engine designer almost always has to bet that a new emerging technology or two will work out as anticipated. Occasionally, this means taking some pretty big risks. Risks that usually turn into problems that get widely reported in the media. For example, engine-development problems in the mid-1950s almost wrecked major aircraft companies, when airframes like the McDonnell F-3H Demon and Vought F-5U Cutlass had to wait months—or even years—for their engines to be developed. So, just how far has jet engine performance come along in the past forty years? Let’s take a quick look.
In the mid-1950s, the U.S. Air Force began operating the North American F-100 Super Sabre, nicknamed the “Hun.” Powered by a single Pratt & Whitney J57-P-7 engine, an axial-flow turbojet generating up to 16,000 lb./ 7,272.7 kg. of thrust, and aided by the newly developed afterburner, it was the first supersonic fighter, achieving a top speed of Mach 1.25. With confidence growing in the axial-flow turbojet engine, new fighter designs quickly showed up, and in 1958 the first McDonnell F-4 Phantom II flew. In the world of combat aircraft, the F-4 is legendary. During the Vietnam War it proved to be a formidable fighter bomber, and it still serves in some air forces. Powered by two giant General Electric J79-GE-15 turbojet engines, each generating up to 17,900lb./8,136kg. of thrust, the Phantom, or the “Rhino” as it was affectionately called, could reach speeds up to Mach 2.2 at high altitudes.
To illustrate the axial-flow turbojet, consider the J79 engine and its five major sections:
A schematic cutaway of a typical turbojet engine, such as the Pratt & Whitney J57. Jack Ryan Enterprises, Ltd., by Laura Alpher
At the front of the J79 is the compressor section. Here, air is sucked into the engine and compacted in a series of seventeen axial compressor stages. Each stage is like a pinwheel with dozens of small turbine blades (they look like small curved fins) that push air through the engine, compressing it. The compressed air then passes into the combustor section, where it mixes with fuel and ignites. Combustion produces a mass of hot high-pressure gas that is packed with energy. The hot gas escapes through a nozzle onto the three turbine stages of the engine’s hot section (so-called because this is where you find the highest temperatures). The stubby fan-like turbine blades are pushed by the hot gas as it strikes them. This causes the turbine wheel to spin at very high speed and with great power. The turbine wheel is connected by a shaft which spins the compressor stages which compact the air flow even further. The hot gas then escapes out the back of the turbojet and this flow pushes the aircraft through the air. When the afterburner (or augmentor) is used, additional fuel is sprayed directly into the exhaust gases in a final combustion chamber, or “burner can” as it is known. This provides a 50% increase in the final thrust of the engine. An afterburner is required for a turbojet to reach supersonic speeds. Unfortunately, using an afterburner gobbles fuel at roughly three to four times the rate of non-afterburning “dry”-thrust settings. For example, using full afterburner in the F-4 Phantom II would drain its tanks dry in just under eight minutes. This thirst for fuel was the next problem the engine designers had to overcome.
The axial flow turbojet became the dominant aircraft propulsion plant in the late 1950s because it could sustain supersonic flight for as long as the aircraft’s fuel supply held up. The term “axial” means along a straight line, which is how the air flows in these engines. Up until that time, centrifugal (circular) flow engines were the military engines of choice—they were actually more powerful than early axial flow turbojets. But centrifugal flow engines could not support supersonic speeds.
Instead of a multiple stage compressor, centrifugal flow engines used a single stage, pump-like impeller to compress the incoming air flow. This drastically limited the pressure (or compression) ratio of the early jet engines, and therefore the maximum amount of thrust they could produce. The comparison between the air pressure leaving the last compressor stage of a jet engine and the air pressure at the inlet of the compressor section is how the pressure ratio is defined. Because the pressure ratio is the key performance characteristic of any jet engine, the axial flow designs had more growth potential than other designs of the period. Therefore, the major reasons why axial flow engines replaced centrifugal flow designs was that they could achieve higher pressure ratios and could also accommodate an afterburner. Centrifugal flow simply could not move enough air through the engine to keep an afterburner lit. By the mid-
1960s, it became apparent that turbojet engines had reached their practical limitations, especially at subsonic speeds. If combat aircraft were going to carry heavier payloads with greater range, then a new engine with greater takeoff thrust and better fuel economy would have to be designed. The engine that finally emerged from the design labs in the 1960s was called a high-bypass turbofan.
At first glance, a turbofan doesn’t look all that much different from a turbojet. There are, in fact, many differences, the most obvious being the presence of the fan section and the bypass duct. The fan section is a large, low-pressure compressor which pushes part of the air flow into the main compressor. The rest of the air goes down a separate channel called the bypass duct. The ratio between the amount of air pushed down the bypass duct and the amount that goes into the compressor is called the bypass ratio. For high bypass turbofans, about 40% to 60% of the air is diverted down the bypass duct. But in some designs, the bypass ratio can go as high as 97%.
A schematic cutaway of a typical turbofan engine, such as the Pratt & Whitney F-100. Jack Ryan Enterprises, Ltd., by Laura Alpher
I know this doesn’t appear to make a whole lot of sense. Don’t you need more air, not less, to make a jet engine more powerful? In the case of turbofans, not so. More air is definitely not better. To repeat, pressure ratio is the key performance characteristic of a jet engine. Therefore the designers of the first turbofans put a lot of effort into increasing this pressure ratio. The result was the bypass concept.
