Fighter Wing: A Guided Tour of an Air Force Combat Wing

their reflectivity through the use of RAM coatings and radar-absorbing structures (RAS). RAM materials absorb radar energy and convert it into heat or small magnetic fields. The physical mechanism that accomplishes this is very complex: The material resonates with the incoming radar energy and then changes it by vibration into heat or by electrical induction into weak magnetic fields. RAM can absorb about 90 % to 95 % of the incident radar energy, depending on composition and thickness. For existing non-stealth aircraft designs, like the F-15 or F-16, RAM coatings (the U.S. Air Force reportedly has a radar-absorbing paint called 'Iron Ball') can cut their RCS by as much as 70 % to 80 %.

Radar-absorbing structures, on the other hand, are only used by aircraft designed specifically to be stealthy, as they must be carefully built into the aircraft's framework. Modern RAS designs use strong, radar-transparent composites to build a rigid hollow structure which is then filled with RAM. Because the RAM can be quite thick under the composite shell, most of the radar beam's energy is absorbed before it hits one of the metallic components of the aircraft's structure. Older RAS structure designs, like those on the SR-71, are made of radar- reflective metals in a triangular shape, with a RAM filling in the triangle cavity. When a radar beam hits such a structure, it is reflected back and forth between the reflector plates. With each bounce, the radar beam passes through the RAM, and more of the energy is absorbed. Eventually, the radar signal becomes too weak to show up on a radar screen, and that is that! On stealth aircraft like the B-2 and F-22, radar-absorbing structures are used extensively on hard-to-shape spots like the leading and trailing edges of the wings, control surfaces, and the inlets to the engines. A well-designed RAS can absorb up to 99.9 % of an incoming radar beam's energy.

Consider a hypothetical air-search radar with a detection range of 200 nm./365.7 km. against a B-52, which looks like the broad side of a barn to a radar. With extensive use of stealth technologies, the B-2A's RCS is 1/10,000 that of the B-52, and the detection range drops to less than 20 nm./36.6 km.! This reduction in a radar's range leaves massive gaps in a hostile nation's early warning net, which an aircraft like the B-2 can easily fly through.

In sum, the B-2A, or for that matter the F-117A or the F-22A, isn't invisible to a radar; but the effective range against these aircraft is so short that they can fly around radar warning sites with relative impunity. And this is exactly what the F-117s of the 37th Tactical Fighter Wing (Provisional) did to Iraq during Desert Storm.

While radar is the primary sensor used to detect aircraft, infrared (IR) sensors are becoming increasingly sensitive. The frequency of the IR portion of the electromagnetic spectrum is just below that of visible light and well above that of radar. Since most infrared energy is absorbed by water vapor and carbon dioxide gas in the atmosphere, there are only two 'windows' in the infrared band where detection of an aircraft is likely. One window ('mid-IR') occurs at a wavelength of 2 to 5 microns. Mid-IR is used by current IR-HOMING air-to-air missiles like the AIM-9 Sidewinder series. Infrared radiation from the heat of an aircraft's engine parts and exhaust falls in this mid- IR region. The other window is in the long IR band, at a wavelength of 8 to 15 microns. The long IR signature of an aircraft is caused by solar heating or by air friction on the fuselage of the aircraft. Modern Infrared Search and Track (IRST) and Forward-Looking Infrared (FLIR) systems (which have become more significant as air-to-air sensors since radar-stealthy aircraft became operational) can look for targets in both windows.

To decrease an aircraft's IR signature, the designer must find ways to cool the engine exhaust, where most of the IR radiation is generated. A good start is eliminating the afterburner, which creates a large IR bright spot or 'bloom.' Though this reduces the aircraft's flight performance, if high speed is not a requirement (as in the design of the F-117A and the B-2A), then the afterburner can be discarded. Both the F-117A and the B-2A have non- afterburning versions of turbofan engines used on other aircraft. The next step in IR suppression is to design the engine inlet so that cool ambient air goes around the engine and mixes with the hot exhaust gases before they are expelled from the aircraft. Cooling the exhaust by even 100deg or 200degF significantly reduces the aircraft's IR signature.

Since it is impossible to completely cool the engine exhaust to ambient air temperature, the aircraft designer must reduce the detectability of the hot exhaust. Wide, thin nozzles can flatten out the exhaust plume so it mixes more rapidly with the ambient air. This rapid mixing quickly dissipates the exhaust plume, reducing its detectability by IR sensors. Both the F-117A and B-2 have exotic nozzles that not only rapidly dissipate the exhaust plume, but also block the line of sight to the hotter parts of the engine itself. In the case of the F-117A, the nozzles were coated with a ceramic material, similar to that used on the Space Shuttle, to help deal with the heat erosion of the hot exhaust.

Although a lot can be done to reduce the medium IR band signature from the engines, little can be done about solar or friction heating of the aircraft's outer skin. At best, one could make greater use of carbon-carbon composite materials, which have good IR-dissipation qualities, in the aircraft's fuselage and wing surfaces. Some special paints have modest effects on the long IR signature, but this is a limited modification at best. Short of an expensive and complex active cooling system, this exhausts the limited list of useful options. Fortunately, current IRSTs do not provide greater detection ranges than radar, even against a stealth aircraft, though this could change in the future.

Detection technologies are moving forward rapidly, and today's stealth jet could be tomorrow's sitting duck if designers remain complacent. My friend Steve Coonts used a concept of 'active' stealth in his novel The Minotaur a few years ago. Computer-controlled 'cloaking' systems are just science fiction right now, but with the continuing improvements in computer and signal-processing technology, we may be only a generation away from an aircraft with the ability to hide behind an electronic cloak of its own making. Millions of years ago, natural selection taught a little reptile called the chameleon that the way to become invisible to a predator is to look exactly like your background.

AVIONICS

In Submarine and Armored Cav, we saw how advances in computer hardware and software revolutionized a fighting machine's ability to find and kill targets. Because the crew is often made up of only one person, modern high-performance aircraft place heavier than ever requirements on fast, high-data-rate computers. You can think of sensors as the eyes and ears of an aircraft, computers as its brain, and displays as its voice — the way it communicates with the human in the cockpit. Sensors, computers, and displays are all components of the aircraft's electronic nervous system or 'avionics.'

In older aircraft, such as the F-15A Eagle, the only search sensor available was a radar, and almost all of the system indicators were analog gauges. In combat, the pilot of an early-model Eagle had a first-generation Heads-Up Display (HUD) which showed him what he needed to fly and fight the aircraft. When you are through counting everything, the F-15A pilot still had over a hundred dials, switches, and screens to worry about. As computer technology improved, and as more capable sensors were added, the amount of data that became available to the pilot increased dramatically. To avoid overloading the pilot, multi-function displays (which look like small computer monitors surrounded by buttons) started replacing many of the single-purpose displays and gauges. In some aircraft, such as the F-15E Strike Eagle, there was now so much data available that to employ the aircraft to its full potential, both a pilot and a weapon systems officer (WSO) had to man it. The Air Force's new F-22 fighter will incorporate even greater advances in sensor and computer capabilities. In comparison to the F-15E Strike Eagle, which has, at best, the equivalent of two or three IBM PC-AT computers (based on Intel 80286 microprocessors), the F-22 will take to the skies with the equivalent of two Cray mainframe supercomputers in her belly, and there is room for a third! To keep up with this vast increase in processing power, data rates on the network or 'bus' connecting various aircraft subsystems have increased from one million characters per second (1 Mb/sec.) to over 50 Mb/sec. There has been a similar increase in computer memory and data-storage capacity.

A pilot simply cannot fly the F-22 without the assistance of a computer. In fact, all U.S. combat aircraft produced since the F-16 have been designed with inherently unstable flight characteristics. The only way for such a machine to stay in the air is for a computer-controlled flight control system, with reaction time and agility measured in milliseconds, to control things (human reaction times are typically measured in tenths of seconds, a hundred times longer). Usually the automated systems process and filter the pilot's 'stick and rudder' control inputs, preventing any 'pilot-induced oscillations' that might cause the aircraft to 'depart controlled flight.' A nightmarish phrase sometimes occurs in accident reports: 'controlled flight into terrain.' The English translation is that some poor bastard drilled a crater right into the ground and never knew it. The dream of every flight-control avionics