by Tom Clancy
Of the three factors that determine RCS, geometric cross section is the least worrisome to designers. Compare the RCS of the B-2 bomber with your average duck. A duck is physically much, much smaller than a stealth bomber. However, to a long-range search radar, a duck is actually five times larger than the B-2A! The common sparrow or finch would be a closer match from a search radar’s perspective. Since physical size isn’t critical for RCS reduction, designers are mainly concerned with the reflectivity and directivity, and as we will see, a lot can be done with these.
And of the two, directivity is by far the part of the RCS equation that has the greatest effect. Reducing the directivity component is why the F-117A and the B-2 have shapes that make them look so odd. Shaping can lower the directivity component by orienting target surfaces and edges so that the incoming radar energy is deflected away from the radar antenna, like the many mirrored faces of a dance club “disco ball.” The F-117A is “faceted” into a series of flat plates, while the smoothly contoured B-2 uses a technique called planform shaping. Both techniques present surfaces that are angled about 30° away from the incoming radar signals. Smaller angles, however, can also have a significant effect on RCS. Consider three metal plates with different angles with respect to the radar beam. If the first plate is perpendicular (90°) to the radar beam, most of the energy is reflected back towards the radar antenna, maximizing the plate’s RCS with respect to the radar. Now, imagine a second plate that is tilted back by 10°. About 97% of the energy is deflected away from the direction of the radar. This is better. Now, think about a third plate, tilted back by 30°. Almost 99.9% of the incoming radar energy is deflected away from the radar!
Even though shaping is the best way to reduce RCS, it is virtually impossible to eliminate all the surfaces or edges which reflect radar energy. Examples of such reflectors are engine inlets, leading edges of wings, canopy rails, or even access-panel join lines on the aircraft’s fuselage. These trouble spots are taken care of by reducing 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 100° or 200°F 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 ye
ars 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 designer and programmer is to make that impossible.
To help the pilot make practical use of all this greatly expanded tactical information, the F-22 will incorporate decision-aid and management software which will help him or her to drive and fight the aircraft to its limits. In essence, the functions of the human WSO of the F-15E have been delegated to electronic systems rather than flesh and blood. But whether the extra help is human or machine, there is no doubt future pilots will need plenty of it to handle all of the information collected by integrated sensor suites and multiple off-board assets while still flying the plane. Automation is an absolute necessity if future combat aircraft are going to be manned by just one person. It costs over a million dollars to train a pilot or WSO, and personnel costs are the biggest single factor in the defense budget, so it is easy to understand the desire to minimize the aircrew required. The trick is to figure out just what the machines are capable of doing on their own, and what requires the pilot’s human judgment. The key to this relationship is a cockpit design that lets the pilot glance at no more than four or five display panels to know exactly what’s going on inside and outside the cockpit (“situational awareness”).
An overview of recent advances in computer technology is beyond the scope of this book, but two areas are critical to our understanding of how an aircraft finds its target, destroys it, and leaves before the enemy can do anything about it. These areas are sensors and “man-machine interfaces” or displays. In sensors, we’ll look at the advances in the performance of radar, IR, and electronic-support-measures (ESM) systems made possible by the massive number-crunching power of today’s computers. In displays, we’ll look at how information is conveyed to the pilot so that he or she can use it to make better tactical decisions under the stress of combat.
SENSORS
Radar has been the most important sensor for fighter and ground-attack aircraft since the Korean War. And the operating principles of airborne systems haven’t changed fundamentally since World War II. Until the 1970s, airborne radar systems, were mostly single-purpose air-intercept or ground-mapping /navigation systems. In 1975 the F-15A Eagle, equipped with the powerful Hughes APG-63, introduced a new era of multi-mode radars.
The APG-63 radar was the first all-weather, programmable, multi-mode, Pulse-Doppler radar designed to be used by a single pilot. Pulse-Doppler radars rely on the principle that the frequency of waves reflected from a moving object will be slightly shifted upward or downward, depending on whether the object is moving toward or away from the observer. Precise measurement of this Doppler shift allows the radar’s signal-processing computer to determine the target’s relative speed and direction with great precision. With a detection range of greater than 100 nm./182.8 km. against a large RCS target (like a Tu-95 BEAR bomber), the APG-63 combined long range with features such as automatic detection and lock-on. By allowing a digital computer to control most radar operations, the pilot was left free to concentrate on getting into position to make an effective attack. This computer, by the way, was just slightly more powerful than your standard first generation IBM PC (equipped with an Intel 8-Bit 8086/8088 processor; today many home appliances like refrigerators use a more powerful computer chip!). The most impressive aspect of the APG-63 radar system was the first-generation programmable signal processor (PSP), which effectively filtered out ground clutter, giving the radar “look-down, shoot-down” capability. This meant that in broken terrain the pilot could successfully track and engage targets flying at low altitude, which previously were able to “hide” amid the clutter of returns from trees, hills, rocks, and buildings. With some modifications to the PSP’s hardware and software, the APG-63 could also provide real-time, high-resolution ground maps, allowing navigation in poor weather or at night. The radar ground maps were good enough for an experienced pilot to pick out vehicles, bunkers, and other targets. This ability would be further enhanced in the F-15E Strike Eagle fighter-bomber variant. Finally, the APG-63 can track one target while searching for others (track-while-scan or TWS).
An overhead view of the coverage obtained by a typical airborne fighter radar, the APG-63/70.
Jack Ryan Enterprises, Ltd., by Laura Alpher
The hardware of the APG-63 was as revolutionary as its software. The antenna is a flat, circular planar array, gimbaled in two axes so that it can maintain target lock-on during high-G maneuvers. This means that the F-15 can launch an air-to-air missile, turn up to 60° away from the target (called off-boresight), and still maintain the track, even while the target pulls evasive maneuvers. The APG-63’s subsystems, such as the power supply, transmitter, and signal processor, are packaged as individual line-replaceable units (LRUs), which reduces maintenance and repair time. An LRU is a box of system electronics (usually small enough to be handled, removed, and rapidly replaced by a single mechanic) that contains a major electronic or mechanical subsystem of an aircraft. When something inside an LRU fails, the entir
e box is sent back to the factory or a base/depot-level maintenance facility for repair.
The radar’s horizontal or azimuth scan has three selectable arcs, 30°, 60°, or 120°, centered directly in front of the aircraft. The vertical or elevation scan has three selectable “bars” (a bar is a slice of airspace with a vertical depth of 1 1/2° per bar)—2 bar (3°), 4 bar (6°), or 6 bar (90°)—for varying vertical coverage. To cover a specific search pattern, the gimbaled antenna scans from left to right over the selected arc. At the end of the arc, the radar beam drops down one bar and scans back right to left. This continues until the entire bar scan is completed. With an antenna sweep speed of around 70°/sec./bar, the largest search pattern (a 120°, 6-bar scan) can take up to fourteen seconds to complete. Early Eagle drivers were very happy with their new aircraft’s radar because, after years of peering into fuzzy, cluttered radar screens as if they were crystal balls, struggling to glean target data, the APG- 63 was a revelation. But the ultimate proof of a system only comes in combat. The USAF F-15Cs in Desert Storm, as well as those in Saudi and Israeli service, have proved the value of the APG-63 radar system. The F-15 has at least 96.5 “kills” of enemy aircraft to its credit, with no losses.
