Stealth Aircraft

Prior to the disclosure in November 1988 that the F-117 stealth fighter existed, very little was publicly discussed or known about the capability of stealth technology. Low Observable (LO) technology concentrates upon the reduction of radar/electromagnetic, infra-red, aural, visual and additional signatures of weapon systems. Balanced LO results in the reduction of allaspects of signature to as near equal a degree as possible. In Research in signature reduction techniques on aircraft have made major advances in reducing acoustic, optical, electronic, radar, and Infra-Red signatures. Strikingly, although stealth aircraft are nearly invisible to radar, they operate only at night because they are among the most visible of aircraft during the day.

One common misconception is that stealth aircraft are totally invisible. Although not invisible, stealth's low observability allows it to penetrate an integratpd air defense system (IADS) by reducing the effectiveness of the three basic air defense functions -- surveillance, fire control, and target destruction. Its reduced radar return weakens the defensive system's ability to consistently detect, track, and engage stealth aircraft, thereby enhancing their survivability.

Radar, an acronym for "Radio Detection and Ranging", systems was originally developed many years ago but did not turn into a useful technology until World War II. One component of a basic radar system is typically a transmitter subsystem which sends out pulse of high frequency electromagnetic energy for a short duration. The frequencies are typically in the Gigahertz (GHz) range of billions of cycles per second. When such a pulse encounters a vehicle made of conducting material (such as metal), a portion of the energy from the incoming pulse is reflected back. If this reflected energy is of a sufficient magnitude, it may be detected by the receiver subsystem of the radar. The computer subsystem which controls the radar system knows when the pulse was transmitted and when the reflected pulse is received. This computer is capable of calculating the round-trip time, t, between the transmitted and received pulses of this electromagnetic energy. These pulses travel at roughly the speed of light, c, which is approximately 186,000 miles/sec (299,999 km/sec).

This apparent size of the target at a given radar wavelength (or frequency) is referred to as the "Radar Cross Section" or RCS. All other things being equal, it is the RCS that dictates the strength of the reflected electromagnetic pulse from a target at a specified distance from the radar transmitter. From a practical standpoint, the RCS is the sole characteristic of the target which dictates whether the target is detected or not.

The current generation of Stealth technologies relies on five elements used in combination to minimize the size of the RCS of a target: Radar Absorbent Material (RAM), Internal Radar-Absorbent Construction (IRAC), External Low Observable Geometry (ELOG), Infrared Red (IR) Emissions Control, and Specialized Mission Profile

1. The Radar Absorbent Material (RAM) approach to Stealth incorporates the use of coatings containing iron ferrite material which basically transforms the electric component of the incoming radar wave into a magnetic field. Consequently, the energy of the incoming radar wave is allowed to dissipate. This is an undesirable outcome of the RAM approach.
2. The Internal Radar-Absorbent Construction (IRAC) approach creates special structure known as "re-entrant triangles" within the outer skin covering the airframe of the Stealth aircraft. These structures capture energy from the incoming radar wave within spaces that approximate the size of the wavelength of a particular radar frequency. The problem with this approach is that the triangles can only protect against a particular radar frequency, so that multiple triangles are required or the aircraft can be detected by different frequencies.
3. The External Low Observable Geometry (ELOG) approach is what gives Stealth aircraft the characteristic angular geometry clearly visible to even a lay observer. This flat, angled shape allows incoming radar waves to reflect or "skip" off the external geometry in all directions. Such a geometric design limits the design possibilities for the aircraft.
4. IR emissions control techniques deal with the heat (IR) signature of vehicular engine output but this requires a different control technique for each different engine signature.
5. The combination of the above four techniques is highly effective in reducing the RCS of Stealth aircraft in their own right. Additionally, each Stealth mission is carefully laid out so as to present only the minimized RCS to threat detection radars which have been identified and located prior to the mission. Thus a very specific and well-choreographed flight profile incorporating altitude, airspeed, angle-of attack and other flight parameters is flown by Stealthy aircraft on each and every mission. This causes complication of the mission so that improvements are desirable.

Aircraft survivability depends on a complex mix of design features, performance, mission planning, and tactics. The effort to make aircraft harder to shoot down has consumed a large share of the resources dedicated to military aircraft design in the 20th century. Since the 1970s, the US Department of Defense has focused on research, development, testing, and production of stealth aircraft, designed to blunt the power of defenders to detect them and thus defeat and/or destroy them.

The SR-71 was an example of where designers took the aerodynamic design and then added some radar absorbing material to the airplane to make it slightly stealthy. The first generation stealth airplanes focused the low observable technology in the front quarter at certain frequencies on the radar spectrum, mostly in the target tracking or X-band area. That's the area that SAMs normally do their target tracking in on airplanes like a MiG-29 or an F-15 have their air to air radar in. And there's a slight degradation in the capability of that SAM as that airplane is coming toward it. However, in the back, it's about the same area.

The second generation of airplanes, the F-117, was designed essentially from the bottom up to be stealthy. With an airplane such as the F-117, designed from the bottom up and used shaping optimally to lower its signature, there is a significantly reduced signature. It's not invisible. It never has been invisible. Radars can track our stealthy airplanes. They can sometimes find them. The key is that that zone of detectability or lethality is shrunk by orders of magnitude, but it's still not invisible. It was crude technology. It was developed at a time when designers didn't have the modeling and computer power needed to make the kind of aerodynamic design that they would have liked, but it was very stealthy. And of course, the night that Desert Storm opened the quote from Col. Al Whitley still is famous in the Air Force: "Boy, I hope this stuff really works." And of course, it did [that isn't exactly what he said].

The third generation of stealth airplanes was represented by the B-2. By that time, there were the modeling tools and the design tools and the computing power to make an aerodynamic design that was optimum. And this airplane is a much higher altitude, much better performing airplane than the F-117. Designers were able to eliminate a lot of the radar absorbing material from the structure. And by the fourth generation, designeres were able to add supersonic speed, the agility of an F-15, F-16 class airplane and do that with no degradation to the stealth. In addition to that, designers were able to add a number of apertures, in other words, openings in the airplane's surface for antennas, radars and other sensors. And in the F-22, as an example, there are over a hundred of those apertures on the airplane, where back a couple of generations to the F-117, there are essentially a couple of aperture openings and the rest of them were hidden when it went into combat.

Observability and stealth technology have been researched for decades. Most aircraft manufacturers have taken stealth factors into consideration in new aircraft design and implemented appropriate solutions in older fighters� modification programes, as applicable. It is safe to assume that several 4+ generation fighters incorporate the results of stealth research and that appropriate low-observable technology and design solutions have been retrofitted into them. The term stealth aircraft is normally only used in conjunction with such aircraft which were specifically designed and built as low-observable aircraft. Such aircraft include, among others, the US F-117, F-22 and B-2.

Solutions geared at improving air combat performance, such as increased payload, range, sensor range, and having several engines, increase the physical size and emission level of an aircraft. Since air combat performance and observability, in practice, require conflicting design solutions, modern fighters incorporate several compromises which, depending on the manufacturer, have resulted in clearly dissimilar design concepts as regards electronic attack (EA) systems and stealth technology, among other things.

A target�s detectability on radar depends on the radar system in use. The radar cross section (RCS) is a measure of how detectable an object is with radar. For example, the RCS of a legacy fighter can be estimated at 3-20 m2, that of a bomber or a transport aircraft 20-100 m2 and the RCS of a cruise missile less than 1 m2, depending on the physical size of the target, radar frequency or the incident angle (orientation of the target to the radar source). According to some estimates stealth technology can reduce the RCS by a factor of 10-1000. As the change in RCS is not directly proportional to the change in the detection range, it is possible at best to reduce the detection range of an aircraft to less than a tenth from normal through stealth design and technology. A reduction of such magnitude makes it significantly more difficult for the adversary to employ his radar homing weapons. In a practical combat situation this generates a decisive advantage to the stealth aircraft.

Solutions used in reducing the RCS are not a panacea against different radar systems. Thus far the employment of stealth technology has focused on making it more difficult for the adversary to use his radar homing weapons, i.e. prevent a shooting solution. Even if the effect of stealth technology solutions used in fighters can be diminished by integrating the air defence system and by new, longer wavelength radar systems, the advantage achieved through stealth technology will not entirely disappear. Fire control radars used in weapon systems will continue to operate well into the future in the spectrum for which stealth fighters� low-observable technology was designed. Although the capability for stealth fighter detection will improve, vulnerability to ground and air-launched radar homing missiles will not significantly increase.

It is impossible to eliminate the thermal footprint of an aircraft or its weapons. In addition to the engine plume, the friction caused by high airspeed generates an apparent temperature difference, clearly detectable from the background. Even though infrared (IR) technology incorporates certain weatherrelated constraints, IR systems are the most promising sensors suited to replace and complement radar systems, and to detect stealth targets and small targets such as cruise missile missiles.