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                                                                                  CHAPTER 1


       Stealth refers to the act of trying to hide or evade detection. It is not a single
technology, rather it incorporates a broad series of technologies and design features. In
simple terms, stealth technology allows an aircraft to be partially invisible to Radar or any
other means of detection. This doesn't allow the aircraft to be fully invisible on radar. Stealth
technology cannot make the aircraft invisible to enemy or friendly radar. All it can do is to
reduce the detection range or an aircraft. This is similar to the camouflage tactics used by
soldiers in jungle warfare. Unless the soldier comes near you, you can't see him.

      As a concept, stealth is nothing new, having been invented by the first caveman to
cover himself with leaves so that he could sneak up on a dim-witted antelope. Soldiers hid
behind trees. Submarines hid under the waves to sneak up on ships, and it was submarines
that first used special coatings on their periscopes to avoid radar detection during World War
II. According to conventional military wisdom, surprise is the best form of attack. With
sophisticated methods of detection, however, catching the enemy unaware is becoming
increasingly difficult, which is perhaps why the United States Air Force decided to re-invent
the airplane. For airplanes, stealth first meant hiding from radar. After World War II, various
aircraft designers and strategists recognized the need to design planes that did not have large
radar signatures (a radar signature is how big the airplane appears on radar from a specific
angle and distance; it is often referred to as the "radar cross section"). But their ability to hide
from radar was limited for many years for several reasons. One major limitation was aircraft
designers inability to determine exactly how radar reflected off an airplane.

       In the nineteenth century, Scottish physicist James Clerk Maxwell developed a series
of mathematical formulas to predict how electromagnetic radiation would scatter when
reflected from a specific geometric shape. . His equations were later refined by the German
scientist, Arnold Johannes Sommerfield. But for a long time, even after aircraft designers
attempted to reduce radar signatures for aircraft like the U-2 and A-12 OXCART in the late
1950s, the biggest obstacle to success was the lack of theoretical models on how radar
reflected off a surface. In the 1960s, Russian scientist Pyotr Ufimtsev began developing
equations for predicting the reflection of electromagnetic waves from simple two dimensional

NMAMIT, Nitte                                                                                          1

shapes. His work was regularly collected and translated into English and provided to U.S.
scientists. By the early 1970s, a few U.S. scientists, mathematicians, and aircraft designers
began to realize that it was possible to use these theories to design aircraft with substantially
reduced radar signatures. Lockheed Martin, working under a contract to the Defense
Advanced Research Projects Agency (DARPA), soon began development of the F-117 stealth
fighter. The first stealth aircraft was the F-117 developed by Lockheed Martin.

       Thus, Stealth Technology essentially deals with designs and materials engineered for
the military purpose of avoiding detection by radar or any other electronic system. Stealth, or
anti-detection, technology is applied to vehicles (e.g., tanks), missiles, ships, and aircraft with
the goal of making the object more difficult to detect at closer and closer ranges.

       Since radar is the most difficult form of detection to elude, avoidance is generally
accomplished by reducing the radar cross section (RCS) of the object to within the level of
background noise; for example, the reported goal of U.S. military designers is to make a
fighter plane with an RCS the size of a bird. The RCS is the area of an imaginary perfect
reflector that would reflect the same amount of energy back to the receiving radar antenna as
does the actual target, which may be much larger or even smaller than the RCS. The RCS of
any given object, however, differs at various angles and radar frequencies. Much about
stealth technology remains classified, but among the anti-detection techniques used in the
U.S. Air Force F-117 Stealth fighter plane (which probably has an RCS of 1 sq m or less) are
a low profile with no flat surfaces to reflect radar directly back, the intensive substitution of
radar opaque composites in place of metals, and an overall coating of radar absorbing
material (RAM). The implementation of stealth technology usually requires such
compromises as reduced payload capacity, aerodynamic instability, and high design,
production, and maintenance expenses.

       In order to achieve optimal stealth properties, the following signatures call for special

       • Radar cross-section (RCS)

       • Infrared signature (IR)

       • Acoustic signature (airborne noise)

NMAMIT, Nitte                                                                                         2

       • Visual signature

       • RF emissions

       The safety and survivability of an aircraft depends on its ability to deceive its enemy
detectors. The first two signatures are the most important in explicitly determining the
position of the stealth aircraft in 3D space. RADAR detectors being active devices, measures
to deceive them are easily realizable. IR detectors on the other hand are passive in nature. The
RADAR and IR guided missiles pose a major threat to the aircrafts. The IR guided missiles
use IR detectors to detect the target passively by sensing the IR/Thermal/Heat signatures. The
only way to deceive the IR detectors is to reduce the intensity of the IR radiation in the
wavelength band of the detector that is incident on the detector to a level below which the IR
detector cannot detect the source.

NMAMIT, Nitte                                                                                      3

                                                                                CHAPTER 2

       Design for low observability, and specifically for low radar cross section (RCS),
began almost as soon as radar was invented. The predominantly wooden deHavilland
Mosquito was one of the first aircraft to be designed with this capability in mind.

       Against World War II radar systems, that approach was fairly successful, but it would
not be appropriate today. First, wood and, by extension, composite materials, are not
transparent to radar, although they may be less reflective than metal; and second, the degree
to which they are transparent merely amplifies the components that are normally hidden by
the outer skin. These include engines, fuel, avionics packages, electrical and hydraulic
circuits, and people.

       In the late 1950s, radar absorbing materials were incorporated into the design of
otherwise conventionally designed aircraft. These materials had two purposes: to reduce the
aircraft cross section against specific threats, and to isolate multiple antennas on aircraft to
prevent cross talk. The Lockheed U-2 reconnaissance airplane is an example in this category.

       By the 1960s, sufficient analytical knowledge had disseminated into the design
community that the gross effects of different shapes and components could be assessed. It
was quickly realized that a flat plate at right angles to an impinging radar wave has a very
large radar signal, and a cavity, similarly located, also has a large return.

       Thus, the inlet and exhaust systems of a jet aircraft would be expected to be dominant
contributors to radar cross section in the nose on and tail on viewing directions, and the
vertical tail dominates the side on signature.

       Airplanes could now be designed with appropriate shaping and materials to reduce
their radar cross sections, but as good numerical design procedures were not available, it was
unlikely that a completely balanced design would result In other words, there was always
likely to be a component that dominated the return in a particular direction. This was the era
of the Lockheed SR-71 'Blackbird'.

NMAMIT, Nitte                                                                                      4

        Ten years later, numerical methods were developed that allowed a quantitative
assessment of contributions from different parts of a body. It was thus possible to design an
aircraft with a balanced radar cross section and to minimize the return from dominant
scatterers. This approach led to the design of the Lockheed F-117A and Northrop B-2 stealth

        Since then there has been continuous improvement in both analytical and
experimental methods, particularly with respect to integration of shaping and materials. At
the same time, the counter stealth faction is developing an increasing understanding of its
requirements, forcing the stealth community into another round of improvements. The
message is that with all the dramatic improvements of the last two decades, there is little
evidence of leveling off in capability.

NMAMIT, Nitte                                                                                   5

                                                                              CHAPTER 3

                                   RADAR BASICS

Stealth does not always refer to radar. But before going through the principle behind stealth,
let us have the bird’s eye view of radar.


       Radar is a system that allows the location, speed, and/or direction of a vehicle to be
tracked. The word "radar" is actually an acronym standing for Radio Detection and Ranging
since the device uses radio waves to detect targets. Radar works by sending out pulses of
electromagnetic waves and then "listening" for echoes bounced back by targets of interest.

                               Fig: 3.1 concept of pulse radar

NMAMIT, Nitte                                                                                    6


       Even though a radar may transmit megawatts of power in a single pulse, only a tiny
fraction of that energy is typically bounced back to be received by the radar antenna. The
amount of power returned from a target to the transmitting radar depends on four major

   1. The power transmitted in the direction of the target
   2. The amount of power that impacts the target and is reflected back in the direction of
       the radar
   3. The amount of reflected power that is intercepted by the radar antenna
   4. The length of time in which the radar is pointed at the target

                Fig: 3.2 Factors that determine the energy returned by a target

A term used to describe the relationship between these variables is power density, sometimes
also called power flux. To understand power density, consider the following diagram. The
power transmitted by a radar is dissipated the further it travels because it is spread over an
increasingly larger area. The area over which the power is spread is proportional to the square
of the distance, or range (R), from the transmitting radar. The amount of power spread over a

NMAMIT, Nitte                                                                                     7

given area is called the power density, and this quantity decreases by the square of the range.
The power density of the transmitted radar wave at the range of the target has a special name
called the incident power density (Pincident).

       Fig: 3.3 Effect of distance from the radar to the target on the power density

Once the radar power reaches the target, some portion of that power will be reflected back to
its source. However, this reflected power also dissipates and spreads out as it echoes back to
the radar receiver. Since the power density has already been reduced by a factor of 1/R2 by
the time it reaches the target and is again reduced by 1/R 2 on the return trip, the final power
density of the energy received by the radar is proportional to 1/R 4. The ability of a radar to
detect the target depends on whether the amount of power returned is large enough to be
differentiated from internal noise, ground clutter, background radiation, and other sources of

NMAMIT, Nitte                                                                                      8

interference. The goal of stealth techniques is to bounce so little radar power back to its
source that the target is nearly impossible to detect or track.


The amount of power that is reflected back to the radar depends largely on a quantity called
the radar cross section (RCS.) Although RCS is technically an area and typically expressed in
square meters (m2), it is helpful to break the term apart to better understand what it means.
Radar cross section is usually represented by the Greek letter  (pronounced "sigma"), and
the quantity depends on three factors.

(i)Geometric cross section:

The geometric cross section refers to the area the target presents to the radar, or its projected
area. This area will vary depending on the angle, or aspect, the target presents to the radar. In
other words, the target will probably present the smallest projected area to radar if it is flying
directly toward the radar and is viewed head-on. A view from the side, top, or underneath
will present a much larger projected area. The geometric cross section (A) determines how
much power transmitted by the radar (Pincident) is intercepted by the target (Pintercepted)
according to the following relationship:

Reflectivity refers to the fraction of the intercepted power that is reflected by the target,
regardless of direction. Radar power does not necessarily reflect equally from all parts of an
aircraft, and some components produce stronger radar reflections than others. In addition,
some radar power is usually absorbed by the target. This absorption is especially true of
aircraft coated with special substances called Radar Absorbent Materials (RAM) or those
using internal reflectors called Radar Absorbent Structures (RAS) that trap incoming radar
waves. Regardless, the power that is reradiated, or scattered, after reflecting off the target is
equal to the intercepted power less whatever portion of that power is absorbed by the target.

NMAMIT, Nitte                                                                                        9

Reflectivity is defined as the ratio of power scattered by the target (P scatter) to the power
intercepted by the target (Pintercepted).

Directivity is related to reflectivity but refers to the power scattered back in the direction of
the transmitting radar. The power that is reflected toward the radar is called the backscattered
power (Pbackscatter). We've already noted that radar energy is not reflected evenly, but
directivity is defined as the ratio of the power that is backscattered in the direction of the
radar to the power that would have been scattered in that direction if the scattering were in
fact uniform in all directions. If the power were to scatter equally, it would form a sphere
expanding uniformly in all directions from the target. This type of behavior is called isotropic
expansion. Isotropic power (Pisotropic) is defined as the power that is scattered in a perfect
sphere over a unit solid angle of that sphere, as shown in the following equation.

We have mentioned that the power reflected by the target can be much stronger in some
directions than in others. As a result, that reflected power will be much greater or much
smaller than the isotropic power depending on how the target is oriented to the transmitting
radar. The directivity, therefore, will be much greater than 1 when the target returns a strong
backscatter in the direction of the radar and much less than 1 when the backscatter is small.

These three factors can be combined to determine the complete radar cross section (RCS) for
a target.

NMAMIT, Nitte                                                                                       10

Simplifying that expression yields the following relationship for radar cross section.

The importance of radar cross section can best be understood by looking at an equation
relating the RCS of the target to the energy received by the radar.


S = signal energy received by the radar
Pavg = average power transmitted by the radar
G = gain of the radar antenna
= radar cross section of the target
Ae = effective area of the radar antenna, or "aperture efficiency"
tot = time the radar antenna is pointed at the target (time on target)
R = range to the target

The following graph gives some understanding of just how little radar power is typically
reflected back from the target and received by the radar. In this case, the target presents the
same aspect to the radar at ranges from 1 to 50 miles. At a range of 50 miles, the relative
power received by the radar is only 0.00000016, or 1.6 x 10-5 % of the strength at one mile.
This diagram graphically illustrates how significant the effect of energy dissipation is with
distance, and how sensitive radars must be to detect targets at even short ranges.

NMAMIT, Nitte                                                                                      11

            Graph: 3.1 Reduction in the strength of target echoes with range

Furthermore, every radar has a minimum signal energy that it can detect, a quantity we will
call Smin. This minimum signal energy determines the maximum range (R max) at which a
given radar can detect a given target.

According to this relationship, reducing the radar cross section of a vehicle to 1/10th of its
original value will reduce the maximum range at which the target can be detected by nearly
44%! While that reduction alone is significant, even greater reductions in RCS are possible.

At this point, you may be wondering what terms like gain and aperture mean, but we will
address those in a future article that discusses the principles of radar in greater detail. For
now, let us return to radar cross section and describe how it is measured.

The greatest challenge aircraft designers have traditionally faced in creating a vehicle
difficult to detect by radar is the ability to predict what the RCS will be for a complicated
shape from any given direction. That difficulty was only overcome in recent decades when
computers became powerful enough to solve a series of equations describing how radar

NMAMIT, Nitte                                                                                     12

waves scatter off complicated shapes. These equations are based on Maxwell's equations
developed by James Clerk Maxwell in the mid-1800s. Maxwell's equations describe the
behavior of electric and magnetic fields and are at the heart of a branch of physics called
electrodynamics. While the four equations Maxwell derived are relatively simple, they can
become quite complex when trying to predict the electromagnetic properties of shapes
reflecting radar energy.


                                           Fig: 3.5

For comparison, the above figure 3.5, illustrates typical RCS values for aircraft and other
objects, ranging from insects and birds up to large ground vehicles and ships.

NMAMIT, Nitte                                                                                 13

                                                                                 CHAPTER 4


        As we have seen in the previous section, Stealth, or anti-detection, technology is
applied to vehicles (e.g., tanks), missiles, ships, and aircraft with the goal of making the
object more difficult to detect at closer and closer ranges. In this chapter we shall look at the
principles and design features which are involved in making an aircraft stealthy.


        The concept behind the stealth technology is very simple. As a matter of fact, with
respect to the radar, it is totally the principle of reflection and absorption that makes aircraft
"stealthy". Deflecting the incoming radar waves into another direction and thus reducing the
number of waves does this, which returns to the radar. Another concept that is followed is to
absorb the incoming radar waves totally and to redirect the absorbed electromagnetic energy
in another direction. With respect to infrared, the objective of making an aircraft stealthy is
achieved by minimizing heat from its engines and other surrounding heat-emitting spots.
What ever may be the method used, the level of stealth an aircraft can achieve depends totally
on the design and the substance with which it is made of.

     The goal of stealth technology is to make an airplane invisible to radar. There are two
different ways to create invisibility:

        The airplane can be shaped so that any radar signals it reflects are reflected away from
         the radar equipment.
        The airplane can be covered in materials that absorb radar signals.

     Most conventional aircraft have a rounded shape. This shape makes them aerodynamic,
but it also creates a very efficient radar reflector. The round shape means that no matter
where the radar signal hits the plane, some of the signal gets reflected back:

NMAMIT, Nitte                                                                                        14

                     Fig: 4.1 Reflection of radar waves from a normal aircraft

        A stealth aircraft, on the other hand, is made up of completely flat surfaces and very
sharp edges. When a radar signal hits a stealth plane, the signal reflects away at an angle, like

                     Fig: 4.2 Scattering of radar waves from a F-117 aircraft

        In addition, surfaces on a stealth aircraft can be treated so they absorb radar energy as
well. The overall result is that a stealth aircraft like an F-117A can have the radar signature of

NMAMIT, Nitte                                                                                        15

a small bird rather than an airplane. The only exception is when the plane banks -- there will
often be a moment when one of the panels of the plane will perfectly reflect a burst of radar
energy back to the antenna.

        Now let us focus on the various design features that play a major role in making an aircraft


        The possibility of designing aircraft in such a manner as to reduce their radar cross-
section was recognized in the late 1930s, when the first radar tracking systems were
employed, and it has been known since at least the 1960s that aircraft shape makes a very
significant difference in how well an aircraft can be detected by a radar.

4.2.1 Body Panel Design

        The revolution in radar stealth came in the 1970s when computers were powerful
enough to solve the Maxwell electromagnetic equations for reasonably complicated shapes.
These equations determine how radar waves are reflected and scattered, and by developing
the capability to analytically predict the RCS of an entire aircraft from different angles,
designers were able to drastically reduce the RCS. The major limitation of this early method
was that it could only analyze flat panels. As a result, the F-117 and its Have Blue prototypes
were composed of a number of faceted panels.

                              Fig: 4.3 F-117 with faceted panels

NMAMIT, Nitte                                                                                          16

        The massive improvements in computer technology over the next two decades
allowed the same basic method to be applied to smooth, contoured surfaces. These improved
codes have been instrumental in designing aircraft like the B-2 and F-22.

4.2.2 Canted Vertical Tails

        Lockheed designers included canted tails based on early research into stealth
technology. As far back as the 1940s, engineers had realized that perpendicular surfaces, like
vertical tails, generated strong radar returns. By canting the tails away from 90°, the radar
cross-section (RCS) of an aircraft could be considerably reduced. Kelly Johnson of Lockheed
included canted tails in the SR-71 design since the original CIA requirements called for an
aircraft as difficult to detect as possible, given the limited techniques of the day.

        Northrop and McDonnell Douglas included canted tails on the YF-17, F-18, and F-
18E/F for similar reasons. Though none of these planes is a true stealth aircraft by any means,
simply canting the tails away from the vertical reduces the RCS to the same levels as a
smaller aircraft.

        Since canting the vertical stabilizers has such an important impact on stealth
characteristics, it is not surprising that every stealth aircraft today employs them, with the
obvious exception of tailless aircraft like the B-2 Spirit.

            Fig: 4.4 SR-71 (left) and F-22 Raptor with Canted Vertical Stabilizers

NMAMIT, Nitte                                                                                     17

4.2.3 Planform Alignment

       Planform alignment is often used in stealth designs. Planform alignment involves
using a small number of surface orientations in the shape of the structure. For example, on the
F-22A Raptor, the leading edges of the wing and the tail surfaces are set at the same angle.
Careful inspection shows that many small structures, such as the air intake bypass doors and
the air refueling aperture, also use the same angles. The effect of planform alignment is to
return a radar signal in a very specific direction away from the radar emitter rather than
returning a diffuse signal detectable at many angles.

       Stealth airframes sometimes display distinctive serrations on some exposed edges,
such as the engine ports. The YF-23 has such serrations on the exhaust ports. This is another
example in the use of planform alignment, this time on the external airframe.

4.2.4 Internal Radar Absorbent Construction

       Behind the skin of some aircraft are structures known as re-entrant triangles. Radar
waves penetrating the skin of the aircraft get trapped in these structures, bouncing off the
internal faces and losing energy. This approach was first used on SR-71.

                        Fig: 4.5 Working of re-entrant triangles

NMAMIT, Nitte                                                                                     18

4.2.5 Aircraft Component Design

       In addition to the overall shaping of the aircraft, a number of other aircraft
components can produce high radar returns. These include:

Engines: The fan blades at the front of a jet engine and the turbine blades on the back are
very radar reflective. Most stealth aircraft place the engine ducts above the wing or fuselage
to help block the engine interiors from radar sources below the aircraft. The F-117 also makes
use of special screens on the engine inlets that block radar waves from reaching these
surfaces. However, these screens are difficult to design because of their adverse impact on
engine performance and have been abandoned in later stealth aircraft. More recent stealth
designs use S-ducted inlets that bend off center to hide the blades from being seen. The F-117
also makes use of a special "platypus" nozzle that effectively hides the turbine blades from
any radar source behind the aircraft.

Cockpit and other windows: The interior of the cockpit is full of sharp corners and
reflective metal objects. Even the pilot's helmet is radar reflective. The F-117 uses flat
window panels and radar-absorbing treatments on the cockpit windows. The same methods
are also used on the windows housing the bomb laser-guidance systems. These measures
block radar waves from entering these areas.

Sharp perpendicular edges: Any kind of edge perpendicular to radar waves causes them to
be diffracted and reflected. In particular, the edges of landing gear doors and other access
doors as well as the trailing edges of the wings produce strong radar returns. This effect can
be minimized by sweeping the edges so they are not perpendicular to the radar waves. Thus,
the edges of doors on the F-117 and other stealth aircraft are covered with small saw-tooths,
or diamond-shaped edges that dissipate the radar energy in many directions.

        Fig: 4.6 Saw-toothed edges used on the B-2 Spirit nose landing gear door

NMAMIT, Nitte                                                                                    19


       Dielectric composites are relatively transparent to radar, whereas electrically
conductive materials such as metals and carbon fibers reflect electromagnetic energy incident
on the material's surface. Composites used may contain ferrites to optimize the dielectric and
magnetic properties of the material for its application.

                     Fig: 4.7 Use of various composite materials on B-2


       RAS or Radar absorbent surfaces are the surfaces on the aircraft, which can deflect
the incoming radar waves and reduce the detection range. RAS works due to the angles at
which the structures on the aircraft's fuselage or the fuselage itself are placed. These
structures can be anything from wings to a refueling boom on the aircraft. The extensive use
of RAS is clearly visible in the F-117 "Night Hawk". Due to the facets (as they are called) on
the fuselage, most of the incoming radar waves are reflected to another direction. Due to
these facets on the fuselage, the F-117 is a very unstable aircraft.

       The concept       behind the RAS is that of reflecting a light beam from a torch
with a mirror. The angle at    which the reflection takes place is also more important. When
we consider a mirror being rotated from 0o to 90o, the amount of light that is reflected in the
direction of the light      beam is more. At 90o, maximum amount of light that is reflected
back to same direction as the light beam's source. On the other hand when the mirror is tilted

NMAMIT, Nitte                                                                                     20

above 90o and as it      proceeds to 180o, the amount of light reflected in the same direction
decreases       drastically.   This     makes      the     aircraft    like     F-117      stealthy.


        Radar absorbing paint or radar absorbent material (RAM) is used especially on the
edges of metal surfaces. The RAM coating, known also as iron ball paint, contains tiny
spheres coated with carbonyl iron ferrite. Radar waves induce alternating magnetic field in
this material, which leads to conversion of their energy into heat. Early versions of F-117A
planes were covered with neoprene-like tiles with ferrite grains embedded in the polymer
matrix, current models have RAM paint applied directly. The paint must be applied by robots
because of issues relating to solvent toxicity and tight tolerances on layer thickness.

        In a similar vein, coating the cockpit canopy with a thin film transparent conductor
(vapor-deposited gold or indium tin oxide) helps to reduce the aircraft's radar profile because
radar waves would normally enter the cockpit, bounce off something random (the inside of
the cockpit has a complex shape), and possibly return to the radar — but the conductive
coating creates a controlled shape that deflects the incoming radar waves away from the
radar. The coating is thin enough that it has no adverse effect on the pilot's vision.


        Infrared Radiation has become an important source of detectabilty of any modern
weapon platform, thanks to its passive nature. It is an important factor that influences the
stealth capability of an aircraft is the IR (infrared) signature given out by the plane. Usually
planes are visible in thermal imaging systems because of the high temperature exhaust they
give out. This is a great disadvantage to stealth aircraft as missiles also have IR guidance
system. The IR signatures of stealth aircraft are minute when compared to the signature of a
conventional fighter or any other military aircraft. If reducing the radar signature of an
aircraft is tough, then reducing the IR signature of the aircraft is tougher. It will be like flying
a plane with no engines. The reduced IR signature totally depends on the engine and where
the engine is placed in an aircraft.

NMAMIT, Nitte                                                                                          21

       Engines for stealth aircraft are specifically built to have a very low IR signature.
The technology behind this is top secret like others in stealth aircraft. Another main aspect
that reduces the IR signature of a stealth      aircraft   is to place the engines deep into the
fuselage. This is done in stealth aircraft like the B-2, F-22 and the JSF. The IR reduction
scheme used in F-117 is very much different from the others. The engines are placed deep
within the aircraft like any stealth aircraft and at the outlet, a section of the fuselage deflects
the exhaust to another direction. This is useful for deflecting the hot exhaust gases in another

       The major sources of IR signature of a weapon platform like an aircraft are the
airframe and the jet. The airframe radiates largely in the 8-14 micron band due to the nozzle.
The airframe radiates heat due to the kinetic heating experienced by it in flight. The jet is the
external expansion of flow emanating from the nozzle. Jet radiation depends on its size, shape
and species composition. Radiation by the hot combustion gases inside the nozzle and its
surface geometry also need to be considered.

       The passive detection capability of an IR system makes it possible to track an aircraft
without giving any prior warning to the aircraft like in the case of RADAR. Hence
countermeasures to avoid IR seeking missiles become inevitable. Countermeasures such as
Infrared jammers and Infrared flares or decoys are frequently employed. Infrared jamming
systems are active, continuous operating devices producing high intensity radiations in all
directions so that the detector gets mis-leaded. Infrared flares are usually expandable devices
that are strong IR sources. When ejected, these decoys confuse and shift the missiles away
from the aircraft. Some of the advanced missiles employ techniques to counter such decoys.
These advanced missiles work on the principle that a flare which comes into the field of view
of the seeker may be rejected while the seeker continues to aim at the target.

       The key to reducing an aircraft's infrared signature is to cool the exhaust air rapidly
using long nozzle ducts or mixing the exhaust with cooler air. The F-117 platypus nozzle, one
of the more difficult items to construct, does both of these things. The Stealth Fighter also
uses a high-bypass turbofan engine that mixes the hot jet exhaust with cooler bypass air. The
B-2 and YF-23 also route the hot exhaust through long troughs coated with heat-absorbing
material that not only help cool the air but block the hot gases from being seen from below.

NMAMIT, Nitte                                                                                         22

                    Fig: 4.8 Flares to confuse IR seeking missiles.

                          Fig: 4.9 Platypus nozzle of F-117.

  Fig: 4.10 Overhead view of the B-2 stealth bomber showing its long nozzle troughs.

NMAMIT, Nitte                                                                           23


       The table below of radar cross sections is based on values provided in the book
Brassey's Modern Fighters and other open sources. Although it is not stated what the aspect
angle is, these are almost surely head-on views. Regardless of the source, any radar cross
section engineer will tell you that RCS varies wildly depending on aspect, radar frequency
and wavelength, and the fidelity of the receiver. As a result, these numbers can only provide a
very crude relative comparison between different vehicles and should be treated accordingly.

                                          RCS          RCS               RCS
                                          [dBsm]       [m2]              [ft2]

       F-15 Eagle                         +26          400               4,305

       F-4 Phantom II                     +20          100               1,076

       B-52 Stratofortress                +20          100               1,076

       Su-27                              +12          15                161

       B-1A                               +10          10                108

       F-16 Fighting Falcon               +7           5                 54

       B-1B Lancer                        0            1                 11

       F-18E/F Super Hornet               0            1                 11

       Rafale                             0            1                 11

       Typhoon                            -3           0.5               5.5

       AGM-86 ALCM                        -6           0.25              2.5

       BGM-109 Tomahawk                   -13          0.05              0.5

       SR-71 Blackbird                    -18          0.015             0.15

       F-22 Raptor                        -22          0.0065            0.07

       F-117 Nighthawk                    -25          0.003             0.03

       B-2 Spirit                         -28          0.0015            0.02

       AGM-129 ACM                        -30          0.001             0.01

       Boeing Bird of Prey                -70          0.0000001         0.000008

NMAMIT, Nitte                                                                                     24

       Note that the RCS for the F-15 seems extraordinarily high and likely represents a
large but very narrow peak from the head-on aspect as opposed to a more typical medianized
value. Other sources suggest an RCS between 5 and 25 m2, which seem much more
reasonable in comparison to similar aircraft like the F-4 and Su-27. Measurements for the F-
22, F-117, B-2, Advanced Cruise Missile (ACM), and Bird of Prey are highly classified, as
the SR-71 may also still be, so the values listed above are educated guesses, at best.
Regardless of these deficiencies, the table does a good job of exemplifying relative trends in
lowering RCS and the significant improvements made since the advent of stealth aircraft.


       Stealth aircraft that stay subsonic can avoid being tracked by sonic boom. Some early
stealth observation aircraft utilized very slow-turning propellers in order to be able to orbit
above enemy troops without being heard. The B-2 Spirit is said to have acoustic stealth
features, as is the Boeing Bird of Prey.

       The presence of supersonic and jet-powered 'stealth' aircraft such as the SR-71
Blackbird indicates that acoustic signature is not always a major driver in aircraft design,
although the Blackbird relied more on its high speed and altitude and had very poor stealth
capabilities by modern standards.


       Most stealth aircraft use matte paint and dark colors, and operate only at night. Lately,
interest on daylight Stealth (especially by the USAF) has emphasized the use of gray paint in
disruptive schemes, and it is assumed that some sort of lighting could be used in the future to
mask shadows in the airframe (in daylight, against the clear background of the sky, dark
tones are easier to detect than light ones) or as a sort of active camouflage. The B-2 has wing
tanks for a contrail-inhibiting chemical, alleged by some to be chlorofluorosulphonic acid and
mission planning also considers altitudes where the probability of their formation is

NMAMIT, Nitte                                                                                      25


       Infrared emissions and sound aren't the only detectable emissions generated by ships
or aircraft. The stealth vehicle must not radiate any energy which can be detected by the
enemy, such as from onboard radars, communications systems, or RF leakage from
electronics enclosures. The F-117 uses passive infra-red and "low light level TV" sensor
systems to aim its weapons and the F-22 Raptor has advanced LPI radar which can illuminate
enemy aircraft without triggering a radar warning receiver response.

          Fig: 4.11 Dark coloured paint used on F-117 to camouflage at night.

NMAMIT, Nitte                                                                                 26

                                                                            CHAPTER 5

                      LIST OF STEALTH AIRCRAFTS


5.1.1 Fully stealth designs:

       F-22 Raptor - Lockheed-Martin / Boeing (in service)
       YF-23 Black Widow II - Northrop / MDD (prototype built, lost competition to YF-22,
       almost full stealth, may resurrect as a fast bomber)
       F-35 Joint Strike Fighter - Lockheed-Martin (under development)
       Have Blue - Lockheed (developed into F-117)
       Tacit Blue - Northrop (technology demonstrator reconnaissance plane)
       F-117 Nighthawk - Lockheed - fighter-bomber (in service)
       B-2 Spirit - Northrop-Grumman - strategic bomber (in service)
       A-12 Avenger II - McDonnell-Douglas / General Dynamics (cancelled)
       MBB Lampyridae - West German stealth fighter prototype (cancelled during wind
       tunnel tests in 1988)
       Bird of Prey - Boeing (technology demonstrator)

5.1.2 Reduced RCS designs:

       Horten Ho 229 - a German design of 1944, and perhaps the first basic stealth design
       Northrop YB-49 - like the Ho 229, this USAF bomber's stealthy characteristics were
       not the result of intentional design
       De Havilland Mosquito - British light bomber and ground attack plane of wooden
       construction, low RCS against early radars.
       Antonov An-2 - Wooden propeller and canvas wings give it a minimal radar
       SR-71 Blackbird - Lockheed Advanced Development Projects High-speed
       reconnaissance aircraft. RCS equal to or better than the B-1B
       Eurofighter - EADS (in service)
       Dassault Rafale - French air force and naval fighter bomber

NMAMIT, Nitte                                                                                 27

       Medium Combat Aircraft - Hindustan Aeronautics Limited (under development)
       Indian Air Force stealthy 5th generation combat aircraft optimized for strike missions.
       MiG Project 1.44 "Flatpack" - Mikoyan-Gurevich (prototype), possibly full stealth
       with plasma shield
       J-10 - Chengdu Aircraft Industry Corporation (projected twin engine design for
       stealth missions)
       T-50 / PAK-FA - Sukhoi (under development, Russian counterpart of US F-22
       Raptor, possibly plasma shielded)
       F-16 C/D and E/F - from Block 30 has got reduced RCS to about 1 m2
       F/A-18 C/D and E/F - both have reduced RCS, believed be to similar to F-16C's, but
       F/A-18 E/F is believed to have more advanced technology, but the aircraft is larger so
       the aircraft might (and might not) have the same RCS as F/A-18C/D
       MiG-29 SMT - has got similar to F-16C/D reduced RCS


       Boeing X-45 - Boeing - based on the manned Boeing Bird of Prey demonstrator
       (technology demonstrator)
       RQ-3 Dark Star - Lockheed / Skunk Works (cancelled)
       Dassault AVE-D Petit Duc - Dassault Aviation (tactical UAV)
       Dassault nEUROn - Dassault / Saab / EAB / Alenia / EADS CASA / RUAG / Thales
       (technology demonstrator)

NMAMIT, Nitte                                                                                    28

                                                                                CHAPTER 6

                            COUNTERING STEALTH

       A number of methods to detect stealth aircraft at long range have been developed.
Both Australia and Russia have announced that they have developed processing techniques
that allow them to detect the turbulence of aircraft at reasonably long ranges (possibly
negating the stealth technology).

       Passive (multistatic) radar and bistatic radar systems are believed to detect stealth
aircraft better than conventional monostatic radars, since stealth technology reflects energy
away from the transmitter's line of sight, effectively increasing the radar cross section (RCS)
in other directions, which the passive radars monitor. In addition, it has been suggested that
use of low frequency broadcast TV and FM radio signals as the illuminating source produces
a much higher RCS than high frequency monostatic radars as the long wavelengths cause
whole structural portions of the targets to resonate. Researchers at the University of Illinois at
Urbana-Champaign with support of DARPA, have shown that it is possible to build a
synthetic aperture radar image of an aircraft target using passive multistatic radar, possibly
detailed enough to enable Automatic Target Recognition (ATR).

       Stealth aircraft could also be passively detected from their electromagnetic emissions
(terrain-following radar, radio communications, missile guidance communications etc.) if
they broadcast such emissions. Stealth aircraft typically attempt to minimize these emissions
(using low probability of intercept radars, satellite communications etc.).

       The problem of successfully countering stealth aircraft on the battlefield remains
essentially unsolved. To this date, the only systems that have been shown to successfully
detect stealth aircraft are very old, and use long wave radar systems that have a low

NMAMIT, Nitte                                                                                        29

                                                                             CHAPTER 7


       Every technology will have its own benefits and drawbacks and in this section we
shall look at the benefits, drawbacks and uses of Stealth air craft.


       A smaller number of stealth aircraft may replace a large fleet of conventional attack
jets with the same or increased combat efficiency, possibly resulting in longer term savings in
the military budget.

       A stealth aircraft strike capability may deter potential opponents from taking action
and keep them in constant fear of strikes, since they can never know if the attack planes are
already underway. This may make them more willing to accept a diplomatic solution,
although the moral reasoning behind this is disputed.

       Raids on important point targets, while maintaining a cover of plausible denial. Since
no-one could detect the attackers or at least identify them, the stealth operator would simply
refuse to comment and hope to avoid war.

       The production and fielding of stealth combat aircraft design may force an opponent
to pursue the same aim, possibly resulting in significant weakening of the economically
inferior party. The 1980s American Strategic Defense Initiative ("Star Wars") program served
a similar purpose against the USSR.

       Stationing stealth aircraft in a friendly country is a powerful diplomatic gesture. It
emphasizes close relations between the allies and expresses high confidence in their
governments and competence of security services, as stealth planes incorporate high
technology and military secrets. The USA has stationed squadrons of F-117 Nighthawks in
the United Kingdom.

NMAMIT, Nitte                                                                                     30

        Another important benefit is the advantage of benefit of surprise and its effect in cases where
shrinking the enemy’s available reaction time is of the essence. A example of such a situation is a
typical OCA strike against an airfield. If non-stealthy strike aircraft or stand-off weapons are used, it
is quite likely that will be detected far enough out that the enemy will have some time available to get
as many of his ready-to-fly aircraft in the air and fly them somewhere else to preserve them.


        Stealth technology has its own disadvantages like other technologies. Stealth aircraft
cannot fly as fast or is not maneuverable like conventional aircraft. The F-22 and the aircraft
of its category proved this wrong up to an extent. Though the F-22 may be fast or
maneuverable or fast, it can't go beyond Mach 2 and cannot make turns like the Su-37.
Stealth aircraft are designed with a focus on minimal RCS (radar cross section) rather than
aerodynamic perfection. Highly stealth aircraft (the F-117 and B-2) are aerodynamically
unstable on all three axes and require constant flight corrections from the fly-by-wire system
to stay airborne. Most modern non-stealth fighter aircraft (F-16, Su-27, Eurofighter Typhoon,
Gripen, Rafale) are unstable on one or two axes only. Stealth aircraft need to have highly
redundant fly-by-wire systems for safety, which adds extra cost and weight to the design. In
case of a strong electromagnetic pulse (e.g. atmospheric nuclear explosion), loss of flight
control computers would affect stealth aircraft more seriously, possibly causing them to
crash, but this is highly unlikely due to electronic hardening that is implemented by the
United States Air Force.

        Stealth aircraft are seriously handicapped in combat once located by the enemy.
Existing fully stealth designs (namely the F-117 and B-2) lack afterburners, whose hot
exhaust would increase the RCS and infrared footprint of the plane. Stealth aircraft are thus
unable to exceed the speed of sound and flee rapidly. This makes them vulnerable to fighter
interceptors, which can reach Mach 2 or higher speeds using afterburners. The peculiar shape
of stealth aircraft reduces their agility in a dogfight, thus they may be destroyed by
autocannon fire from a traditional jetfighter, even if their low RCS and effective infrared
shielding prevents a successful missile lock. While the F-117 can carry two air-to-air missiles
for self-defense, they aren't typically very effective, a consequence of the low
maneuverability. The B-2, meanwhile, cannot carry any form of air-to-air weapon.

NMAMIT, Nitte                                                                                               31

        The high level of computerization and large amount of electronic equipment found
inside stealth aircraft makes them vulnerable to passive detection.Another serious
disadvantage with the stealth aircraft is the reduced amount of payload it can carry. As most
of the payload is carried internally in a stealth aircraft to reduce the radar signature, weapons
can only occupy a less amount of space internally. On the other hand a conventional aircraft
can carry much more payload than any stealth aircraft of its class.

        Stealth aircraft are high-maintenance equipment. The condition of the aircraft's skin
determines stealth efficiency, either by diverting radar impulses due to specific geometry of
the airframe and/or absorbing electromagnetic waves in graphite-ferrite microspheres based
surface paint layer. The cockpit windows are shielded with delicate gold and indium foil
layers. If the plane's skin is punctured by a pebble thrown from the runway or heavy rain
damages      the     paint     layers,    the    RCS       could     be     dramatically      increased

        Whatever may be the disadvantage a stealth aircraft can have, the biggest of all
disadvantages that it faces is its sheer cost. Stealth aircraft literally costs its weight in gold.
Fighters in service and in development for the USAF like the B-2 ($2 billion), F-117 ($70
million) and the F-22 ($100 million) are the costliest planes in the world. After the cold war,
the number of B-2 bombers was reduced sharply because of its staggering price tag and
maintenance charges. There is a possible solution for this problem. In the recent past the
Russian design firms Sukhoi and Mikhoyan Gurevich (MiG) have developed fighters which
will have a price tag similar to that of the Su-30MKI. This can be a positive step to make
stealth technology affordable for third world countries.


        To date, stealth aircraft have been used in several low- and moderate-intensity conflicts,
including Operation Desert Storm, Operation Allied Force and the 2003 invasion of Iraq. In each case
they were employed to strike high-value targets which were either out of range of conventional
aircraft in the theater or which were too heavily defended for conventional aircraft to strike without a
high risk of loss. In addition, because the stealth aircraft aren't going to be dodging surface-to-air
missiles and anti-aircraft artillery over the target they can aim more carefully and thus are more likely
to hit the target and not cause as much collateral damage. In many cases they were used to hit the high
value targets early in the campaign (or even before it), before other aircraft had the opportunity to

NMAMIT, Nitte                                                                                               32

degrade the opposing air defense to the point where other aircraft had a good chance of reaching those
critical targets.

         Stealth aircraft in future low- and moderate-intensity conflicts are likely to have similar roles.
However, given the increasing prevalence of excellent Russian-built surface-to-air missile systems on
the open market (such as the SA-10, SA-12 and SA-20 (S-300P/V/PMU) and SA-15 (9K331/332)),
stealth aircraft are likely to be very important in a high-intensity conflict in order to gain and maintain
air supremacy, especially to the United States who is likely to face these types of systems. It is
possible to cover one's airspace with so many air defences with such long range and capability that
conventional aircraft would find it very difficult "clearing the way" for deeper strikes. The surprise of
a stealth attack, and the ability to penetrate the air defences and survive, may become the only
reasonable way of making a safe corridor through which conventional bombers and other aircraft can
enter the enemy's airspace.

NMAMIT, Nitte                                                                                                 33

                                                                            CHAPTER 8

       Stealth technology is clearly the future of air combat. In the future, as air defense
systems grow more accurate and deadly, stealth technology can be a factor for a decisive by a
country over the other. In the future, stealth technology will not only be incorporated in
fighters and bombers but also in ships, helicopters, tanks and transport planes. Ever
since the Wright brothers flew the first powered flight, the advancements in this
particular field of technology has seen staggering heights. Stealth technology is just one
of the advancements that we have seen. In due course of time we can see many improvements
in the field of military aviation which would one-day even make stealth technology obsolete.

NMAMIT, Nitte                                                                                   34

                                                                               CHAPTER 9

                       SUMMARY AND CONCLUSIONS
            Stealth is not a single technology, rather it comprises of many technologies
                that enable an aircraft/ship/missile to be undetected by the enemy sensors.

            Stealth capabilities can be achieved by reducing the following signatures,
                         Radar Cross Section (RCS).
                         Infrared Signature.
                         Acoustic Signature.
                         Visual Signature.
                         RF emissions.

            RCS can be reduced by using faceted panels, canted vertical tails, re-entrant
                triangles, planform alignment, non metallic airframe, and radar absorbing
                materials (RAM).

            IR signature can be reduced using long nozzles, mixing exhaust gas with
                cooler air etc.

            Although a number of theories and techniques have been proposed in
                detecting a stealth aircraft, till now countering stealth at the battle field
                remains unsolved.

NMAMIT, Nitte                                                                                    35


Tom Harris. How Stealth Bombers Work.
Stealth Technology.
How does stealth technology work?
Stealth Technology.
Gary Wollenhaupt. Stealth Capability: The Raptor.
Radar Cross Section.
Stealth Aircraft.
Canted Vertical Tails.
Northrop Grumman B-2 Spirit Intercontinental Strategic Bomber.
Martin F-117 Nighthawk Precision Attack Bomber.
Lockheed Martin F-22 Raptor Air Superiority Fighter.
F-22 Raptor Stealth.
Stealth Technology.
Stealth Technology

NMAMIT, Nitte                                                                                   36

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