A quick look at the F-22 reveals an adherence to fundamental shaping principles
of a stealthy design.
The leading and trailing edges of the wing and tail have identical sweep
angles (a design technique called
planform alignment). The fuselage and canopy have sloping sides. The canopy
seam, bay doors, and
other surface interfaces are sawtoothed. The vertical tails are canted.
The engine face is deeply hidden by
a serpentine inlet duct and weapons are carried internally.
The following article was written by Alan Brown, who retired as Director
of Engineering at Lockheed
Corporate Headquarters in 1991. He is generally regarded as one of the
'founding fathers' of stealth, or
low observable technology. He served for several years as director of low
observables technology at
Lockheed Aeronautical Systems Co. in Marietta, Ga. From 1978 to 1982, he
was the program manager
and chief engineer for the F-117 stealth fighter and had been active in
stealth programs since 1975. This
article first appeared in 1992.
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'.
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 aircraft.
Over the past 15 years [now 20] 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.
This article, consequently, must be seen only as a snapshot in time.
There are two basic approaches to passive radar cross section reduction:
shaping to minimize
backscatter, and coating for energy absorption and cancellation. Both of
these approaches have to be
used coherently in aircraft design to achieve the required low observable
levels over the appropriate
frequency range in the electromagnetic spectrum.
There is a tremendous advantage to positioning surfaces so that the radar
wave strikes them at close
to tangential angles and far from right angles to edges, as will now be
illustrated.
To a first approximation, when the diameter of a sphere is significantly
larger than the radar
wavelength, its radar cross section is equal to its geometric frontal area.
The return of a one-square-meter sphere is compared to that from a one-meter-square
plate at
different look angles. One case to consider is a rotation of the plate
from normal incidence to a shallow
angle, with the radar beam at right angles to a pair of edges. The other
is with the radar beam at 45
degrees to the edges. The frequency is selected so that the wavelength
is about 1/10 of the length of the
plate, in this case very typical of acquisition radars on surface to air
missile systems.
At normal incidence, the flat plate acts like a mirror, and its return
is 30 decibels (dB) above (or
1,000 times) the return from the sphere. If we now rotate the plate about
one edge so that the edge is
always normal to the incoming wave, we find that the cross section drops
by a factor of 1,000, equal to
that of the sphere, when the look angle reaches 30 degrees off normal to
the plate.
As the angle is increased, the locus of maxima falls by about another factor
Of 50, for a total change
of 50,000 from the normal look angle.
Now if you go back to the normal incidence case and rotate the plate about
a diagonal relative to the
incoming wave, there is a remarkable difference. In this case, the cross
section drops by 30 dB when the
plate is only eight degrees off normal, and drops another 40 dB by the
time the plate is at a shallow angle
to the incoming radar beam. This is a total change in radar cross section
of 10,000,000!
From this, it would seem that it is fairly easy to decrease the radar cross
section substantially by
merely avoiding obviously high-return shapes and attitude angles.
However, multiple-reflection cases have not yet been looked at, which change
the situation
considerably. It is fairly obvious that energy aimed into a long, narrow,
closed cavity, which is a perfect
reflector internally, will bounce back in the general direction of its
source. Furthermore, the shape of the
cavity downstream of the entrance clearly does not influence this conclusion.
However, the energy reflected from a straight duct will be reflected in
one or two bounces, while that
from a curved duct will require four or five bounces. It can be imagined
that with a little skill, the number
of bounces can be increased significantly without sacrificing aerodynamic
performance. For example, a
cavity might be designed with a high-cross-sectional aspect ratio to maximize
the length-to-height ratio. If
we can attenuate the signal to some extent with each bounce, then clearly
there is a significant advantage
to a multi-bounce design. The SR-71 inlet follows these design practices.
However, there is a little more to the story than just the so called ray
tracing approach.
When energy strikes a plate that is smooth compared to wavelength, it does
not reflect totally in the
optical approximation sense, i.e., the energy is not confined to a reflected
wave at a complementary angle
to the incoming wave.
The radiated energy, in fact, takes a pattern like a typical reflected
wave structure. The width of the
main forward scattered spike is proportional to the ratio of the wavelength
to the dimension of the
reradiating surface, as are the magnitudes of the secondary and tertiary
spikes. The classical optical
approximation applies when this ratio approaches zero. Thus, the backscatter
- the energy radiated
directly back to the transmitter increases as the wavelength goes up, or
the frequency decreases.
When designing a cavity for minimum return, it is important to balance
the forward scatter associated
with ray tracing with the backscatter from interactions with the first
surfaces. Clearly, an accurate
calculation of the total energy returned to the transmitter is very complicated,
and generally has to be
done on a supercomputer.
It is fairly clear that although surface alignment is very important for
external surfaces and inlet and
exhaust edges, the return from the inside of a cavity is heavily dependent
on attenuating materials. It is
noted that the radar-frequency range of interest covers between two and
three orders of magnitude.
Permeability and dielectric constant are two properties that are closely
associated with the effectivity of
an attenuating material. They both vary considerably with frequency in
different ways for different
materials. Also, for a coating to be effective, it should have a thickness
that is close to a quarter
wavelength at the frequency of interest.
Reduction of radar cross section of engine nozzles is also very important,
and is complicated by high
material temperatures. The electromagnetic design requirements for coatings
are not different from those
for low temperatures, but structural integrity is a much bigger issue.
The driver determining radar return from a jet wake is the ionization present.
Return from resistive
particles, such as carbon, is seldom a significant factor. It Is important
in calculating the return from an
ionized wake to use nonequilibrium mathematics, particularly for medium
and high altitude cases.
The very strong ion density dependency on maximum gas temperature quickly
leads to the conclusion
that the radar return from the jet wake of an engine running in dry power
is insignificant, while that from
an afterburning wake could be dominant.
When the basic aircraft signature is reduced to a very low level, detail
design becomes very
important. Access panel and door edges, for example, have the potential
to be major contributors to
radar cross section unless measures are taken to suppress them.
Based on the discussion of simple flat plates, it is clear that it is generally
unsatisfactory to have a door
edge at right angles to the direction of flight. This would result in a
noticeable signal in a nose on aspect.
Thus, conventional rectangular doors and access panels are unacceptable.
The solution is not only to sweep the panel edges, but to align those edges
with other major edges on
the aircraft.
The pilot's head, complete with helmet, is a major source of radar return.
It is augmented by the
bounce path returns associated with internal bulkheads and frame members.
The solution is to design the
cockpit so that its external shape conforms to good low radar cross section
design rules, and then plate
the glass with a film similar to that used for temperature control in commercial
buildings.
Here, the requirements are more stringent: it should pass at least 85%
of the visible energy and reflect
essentially all of the radar energy. At the same time, a pilot would prefer
not to have noticeable
instrument-panel reflection during night flying.
On an unstable, fly by wire aircraft, it is extremely important to have
redundant sources of
aerodynamic data. These must be very accurate with respect to flow direction,
and they must operate ice
free at all times. Static and total pressure probes have been used, but
they clearly represent compromises
with stealth requirements. Several quite different techniques are in various
stages of development.
On board antennas and radar systems are a major potential source of high
radar visibility for two
reasons. One is that it is obviously difficult to hide something that is
designed to transmit with very high
efficiency, so the so called in band radar cross section is liable to be
significant. The other is that even if
this problem is solved satisfactorily, the energy emitted by these systems
can normally be readily
detected. The work being done to reduce these signatures cannot be described
here.
There are two significant sources of infrared radiation from air breathing
propulsion systems: hot parts
and jet wakes. The fundamental variables available for reducing radiation
are temperature and emissivity,
and the basic tool available is line of sight masking.
Recently some interesting progress has been made in directed energy, particularly
for multiple bounce
situations, but that subject will not be discussed further here. Emissivity
can be a double edged sword,
particularly inside a duct.
While a low emissivity surface will reduce the emitted energy, it will
also enhance reflected energy that
may be coming from a hotter internal region. Thus, a careful optimization
must be made to determine the
preferred emissivity pattern inside a jet engine exhaust pipe.
This pattern must be played against the frequency range available to detectors,
which typically covers
a band from one to 12 microns.
The short wavelengths are particularly effective at high temperatures,
while the long wavelengths are
most effective at typical ambient atmospheric temperatures. The required
emissivity pattern as a function
of both frequency and spatial dispersion having been determined, the next
issue is how to make materials
that fit the bill.
The first inclination of the infrared coating designer is to throw some
metal flakes into a transparent
binder. Coming up with a transparent binder over the frequency range of
interest is not easy, and the
radar coating man probably wonít like the effects of the metal particles
on his favorite observable.
The next move is usually to come up with a multi layer material, where
the same cancellation
approach that was discussed earlier regarding radar suppressant coatings
is used. The dimensions now
are in angstroms rather than millimeters.
The big push at present is in moving from metal layers in the films to
metal oxides for radar cross
section compatibility. Getting the required performance as a function of
frequency is not easy, and it is a
significant feat to get down to an emissivity of 0.1, particularly over
a sustained frequency range. Thus,
the biggest practical ratio of emissivities is liable to be one order of
magnitude.
Everyone can recognize that all of this discussion is meaningless if engines
continue to deposit carbon
(one of the highest emissivity materials known) on duct walls. For the
infrared coating to be effective, it is
not sufficient to have a very low particulate ratio in the engine exhaust,
but to have one that is essentially
zero.
Carbon buildup on hot engine parts is a cumulative situation, and there
are very few bright, shiny parts
inside exhaust nozzles after a number of hours of operation. For this reason
alone, it is likely that
emissivity control will predominantly be employed on surfaces other than
those exposed to engine
exhaust gases, i.e., inlets and aircraft external parts.
The other available variable is temperature. This, in principle, gives
a great deal more opportunity for
radiation reduction than emissivity, because of the large exponential dependence.
The general equation
for emitted radiation is that it varies with the product of emissivity
and temperature to the fourth power.
However, this is a great simplification, because it does not account for
the frequency shift of radiation
with temperature. In the frequency range at which most simple detectors
work (one to five microns), and
at typical hot-metal temperatures, the exponential dependency will be typically
near eight rather than four,
and so at a particular frequency corresponding to a specific detector,
the radiation will be proportional to
the product of the emissivity and temperature to the eighth power. It is
fairly clear that a small reduction
in temperature can have a much greater effect than any reasonably anticipated
reduction in emissivity.
The third approach is masking. This is clearly much easier to do when the
majority of the power is
taken off by the turbine, as in a propjet or helicopter application, than
when the jet provides the basic
propulsive force.
The former community has been using this approach to infrared suppression
for many years, but it is
only recently that the jet-propulsion crowd has tackled this problem. The
Lockheed F 117A and the
Northrop B 2 both use a similar approach of masking to prevent any hot
parts being visible in the lower
hemisphere.
In summary, infrared radiation should be tackled by a combination of temperature
reduction and
masking, although there is no point in doing these past the point where
the hot parts are no longer the
dominant terms in the radiation equation.
The main body of the airplane has its own radiation, heavily dependent
on speed and altitude, and the
jet plume can be a most significant factor, particularly in afterburning
operation. Strong cooperation
between engine and airframe manufacturers in the early stages of design
is extremely important. The
choice of engine bypass ratio, for example, should not be made solely on
the basis of performance, but
on a combination of that and survivability for maximum system effectiveness.
The jet-wake radiation follows the same laws as the engine hot parts, a
very strong dependency on
temperature and a multiplicative factor of emissivity. Air has a very low
emissivity, carbon particles have
a high broadband emissivity, and water vapor emits in very specific bands.
Infrared seekers have mixed feelings about water vapor wavelengths, because,
while they help in
locating jet plumes, they hinder in terms of the general attenuation due
to moisture content in the
atmosphere. There is no reason, however, why smart seekers shouldnít
be able to make an instant
decision about whether conditions are favorable for using water-vapor bands
for detection.