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 of 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 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 Aircraft, working under a
contract to the Defence Advanced Research Projects Agency, soon began
development of the F-117 stealth fighter.
Aircraft designers generally describe an
airplane's radar cross section in terms of "decibel square meters," or
dBsm. This is an analogy that compares the plane's radar reflectivity to
the radar reflectivity of an aluminium sphere of a certain size. The B-2
reportedly has a radar signature of an aluminium marble. The F-22 Raptor
interceptor is roughly the same, and the F-117 is only slightly less
stealthy. The newer Joint Strike Fighter has the signature of an aluminium
golf ball. The older B-1 bomber, designed during the 1970s and 1980s, is
about the size of a three-foot (one-meter)-diameter sphere, whereas the
1950s-era B-52 Stratofortress, a monstrously non-stealthy airplane, has an
enormous radar cross section of a 170-foot (52-meter)-diameter sphere. The
size of an aircraft has little relationship to its radar cross section,
but its shape certainly does.
When designing a stealth aircraft,
engineers try to either absorb radar energy or deflect it away from the
radar receiver. They absorb it with special materials or "trap" it within
the airplane's structure. They deflect it by carefully designing the
structure. Certain parts of an aircraft structure are notorious for
reflecting radar energy. Cockpits, for instance, bounce radar straight
back to the source, so they must be carefully designed and coated with
special materials. Engine inlets are often designed so that the radar
energy cannot go straight into them and reach the face of the turbine
blades. Instead, the radar energy is bounced back and forth inside the
inlet. Tail surfaces are sharply angled, rather than vertical, so that
they bounce radar in a different direction.
Stealth does not always refer to radar.
Reducing an aircraft's heat signature is also important. This is usually
done by channelling the engine exhaust through long tubes and mixing it
with cooler outside air.
Because radar can still detect very
small radar signatures, stealth aircraft are also operated in a careful
manner and assisted by other aircraft. For instance, they try to avoid
certain radars and operate in conjunction with aircraft designed to jam
enemy radar. They try to hide in the electromagnetic "noise" of the
battlefield.
While stealth was a major effort of
aircraft designers of the 1980s and 1990s, the widespread availability of
powerful computers and knowledge of stealth techniques has meant that it
is no longer difficult to design an aircraft with some
stealth characteristics, although achieving the degree of
stealth incorporated into the F-117 or the B-2 is still difficult. Today,
the research emphasis has shifted to developing various systems that can
be used with a stealth aircraft, such as radar and weapons that will not
be easily detected. Naturally, there is also an effort among missile and
radar designers to develop systems that can detect stealth aircraft.
Low-frequency radar will spot virtually any stealthy aircraft but is bad
at determining its exact location. Communications networks enabling a
defensive system to combine information and locate a target also connect
these and other radars. Other systems attempt to pick up radio and
television signals that may bounce off a stealthy airplane.
The development of stealthy airplanes
teaches several important lessons about technology. The first is that
often many different technologies must be combined to achieve a desired
outcome. An advance in one field, such as materials or aerodynamics, must
be accompanied by advances in other fields, such as computing or
electromagnetic theory. The second lesson is that sometimes trial and
error techniques are insufficient and advances in mathematical theory are
necessary in order to achieve significant advances. Finally, stealth
teaches the lesson that technology is never static - a "stealth
breakthrough" may only last for a few years before an adversary finds a
means of countering it.