Page 303 - Carrahers_Polymer_Chemistry,_Eighth_Edition

266 Carraher’s Polymer Chemistry

carbon graphite epoxy composites. The Stealth Bomber was originally slated to hunt Russian mobile
missiles that were built in the 1980s. It was deployed in 1993 and has a wingspan of more than 52 m,
a length of 21 m, and is more than 5 m high. The reported cost of a single B-2 is varied ranging from
about $2.4 billion to a low of $500 million. In any case, it is expensive and only about 21 have been
produced with no plans to build more. The technology associated with the B-2 has been continuously
upgraded, allowing it to perform tasks with even greater accuracy and stealthiness.
The word stealth comes from ancient roots meaning “to steal.” There are a number of stealth
aircraft in operation today with others coming on line. To be “stealthy,” a plane should

• Be difficult to see with the eye

• Make little or no noise (achieved by muffling the engines)

• Display little heat from engines and other moving parts
• Minimize a production of contrails and other signs, and
• Absorb and scatter radar beams

The most common mode of aircraft detection is radar. Essentially, radar is the detection of
radio waves that have been “thrown out” and which bounce off objects returning to the site of
origin. Today’s radar, if properly used, can help identify the location, speed, and identity of the
aircraft. The radar cross-section (RCS) of an aircraft is how much echo the plane sends from
2
2
radar. Birds have an RCS of about 0.01 m . The Stealth Bomber has a RCS of 0.75 m . The
Stealth Bomber, and many stealth aircraft, gains their stealth character mainly from both the
shape of the aircraft and the presence of radar absorbing material (RAM). The RAM is made to
absorb and eliminate radio waves rather than reflecting them. Most of the RAM materials are

polymeric.
The Stealth Bomber has a covering of RAM, skin. The basic elements of the composite cover

consist of graphite carbon fibers embedded in an epoxy resin. The carbon fibers dissipate some of

the absorbed radiation by ohmic heating. The material also contains ferrite particles that are impor-
tant because they are good “lousy” radar absorbers. The good absorbency of radar signals is depen-
dent on a low dielectric constant. In principle, the thicker the coating the greater the absorption of
signals. Thick coatings increase cost and weight and decrease the aerodynamics of the aircraft. (In
truth, the fl ying wing is aerodynamically unstable and is flown with a quadruple-redundant fl y-by-

wire [FBW] system.) The thickness of the layers needed to provide the necessary radar signature
can be reduced through use of materials with intermediate absorption behavior, ferrite particles,
to smooth the interference between the aircraft and the surrounding air since extreme differences
cause strong radar refl ections.
The graphite resin layers are laid in a manner similar to that employed in making the hull of a
fiberglass boat hull. This layering, over a frame or mold, is accomplished using computer guided

assistance. Thin layers of gold foil are also applied to the canopy to give radar refl ecting character-
istics similar to the rest of the aircraft.
The Stealth Bomber is shaped like a wafer-thin Batman boomerang with smooth, knife-like

curved surfaces that lack right angles. The reflective nature of the design can be understood by con-

sidering looking straight on at a mirror. Your image will be reflected back at you. Tilt the mirror

45 degrees and it will reflect your image upward. The bomber design is a large system of triangles

with flat surfaces angled to deflect radar waves away from the radar emitter. It scrambles the radar
all about but not back to the receiver. This shape is also intended to offer a minimum of disruption
to the air, that is, the shape brings about a minimum amount of air resistance, resulting in a mini-
mization of radar difference between the aircraft and its surroundings.
The use of composites allow there to be few seams, thus reducing radar reflections. For years,

seams were believed necessary for large aircraft. For instance, the wings of the large bombers actu-

ally “flap” in the wind, contributing to the ability of the aircraft to remain in the air for long periods

of time. Without seams and some flexibility these large metal-based aircraft would literally snap

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