The Science of Stealth

A technician guides a paint sprayer to coat an F-117 Nighthawk. Painting the first F-117 took five workers five days; thereafter, a team at Sandia National Laboratories in New Mexico developed a precise, machine-delivery system. The applicator, which rode on a 30-foot rail, had sensors that compared details of the airframe, such as the intersection of angular facets, to a three-dimensional computer model.

Stealth coatings are now applied by robot. Sensors report any condition that could cause flaws in the finish, such as variations in flow rates.

A fiberglass mockup of the F-35 served as a test bed for a robotic aircraft finishing system at Lockheed Martin’s Fort Worth plant.

Radar returns enable trackers to estimate the type and size of an aircraft. An ideal stealthy aircraft would have no discernible shape on a radar screen; its return might resemble a small fuzzy ball. How an airplane appears on a radar screen depends on the side presented to the radar beam. The stelathiest aspect of the F-22 is nose-on.

The open bomb bay of an F-22 Raptor after a strike is more reflective than the airplane’s underside with doors closed and much more reflective than its nose-on aspect.

Radar returns are classified according to shape. Two shapes that frequently appear are the PacMan and Bow Tie.

Today’s champ in “all-aspect” stealth throughout its range of missions and the flight profiles it presents to an enemy is the F-22 Raptor. Its RAM reduces its radar signature, but its small return is mainly due to an airframe design that eliminates right angles in favor of curved shapes of varying radii.

The radar cross section of the F-22 has been compared to that of a bumblebee, about one square centimeter. The B-1B Lancer has an RCS of roughly one square meter, and the wholly un-stealthy B-52 Stratofortress has one as large as 100 square meters.

A radar cross section, or RCS, is a measure of reflectivity, expressed in square meters, often generalized from the return from the head-on and level radar illumination of the airplane’s nose. A complex mathematical formula converts that return to an estimate of the size of a sphere that would return the same measure of energy.

The B-2 Spirit is also shaped for stealth and now has an improved radar-absorbent coating. Maintaining the B-2’s stealthy skin once required removing and meticulously reapplying nearly 3,000 feet of special tapes and caulk by hand, to seal the radar-reflective gaps between its panels. Northrop Grumman has developed a new coating, “alternate high-frequency material,” now applied by robot to reduce maintenance from days to hours. As far as is known, the B-2 has never been detected by enemy radar.

The F-117 Nighthawk was the world’s first stealthy aircraft. Composed of flat panels, each angled so that no one is likely to be at a right angle to a radar, the F-117 was a master of deflection. Retired in 2008, F-117 Nighthawks first saw action in 1989, when the United States invaded Panama.

Although some features of the SR-71 Blackbird were designed to reduce its radar cross section—smoothing the sides of the fuselage into chines and canting the vertical stabilizers inward—it cannot be considered a stealthy aircraft. It did employ a type of RAM, known as “iron ball,” containing molecules of carbonyl iron ferrite.

Although the composition of coatings on stealth aircraft flying today is classified, the B-2 Spirit bomber, F-22 Raptor, and F-35 Joint Strike Fighter have skins that are complex and costly variations on the formulas, materials, and application techniques designed generations ago for the SR-71 Blackbird.

The Lockheed Martin F-35 has the most advanced stealth coating yet developed; it is more durable than that of previous generations. (The new coating is now being applied to F-22 Raptor airframes.) Like most stealth aircraft, the F-35 embeds within its airframe detectable energy sources such as radio antennas and carries inside radar-bright equipment like bombs. F-35’s skin sections are joined in sawtooth or “W” patterns rather than straight lines, to redirect a radar contact. Because of its “cleaner” shape, the prototype X-35 was stealthier than the production aircraft.

The F-35 has a radar signature greater than that of the larger F-22. The production model F-35C has a deeply sculpted weapons bay and wing joint, and critics say that any aspect of the fighter other than the nose-on view has an undesirably large radar signature.