New Look at Stealth

"Stealth" aircraft are designed to confuse radar. The aircraft's shape and special paint absorb radio waves, reducing the radar signature to levels indistinguishable from that of a bird in flight.
Multiplicity is central to U.S. military plans for future generations of unmanned air vehicles (UAVs). A large number of low-cost UAVs will be used in wartime and surveillance operations, with any individual aircraft in this fleet being replaceable. If one aircraft malfunctions or is brought down by hostile fire, the network of other UAVs must be able to "self-heal" by altering the path of inter-craft communication, and continuing to send uninterrupted transmissions.
To make fleets of such small unmanned aircraft feasible, however, the cost of replacement for any one UAV must be as small as possible. This means conventional stealth technology is cost-prohibitive for these craft. Designers are working on new methods of masking the radar trail, thermal signature, and sound emitted by small aircraft.
Data must be transmitted between the UAV and back to a command base. To make data interception by an enemy more difficult, these UAVs must have the ability to precisely control and rapidly change the power, direction, and frequency of signals being transmitted.
The Army's Unmanned Combat Armed Rotorcraft (UCAR) program is considering these design elements for helicopters, which must be able to hover at relatively low altitudes over a battlefield or surveillance area without being detected. The distinctive sound from rotor blades can be reduced by half with a design eliminating the smaller tail rotor, but the sound must be reduced further to make these craft feasible in practice.
Plans for future ground vehicles must meet similar goals. A new design for much quieter hybrid electric engines is expected to make tanks and other wheeled or tracked vehicles more stealthy.

Stealth Retains Value, But Its Monopoly Wanes

Stealth--the ability of an aircraft to evade radar--is no longer the dominant focus for warfighters. For the U.S. and its allies, stealth is now considered only one of several factors that must be controlled and manipulated to make warplanes survivable on future battlefields.
The need for improved stealth is largely driven by the increasing ability of foes to network sensors and find the elusive clues that can give away the position of radar-evading aircraft. Because of the evidence of improving antiaircraft technology, the U.S. will keep developing stealthier aircraft. The ultimate goal is development of a reliable, precise, all-weather automatic target recognition system.
Pentagon planners assume knowledge about stealth technology is proliferating quickly, and U.S. industry can count only on staying a few years ahead of its competitors. Countries most likely to become a foe of the U.S. are not yet manufacturing stealth aircraft and missiles. Even leaks to Russia of algorithms used in stealth designs do not seem troublesome. Radar cross section computer codes cannot be used to produce and then thwart stealth designs without the exact knowledge of the shape of the vehicle and precise material specifications. Knowing about stealth is not difficult, but countering it is.
Two possible stealth-enhancing options interest scientists. One is putting magnetic materials in aircraft coatings to absorb incoming radar signals. The second is to apply coatings that conduct an electrical current aimed at disrupting or canceling incoming radar signals.
Electronic countermeasures or jamming will be a growing need for stealth aircraft as enemy radar systems become more sophisticated in picking out the electronic whispers from low-observable designs. Stealth aircraft already count on a certain amount of help from thermal noise, clutter, and externally generated interference to create an electronic floor or noise under which they can operate. Without the competition of noise and interference, even stealth aircraft would be detectable.

Stealth Engine Advances Revealed in JSF Designs

Several aviation companies are competing for the ultimate in stealth propulsion in the Joint Strike Fighter (JSF) program. Stealth aircraft avoid radar signatures from designs that keep radar beams from striking the engine or radar-absorbing materials (RAM). The Pentagon, by extending the JSF competition, allowed more opportunities for stealth technology and engine design leaks. Industry's concerns about their competition-sensitive technology are justified with much already known about recent JSF improvements and the two distinct designs of Boeing and Lockheed Martin.
Engine faces are sources of large, distinct radar reflections that identify the engine and aircraft. Boeing and Pratt & Whitney, two engine manufacturers that cooperated, added stealth to the inlet guide by masking inlet vanes. Lockheed Martin and McDonnell Douglas diverted radar beams using air duct surfaces coated with RAM. Radar blockers in the latest JSF designs are an integral part of the engines. In contrast, radar blockers for the Boeing's F/A-18E/F and F-22 are separate add-on devices. The Lockheed F-117 has inlet screens that divert radar waves around the aircraft. The tradeoff in new stealth technology is one between costs and aerodynamic performance. Smaller aircraft use integrated serpentine ducts rather than large inlets in their stealth designs. In larger aircraft, a larger radar blocker is sometimes more efficient. A large complex inlet has integration costs and will put demand on power systems.
The Pentagon will have choices between short takeoff and vertical landings (STOVL) systems, inlet designs, weapon bays positioning, and sensor arrays in the two Boeing and Lockheed Martin designs. Infrared and optical sensors are used in newer antiaircraft missiles. Hiding hot elements and weapon systems are critical in addition to radar concealment. The Pentagon wants a stealth technology with radar and infrared blockers that are simple, inexpensive, and sturdy.

Invisibility Rules the Waves

Stealth technology strives to keep military ships invisible to modern detection systems including radar, magnetic sensors, heat sensors, and acoustic detectors. There are many different types of stealth technology. For example, dazzle camouflage involves painting a ship with a camouflage pattern. Other methods help make ships inconspicuous by matching the brightness of the craft to its background via isoluminance, or using outward-facing bright lights. These techniques work best in open seas. In recent years, naval ships have been deployed to conflicts taking place in coastal waters, leading to the need for other stealth techniques.
If the enemy has radar, a camouflaged ship can be spotted. Soon after radar was invented, radar stealth technology began attempting to make ships invisible to radar. To avoid detection, ships must either absorb the emitted radio pulses, deflect the reflections away from the enemy's receiver, or cancel them by sending out radio waves that are out of phase with the actual radar echo. Achieving any of these is difficult owing to how radio pulses interact with ships. Vessels have both a radar cross section (RCS), a representation of its overall echo intensity, and a radar signature, or the waveform of a detected radar echo used to discriminate between targets. The RCS is determined by the size and shape of a ship and the angle at which the radar pulse hits. A smaller RCS allows a ship to be closer to the enemy without detection. Since most ships have a large RCS, designers try to reduce it by using decoys, firing bits of metal into the air to generate a larger RCS and a more attractive target to the enemy.
A ship's RCS can be reduced by decreasing its reflected energy by covering it with radar absorbent paints or foam. A ship can also be built of low radar reflectivity materials such as fiberglass, carbon (C)-fiber composites, and glass-reinforced plastic. Since a ship's sharp angles reflect energy back towards the receiver, reducing these also reduces detection.
Using a combination of techniques can make ships much more difficult to detect although never completely invisible.