Stealth

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Stealth is the common name for not any specific system, but a collective term for a range of military low observability techniques. These techniques focus on reducing the signature of "stealthy" platforms — aircraft, missiles, ships, and ground vehicles — as detected, or not detected, by hostile sensors. While it is sometimes the case that a stealthy platform is not detected, that is not always achievable. Realistic goals, however, include making the platform hard to locate, and especially hard to "lock up" in the fire control systems of weapons that could be used against it.

For example, recent U.S. Navy ships, such as the Burke-class destroyers, have reduced observability compared to earlier classes. It is not seriously expected that an enemy would not know the ship was present, but its radar signature is sufficiently reduced to make it very hard for the final guidance radar of an anti-shipping missile to discriminate the ship from decoys and electronic self-protection signals intended to confuse the missile.

Radar signature reduction is the best-known aspect of low observability. By no means, however, is radar the only technology that the stealth technologies try to defeat. Low-observability aircraft also reduce their infrared signatures and reduce their acoustic signatures if operating on or close to the ground. They prefer to operate in the darkest of night, and are painted black or a very dark gray even harder to see.

Indeed, while the first "stealthy" aircraft was the F-117 Nighthawk light bomber, this claim is met, with some amusement, by the submarine services of any advanced navy. The U.S. Navy submarine force calls itself the "silent service". Noise reduction to defeat passive sonar is an obsession with submariners who desire to be old submariners.

There is little question that submarines were the first low observability platforms. Early modern submarines, in the First World War, were primitive by today's standards — but they struck without warning, for the simple reason that humans do not see deep underwater. Understanding submarine quieting technologies is important to getting a full understanding of stealth.

  • Techniques applicable to aircraft
    • Reducing radar signatures
    • Reducing infrared signatures
    • Reducing airborne radiated acoustic signatures
  • Techniques applicable to ships
    • Reducing radar signatures
    • Reducing underwater acoustic signatures
    • Reducing magnetic signatures
    • Reducing infrared signatures
  • Techniques applicable to ground vehicles
    • Reducing radar signatures
    • Reducing infrared signatures
    • Reducing magnetic signatures
  • Techniques applicable to submarines
    • Reducing underwater acoustic signatures
    • Reducing magnetic signatures

Reducing radar signatures

While may assume the first low-observability military use was the F-117 Nighthawk, the "Black Jet" that was the only manned aircraft allowed to overfly Baghdad in the Gulf War, attempts to reduce radar signatures began decades earlier. During the Second World War, Germany experimented with surface coatings that absorbed radar energy rather than reflecting it. [1] German work was preliminary, but seemed to make the most progress on protecting surfaced submarines from radar.

Early retrofits of reduced signature technology

After WWII, both the U.K. and U.S. continued work on radar absorbing materials, that being the only approach known, at the time, to reducing radar observability. The U.K. concentrated on naval applications, while the U.S. focused on aircraft uses. Specifically, the U.S. priority was reducing the radar signature of the U-2 high-altitude reconnaissance aircraft that were overflying the Soviet Union. Both radar absorbing material, and a system of wires on the fuselage, to act as antennas that scattered radar energy were used. The wires decreased aerodynamic performance, and may have increased vulnerability.

Intentional design begins

Even before the U-2 was shot down in 1960, work was beginning on an alternative, which would both be extremely fast, but have reduced observability built in. This was to become the SR-71 Blackbird. An assumption was that a sufficiently fast aircraft, with reduced signature, would be seen as bursts of noise, at impossibly widely separated distances, to be a real airplane. Such a radar target would be rejected by the radar's signal processing.

While the complex diffraction theory was not yet known, Lockheed's Skunk Works were aware that the engine and certain flat control surfaces would be highly radar-reflective. Considerable use was made both of radar absorbing paint, and nonmetallic control surfaces that were inherently less reflective. The engine was partially covered by shrouds, which both had a high-speed airflow direction role, but also hid some of the engine from radar. The SR-71 had significantly reduced RCS, but not as much as was achieved with theoretical breakthroughs.

Diffraction concept

Somewhat ironically to the West, the fundamental theory of low radar observability came from an unclassified Soviet publication.[2]

  • Diffraction
  • Radar absorption
  • traveling waves; surface propagation and re-radiation
  • multiple scattering and diffusion

Complements to reduced radar signatures

There is a perception that stealth aircraft such as the F-117 Nighthawk move in electronic and infrared silence, but, in complex strikes, that is not the case. If a stealthy aircraft is hard to find on radar when there are no other targets in the sky, imagine how much it is harder to find if there are radar jammers, blasting away at one's radar systems. The problem is rather like hearing the squeak of a timid mouse during a very noisy celebration.

Countermeasures to radar signature reduction

Radar signature reduction is, to a significant extent, dependent on the wavelength of the radar. The reflection and diffraction techniques are optimized first against the most likely weapons control frequencies, and second against the most likely search radar frequencies.

Reducing infrared signatures

Stealth aircraft mix their jet exhaust with cooler air, and vent it through a series of baffles.

Reducing acoustic signatures in air

Reducing underwater acoustic signatures

Acoustic detectors are the primary threat to modern submarines. While some of the techniques are highly classified, some of the method known are covering the decks with rubberized tiles that absorb acoustic energy. The noisier parts of the propulsion system, such as reactor pumps and turbines, are mounted on acoustic isolation "rafts" using both springs and elastics.

Extreme design and maintenance are applied to propellers to reduce cavitation, a phenomenon in which bubbles form, and then noisily collapse. There is a trend to move to pump-jets and other means of transferring power to water, much less noisily than with propellers.

While it is not a stealth technique, both surface ships and submarines use acoustic decoys, often noisier than the ship itself.

Reducing magnetic signatures

References

  1. Hott, Bartholomew & George E. Pollock, The Advent, Evolution, and New Horizons of United States Stealth Aircraft
  2. Browne, Malcolm W. (14 May 1991), "2 Rival Designers Led the Way To Stealthy Warplanes", New York Times