Stealth 101

EVER SINCE THEIR INTRODUCTION IN THE 1980S, stealth aircraft have captured the public’s imagination. Commonly believed to be “invisible” to radar, stealth aircraft have provided the U.S. military with an unmatched advantage since their first use in 1989. These aircraft are beginning to proliferate around the world with Russia, China, India, Japan, South Korea, the UK, France, and the United States all having stealth aircraft programs at various stages of development. To date, the United States is the only nation that has used stealth aircraft in combat and currently operates at least four different stealth platforms: the B-2 Spirit bomber, the F-22 Raptor fighter, the F-35 Lightning II multi-role fighter, and the RQ-170 Sentinel UAV. As stealth aircraft are set to represent increasing share of the global combat aircraft inventory, it is vital to understand this technology, its benefits, and its limitations.

What is Stealth

Jeremy M. Wilson/U.S. Air Force

Right off the bat, it is important to recognize that stealth is not a magic bullet; it is not a Harry Potter-esque invisibility cloak that renders an aircraft immune to any form of detection. Instead, it is best to think about stealth as what it really is, signature management. In this case, signature refers to how visible an aircraft is to a variety of different sensors. Stealth is the novel application of materials and aircraft geometry to reduce its radar cross section (RCS), how visible an aicraft is to radar. For example, the B-2’s RCS is 1,000 times smaller than that of the B-52 despite both aircraft being of comparable size. Many stealth aircraft also include technologies to reduce their thermal signature. The B-2 is an excellent example of this approach as well.

Lockheed Martin

While not rendering an aircraft fully invisible, stealth technologies make it considerably more difficult for an adversary to detect and then track a given target. Stealth technologies are most commonly applied to aircraft, but they have also been utilized on several different cruise missiles and naval surface vessels. It is important to recognize that other, active measures exist to prevent an aircraft from being detected. The best known of which is jamming, the transmission of electromagnetic energy of specific frequencies to prevent radar systems from working as designed. These technologies will not be covered here as this section only focuses on signature management, sometimes referred to as “passive” stealth.
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How Stealth Works

Brian Ferguson/U.S. Air Force photo

In order to reduce an aircraft’s RCS, engineers have two primary tools at their disposal: advanced materials / coatings and aircraft geometry. The first category is generally referred to as Radar Absorbing Material (RAM). This class of materials radiates less radar energy than it receives. In this way, it reduces the expected radar signature for a given object. While offering significant reductions in RCS, RAM by itself does not make a stealth aircraft. For example, the rotating blades of a jet engine produce a massive radar signature. RAM can also be maintenance intensive, requiring constant care and special facilities in order to maintain its absorptive characteristics.

While RAM is important, the defining characteristic of stealth aircraft is their geometry. Unique shapes and distinctive design features enabled the radical reduction in RCS first observed on the F-117A Nighthawk in the 1980s. Stealth aircraft are designed to scatter incoming radar energy rather than reflect it back to the receiver. This explains the hard, angular look of the F-117A as these surfaces were intended to reflect radar energy away from the receiver. Subsequent designs have taken a more organic approach utilising compound curves to achieve the same result.

It must be noted that all stealth aircraft are not created equal. Most designs are optimised against radars in the L-X Bands as they represent the most common bands for search and fire control radars worldwide. Broadband stealth, that is stealth against a wider array of radar frequencies, requires more dramatic designs. This requires some form of tail-less flying wing such as the B-2. While exceptionally stealthy, these aircraft require complex flight control systems to maintain stable flight.

In addition to clever airframe design, all stealth aircraft go to great lengths to conceal their engine intakes. As noted above, a jet engine’s rotating fan blades create a tremendous radar signature. Concealing these elements of the engine while adequately managing airflow across the aircraft’s entire flight envelop is one of the most challenging aspects of designing a stealth aircraft, particularly one capable of supersonic flight.

David Salanitri/U.S. Air Force

Heat management is another piece of the stealth puzzle. This is an increasingly important element to a stealth aircraft design as infrared search and track (IRST) capabilities and advanced infrared air to air missiles become increasingly common. While not all stealth aircraft go to aggressive lengths to reduce their thermal signature, the most advanced aircraft, namely the B-2 and the F-22, include technologies to reduce their thermal signature.

The B-2 is particularly aggressive in this regard with its engines concealed deep within the airframe. In addition, the rear of the aircraft includes engine nozzles designed to cool exhaust gasses by mixing cool air with the hot exhaust. Furthermore, there is an engine “trench” coated in thermal tiles designed to eliminate the thermal signature when viewed from below. The F-22 attempts to manage its thermal signature by including supercruise technology. This means that the aircraft does not need to use its afterburners to achieve supersonic flight. Afterburners produce a huge thermal signature, the equivalent of a lighthouse on a dark evening. Different combinations and applications of these different stealth elements can create a range of aircraft that while all deserving the moniker  “stealth” provide different levels of signature management.
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The Future of Stealth

Stealth technology is becoming even more valuable as modern air defense and air combat systems grow in lethality and ubiquity. For this reason, stealth is being pursued by a number of global air forces. As alluded to above, not all stealth is created equal. Some nations, particularly Russia, have developed stealth aircraft that maximize stealth characteristics when observed from the front. This is in comparison to other design philosophies that attempts to maximize stealth from all aspects. It is highly likely that aircraft with some degree of frontal aspect RCS reduction will become increasingly commonplace. This approach is less technically challenging, lower in cost, and provides considerable capability for nations primarily interested in territorial defense. This approach can be seen in upgraded fourth generation fighters such as the Advanced Super Hornet or the F-15SE Silent Eagle.

In the long term, the efficacy of stealth may decline as new radar and optical sensing techniques are developed. This trend can already be observed at any airshow featuring Russian or Chinese radars. There are a variety of systems that claim “counter-stealth” capabilities. While it is unclear how well these systems would work in an actual combat environment, it is clear that researchers in the United States, Russia, and China understand that stealth can be defeated by the combination of novel tactics and specific technologies. Stealth will remain a vital element of advanced combat aircraft for the years to come; however, it may never regain the powerful effects it had when introduced to the world in the 1980s and early 1990s.
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United States

  • F-117A Nighthawk – Retired
  • B-2 Spirit – Active
  • F-22 Raptor –  Active
  • F-35 Lightening II – Active
  • RQ-170 Sentinel – Active
  • B-21 Raider – In Development

Russia

  • Su-57 – In Testing
  • PAK-DA – In Development

China

  • J-20 – Active
  • J-31 – In Testing
  • H-XX – In Development

Japan

  • X-2 Shinshin – In Testing

South Korea

  • KFX – In Development

India

  • AMCA – In Development

United Kingdom

  • Taranis (Unmanned) – In Testing

France

  • Neuron (Unmanned) – In Testing