Ever since the revolutionary F-117 Nighthawk made its flashy combat debut during the Gulf War, stealth technology has been a staple of popular media and a symbol of American military prowess. In recent years, other countries have made great strides in stealth as well, with China, Russia, Japan, and others testing stealth aircraft prototypes with varying degrees of maturity, and with stealth ships becoming increasingly commonplace.
However, popular media often oversimplifies or outright misrepresents stealth, inhibiting public understanding of what stealth vehicles bring to the table in terms of military capability. This article will give a brief overview of how stealth works and address common misconceptions, focusing on stealth jet aircraft but touching on principles which apply to stealth vehicles of all types.
Stealth, also known as low-observability, aims to reduce the likelihood an enemy will detect an object (usually a vehicle). Militaries locate and track their targets using a variety of systems, including radars, infrared sensors, visible light sensors, interception of electromagnetic emissions (such as radar pulses and datalinks), and passive sonars. Stealth works by reducing a vehicle’s visibility to these sensors. Visual light signature reduction will not be covered in this article because most people understand the basic principles of camouflage and because visual light sensors are not typically used for long-range detection or targeting.
Radar cross-section reduction
The radar cross-section (RCS) of a vehicle is its “ability to reflect radar signals in the direction of the radar receiver.” Reducing RCS is usually the main goal of stealth, as radars are the primary sensor for long-range detection and targeting of large vehicles such as ships and planes. Radars work by emitting radio waves which bounce off of the target(s) and return to the radar, where the information is processed and used to determine the position of the target(s). By deflecting and/or absorbing these waves so fewer of them return, RCS can be reduced. Assorted images below illustrate some of the principles behind radio wave interactions with aircraft.
As these images suggest, a multitude of factors contribute to an object’s RCS, including resonance interactions with various shapes, the angle at which the radio wave contacts the surface, and diffractions. The radar’s frequency determines which effects will dominate. Stealth vehicles are usually optimized to defeat high-frequency radars, which are used by most aircraft and air defense systems.
In general, circular/conical surfaces, gaps, seams, and discontinuities all reflect radio waves and compromise stealthiness; this is why stealth vehicles appear smooth and angular. Any systems which cannot be optimized for stealthiness without compromising their function, such as turbine engines, weapons, etc. must be mounted so that radio waves do not directly contact them — for this reason, stealth aircraft carry their weapons internally and have S-shaped engine intakes to dissipate radio waves before they can reach the fan blades.
Because geometry plays a large role in RCS, the degree of stealth depends on the direction from which the radio waves are approaching the vehicle. Generally, a low-observable vehicle will be stealthy from the front but may have rounded surfaces and other undesirable characteristics when viewed from the sides.
In addition to geometry optimization, radar absorbing materials (RAMs) are usually applied. The most common type of RAM uses a magnetic field to convert electromagnetic waves into heat. Current RAM coatings are optimized for a specific radar frequency and are not capable of absorbing 100% of the emissions which reach them. RAM coatings are expensive and need to be re-applied periodically.
Infrared signature reduction
Infrared emissions, which are perceived by humans as heat, are another important factor to consider when designing a stealthy vehicle. While radar is the most common long-range detection technique, infrared sensors have made huge leaps in recent decades and are now able to detect vehicles at great distances under certain circumstances. Many types of missiles use infrared guidance and some aircraft are equipped with long-range infrared search and tracking systems.
Vehicles are detectable in the infrared (IR) spectrum because their systems produce heat during operation. The largest source of IR radiation in most vehicles is the exhaust created by combustion engines — this problem is especially acute for jet aircraft, which rely on powerful streams of hot air for propulsion.
There are a number of approaches to minimizing engine-related IR signatures. When possible, the engine can be set further into the fuselage to minimize the visibility of its hot components. Some aircraft such as the B-2 have exhausts on top of their wings, preventing an observer below from viewing the engine or the hottest part of its exhaust. The F-22 uses small holes and ducts in its nozzles to introduce cold air into the exhaust stream and create vortices, cooling it to an extent. While these approaches reduce the signature somewhat, it is virtually impossible to create an aircraft that is truly stealthy in the IR given the sheer quantity of heat produced by a turbine engine. The below video provides an example; despite the F-35B’s considerable IR signature reduction measures, the exhaust stream is clearly visible.
Friction between the air and the vehicle’s skin can also lead to heat buildup at high speeds, compromising infrared stealthiness. To address this, the F-22 uses active cooling of the leading edge (the front of the wing). Most aircraft, however, do not utilize such features.
Land and sea-based vehicles employ similar tactics to those described above, using shrouds and cooling solutions for their exhausts and employing low-emissivity paint.
Many electronics emit signals which can be intercepted, compromising stealthiness. For example, the radio waves emitted by a simple radar are detectable not only to the radar itself but to other antennae which intercept the signal. This information can then be processed and used to get an approximation of the original radar’s location. This risk is present in all electronics that send electromagnetic signals.
In order to prevent detection, stealth vehicles employ low-probability-of-intercept (LPI) radars and links. The advanced electronically scanned LPI radars used aboard modern stealth vehicles search with small, precise, agile beams which are difficult to detect. They can also switch their frequency with each pulse (frequency agility), further frustrating efforts to determine their position. Active electronically scanned array radars can even emit a different frequency from each of their many transmit/receive modules simultaneously, creating a signal which is diffuse at any given frequency and blends with background radiation. Datalinks use similar methods to prevent detection and interception — the Multifunction Advanced Data Link (MADL) is a directional, agile link used to communicate between F-35s. The F-22 uses a similar link, the IFDL.
Of course, another approach is to avoid sending signals entirely. Maintaining radio silence has been a common tactic during high-stakes operations for decades, although the reliance of modern weapons systems and avionics on networked communication makes this increasingly impractical. Plus, aircraft operate more effectively when they can communicate, hence the importance of stealthy links.
Sonic signature reduction
Since water is a good conductor of sound, submarines and (to a lesser degree) ships are concerned with their sonic emissions. Because air is a relatively poor conductor of sound, engineers of planes and land vehicles usually do not treat sonic signature as a primary consideration.
Passive sonars, which are basically powerful arrays of underwater microphones, can detect noisy ships and submarines from tens or even hundreds of miles away. Because sound is produced by physical vibrations, sonic signatures can be reduced by employing machinery which runs smoothly and requires fewer moving parts. For submarines, which are hunted primarily by sonars, sonic signature is a paramount design consideration and technologies such as anechoic tiles, pump-jet propulsors, electric motors, etc. are employed.
Stealth makes vehicles un-detectable
As discussed above, stealth is highly conditional. Rather than completely eliminating a vehicle’s radar and infrared signature, stealth merely reduces it. Thus, while stealth vehicles can get much closer to radars without detection, they are not invisible. As a result, stealth vehicles still need to take into account the position of enemy ships, planes, and ground-based radars when planning their sorties so as to avoid approaching a sensor from an undesirable angle or distance.
Stealth ships are much less “stealthy” than aircraft; they have a large enough signature to be seen on radar, but they appear the size of a much smaller craft (the 14,500-ton Zumwalt-class destroyer is said to have the signature of a fishing boat).
Low-frequency radars make stealth useless
Many a sensational headline has claimed that China and Russia’s low-frequency radars will make America’s stealth aircraft fleet obsolete; unsurprisingly, the real story is far more complicated. It is true that low-frequency radars are impacted less by stealth geometry techniques because radar returns increase drastically as wavelength increases relative to target size. When the wavelength is longer than the shape it is interacting with, geometric stealth techniques lose their effectiveness due to resonance.
However, low-frequency radars suffer from a number of drawbacks, including large size, high clutter pickup, higher power and signal processing requirements, and low accuracy. In order to receive long wavelengths, a large antenna is needed, restricting low-frequency radars to large trucks/trailers and ships. As such, land-based low-frequency radars generally take relatively long relocate and would be vulnerable to cruise missiles and laser-guided bombs in the opening stages of a conflict.
Current low-frequency radars are generally not accurate enough to guide missiles, being useful only for early warning and determining the general location of an enemy contact. Upon detection, a higher-frequency targeting radar (which could be foiled by stealth geometry) would be necessary to launch radar-guided anti-air missiles. Despite this limitation, the imprecise information gathered by a low-frequency radar is still useful for telling fighters equipped with infrared search-and-track systems where to look and helping short-range infrared missile units position themselves appropriately.
In summary, while low-frequency radars can provide valuable information concerning the presence and approximate location of stealth aircraft, the size and poor precision of low-frequency radars preclude them from being a complete anti-stealth solution.
All stealth aircraft are created equal
While aircraft are generally slotted into a stealth vs. non-stealth binary, low-observability is more of a spectrum. The F-22, for example, is believed to have a radar signature many times smaller than the cheaper F-35, despite both being “stealth” aircraft. Size plays a significant role as well — flying-wing stealth bombers such as the B-21 and B-2 have large enough features that even low-frequency radars have trouble detecting them with resonance, giving such designs an edge over smaller stealth aircraft. There is significant variance between non-stealth aircraft as well, with large, early-fourth-generation aircraft such as the F-15 having radar signatures many times larger than newer aircraft; modern designs such as the F/A-18E/F Super Hornet incorporate some “stealthy” design features despite low-observability not being a primary design consideration.