In the recent decades the world has invested a great amount of time and energy on stealth technology in the endeavour to delay the military airborne platforms to be seen by the adversary. Most new platforms are designed by incorporating some level of ‘stealth’ or ‘low observable’ (LO) technology at the initial design stage itself. Even existing jet fighters are at times considered for modification in order to reduce their signature. Creating a stealthy design comes at a price. It could be in terms of aerodynamic and other design compromises, long period to perfect the design, or high developmental financial costs. Some stealth features require regular special maintenance. Specific aircraft shapes may also limit weapon and fuel carriage or may not be the best for flight efficiency. Any pylon, tank, missile or pod carried externally increases the radar cross section (RCS). Lockheed F-117 Nighthawk was the first operational aircraft specifically designed around stealth technology. Other current stealth aircraft include the B-2 Spirit, the F-22 Raptor, the F-35 Lightning II, the Chengdu J-20, Shenyang FC-31 and the Sukhoi Su-57. While air forces and designers concentrate on stealth features, they are also evolving counters to beat the stealth effect. Passive IR sensors, multi-static radars, very low frequency radars, and over-the-horizon radars are being designed.
Stealth or LO is essentially a combination of many technologies, all of which combine to greatly reduce the distances at which an aircraft can be detected. It involves RCS, acoustic signature, thermal imprint and many other aspects to be reduced. The term ‘Stealth’ became popular during the late eighties when the F-117 stealth fighter was deployed in the Gulf War in 1991.
Aircraft parts that contribute significantly to the echo at any aspect is the vertical stabliser. Aircraft like the F-117 have tilted tail surfaces to reduce corner reflections. A more radical method employed is to omit the tail, as in the B-2 Spirit and achieve near perfect stealth shape, as it would have no angles to reflect back radar waves. The propellers and the jet turbine blades produce a bright radar image. Stealth design must bury the engines within the wing or fuselage, or at least install baffles in the air intakes, so that the compressor blades are not visible to radar. Forward facing radar aircraft cone acts as a reflector. Leading edge of the wing also reflects radar waves, and require radar absorbing materials to trap the waves. There should be no complex bumps or protrusions of any kind on the aircraft. The weapons, fuel tanks, and other stores must not be carried externally. Stealthy becomes un-stealthy even when a door or hatch opens.
The leading edges of the wing and the tail planes when set at the same angle in F-22 Raptor reduced reflections. Other smaller structures, such as the air intake bypass doors and the air refueling probe also use the same angles. Coating the cockpit canopy with a thin film of transparent conductor helps to reduce the radar reflections from the cockpit and even the pilots helmet. The coating is thin enough that it has no adverse effect on pilot vision. Dielectric composite materials are more transparent to radar, whereas electrically conductive materials such as metals and carbon fibers reflect electromagnetic energy incident on the material’s surface.
Radiation Absorbent Materials (RAM)
RAM paints are used especially on the edges of metal surfaces. Paint comprises depositing pyramid structures on the reflecting surfaces. These structures deflect the incident radar energy in the maze of RAM. Initially radar-absorbent paints resulted in high weight and drag penalty and had to be rejected.
Reducing Radio Frequency (RF) Emissions
Aircraft must avoid radiating any detectable energy, such as from onboard radars, communications systems, or RF leakage from electronics enclosures. Many aircraft use passive infrared and low light TV sensors to track enemy aircraft and aim weapons. The F-22 reportedly has an advanced low-probability-of-intercept radar (LPIR) which can illuminate enemy aircraft without triggering the radar warning receiver.
The RCS is traditionally expressed in square meters. This does not equal geometric area. It represents equivalent reflectivity. At off-normal incident angles, energy is reflected away from the receiver, reducing the RCS. Modern stealth aircraft are said to have an RCS comparable with small birds or large insects.
Radar Stealth Countermeasures
All current stealth aircraft were designed to counter X-Band radars, but those shapes are getting ineffective if a radar operates in S-band and even more ineffective when the radar operates in L-band. The reason for the stealth aircraft to be detected is the wavelength of the radar, a radar operating in L-band produces wavelengths with comparable size to the aircraft itself and should exhibit scattering in the resonance region rather than the optical region, so that most of the existing stealth aircraft will turn from invisible, to visible. Shaping alone offers very few stealth advantages against low-frequency radars. Low frequency radars even with signal wavelength more than twice the size of the aircraft can still generate a significant return. However, low-frequency radars lack accuracy, and because of large size, are difficult to transport. Multiple emitters is another option. Detection can be better achieved if emitters are separate from receivers. Bistatic or multi-static radars have emitters separate from receivers.
Early stealth observation aircraft used slow-turning propellers to avoid being heard by enemy troops. The supersonic aircraft have a sonic boom. Modern aircraft engines are more efficient and a little less noisy. Standard rotor blades in a helicopter are evenly spaced, and produce greater noise at a given frequency and its harmonics. Helicopter rotor noise can be reduced by varying spacing between the blades that will spread the rotor noise over a greater range of frequencies.
The visual signature is best reduced through camouflage paint or other materials to color and break up the lines of the aircraft. Most aircraft use matte paint and dark colors. Gray paint disruptive schemes are more effective. Things like preventing sunlight glinting from the aircraft cockpit canopy had to be addressed. The original B-2 design had wing tanks for a contrail-inhibiting chemical. Later they had a contrail sensor that alerts the pilot when he should change altitude.
Reducing aircraft heat signature is required to prevent IR sensor tracking. The exhaust plume contributes a significant infrared signature. One means to reduce IR signature is to have a non-circular slit-shape tail pipe, to reduce the exhaust cross section area and maximize the mixing of hot exhaust with cool ambient air as is the case in F-117. Often, cool air is deliberately injected into the exhaust flow to boost this process. In some aircraft, the jet exhaust is vented above the wing surface to shield it from missiles observers below. Another way to reduce the exhaust temperature is to circulate coolant fluids such as fuel inside the exhaust pipe, where the fuel tanks serve as heat sinks cooled by the flow of air along the wings.
Infrared Search and Track (IRST)
IRST systems may be effective even against stealth aircraft, because any aircraft surface heats up due to air friction and a two channel IRST can compare difference between the low and high channel. Russia had IRST systems on MiG-29s and SU-27 in 1980s. There is a resurgence of ISRT systems. The MiG-35 is equipped with a new Optical Locator System with more advanced IRST capabilities. The French Rafale, the European Eurofighter, and Swedish Gripen make extensive use of IRST. Typically, the IRST allows detection of non-afterburning aerial target at 45 km range. The Lockheed F-21b on offer to India will have a long-range IRST.
Vulnerable Modes of Flight
Stealth aircraft are still vulnerable to detection during, and immediately after using their weaponry. Since stealth aircraft carry all armaments internally, as soon as weapons bay doors open, the plane’s RCS multiplies. While the aircraft will reacquire its stealth as soon as the bay doors are closed, a fast response defensive weapons system has a short opportunity to engage the aircraft. The F-22 and F-35 claim that they can open their bays, release munitions and return to stealthy flight in very short period. But some weapons require that the weapon’s guidance system acquire the target while the weapon is still attached to the aircraft. This forces relatively extended operations with the bay doors open.
Fully stealth aircraft carry all fuel and armament internally, which limits the payload. The F-117 carries only two laser or GPS-guided bombs, while a non-stealth attack aircraft can carry several times more. This requires the deployment of additional aircraft to engage targets that would normally require a single non-stealth attack aircraft. This apparent disadvantage however is offset by the reduction in fewer supporting aircraft that are required to provide air cover, air-defense suppression and electronic counter measures.
A heavily defended site normally has overlapping radar coverage, making undetected entry by conventional aircraft delayed or difficult. Aircraft detection can be delayed by exploiting ground radars Doppler slot. With knowledge of enemy radar locations and the RCS pattern of own aircraft a flight route can be flown that minimizes radial speed while presenting the lowest-RCS aspects of the aircraft to the threat radar. There are other flight tactics such as manouvres combined with chaff dispensation to confuse enemy radars.
Operational usage of stealth aircraft
He who can see first and shoot from farther will win the air combat. Stealth will delay being seen. The U.S. and Israel are the only countries to have used stealth aircraft in combat. In 1990 Gulf War, the F-117s flew 1,300 sorties (6905 hours) and scored direct hits on 1,600 high-value targets in Iraq. Only 2.5% of the American aircraft in Iraq were F-117s, yet they struck 40% of the strategic targets with an 80% success rate. In Yugoslavia in 1999, F-117 and the B-2 Spirit strategic stealth bomber were used. One F-117 was shot down by a Serbian S-125 ‘Neva-M’ missile. The B-2 Spirit destroyed 33% of selected Serbian bombing targets in the first eight weeks of U.S. engagement in the War. B-2s flew non-stop to Kosovo from their home base in Missouri and back. In the 2003 invasion of Iraq, F-117 Nighthawks and B-2 Spirits were used, and this was the last time the F-117 would see combat. Sikorsky UH-60 Black Hawks helicopter used for the May 2011 operation to kill Osama bin Laden, was heavily modified for quieter operations and employing stealth technology to be less visible to radar. The F-22 made its combat debut over Syria in September 2014 as part of the U.S. led coalition against ISIS. In 2018, Israeli F-35I stealth fighters conducted a number of missions in Syria and even infiltrated Iranian airspace without detection. The Chinese J-20 entered service with the People’s Liberation Army Air Force (PLAAF) in 2017. The FC-31 is still under flight testing. The Russian Sukhoi Su-57 stealth multi-role fighter is scheduled to enter service in 2020.
Future Stealth Initiatives
Shaping the aircraft for stealth often compromises the performance in terms of aerodynamics. After the invention of metasurfaces, the conventional means of reducing RCS have been improved significantly. Metasurfaces are thin two-dimensional meta-material layers that allow or inhibit the propagation of electromagnetic waves in desired directions. Metasurfaces can redirect scattered waves without altering the geometry of a target. Plasma stealth is a phenomenon proposed to use ionized gas (plasma) to reduce RCS. Interactions between electromagnetic radiation and ionized gas might create a layer or cloud of plasma around the platform to deflect or absorb radar. There is research to integrate the functions of aircraft flight controls such as ailerons, elevators, flaps etc. into wings to lower RCS by reducing moving parts, weight and cost. The concept of a flexible wing that can change shape in flight to deflect air flow is evolving. Adaptive Aeroelastic wings are being researched. Fluidics, is essentially fluid injection in aircraft to control direction, through circulation control and thrust vectoring. Fluidic systems, in which larger fluid forces are diverted by smaller jets intermittently to change the direction. BAE Systems has tested two fluidically controlled unmanned aircraft.
Under Development Stealth Aircraft
Many stealth aircraft are under development. The Russian MiG-41 is planned to replace the MiG-31. MiG LMFS has evolved from now cancelled Mikoyan project 1.44. Tupolev PAK DA is to be stealth bomber to replace Tu-95. Xian H-20 would be a subsonic stealth bomber. Shenyang J-18 is VSTOL stealth aircraft, similar to the U.S. F-35B. Indian Advanced Medium Combat Aircraft (AMCA) will be a stealth fifth generation aircraft. Northrop Grumman B-21 Radar will be a long range strike stealth bomber. BAE Systems Tempest, Saab’s Flygsystem 2020, the multi-nation European New Generation Fighter are some more serious programs. KAI KF-X is a joint program between Indonesia and South Korea. HESA Shafaq is an Iranian stealth aircraft project. Turkey plans to develop TAI TFX to replace F-16s one day. Project AZM is a Pakistani attempt to develop a fifth-generation fighter jet technology. Among the helicopters is a stealthy unnamed Kamov helicopter. Hindustan Aeronautics Ltd (HAL) Light Combat Helicopter (LCH) and Eurocopter Tiger are known to have stealth features.
Is Stealth Overrated
Radar-evading warplanes require careful design work, extensive testing and exotic materials for their construction, all features that can double or triple their cost compared to conventional, non-stealthy planes, say experts. They argue that stealth is overrated and it’s better to buy greater numbers of cheaper, non-stealthy planes. The high expense of developing, buying and maintaining stealth jets means they may be used sparingly. Many stealth aircraft still have questionable ability. Many long range AAMs are difficult to carry internally in most fighters. Canard controls and aircraft external hard-points are ‘stealth killers’. Also cost-to-benefit ratio of stealth is still in question. If a modern air force wants to attack an adversary with significant anti-aircraft defences, it needs an effective Suppression of Enemy Air Defenses (SEAD) to avoid unacceptable losses. A fast jet that is well armed and highly maneuverable, ‘electronic attack’ aircraft of the class of Boeing EA-18G Growler have ability to jam across the entire spectrum. SEAD maybe much cheaper than stealth.
Technologies are already fast evolving to counter stealth. At US $ 100 million even a small aircraft like F-35 is not cheap. The F-22 Raptor costs nearly US $ 150 million. U.S. Air Force had to shut down the F-22 assembly line after just 187 aircraft. All countries are developing advanced radar systems that will be able to detect these aircraft in the near-future. The Russian T-50 is known to be less stealthy and has faced technology and cost issues. The Chinese J-20 and J-31 parallel stealth programs are shrouded in secrecy, and China was forced to purchase the expensive Russian Su-35 jet fighter. Stealth aircraft also require high maintenance costs and time. Most stealth fleets are known to have high down time. Does Stealth mainly have psychological value, only time will tell.
This article was published in SP’s Aviation, and has since been updated