In the recent decades, the world has invested heavily in terms of time and energy on stealth technology in an endeavour to delay detection of military airborne platforms by the adversary. Most new platforms are designed by incorporating some level of ‘stealth’ technology at the initial design stage itself. Even existing jet fighters are, at times, are modified to reduce their signature. Creating a stealthy design comes at a price in terms of aerodynamic and other design compromises, long period to perfect the design or high cost of development. Some stealth features require special maintenance. Specific aircraft shapes may also limit weapon and fuel carriage capability or may not be the best for flight efficiency. Any drop tank or weapon load carried externally, increases the radar cross section (RCS). Lockheed F-117 Nighthawk was the first operational aircraft specifically designed with stealth technology. Other current stealth aircraft include the B-2 Spirit, F-22 Raptor, F-35 Lightning II, Chengdu J-20, Shenyang FC-31 and 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 TECHNOLOGY

Stealth is essentially a combination of several technologies all of which together greatly reduce the distances at which an aircraft can be detected. It involves reduction of RCS, acoustic signature, thermal imprint and other aspects. The term ‘Stealth’ became popular when the F-117 stealth fighter was deployed in the Gulf War in 1991.

Aircraft part that contribute significantly to the echo is the vertical stabliser. The F-117 has tilted tail surfaces to reduce 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 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 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 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 edge of the wing and the tail plane when set at the same angle in F-22 Raptor, reduced reflections. 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 to have 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.

The F-22 and F-35 claim that they can open their bays, release munitions and return to stealthy flight in very short period

REDUCING RADIO FREQUENCY (RF) EMISSIONS

Aircraft must avoid radiating any detectable energy such as from onboard radars, communications systems or RF leakage. Many aircraft use passive infrared and low light TV sensors to track enemy aircraft and aim weapons. The F-22 has an advanced low-probability-of-intercept radar (LPIR) which can illuminate enemy aircraft without triggering the radar warning receiver.

RCS

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 have an RCS comparable with small birds or large insects.

RADAR STEALTH COUNTERMEASURES

Shaping alone offers very few stealth advantages against low-frequency radars that 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 as in bistatic or multi-static radars.

ACOUSTIC SIGNATURE

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 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.

VISUAL SIGNATURE

The visual signature is best reduced through camouflage paint or other materials to colour and break up the lines of the aircraft. Most aircraft use matte paint and dark colours. 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.

INFRARED SIGNATURE

Reducing aircraft heat signature is required to prevent IR sensor tracking. The exhaust plume contributes a significant infrared signature. One way to reduce IR signature is to have a non-circular slit-shape tail pipe, to reduce the exhaust cross section area and maximise 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.

Radar-evading warplanes require careful design work, extensive testing and exotic materials for their construction, features that can triple their cost compared to nonstealthy planes

INFRARED SEARCH AND TRACK (IRST)

IRST systems may be effective even against stealth aircraft as its surface heats up due to air friction and a twochannel IRST can compare difference between the low and high channels. Russia had IRST systems on MiG-29 and Su-27 in 1980s. 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, IRST allows detection of non-afterburning aerial target at 45 km range. The Lockheed F-21 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 brief 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.

REDUCED PAYLOAD

Stealth aircraft carry 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 aircraft. This apparent disadvantage however, is offset by the reduction in fewer supporting aircraft that are required to provide air-defence suppression and electronic counter measures.

TACTICS

A heavily defended site normally has overlapping radar coverage, making undetected entry by conventional aircraft 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 minimises radial speed while presenting the lowest-RCS aspects of the aircraft to the threat radar. There are other flight tactics such as manoeuvres combined with chaff dispensation to confuse enemy radars.

OPERATIONAL USAGE OF STEALTH AIRCRAFT

The US and Israel are the only countries to have used stealth aircraft in combat. In 1990 Gulf War, the F-117s flew 1,300 sorties and scored direct hits on 1,600 high-value targets in Iraq. Only 2.5 per cent of the American aircraft in Iraq were F-117s, yet they struck 40 per cent of the strategic targets with 80 per cent success rate. In Yugoslavia in 1999, F-117 and the B-2 Spirit stealth bomber were used. One F-117 was shot down by a Serbian S-125 ‘Neva-M’ missile. The B-2 Spirit destroyed 33 per cent of selected Serbian targets in the first eight weeks of US 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 and B-2 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 US-led coalition against ISIS. In 2018, Israeli F-35I stealth fighters conducted a number of missions in Syria and even infiltrated Iranian airspace undetected. The Chinese J-20 entered service with their Air Force in 2017. The FC-31 is still under development. The Russian Su-57 stealth 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 ionised gas to reduce RCS. Interaction between electromagnetic radiation and ionised 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 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 airflow, 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

Several stealth aircraft are under development. The Russian MiG-41 is 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 US F-35B. Indian Advanced Medium Combat Aircraft will be a stealth fifth-generation aircraft. Northrop Grumman B-21 Raider 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 programmes. KAI KF-X is a joint programme between Indonesia and South Korea. HESA Shafaq is an Iranian stealth aircraft project. Turkey plans to develop TAI TFX to replace F-16s. 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 Light Combat Helicopter and Eurocopter Tiger are reported to have stealth features.

Technologies are already fast evolving to counter stealth. All countries are developing advanced radar systems that will be able to detect these aircraft in the near-future.

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 hardpoints 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 Defences (SEAD) to avoid losses. A fast jet that is well armed and highly maneuverable, ‘electronic attack’ aircraft of the class of Boeing EA-18G Growler has ability to jam across the entire spectrum. SEAD may be much cheaper than stealth.

Technologies are already fast evolving to counter stealth. At $100 million even a small aircraft like F-35 is not cheap. The F-22 Raptor costs nearly $150 million. US 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 programmes are shrouded in secrecy and China was forced to purchase the expensive Russian Su-35. Stealth aircraft also require high maintenance costs and time. Most stealth fleets are known to have high down time. Whether or not stealth primarily has a psychological value, only time will tell.