INS Shivalik
Providing stealth features in the design of a warship will entail additional costs. It has been reported that Visby stealth Corvette costs 50 per cent more than a conventionally built Corvette of the same dimensions. The projected cost of the future US Navy DD(X) Zumwalt is $2.8 billion which represents a significant increase. However, the DD(X) will offer full-spectrum signature management to cloak it from a variety of detection and targeting methods. In spite of the higher costs, stealth features are now considered amongst the key requirements around which a warship will be designed and built.
The primary aim of the stealth technology is to render the warship invisible to enemy detection systems…
There has been a significant shift in Warship Design in the past two decades to incorporate stealth. For example, the Shivalik Class frigates in India, Type 45 destroyers in United Kingdom, La-Fayette Class frigates in France, Visby Class corvettes in Sweden and many more. On the other hand, in case of DD(X) Zumwalt Class destroyers in the United States, the very cost of stealth led to decisions in favour of a more traditional tried-and-tested warship. It may be appreciated that a stealth warship cannot provide intimidating power projection off the coast to potential enemies if they do not find/detect that you are there. Moreover, stealth cannot guarantee platform safety and integrity, once the first salvo is fired. We cannot forget the threat from small stealthy fast boats manned by pirates/terrorists as in the instance of the damage done to the destroyer USS Cole.
Nevertheless, there is growing demand to evolve stealthy warships to enhance their survival. The primary aim of stealth is to reduce platform susceptibility, increase survivability and therefore, to prevent damage and reduce demands on defensive systems. Low signatures make the detection of a ship more difficult to locate and lends it more reaction time.
Stealth Technology Aspects
The primary aim of the stealth technology is to render the warship invisible to enemy detection systems such as radar and magnetic sensors by reducing levels of reflected radiation and at the same time, lowering the warship’s own emissions. The various signatures and cross-sections include radar, thermal, acoustic, magnetic, communications, optical & bioluminescence and research is discovering more. Radar cross-section can be reduced by use of radar absorbent materials, use of low probability of intercept transmission, remove corner reflectors, break-up of superstructure and meta-materials.
Meta-materials are artificial composite materials with extraordinary physical properties hitherto not observed in natural materials…
Thermal signatures can be reduced by controlling/cooling the plume and heated parts of the stack, insulating equipment and low emissivity paints. Acoustic signatures can be reduced by resilient mounts, double mounts, rafts, flexible coupling, quiet propellers, sound insulation and streamlined hull design. Magnetic signatures can be minimised by use of non-magnetic materials, GRP and plastics. Communication signature can be reduced by minimising ship’s broadcasts and use of low probability of intercept transmission. Optical and bioluminescence signatures can be reduced by minimising the wake disturbance, lower freeboard, dazzle camouflage and meta-materials.
Radar Aspects
The basic principle of a radar is very simple. Visible light (wavelength 400 nm to 700 nm) is part of Electromagnetic Spectrum covering waves such as X-rays, Gamma rays, ultraviolet radiation which all have more energy than visible light and also waves of greater wavelength such as infra-red, radar and Very Low Frequency (VLF) waves used to communicate with submerged submarines. If these electro-magnetic waves are sent in the form of short pulses which strike an object with flat surface, some of the wave energy will be reflected back to the radar receiver. If the elapsed time from the transmission of a short radar pulse to the time the echo is received is measured, the wave speed allows the contact range to be calculated. All electro-magnetic waves travel at the same speed as the speed of light in vacuum i.e. 3×10 (8 times)m/s. If it is 2ms, the range will be 300km. The size of the target’s image is measured by its RCS and is expressed in square metres. However, this area does not equal its geometric area.
A perfectly conducting sphere of projected cross-sectional area of one metre square i.e. 1.13m diameter, when measured in a test rig will have an RCS of one square metre. Those materials should be incorporated into warship superstructure and outside surfaces which have a very low radar reflection coefficient such as plastics, carbon composites or GRP. RAMs, usually foams which can be overlaid with specified paints, are also included wherever possible. RAM is normally composed of a combination of dielectric and ferromagnetic materials. Dielectrics can be thought of as slowing waves down whilst, ferromagnetic materials will absorb them. RAM has both narrow and wide band absorbing features. Ship geometry is of vital importance. Dihedrals and trihedrals (where two or three surfaces meet together at 90 degrees) must be eliminated at all costs. Both of these geometries will strongly reflect radar energy over a wide angular range directly back to the search radar.
Infra-red stealth is vital in warship design…
Meta-materials are a very recent area of research around the world. Meta-materials are artificial composite materials with extraordinary physical properties hitherto not observed in natural materials. Meta-material engineered composites are tailor-made to have EM properties not found in nature and they share many similarities with photonic crystals, which have periodic structures that allow only certain wavelengths to pass through them. Meta-materials have large potential in stealth applications.
The USA is developing Adaptive Water Curtain Technology (AWCT) to deflect and scatter enemy radar waves from the searching radar system thus reducing the ship’s RCS. The AWCT system consists of highly conductive sea water pumped up and sprayed in a fashion that effectively creates an angled radar reflective spray curtain around the ship for a short period of time. The use of many adjustable nozzles could allow the overall shape of the curtain to be controlled and permit windows of opportunities to operate the ship’s own radar and sensors, and fire various guns and missiles between descending screening curtains of water. The existing pump technology for the control of multiple pumps after some adaptation could be capable of generating a faceted variable angled curtain that could help to reduce RCS and thus defeat the threat of dangerous sea-skimming anti-ship cruise missiles.
For a modern warship, the importance of minimising emitted sound cannot be underestimated…
Infra-Red Aspects
Ships radiate or lose heat from their exhaust plume, funnels, vents, open hatches, recently fired guns, recently embarked helicopters and other features. These platform emissions are radiated into the environment and by virtue of the atmosphere’s transparency, these heat wavelengths may be detected over very great distances. Thermal heat sensor systems operate passively and provide very little warning. The present generation of heat seeking missiles is non-imaging. It is, therefore, easy to defeat heat seeking missile with a more attractive source of heat. If there is enough time to deploy the decoy source.
Heat seeker missiles will target the Middle Infra-Red (MIR) band of the electro-magnetic spectrum in the range between three to five millionth of a metre whilst the true thermal imagers watch and identify in the range of eight to fourteen millionth of a metre. New generation of three to five millionth of a metre imagers will render current heat seeking counter measures less effective. Infra-red emissions can generally be divided into two parts: the so called Infra-Red Cross-Section (IRCS) which is the total emitted power from a target, and the Infra-Red Signature (IRS), which is the target’s detailed distribution of heat emitters. If the ship’s IRCS is adequately reduced, it will enhance the overall effectiveness of the decoy systems.
Imaging sensors can, however, see both the IRCS and IRS allowing for the possibility of actual image and target identification. To avoid identification, it is thus vital to reduce both IRCS and IRS. IRCS contributors mainly include gas turbine exhaust plumes usually operating between 300 to 500 degrees Celsius. The exhaust gases leave the ship’s funnel forming a vertical plume which as it expands begins to cool. Plume shapes and temperatures will tend to vary with different engines and operating conditions with different platforms having relatively well-defined visible plume characteristics. Although gases are generally quite poor infra-red radiators, as the plumes themselves contain extremely hot carbon particles which can radiate extremely well, they are easy to detect. Cold air blowers are introduced to reduce the metal uptake temperature. And to turbulently mix hot plume gases with further cold air as quickly as possible so the plume disperses quickly. Engine exhausts are often separated from funnel’s outer casing, to surface temperatures further. It is usually practical to position engine compartments near the waterline, so the sea surface can provide some measure of masking against sea–skimming missiles.
As infra-red emission is a surface problem, coating ships with low emissivity paint will reduce IRCS significantly. From the law of infra-red emissivity it can be proved that the radiated Intensity (I) is proportional to the Temperature (T) to the power four times. It can be shown that a 10 per cent increase in temperature will result in 46 per cent increase in radiated intensity. Therefore, infra-red stealth is vital in warship design.
Warships are basically large metal objects which concentrate earth’s relatively weak magnetic field within them…
Acoustic Signature & Allied Aspects
Ships are full of equipment generating significant acoustic noise which can be detected underwater when the sound is transmitted through the hull. Sound can travel very long distances under water so that ships can be detected by submarines, torpedoes and sonar dipped from helicopter. The greatest threat to a warship is from the silent submarine and sonar. There are two main types of sonar technology in naval use – passive sonar listening to sounds generated from surface vessels and submarines and active sonar which emits pulses of sound energy from a transducer and then listens for returning echoes. The time between transmission and reception of echo enables a sonar operator to establish the range of the target.
For a modern warship, the importance of minimising emitted sound cannot be underestimated. Any piece of machinery incorrectly mounted can provide tell-tale signs of its presence to the experienced sonar operator. In a combat situation, an active pulse can be detected and hence, passive sonar is used instead. Propellers are the noisiest part of a warship. As the propeller blades spin, they create a region of partial vacuum at the trailing edges. Cavitation bubbles, which first grow rapidly in size and then collapse equally quickly due to the surrounding high-pressure water, form in the lower pressure region behind the blades. When the bubbles collapse, a great deal of energy is released in the form of acoustic shock waves which can even generate light through sono-luminescence.
One way to reduce the collapsing cavitation bubble noise as practiced by the US Navy is to inject low pressure air into the partial vacuum behind the rotating blades, which reduces the pressure difference between the bubbles and surrounding water so that the bubbles collapse more slowly and more quietly. Also, there is great emphasis placed on propeller blade design to reduce the effect of singing, blade rate noise and cavitation. Modern propulsion units have reduced cavitation at high speeds with all moving parts housed in ducts so very little sound is radiated directly into the marine environment with a more laminar flow.
Radar systems now mounted on satellites are sensitive enough to detect ship wakes…
The key sources of noise that contribute to a warship acoustic signature include its own active sonar transmissions, machinery both propulsion and auxiliary, flow fluid in distributed pipeline systems and cavitation of propellers. Sonar transmissions are minimised by reducing the time and sonar power levels. However, most of the machinery-radiated noise is provided by ship’s diesel engines, gas turbines which cannot be readily turned off. It is necessary to employ great care to incorporate noise suppression measures to achieve low target noise levels. These include resilient mounts, double mounts, rafts, flexible coupling and quiet propellers. In case of submarines, rubber or polyurethane tiles having broad and narrow band characteristics with multiple layer coatings are tailor-made against enemy sonar. One successful method to silence noisy machinery in the future could be to simply drill holes in the casing that surround it. Work is going on to develop this in France.
Magnetic Signature Aspects
Warships are basically large metal objects which concentrate earth’s relatively weak magnetic field within them and create distortion or magnetic anomalies which potentially have the ability to trigger the detonation of magnetic mines. Warships can now reduce these magnetic distortions to low enough levels so that magnetically triggered mines can not detect them. This can be done by magnetising the ship’s hull in the opposite direction to the earth’s magnetic field, cancelling out the effect. Reverse magnetisation is normally achieved by using hull–embedded electro-magnets. It is also possible to design warships from non-magnetic materials such as GRP, vitally important to mine-hunters.
Deperming is now viewed as a procedure for erasing the permanent magnetism from ships and submarines, in order to camouflage them against magnetic detection vessels and enemy marine mines. Sea-going metal hulled ships will nonetheless develop a magnetic signature as they travel due to interaction with earth’s magnetic field and this signature can still be exploited by magnetic mines or facilitate detection of a submarine by ship or aircraft with Magnetic Anomaly Detection (MAD) equipment. Navies use deperming procedures as a counter-measure against this.
A warship’s stealth should not be compromised by operator error…
When a warship is close to a magnetic mine or magnetic torpedo, the magnetic field of the warship actuates the firing mechanism causing the mine or torpedo to explode. Degaussing is thus a fitted electrical installation designed to protect warship against magnetic mines and torpedoes. The purpose of degaussing is to counteract the ship’s magnetic field and establish a condition such that the magnetic field near the ship is as nearly as possible the same as if the ship were not there. Degaussing coils are simply large diameter electrical wires which when carrying a large enough current are able to produce an electro-magnetic field.
The US has developed High Temperature Superconducting (HTS) degaussing coil system which works by encircling the vessel with superconducting ceramic cables whose purpose is to neutralise the ship’s magnetic signature. The main advantage of the HTS Degaussing Coil System is its greatly reduced weight and increased efficiency.
Bio-luminescence Aspects
The night time wakes of the ocean going ships are frequently observed for their bio-luminescence flow fields. Ship-wake bio-luminescence is well known and in fact, the last German U-boats detected in World War I was sunk because it created a significant bio-luminescence footprint. Bio-luminescence can give away submarine position to a vigilant enemy. As such, the bio-luminescence flow simulated light emission is now considered a real threat to maritime stealth operations. Marine bio-luminescence is produced by a vast number of creatures including bacteria, dino-flagellates, radiolarians, jellyfish, hydrozoa, sea pens, sea pansies and comb jellies. Bio-luminescence is especially abundant in warm coastal regions where nutrients are abundant and marine life thrives.
Overall electronic stealth design has become extremely sophisticated, encompassing numerous signatures and cross-sections…
Wake Effect Aspects
A ship on surface will generate a wake which not only persists for a long time but also easy to see at high altitude and even on satellite imagery. In incompressible liquids such as water, a bow wake will be created when a warship moves through the medium. As water cannot be compressed, it is displaced instead, resulting in a wake. The wake spreads outwards from the source until its energy is sufficiently dispersed. The wake bubbles are a real problem which can persist for up to a kilometre behind the warship.
Radar systems now mounted on satellites are sensitive enough to detect ship wakes. The US Navy has recently patented a technique that may make warship harder to detect by eradicating the bubbles as soon as they appear. In a test to remove bubbles, several transducers injected 1 MHz acoustic waves into the water and the bubbles floated to the surface. It is opined that active production of 1MHz acoustic waves into water is perhaps not a practical stealthy solution.
Extremely Low-Frequency Signature Aspects
Galvanic currents flowing in the hull and in the water around the hull can generate underwater electrical potentials. Under certain conditions, these can cause ELF electrical fields to be radiated into the water. Detection of ELF signatures can be prevented by certain basic electrical design measures. Recent research combining ELF electrical signals recorded along with those of hydro-acoustic detection is even now producing a greater level of sophistication in target detection and identification.
Likely Future Cross-Sections in Stealth Technology
Maritime sensing could be revolutionised in the future with developments in ultraviolet spectrum and ELF waves. Moreover, warships may incorporate more natural or synthetic materials such as multiple wavelength coatings that are becoming increasingly necessary notably of visible, infra-red and microwave wavelengths to provide optimum response across all threat wavelengths. Active RAM could be an attractive possibility for the future or a surface skin could act as a large area living ear to listen for long range search radar pulse characteristics and then re-radiate low amplitude but out-of-phase pulses to cancel the enemy radar echoes.
The ideal of a warship which cannot be detected is unlikely to be achieved across all energy bands concurrently…
In the future, technology may also utilise the abundance of water in the maritime environment for various activities such as directed walls of water for close range missile defence, mist sprays to defeat the electro-optics of future sophisticated guided missiles and to provide ice packs on Arctic Patrol vessels to insulate heat from external thermal imagery detection. It is likely that satellite tracking of ships in visible, near infra-red, radar and thermal bands will become more common place over the next decade.
Conclusion
Overall electronic stealth design has become extremely sophisticated, encompassing numerous signatures and cross-sections. The ongoing one upmanship between offensive and defensive capability will continue to encourage this trend, requiring design of further stealthier warships. The ideal of a warship which cannot be detected is unlikely to be achieved across all energy bands concurrently. Warships will be less likely to be detected if signatures are minimised. This also increases the probability that decoys and electronic counter measures will be more effective against enemy threats.
In most cases, the monitoring of emissions is simple, comprising easily applied safety issues which can readily counter any unwitting ignorance such as use of mobile phone on the quarter deck in combat situation. At the same time, sophisticated electronic systems are quite capable of generating fake or pre-recorded radio emissions to saturate the airways and saturate the enemy’s ability to handle the communication traffic.
With so many possible different types of sensor emissions from a warship, be they active signatures or passive cross-sections, it would be imperative to consider the impact of communications equipment used onboard and the overall coordination of all of the warship’s sensors and communications to have a transmission control policy in place at all times to ensure that all emissions are kept as low a level within operational constraints. A warship’s stealth should not be compromised by operator error.
Providing stealth features in the design of a warship will entail additional costs. It has been reported that Visby stealth Corvette costs 50 per cent more than a conventionally built Corvette of the same dimensions. The projected cost of the future US Navy DD(X) Zumwalt is $2.8 billion which represents a significant increase. However, the DD(X) will offer full-spectrum signature management to cloak it from a variety of detection and targeting methods. In spite of the higher costs, stealth features are now considered amongst the key requirements around which a warship will be designed and built.
References
SHIVALIK – India’s New Generation Warship – Vice Admiral Rajeshwer Nath, IDR Apr-Jun 2010
Technological Innovations in the Design of Warships for the 21st Century by Vice Admiral Rajeshwer Nath, IDR Sept. 2004.
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