Stealthy materials - stealth technology for ships
The quest for stealth has required new materials solutions that are increasingly inspired by nature, says Dr Chris Lavers, Head of Sensors and Telecommunications, Plymouth University at the Britannia Royal Naval College, UK.
Surface warships incorporate stealth technology to ensure survival. Stealth’s principal aim is to make ships ‘invisible’ to an array of increasingly smart detection systems such as sonar and radar. It seeks to reduce a platform’s emissions, detection range and vulnerability, and eliminate reflected radiation.
Early stealth ships were developed during the First World War, when Britain attempted to hide ships by painting them grey to blend into their background, with limited success. In 1917, Lieutenant Norman Wilkinson devised a dazzle camouflage paint scheme of coloured blocks and stripes so vessels blurred into the changing sea, sky or coastline. However, this is entirely passive - once painted, patterns cannot be changed. Active methods involve altering appearance in near real-time to confuse enemies.
Encapsulated liquid crystals provide an alternative for ships. Small voltages applied across thin liquid crystal cells changes reflectivity. Combined with natural photonic structures, such as butterfly iridescence, liquid crystals could alter reflectivity in a more controlled manner. Future technologies may use butterfly biomimetics using nanostructured zinc oxide replica wing structures, gold/palladium deposition or heat absorbent materials.
Modern warship radar stealth materials owe much to earlier aircraft. The UK began experimenting with radar absorbent material (RAM) on its warships and Canberra reconnaissance aircraft in the 1950s in response to U-boat radar absorbing snorkel tubes. In 1954 Lockheed, USA, was tasked with developing a CIA spy-plane - leading to the U2 aircraft. The U2 had a network of thin wires with spaced ferrite beads to absorb energy and ceramic wing leading edges. It was composed of fibreglass and honeycomb plastic, skimmed with heat resistant fibreglass.
Lockheed’s F117A Nighthawk was the first operational aircraft to fully exploit stealth. It was based on models of rotating plates in two dimensions. It had honeycomb RAM on wing edges, inlets, exhausts, nozzles, holes, and door seals. There were six skin layers with absorbent adhesives, tapes and putty, and sub-elements of ferromagnetic coatings in a high dielectric plastic.
Dielectrics slow down waves, while ferromagnetic materials absorb them. A high frequency antireflection coating on a low frequency wideband structure absorbs most of the energy, analogous to peacock feathers, where constructive interference coatings rest on black broadband absorbers. The F117’s most obvious stealth feature is its facetted intersecting plated airframe which deflects radar.
Radar stealth consists of three techniques:
• Materials with a low reflection coefficient - plastics, carbon composites and glass-reinforced plastic.
• Radar absorbent materials - foams overlaid with specialist paints. Simple RAM destructively cancels threat reflected waves with quarter wavelength coatings. Surface paint may add carbonyl iron ferrite spheres so waves induce alternating magnetic fields in the paint, converting radar energy into heat. Conductive coatings also create controlled shapes that deflect radar waves.
• Ship geometry. Dihedrals and trihedrals are eliminated as both geometries reflect strongly over wide angular ranges.
Problematically, ships also need to use radar, thus reflecting waves. One solution places radar behind movable panels. Lockheed’s Sea Shadow adopts this approach.
Sweden’s Visby corvette is the first operational naval vessel to use advance analysis tools and fully integrate stealth technology to minimise threat signatures. This includes shaping - inclining the hulls outwards, flattening the superstructure and arranging mast surfaces into truncated pyramids. Flush-mounted retractable antenna with frequency selective surfaces and RAM are used. The external hatches have conductive coatings, while the hull sandwich construction exploits carbon fibres reinforced with plastic for good conductivity and strength.
Active plasma shields can also protect ships. Soviet aircraft have plasma antennae that reduce radar reflections, deflecting waves around platforms. Plasma, like the magnetosphere’s protective shield, can generate invisible shields that extend around ship surfaces, perhaps even protecting them from advanced particle beam weapons.
Invisibility may become possible in parts of the electromagnetic spectrum. David Smith at Duke University, Durham, USA, recently demonstrated the first microwave invisibility ‘cloak’ - preventing radar seeing metallic objects - using metamaterials that could hide ships or missiles from enemy radar. Smith’s metamaterial is composed of peculiar periodic patterns of rings and wires on fibreglass. Metamaterials have a negative refractive index, bending light towards the normal. Microwaves incident on the cloak bend around it, so an observer ‘sees’ waves pass through empty space. Optical and radar metamaterials may help engineer ships that are invisible to human observers and radar, but this remains challenging.
Ships at sea emit significant heat, standing out against cold seas and sky. Thermal sensors operate passively, and, unlike active radar, provide little warning of imminent attack. Non-imaging heat seekers target the middle infrared band (three to five microns) while imagers watch the eight to 14µm range, although developments will see middle infrared imagers. The threat is combatted by reducing heat from diesel and gas turbine engines using oddly shaped air blowers that decrease funnel temperature and mix exhaust gases with cold air. Hot compartments are insulated from the hull and high temperature ducts point up, making them hard to see from the normal viewing direction.
Oil paints are also good infrared emitters so tailored spray-on low emissivity paint is used with added radar absorbent particles.
Another detection risk is the distortion ships makes in the Earth’s magnetic field, which can trigger magnetic mines. Ships can reduce these distortions to levels mines cannot detect by magnetising the hulls in the opposite direction to the Earth’s magnetic field, cancelling the effect. Reverse magnetisation is achieved with hull-embedded electromagnets. It is also possible to design warships from non-magnetic materials such as glass-reinforced plastic.
Nature takes its course
Ultimately, biologically inspired design may solve engineering problems. Plant stems are hollow and complex, yet achieve high stiffness/weight ratios, enabling fabrication of extremely light and stiff strut structures. This could be replicated with pultrusion composites for wind-resisting wind turbines, turbine supports and radar antenna. Radar antenna and aeroplane wings may also benefit from conductive carbon nanofibre coatings to make self-heating composites and prevent icing up.
The lotus leaf’s superhydrophobicity is the perfect model for washing pathogens/chemicals off warship surfaces. Modelling dolphin skin may reduce drag and enhance warship performance while minimising acoustic signature, and intelligent optical fibre surfaces, borrowed from aviation, could warn of cracks and trigger liquid crystals to self-heal, like blood clotting in vascular networks. Liquid crystal drag-line silk converts from liquid to solid as it squeezes down spider spinning ducts, so a similar ‘glue’ could repair damage in ship coatings, turbines and rotary composite blades.