Practically everyone has heard of stealth aircraft which are invisible to radar. Not so widely publicised is how stealth technologies are influencing new warship designs.
Discussions with shipyards and papers presented at the recent Imdex naval exhibition in Singapore shed much light on the present state of this secretive discipline.
British shipyards Yarrow Shipbuilders and Vosper Thornycroft are in the vanguard of naval stealth design, while BaeSEMA is a new entrant to the field.
The largely Yarrow-designed Type 23 frigate in Royal Navy service displays some stealth features, though by no means as dramatic as those included in more recent designs. Vosper recently built two corvettes for Oman, which have many stealth features.
But more innovative concepts have been designed, notably Israel’s US-designed Sa’ar 5 corvettes. Yarrow unveiled its Warship concept for a stealthy vessel at Imdex.
Last year Vosper presented its remarkable Sea Wraith corvette concept and at the Imdex exhibition, it unveiled a 95m `stealth corvette’ which shared many of the Sea Wraith’s features.
Last year BAeSEMA revealed its Cougar corvette, designed for stealth and economy, since it could be built largely to mercantile construction standards.
Vosper and BAeSEMA seem to disagree on what is needed for stealth, so what exactly does a warship need to be stealthy?
Chester Allen Hard, director of naval architecture at the US’s Ingalls Shipbuilding which built Israel’s Sa’ar 5 corvettes, says the devil is in the detail. In a paper for Imdex, he says design for stealth is `primarily a problem in energy management’.
Same spot in space
Most `sensor systems involving radar wavelengths operate with the transmitting and receiving elements virtually co-located in the same spot in space’, Hard wrote. `For every pulse sent out, the resulting indication of an object is the fundamental reflection of that pulse, travelling back to the receiving element’.
To minimise a hostile radar’s ability to process a returning signal accurately `involves managing the energy that first hits the reflective surface’. One approach is to make the reflected energy too weak to be received and discerned beyond background noise.
This can be achieved by special radar-absorbent materials, though Hard believes they are `limited in the amount of energy reduction, or attenuation, that they achieve’. These materials are also separate from the structural materials needed to build the basic ship. `This leads to considerations for attachment, maintenance and durability over time which are very complex problems to solve in the tough operating environment of a surface ship.’
Another way of solving the problem is by managing reflected radar energy so it does not follow the incoming energy’s reciprocal path.
`At one kilometre, a one degree difference in angle means the reflected energy misses the receiving element by almost 35 metres. At 10 degrees, the miss becomes over one third of a kilometre.’ This principle is the essence of recent warship designs with multiple-faceted surfaces to reflect radar signatures far away from the radar.
Another imperative is to muffle noise, whether machinery, propellers or hydrodynamic, created by irregular water flow around a hull.
Other problems highlighted in this and other papers are the shape, arrangement and location of weapons and electronics and necessary ship fittings like boats, anchors and cables. The stealthiest design is one where the crew do not work too much on deck, requiring railings and safety arrangements.
For instance, on the Yarrow warship concept, one innovation is to have retractable platforms for semi-rigid inflatable boats, complete with cranes, installed within the hull just below helicopter flightdeck level.
On the Sa’ar 5, Sea Wraith and Yarrow’s Warship vertical-launch missile systems are fitted either flush with, or below, deck level, while box-launchers for anti-ship missiles are hidden behind shielding and are not directly visible to radar. Even guns are being transformed by new faceted mountings, while anchors are being hidden behind doors.
`It is not uncommon to find a single weapons mount which, when left untreated, has a signature larger than the total signature of a [stealthy] patrol craft,’ the Ingalls paper said. An example is a tracking radar and its pedestal which must be able to turn to `look’ in the direction of the threat it is tracking.
Sweden’s CelsiusTech presented an Imdex paper on reducing the radar cross-section of a tracking radar and its pedestal, producing an interesting idea of putting a frequency-selective surface in front of the antenna. The surface filters incoming radar signals according to frequency and incidence angles and polarisation. In this way, only the ship’s radar signal returning from the target it is tracking will be received by the antenna. The pedestal would also be faceted, minimising radar cross-section.
Another way of reducing a ship’s radar cross-section is for radar aerials to be fixed items on the superstructure. An example is the installation of the SPY-1D phased array radar on recent US cruisers and destroyers. Four fixed-flat plate radar aerials are fitted to different parts of the superstructure to provide all-round coverage.
While these ships were not designed with stealth in mind, such fixed radars, like the fixed active array radar fitted to the mast superstructure on Yarrow’s warship concept, are common on new stealth warship designs.
The Ingalls paper was implicitly dismissive of stealth materials such as radar-absorbent materials, as was a paper by Yard UK, a part of BAeSEMA. The Royal Navy made extensive use of such materials during the Gulf War.
The Yard paper observed that absorbent materials are `rarely used as a primary radar cross-section reduction measure in new warship designs. It is sometimes used as a palliative treatment on existing ships. The addition of radar absorbent materials can lead to stability problems.’
BAeSEMA and Vosper have different approaches to designing for stealth. Vosper’s Sea Wraith concept uses all conceivable means of cutting radar cross-section and infra-red signature. These include an array of water nozzles that spray a fine mist to envelop the ship.
Vosper says tests have shown that a ship’s hot spots `can be reduced considerably by using this innovative method of disguise’.
The mist, using sea water, is also claimed to have the effect of blending the ship’s infra-red signature and visual appearance with the background sea environment. The system does require a lot of internal piping, weighing several hundred tonnes according to Vosper’s Brian Spilman, head of the firm’s future projects group.
Sea Wraith also has a retractable mast for navigation lights, communications and other sensors. This means the ship could selectively vary its radar cross-section as required. Sea Wraith’s complex superstructure, with two asymmetrically positioned towers, `can confuse an incoming missile’, generating `radar disturbance’ to its control system, Vosper says.
Vosper has also gone in for special paints and composite materials with conducting surfaces, while a host of infra-red sensors identify hot spots and alert the mist system and other decoys.
Malcolm Bird, warship prime contract business development manager at BAeSEMA, is dismissive of Sea Wraith’s exotica and says the less extensively stealth-fitted Cougar can do the job just as well.
`We believe that in this design we’ve got a very low signature and to do more is not worth the candle.’ Thus Cougar has no special paints or radar absorbent materials, though it solves the problem of engine and intake emissions by powering the corvette with waterjets whose signatures are below the waterline.
One key aim (and by-product) of some stealthy designs is a small crew, as extensive automation and design for shoreside rather than underway maintenance, economises on crew. Bird presented this consideration in another paper for Imdex, showing how BAeSEMA had demonstrated this in Cougar. `An underlying philosophy with the Cougar corvette is that where possible maintenance should be done ashore,’ the paper said.
So investing in stealth may change the nature of sea warfare because warships will no longer be designed for long, intense operations and `showing the flag’ in peace time, but more for short engagements in which stealth will be crucial.