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Designing surface ships for the next decades.

Designing Surface Ships for the Next Decades

Ships and Weapons for the 1990s

A revolution in surface warfare has been in progress for more than 40 years, yet that revolution is still largely misunderstood. Even those who buy warships and operate them are sometimes puzzled by what has happened. Many commentators question the need for surface warships, suggesting that missiles and submarines have made them obsolete.

The truth is that the surface warship is as important as it ever was. The threat has, however, escalated rapidly, pushing up the cost of defending the surface combatant. This process is nothing new: eighty years ago the torpedo had the same effect on the size and cost of the battleship. Electronics have had an extraordinary impact on ship design, changing the traditional ratio between hull cost and equipment cost. By the 1960s the question was no longer, how fast should a warship be, or how much protection should she have, but how much armament should she carry? An even more important question was, how much command and control equipment should she have?

Current warship design is driven by the same two imperatives which have prevailed since the Second World War: anti-submarine warfare (ASW) and anti-air warfare (AAW). All that has changed is the potency of submarines and standoff anti-ship weapons. The first impels developments in hull forms and propulsion, the second forces the pace in defensive weapons and electronics.

Progress in Propulsion

Ship designers and naval architects are hobbled by the same constraints as other engineers. Expansion of a design in one direction inevitably creates pressure in other directions. Modern warship hulls are deeper, so as to provide more internal volume for weapons and electronics, which in turn dictates increases in beam to maintain stability. This combination makes the hull heavier and more resistant, so that machinery must be more powerful to achieve the designed speed. Time and again designers try to break out of the vicious circle, using different combinations of propulsion system and hull forms, but the laws of physics and hydrodynamics are immutable.

Today's surface combatants, whether designated cruisers, destroyers or frigates, owe much to previous generations of destroyer designs rather than the armoured battleships and cruisers of 50 years ago.

An age-old problem for escorts is the fact that big ships hold their speed in rough weather, whereas smaller ships are forced to slow down to avoid damage. In the 1950s the US Navy continued to build fast air-defence destroyers to screen the fast carrier task forces, but the difficulties of finding sufficient internal volume for fuel proved insuperable. One way out was to adopt lighter steam turbines and boilers, with higher steam pressures and temperatures to reduce fuel consumption. But this proved unsuccessful since reliability decreased when weight-savings were taken too far.

One of the most important discoveries of the post-1945 period was that high speed was of limited tactical value. A modest attainable speed (25--30 knots) was adequate for ASW escorts, and produced significant savings in fuel and maintenance. An important reason for this was the discovery that sonar was of very little use at speeds over 25 knots. False intelligence reports about the high speed of "Whiskey" class submarines had created a requirement for frigates capable of speeds equal to those of destroyers (32--35 knots). Later experience was to show that long-range weapons were more cost-effective than high speed as a means of countering fast submarines.

The correctness of this course was vindicated by the advent of nuclear submarines. Within a few years the USS "Nautilus" and her successors were to invalidate any hope of matching the speed of the hunter to the hunted. Helicopters and standoff weapons are much faster than any submarine. The money is much better spent on improving the platforms for such weapon systems. This means better sea-keeping, enhanced stability and higher endurance.

It is all the more ironic that Western navies lost interest in high speed at about the time when the gas turbine came on the scene. A "navalized" gas turbine, particularly the light and compact aero-derived type, offers a high power-to-weight ratio and is capable of driving warships at impressive speeds. In practice, however, engineers found that driving a modern frigate at 30 knots in Sea State 4 was even more difficult than driving a World War II destroyer at 36 knots in smooth water. The difference is illusory, however, because modern warships are only theoretically slower than their predecessors. Their effective speeds are generally higher, and in fuel consumption, propeller efficiency and quietness modern surface ships are much more efficient.

One noticeable characteristic of modern surface combatants is the way in which they achieve their sea speeds (in normal weather and loaded) on less power than traditional destroyers. This is in part due to improved hull forms and propellers, which in turn reflect changes in mission. Destroyers were long and narrow to maximise speed, so as to be able to keep up with the battle fleet and to overhaul enemy capital ships. Post-1945 escorts were given slow-turning propellers and comparatively beamy and deep hulls to maximise sustained speed in rough weather.

Another factor influencing the design of powerplants is silencing. With slow-turning propellers there is less cavitation. The "singing" of propeller blade tips alerts hostile submarines and drowns out sonars. The classic solution to endurance has long been the diesel engine, but it is unpopular among ASW specialists, who prefer the rotary action of the gas turbine. It is widely felt that radiated noise and vibration are more easily suppressed in gas turbine plants.

The need to silence machinery is one of the many hidden costs which make warships more expensive tonne-for-tonne than merchant ships. It is the elaborate methods used to isolate machinery from the hull, the design of the propellers and the provision of measures to reduce flow noise, as much as the choice of powerplant which complicate the task of the warship designer. An unsilenced ASW ship cannot, for example, operate the latest ASW sensors, whereas a non-ASW ship such as an air defence destroyer or an offshore patrol vessel can dispense with these refinements.

The gas turbine has significantly influenced the design of warships. Its insatiable demands for air dictate large intakes, and equally large uptakes to disperse the exhaust gas. Traditional steam plants absorbed most of the heat generated, leaving comparatively cool waste products to be dispersed through the funnel. Gas turbines, in contrast, absorb only 10 percent of the waste heat. Designers of the first-generation gas turbine ships found radio aerials melting from the heat, and special means were adopted to cool the exhaust gas. There is also the tactical requirement to reduce the ship's infrared signature.

The lightness of aero-derived gas turbines makes for easy replacement. Using carefully planned exit routes (usually the air intakes or exhaust uptakes) it is possible to remove a gas turbine in a few hours. European navies use a Gas Turbine Pool to support the Rolls-Royce Olympus turbine, allowing time-expired or faulty engines to be returned to the pool for refurbishment or repair.

One of the most important advantages of gas turbines and modern diesel engines is the way in which they have simplified the design of ships' propulsion systems. In the heyday of steam naval architects were encouraged to design each successive class with higher speed. This could be achieved more easily with steam turbines and boilers (at a cost), but gas turbines are derived from a restricted range of aero engines. Although they can be uprated to increase output, the volume remains constant, giving the designer one stable element through a whole series of designs. This makes the task of controlling size and cost much easier.

Hull Form

Because there is no wide divergence between the missions of modern cruisers, destroyers and frigates, they all have much in common, within each navy's overall design philosophy. Draught tends to be deeper than it used to be, providing much-needed volume and improving rough weather performance. Attempts were made in the 1950s and 1960s to limit the growth in size by building superstructures of light alloy, but experiences in Vietnam, the Falklands and elsewhere have persuaded designers to revert to the safer traditional expedient of increasing beam.

Different combinations of beam, depth and underwater hull form will not reduce costs dramatically. Assuming equal displacement, the cost of steel and fabrication remain the same, and minor savings achieved by novel methods of assembly will result in only minor savings. Some 90 percent of the total cost will be absorbed by machinery, weapons and systems. Put another way, "more bangs per buck" means greatly increased cost, whatever panacea is offered by designers, shipbuilders and weapon salesmen.

For the future there are several interesting lines of development. The consensus among designers is that if a navy wishes its ships to attain much higher speeds it should invest in something much more radical than a conventional displacement hull.

Hydrofoils and air cushion craft of various kinds exist in considerable numbers. Both rely fundamentally on eliminating hull drag to permit a dramatic increase in speed. But they are expensive, and show no sign of fulfilling their early promise.

More promising is the Small Waterplane Area Twin Hull (SWATH), which combines low drag with high stability. It is favoured as a helicopter platform, and the US Navy's prototype has operated a naval helicopter successfully. SWATHs manoeuvre poorly at high speed, and at cruising speed require more power than conventional ships, but at high speed are much more economical than air cushion craft of hydrofoils.

The Demands of ASW

The submarine remains a major threat to surface warships and it is hardly surprising that ASW requirements continue to dominate the design of most combatants. Hull-mounted sonars have been overtaken by bow-mounted transducers and towed arrays requiring large winches. The helicopter, initially embarked as a weapon delivery system, now functions as an extension of the ship's own weapon systems.

The basic requirement for a helicopter is a flight deck, but as experience shows that the machine is very susceptible to saltwater corrosion, a hangar is desirable, both as protection against the elements and as a workshop and armoury.

For many reasons the after-end of a ship is the most convenient position for a flight deck and hangar, but this creates its own problems for the ship designer. Hangars are wind-traps, and the fuel and weapons they shelter impose safety requirements. The deck must be adequately lit for night operations, fire-fighting equipment must be provided, and some form of mechanical hauldown or securing systems is essential. As helicopters get bigger they encroach further on deck space and internal volume.

The towed sonar array is the latest burden inflicted on ship design. The requirement to tow a long array (up to 1800 metres long) dictates a powerful winch aft. It also generates enormous quantities of data (a pair of towed-array ships working with maritime patrol aircraft can detect contacts across an area of 250 000 square miles). Such vast amounts of data must be processed, dictating massive computer capacity and additional display capacity, i.e. increased internal volume.

Is Air Defence Affordable?

For most navies the air defence cruiser (CG) and destroyer (DLG or DDG) represent the largest type of combatant which can be contemplated. Very few navies can afford the price, so that anti-air warfare ships are comparatively few in numbers.

In the US Navy production is currently centred on the Aegis cruiser and the new, smaller "Arleigh Burke" class destroyer. The Aegis programme now runs to 15 units in commission, nine in various stages of construction and the last three funded. They are divided into five distinct sub-classes, reflecting the priority given to upgrading the Aegis system lying at the heart of the design. Briefly, Aegis uses a multi-function phased array radar, the SPY-1 series, and massive computing power to handle saturation missile attacks.

As the target of 27 "Ticonderoga" (CG-47) class Aegis cruisers by 1993 draws near, production is switching to the smaller "Arleigh Burke" (DDG-51) class destroyer. Although superficially resembling the Aegis cruisers these 8 400-tonne ships incorporate many improvements. Apart from alloy funnels, the superstructure is steel. Some 70 tons of Kevlar armour protects important areas, and they are the first American warships with a "collective protection system" for NBC defence.

Some measure of stealth is provided by sloped surfaces and rounded edges to reduce the radar echoing area. Special measures are also taken to reduce the infrared signature. For the first time for many years the Combat Information Centre (CIC) is below the waterline. All electronics are hardened against electromagnetic pulse.

Although the average cost of $ 700 million per ship is not cheap by any but American standards, they are looked on as all-round anti-submarine warfare and anti-air warfare escorts, packing a powerful offensive punch as well. Armament includes vertically-launched Standard SM-2 medium-range surface-to-air missiles, Tomahawk cruise missiles, Harpoon antiship missiles and the ASROC anti-submarine missile. Propulsion is similar to the installation in the Aegis cruisers i.e. four LM-2500 gas turbines coupled to two shafts.

The lead ship "Arleigh Burke" was delayed by a strike at Bath Iron Works, but was finally launched in September last year. She will be handed over in 1991. Four more are under construction and three were ordered in December 1988.

The only comparable European design is the French F70 type frigate or DLG. Known as the "Cassard" class, only two have been funded so as to allow time to develop the Aster missile under the FAMS programme, with European collaboration. The "Cassard" and "Jean Bart" are armed with the Standard SM-1 missile but this is now out of production and the SM-2 version proved too expensive for the French Navy.

The British Type 23 or "Duke" class frigate is an outstanding example of a modern ASW frigate, combining high capability with comparatively low cost. The major requirement of the design being to optimize towed array operations, great attention has been paid to silencing the machinery. This has led to a unique CODLAG (Combined Diesel Electric and Gas Turbine) design. Rolls-Royce Marine Spey gas turbines provide main drive, while diesel generators drive electric motors for low-speed drive.

The lead ship, HMS "Norfolk", has just been accepted from her builders and six more are in varying stages of construction. Nine more are expected to be ordered by the end of this year.

When France, Great Britain and Italy pulled out of the NFR-90 frigate project last September the three navies were expressing their impatience with the awkward timetable. The German Bundesmarine had already inflicted a near mortal wound on the project by ordering four Type 123 frigates to replace elderly destroyers, and the US Navy has intimated that it will abandon NFR-90 if the number of participants goes below its present figure of five.

The Bundesmarine is reported to be considering the construction of an anti-air warfare version of the Type 23, with an enlarged hull.

The Royal Navy also wants an anti-air warfare design to replace its 12 ageing Type 42 DDGs by the end of the 1990s. The main difficulty, the one which continues to hamstring the NFR-90 project, is the failure to select a new-generation medium-range surface-to-air missile. The FAMS is based on the French Aster, but the alternative NATO AAW System (NAAWS) is built around a non-existent missile.

The Missile Threat

Surface warfare tactics have been totally altered by the anti-ship missile. Cruise missiles can now be launched hundreds of miles away, and even anti-ship missiles like the Harpoon are having their range doubled. These weapons can be launched from submerged submarines, aircraft and surface ships, complicating the tactical picture.

Even ASW escorts must be capable of defending themselves against the missile threat. This means a comprehensive electronic warfare capability and effective "hard kill" weapons, particularly close-in weapon systems (CIWS). It is comparatively easy to retrofit offensive weapons - particularly missiles - but effective modern defensive systems are bulky and expensive.

Technology is coming to the rescue with lighter multi-function radars capable of handling hundreds of tracks. Conventional explosive weapons such as surface-to-air missiles may eventually be replaced by "electronic guns" or laser weapons capable of directing energy at incoming missiles.

Fired by the example of glasnost and perestroika Western governments may deny navies the funds needed to match a seemingly receding Soviet threat. However, the Gulf War showed that potent weapons in the hands of Third World countries pose just as deadly a threat to ships.

The potency of the cruise missile helps to explain why the US Navy has chosen to recommission its four "Iowa" class battleships. Each battleship is armed with 32 Tomahawk cruise missiles in addition to nine 16-inch guns. As the core of a powerful Surface Action Group (SAG), the battleship poses a threat which no hostile force dare ignore. Defence of the flagship rests mainly on her escorts, but she can also defend herself by means of electronic warfare countermeasures and CIWS Gatlings.

In effect the Surface Action Groups strengthen the US Navy's main striking force of 15 Carrier Battle Groups at a fraction of what it would cost to build four more nuclear carriers. The battleship conversions make up for the cancellation of the four nuclear Strike Cruisers (CGSNs) ten years ago.

As this brief survey shows the surface combatant is far from finished. Despite the claims made for air power and submarines, surface ships are the only guaranteed means of commanding the surface of the sea. Shipping must cross the oceans safely in time of peace, and in time of war power in the NATO context can only be projected across the sea. Personnel can fly by air, but fuel, raw materials and heavy weapons can only be transported in large numbers if they go by sea.

It is worth remembering that in 1973 it took the US Airlift Command all its C-5 Galaxy transports to airlift 50 tanks to make good Israel's losses on the Golan Heights, but the ammunition to replenish Israeli war stocks went in one merchant ship.

PHOTO : The NFR-90 project is in trouble owing to the eight partners' failure to resolve the

PHOTO : missile and command systems problems.

PHOTO : Even the design of small support aircraft carriers like the "Principe de Asturias", shown

PHOTO : here, is highly specialised.

PHOTO : The USN's new Wasp (LHD-1) assault ships are quasi-carriers, with large flight decks, but

PHOTO : complicated by the addition of a docking well.

PHOTO : The Soviet missile cruiser "Slava" is driven by two-shaft gas turbines at some 34 knots

PHOTO : and packs 16 SS-N-12 anti-ship missiles.

PHOTO : The change to vertically launched missiles eases the designer's problems by releasing

PHOTO : between-deck space.

PHOTO : RRS "Vigilance" is one of six 62-metre corvettes built by Singapore Technologies Marine.

PHOTO : Strike craft are inexorably growing in size.

PHOTO : Although offshore patrol vessels like the Italian Navy's new "Libra" (above) are cheaper

PHOTO : than ships of the line, they still need room for wartime conversion.

PHOTO : MCM craft are expensive because of the need for complex sensors and silencing. This is

PHOTO : Vosper-Thornycroft's HMS "Sandown".

PHOTO : The USN's "Adventurous", a T-AGOS (Ocean Surveillance) ship uses a towed sonar array to

PHOTO : monitor submarine movements.

PHOTO : Helicopter operations impose special burdens on ship design, especially the larger

PHOTO : machines like this Sea King HAS.

PHOTO : Close-up of Widney Aish's WARRANT 76 mm naval decoy system. Where to put it? Another

PHOTO : design problem.
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Author:Preston, Antony
Publication:Armada International
Date:Dec 1, 1989
Words:3236
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