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Power Growth in Naval Surface Combatants

Commentary, 11 April 2005
Maritime Forces, Technology, Europe
Installed power on naval surface combatants has steadily grown over the past few decades. The principal reasons for this are two fold: ships have got larger and faster, and ships have more power demanding mission-systems.

Until recently only a few nations possessed ships and infrastructure capable of long term naval deployment worldwide, now many nations are committing to global operations albeit mostly in concert with multi-national task forces.  Multi-national global operations bring many demands on navies' ships and their supporting infrastructure but one thing particularly relevant today is `mission unpredictability' and hence the need for multi-mission flexibility. In order for a ship to be of use in the current climate of world-wide security it must be able to reach its destination rapidly and with as much autonomy as possible; it must be effective when it gets there and effective in a multitude of operation types ranging from humanitarian support to embargo enforcement to anti-terrorist operations. A multi-national task force requires a minimum level of mission compatibility if each nation's contribution is to add to the task forces impact rather than detract from it. Ship's sustainable speed, aviation capability and communications are three areas but when operating across the globe, re-supply of equipment and weapons will also be a significant factor.

SINGLE-ROLE COMBATANTS OR MULTI-MISSION

Capabilities demanded of a multi-mission globally-deployed combatant are: to be able to carry a medium to large helicopter (and to be able to refuel, re-arm and maintain it), to provide air- defence and anti-submarine capability to a task group, and to provide land attack functions and support special-forces and boarding operations. To which one can add the frequently called upon civilian duty of engagement in disaster relief operations. This capability mix can be provided in three different ways, single- role ships, multi-role ships or changeable-role ships.

In order to provide the total capability required a number of single role ships would have to be provided for any given situation to cover all eventualities. This provides for greater survivability due to the distributed nature of the full capability as well as the sum being greater than the parts especially in a Network Centric or Network Enabled environment where the multiple nodes can provide a far greater sensor spread and weapon reach. The disadvantages with this approach are the time necessary to deploy a credible and capable task force and the cost of so many more platforms. The single-role ships, while being smaller, will need a great deal of support to deploy world-wide and will need proportionately more crew to man the fleet; an extremely expensive undertaking and very demanding of scarce highly-trained staff.

Single-role but re-configurable ships are intended to change their main role with the fitting of modular mission systems; these types of ships have the ability to rapidly change pre-determined mission system into a ready prepared space. Whilst the ship remains pre-prepared the change of role inevitably requires not only time, especially when the ship is not already in a port where the correct module is situated, but an extensive infrastructure world-wide capable of delivering various mission modules to the appropriate are of operations. Early entry into a crisis situation may therefore not be possible and the expensive elements of a mission system may be in a port half the world away with no means of getting it into theatre: an expensive investment not being fully realised. For most navies then, the most effective ship type is likely to be a multi-mission capable ship.

The additional complexity of this type of ship over a single-role ship adds to the overall ship cost and the likelihood is then that the multi-mission ship becomes a high value unit and as a high value unit it therefore requires to be more survivable with increasing survivability demanding additional space and additional systems. However on a fleet basis overall lower costs and greater flexibility and autonomy are likely to be the result. The multi- mission capable ship is more likely to deliver what is required for the global coalition type operations and it is not considered possible to provide this type of capability in small hulls and neither would is it considered to be cost effective to do so.

SPECIFIC SHIP SIZE DRIVERS

The size of ship is largely determined by four principal characteristics: sensors, weapons, sustainability and ship speed. Ships sensors will be predominantly multi-function radars in addition to the growing use of deployable sensors and vehicles. Deployable sensors such as Unmanned Aerial Vehicles (UAV), Unmanned Surface Vehicles (USV) and Unmanned Underwater Vehicles (UUV) require volume in accessible places in the ship especially on the main deck to allow easy deployment. Large multi-function radars require height above the waterline to ensure capability against sea skimming supersonic threats.

The modern multi-function radar, so important to the ability to counter multiple threats, incorporates the past generation's single- function surveillance radar and single function tracking or illumination radar into one unit but to be effective has still to be mounted high off the waterline. Multi-function radars are heavy and their weight is a key driver on ship's stability: transverse stability of the ship is principally determined by the ship's beam and the waterline. Supporting an effective large multi-function radar will requires a ship's beam at the waterline of at least 18m but more likely to be 20m and probably closer to 22 unless lightweight superstructure construction (aluminium as on the RNT45 or composites perhaps) is adopted, which, if the naval architect is to maintain a hydro-dynamically efficient hull form with length to beam of around 7.5 to 1, gives hull length of 140m to 165m typically giving a ship displacement between 7,000t and 10,000t.See Figure 1 - relationship of ship's displacement with increasing ship's beam.

The weapon systems of the ship will probably consist of a Helicopter, a Vertical Launch System (VLS), a Land-Attack gun system and a Surface-to-Surface Missile (SSM) System. From aft forward the length of the ship is determined by the flight deck and hanger, the SSM system, the uptakes and downtakes for the main propulsion gas turbines together with the Replenishment at Sea (RAS) arrangements, the substantial RCS-reduced main mast, the bridge, the forward VLS and blast areas around it, the gun system and the blast areas around it, the fore end and the mooring and berthing equipment such as capstans and winches. The Flight Deck and Hangar make a considerable contribution to a modern combatant. Helicopters have grown in size from Wasp to Lynx, NH90, Sea King and EH101. Approximately XXm of ship length (flight deck and hangar) were required for a small helicopter such as lynx, approximately XXm is required for an SH60 or NH90 but a large naval helicopter such as EH101 requires about 50m for flight deck and hangar.

Large fast Combatants require shaft horsepower considerably beyond that which is available from the currently available high-speed diesels. The marine gas turbine is used where propulsive power exceeds 26 to 30MW either in a combined diesel and gas turbine arrangement or for the larger vessels in an all gas turbine. Gas Turbines are extremely power-dense using high- speed turbines to develop significant amounts of power from a large throughput of air. For the combatant this leads to a significant impact on the upper deck length from the GT uptakes and downtakes although the move to twin higher-power gas turbines from a quad GT arrangement will considerably reduce the impact on the upper deck as well as reducing the fuel bunkerage. The introduction of the vertical launcher significantly improves the performance of any missile system against multiple threats and has now been almost universally adopted on new-design warships. The VL silo however acts as its own magazine and hence the desire to have more than 32 missiles. For the Mk41 or Sylvar launchers a 61 missile silo gives a deck area of 8.7m by 6.32m but more significantly for a f'c's'l mounted silo (and where else is available?) a launcher depth of about 7m for the tactical launcher (SeaSparrow, Standard, VLASROC, LASM, NTACMS, etc) and 7.7m for the strike length module (Tomohawk, Fasthawk and TBMD). Of course, when considering deck area, one can to some extent trade length for width but the depth of the silo coupled with the silo width and the ships structural integrity requirement, will constrain how far forward the silo can go.Figure 4 - 64 cell Vertical Launch System.

To maintain the launcher flush with the main weather deck requires a considerable main hull girder depth essentially ensuring that these ships are `three-deck ships' of at least 6000t likely more. In summary single-function short-range VL missile launchers are much smaller than the Standard and PAAMS long range AAW missile launchers and adding multi-function capability for SSMs and land-attack missiles, including extending this to Ballistic Missile Defence, means the launcher silo installation becomes a major driver of ship length and main hull depth and hence overall ship size. Any globally capable Area Air Defence Combatant which is to provide a credible capability is likely therefore have a minimum length of in excess of 140m to allow the mission systems to be operationally effective, adding land attack and BMD leads the ship more likely to be 160m or 170m. Ship's length helps a complex combatant because it allows for more separation of key sensors and communications systems, thus reducing Electro Magnetic Interference (EMI) and improving the overall capability of the ship.

In order to provide a sustainable adaptable ship volume and internal deck area is required to carry the complement in 21st century standards of accommodation. More and more navies are adopting higher standards of accommodation to reflect the need to improve retention of the highly trained people so necessary to any modern technologically oriented navy. A direct comparison of a crew of 260 in 1960s standards compared to today's standards shows a substantial increase in area required between the two. Over 1600m2 compared with 900m2. For many navies this is probably due mainly to the improved accommodation standards of Senior Ratings and Junior Ratings now in cabins rather than mess decks (with their provision of separate recreation areas) and increases in the allowances of showers and WCs. Even this probably doesn't tell the whole story as, although there is a "grossing up" allowance for unusable area, this doesn't include access, and the provision of access passageways to these cabins and to the JR's washing cubicles must be more demanding on space than access to large mess decks.

Complements have reduced. For a frigate (Leander type) complement would have been about 250 with an Area Air Defence ship being 300-350 typically. A new design AAW Destroyer would probably now be nearer to 200, with most of the reduction in the Junior rates, but with additional space allocated for special forces and training berths. Overall though the area given to accommodation on a modern multi-mission combatant is likely to be an increase of around 30 to 50% compared to past ship classes. Modern ships with crew sizes of only 60 or so have limited sustainable ocean-going capability even with a high degree of automation in the ship's systems. Global deployment will also require either the constant companionship of a naval auxiliary ship or for the combatant to carry considerable fuel. Any force commander would prefer the ship to have a greater endurance on its own fuel rather have to rely on the presence of an auxiliary. This will allow a more flexible approach to operations, as well as increase the time between replenishment when on Task Force operations reducing the vulnerability of the group as a whole.

A typical endurance for a 1960s frigate was about 4000nm at 12 knots. By the 1980s this was 4000nm at 18 knots. Today's global AAW Combatant needs between 6000 and 8000nm at 20 knots which at a modern high-speed diesel efficiency requires around a 1100t of useable fuel and nearer 1300t of bunkerage. Water compensated fuel tanks are no longer a useable solution to ship stability (MarPol) and hence the impact of carrying this amount of fuel around and the impact on ship stability of using it are significant and can only be mitigated by a larger ship design. The displacement driver of the fuel and the volume driver of the accommodation add to the beam driver for stability and length drivers of the combat system to produce what could be termed a large ship.

One can see this trend in recent procurements. If navies require a globally deployable multi-capable and adaptable ship then it will be large even by recent standards but what may seem counter intuitive is that to make it even larger whilst keeping capability the same will probably make it less expensive to build. With a complex vessel the systems integration will a major cost driver, steel is by comparison relatively cheap even in today's market, whereas production and outfitting time is expensive. The designer can certainly contribute to reducing ship build cost by designing in sufficient space in the right places for systems routing and margin space. Increasing deck- head height will allow more room for the routing of cabling, piping and ventilation without the need for complex pipe runs and should significantly reduce the outfitting costs.

These modern, global-combatant will require power: power for ship speed, power for hotel requirements and power for today's mission systems and future systems. Speed and the `need for speed' is always a hot topic, many commanders will tell you that it is useful but not essential; good intelligence that allows a commander to pre-position or deploy his ships early may be better than high ship speed. These highly capable ships however will be in great demand in any operation and so the ability to re-deploy in-theatre at a high speed will be essential. This is especially true of current operations where the threat direction is not necessarily clear and the need to support forces ashore may require rapid transit to provide the necessary support. A ship speed of 31 to 35 knots would give an edge and an ability to rapidly re-deploy over short distances. For optimum hull form efficiency the trough in the ship's drag curve occurs at a Froude number of about 0.38 with the main resistance hump at Fn=0.54. For 30 knots then the optimum ship length (between perpendiculars), for lowest drag, will be about 165m. This approximates to somewhere around 10,000t displacement but the combatant will still retain good drag characteristics as low as 150m or so.

With many of the drivers identified that drive the trend towards larger combatants, what does this mean for propulsion power? Expectations are that a shaft power requirement of somewhere between 70 - 85MW will be required to meet the demanding role of the new Global Combatant. Installed power however isn't simply shaft power. The ship's mission systems and hotel requirements usually lead to a typical peak power demand of around 2 - 3MW. This though can be significantly increased if hybrid-electric propulsion is adopted or if new `electric weapons' are to be included at an interim point in the ship's life. For a hybrid electric propulsion system it can be expected that shaft powers of 5 - 6MW each (twin shaft 10 - 12 MW) will provide the new global combatant with a speed around 20 knots. The total installed power generation capability for such a ship, powering both ship services and cruise propulsion, will then be around 16 - 18MW, allowing for some redundancy in the generation system. Four Diesel Generators (DG) at about 1.5MW each will then be replaced by four DGs at 4.5MW each incurring additional machinery space, uptakes, and additional maintenance efforts or by four 4.5MW Gas Turbine Alternators (GTA) like the new Rolls-Royce RR4500 genset providing power, low on-board maintenance and frequency stability necessary for the modern multi-function radar as well as providing for future electric mission systems.

SUMMARY

The current operational environment and hence requirements on any destroyer whether it is optimised for Land Attack or Air Warfare will push the size of the ship up. The likely size of the new Global Combatant providing an effective multi-role platform is probably around the 150 - 170m long, 20 - 22mm beam, 9,000 - 10,000 tonne displacement and can achieve 32 knots on about 70MW. This will provide a ship capable of Anti-Air warfare and/or Deep Strike, Land Attack, a Special Forces and/or Unmanned Vehicle Capability and be Survivable against many varied threats.

David J Bricknell
Vice President Systems
Rolls-Royce Naval Marine

Robert Skarda
Manager Systems
Rolls-Royce Naval Marine

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