Shown is an artist’s impression of one of the Royal Navy’s Future Aircraft Carriers. At 65 000 tonnes, the ships will be largest ever built for the RN and are designed to carry over 30 Lockheed Martin F-35 Joint Combat Aircraft plus helicopters and surveillance and control aircraft.

Latest generation aircraft carrier to include diesel and gas turbine engines

The award of a contract to Wärtsilä earlier this year for four, medium-speed, dieselengine generating sets totaling some 40 MW finalizes the long-awaited decision on the drive system for Britain’s Future Aircraft Carriers (CVF). These sets will be combined with two Rolls-Royce MT30 gas turbine-driven generating sets in an Integrated Full Electric Propulsion (IFEP) system, bringing the total output capability to just under 110 MW — giving the vessels a speed reported to be in excess of 25 knots. The CVF carriers, HMS Queen Elizabeth and HMS Prince of Wales, are due to come into service respectively in six and eight years’ time and will be the largest and most powerful vessels ever built for the Royal Navy (RN). Each has a displacement of 65 000 tonnes and a length of 284 m. This is smaller than the largest of the U.S. carriers, but bigger than the current generation of French carriers. They are being designed and built by The Aircraft Carrier Alliance comprising BAE Systems, Thales Naval Division UK, Vosper Thorneycroft (VT) Shipbuilding, Babcock Manufacturing and the UK Ministry of Defence (MoD).

The design of the vessels has been a matter of lively discussion over some 10 years. The reliance on fossil fuels has been controversial in some quarters. It is understood that a nuclear drive system was originally decided against on the grounds of costs. Although the designers admit that with the current price of fuel oils going through the roof, this decision could have been different.Several alternative drive configurations were considered, including Rolls-Royce’s WR21 regenerative gas turbines as used on the RN’s Type 45 frigate, but the additional power density of the MT30 swung the decision in its favor and, in any case, the WR21 has apparently become less economically viable since Northrop Grumman pulled
out of the WR21 consortium.

Podded propulsor drives were also a possibility,but were rejected in the early design stages on grounds of cost, reliability and performance, all unproven at that stage considering the large size required. Also,the high maneuvering capability of pods was not considered a prime factor for a vessel of this type. The IFEP system was decided upon with the gas turbine-powered gen-sets driving two conventional 6.7 m diameter bronze propellers through Converteam 15-phase, 20 MW induction motors and line shafting.

Rolls-Royce’s MT30 gas turbine is a marinized version of the company’s Trent 800 aerospace engine with an output capability of 36 MW. It shares 80% commonality with the Trent family
and has already been accepted to

Layout of the proposed propulsion system.jpg

Layout of the proposed propulsion system for the new aircraft carriers, showing the four Wärtsilä diesel engines and two Rolls-Royce gas turbines supplying electrical power for the Integrated Full Electric Propulsion (IFEP) system. This system drives twin conventional propellers.

power the Lockheed Martin Littoral Combat Ship, USS Freedom, and the first two DDG Zumwalt Class destroyers for the United States Navy. Rolls-Royce pointed out that the potential of the MT30 is substantial. It is claimed to be capable of development to deliver 40 or even 44 MW and is highly compact, with an overall length of just 8.6 m in mechanical drive configuration. Moreover,its fuel efficiency at high power is said to be more than comparable with most diesel engines. It is also the highest powered gas turbine certified to American Bureau of Shipping’s Naval Vessel certification.

As far as diesel engine selection was concerned, it was apparent that for an optimized ropulsion system it would be necessary to provide a minimum installed power of 39 MWe for ambient air temperatures between -20° and 45°C. To meet this need and to ensure that the diesel output met the specific requirements of the selected generator, a tender was issued to all major diesel generator manufacturers capable of meeting Lloyds Register Naval Ship Rules.

Finally, four Wärtsilä, four-stroke, medium-speed engines were chosen, all integrated with Converteam generators mounted on common base plates. Two of these were 16V38B models and two were 12V38B, with the former sets providing 11.3 MWe at 14.126 MVA and the smaller sets delivering 8.5MWe at 10.582 MVA.These diesel power units have bore and stroke configurations of 380 by 475 mm and represent the largest diesel engines ever supplied to the Royal Navy. A fifth Wärtsilä 200 diesel generating set is also included for emergency purposes.

The standard Wärtsilä conditionmonitoring package for the particular engines has been purchased. Wärtsilä engines are currently in use in various Royal Navy vessels, including the Daring Class Type 45 Destroyer. These engines have reportedly been reliable and cost-effective, with the additional advantage to the RN that generic maintenance and operating routines for the new engines are generally in line with existing power units.

RN engineers make it clear that these factors, coupled with Wärtsilä’s large customer base for the selected engine type, help reduce through-life costs for the equipment and maintain a high level of equipment availability. This electrical installation will provide power for propulsion, as well as all the ship’s services and is generated at a voltage of 11 kV — more than twice that used for the Type 45 frigates and the highest ever used for a Royal Naval warship.The services, amounting to some 10 MW, will be distributed throughout the ship at 440 V into five independent zones.

Twelve switchboards enable rerouting of power in the event of damage. The prime movers will be widely dispersed throughout the vessel, with the diesel engines sited deep in the ship. The gas turbines will be installed in sponsons high up on the starboard side of deck four. This avoids hangar space being obstructed by ducting from the gas turbine outlets and provides ready access for installation or removal of the MT30s during servicing. For this application, the MT30s have been integrated on a common base frame with the alternator supplied by Converteam. Thus,the units are installed and removed for servicing with a single lift.

The overall philosophy behind the choice of IFEP was explained during a briefing at the Ministry of Defence’s Abbey Wood establishment in North Bristol. Here, it was pointed out that the combination of diesel and gas turbine power generation provides an optimum

CVF compared.jpg

The new CVF compared with other main aircraft carriers.

configuration for economic cruising and high-speed “sprinting” for aircraft launch and recovery. On diesel power alone, the cruise speed is impressive, believed to be about 18 knots, giving the ship an extended area of operation or time at sea without the need to replenish fuel. The 70 MWe of gas turbine power, on the other hand, is used for most flying operations and to provide enhanced resilience and survivability. Overall, a prime advantage of IFEP over pure mechanical or hybrid mechanical/electrical systems is that all prime movers can contribute to the power delivered to the vessel’s propulsion and as such can be operated at their most efficient point.

Hydraulic start has been initially chosen for the MT30s, but with the growing interest in electric starting, this may eventually be adopted. Electric starting has the potential to reduce through-life costs and increase safety by obviating the need for high-pressure hydraulics. A possible solution would see a power-dense permanent-magnet motor fitted to the MT30 accessory gearbox as a direct replacement for the current hydraulic motor. A power electronics cabinet containing the motor drive system would then be mounted on either side of the GT enclosure or close by in the machinery space.

In conclusion, the authors of the MoD briefing emphasized that the CVF project provides an ideal base to allow for full utilization of the benefits of the IFEP system. “IFEP really comes into its own on larger vessels where the space allows an optimum layout of equipment,” noted presenter, Simon Newman, Rolls-Royce’s MT30 technical lead on the Aircraft Carrier Alliance. “The size of the ship necessitates a power set of prime movers and these established gas turbines and diesel engines are tried and tested power units with proven reliability and demonstrated supportability. The continued close working relationship developed by the ACA with Rolls-Royce, Wärtsilä and Converteam and the individual expertise of these companies will deliver a truly integrated propulsion plant capable of the continued projection of maritime power well into the future.

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