The first of Britain’s much-heralded new aircraft carriers is moving towards sea trials this year, to be followed by flight trials in 2018 and operational service starting in 2020. The ships have demanded innovation in many fields, not least in the development of a completely new protective coating for their flight decks.
The carriers will be host to a new generation of multi-role aircraft. The vertical landings these will perform on the carrier decks create blast furnace temperatures from their jet exhausts, and extreme jetwash. Besides surviving this heat and pressure, the landing areas have to maintain a non-slip surface to prevent the aircraft skidding as the ships pitch and yaw at sea. These two demands, seemingly in direct conflict, had to be solved with a new type of coating for the ships to operate as intended.
Queen and Prince
The HMS Queen Elizabeth and HMS Prince of Wales will be the Royal Navy’s biggest ships in history, each built with 40,000 tonnes of steel and requiring a total of 1.5 million square metres of paintwork, equivalent to slightly more than the size of Hyde Park.
Their flight decks are the size of three football pitches (19,500m²). This is the business area, where Lockheed Martin F-35 Lightning II jets, operated in combination by the Royal Navy and the RAF, will take off and land. Also known as the Joint Strike Fighter (JSF), these are single-seat, single-engine, all-weather stealth multirole fighters, designed to perform ground attack and air defense missions. For carrier-based operations they will be using their Short Take-Off and Vertical Landing (STOVL) capability.
Rock and roll
Normally the flight decks of aircraft carriers are coated with an anti-slip paint to prevent aircraft from losing control as they take off and land, and also keeping the crew safe. Those coatings are adequate for the short take-offs of the Joint Strike Fighters, which neverthless involve powerful downward jet efflux and high temperatures – much more than created by the old Harrier jump jet. But on the new carriers the aircraft landings will be vertical, and the aero-thermal effects would far exceed the tolerances of normal flight deck coatings.
The requirements demanded that a completely new coating solution be developed, and with it a completely new application system. To add to the difficulty, the coatings were to be applied outdoors at the naval yards in Fife, Scotland – where the weather conditions alone present a significant challenge.
The new coating, developed by Monitor Coatings of North Shields specifically for the carriers, is an innovative thermal metal material. It can withstand the high temperature and pressure from aircraft exhausts, yet still provide sufficient friction for the aircraft undercarriage and safety for ground crews. It also protects the decks from the weather and contains the flight deck markings.
The JSF’s jet exhaust gases reach temperatures of more than 920°C and generate immense thrust and efflux. Built around a combination of aluminium and titanium, the new thermal metal coating goes even further – it can withstand temperatures of up to 1,500°C (2,700°F).
The coating is being applied to the designated landing spots using a specially developed robotic spray, which fires metal atomised from wire through a jet of plasma at temperatures of almost 10,000°C (18,000°F). The molten droplets then flatten and quickly solidify, creating a tough but rough coating 1.2mm thick that is bonded to the steel beneath.
Monitor Coatings, a technology leader for surface engineering in extreme environments, were invited to tender in a competitive bid along with many other coatings companies in the UK. The company is part of Castolin Eutectic and provides wear protection services for aerospace, oil & gas, steel and other industries from facilities in the UK, Singapore and China. Monitor say: “Our starting point in the coating design was to understand the parameters and operating conditions from a very broad brief offered by Thales. With our knowledge of Aerospace coatings we designed a system that could cope with strength, thermal shock, marine environments and the ability to be textured to provide the anti-skid characteristics required.
“The initial design parameters were very broad: a replacement for conventional paint that could withstand the gas wash effect generated from the F32B MkII Lightning Strike while operating in a marine environment. The thickness of the coating and final surface texture elements were developed jointly with the Aircraft Carrier Alliance (ACA) and Monitor.
“Once we established the base parameters, primarily from experience, we used a finite element and finite volume analysis model to predict the results we needed. These were then added to the system and fine-tuned to compensate for off-site working. A digital approach meant that we could save time and money and avoid a vast number of iterations with multiple variables.
“Work started back in mid-2009 with a break between summer 2010 and summer 2011. The development process was however gated. First we had to submit standard coatings for third party testing. The tender process was then won on the back of the test results. Following this we conducted the modelling phase, proofed on a pilot deck situated at Monitor. The first real transition to ship deck production was in August 2015.”
On the deck
Working for the Aircraft Carrier Alliance, Flight Deck and Coating Manager David Thomson, and Monitor Coatings Project Manager Chris Castiglione had to make sure that the new coating could successfully be applied, and that it would work in operational conditions. They told PCE: “The thermal coating is applied on three of the carrier’s five landing spots to provide operational capabilities for the JSF. It was determined that at takeoff speed using the carrier’s ramp, the regular non-slip coatings won’t be adversely affected. The aero-thermal effects on the landing spots however would survive only a few aircraft evolutions.
“The rest of the flight deck is covered with a non-slip coating similar to that used on any other warship. But for these carriers the JSF is configured as STVL – Short Takeoff Vertical Landing. It creates a combination of temperature, pressure and velocity that would dramatically reduce the longevity of a normal non-slip coating.”
After passing laboratory trials, the solution was tested to see that it would achieve the required coefficient of friction through rolling drum trials. It was also necessary to demonstrate that the coating could practically be applied to large areas of the flight deck.
A trial area on the deck was used to prove the application technology. The tests included the tenting and containment needed to ensure that the environmental conditions were suitable for the application, which was undertaken during one of the wettest and stormiest winters on record.
Application of the coating is via an Arc Spray system, similar in concept to an inkjet printer. The concept of Arc Spray is not new in the building of large vessels such as oil rigs, bridges and steel hull boats, but generally involves standard coatings such as aluminium or zinc which are often applied manually. Monitor say this new type of use is a first.
Development work culminated in a two day demonstration of the process application, quality system and in-service repair program of the Thermal Metal Spray to the Aircraft Carrier Alliance and stakeholders, well received by senior naval officers and civilians from the marine engineering community.
Thompson and Castiglione continue: “Preps for the coating is by far the hardest part of the coating operation. Initially we had to prove that the coating could be successfully applied in a factory environment, then in 2015 we proved that it could successfully be applied on a non-critical area of the deck by laying a runway-sized area on a non-operational section.
“The application requires a dry, clean temperature controlled area. Preparing the steel deck was the longest part of the operation. Specific blast criteria had to be achieved, and the blast profile was tested at the BAE testing facility in Warton, Lancashire. The blast profile details are proprietary knowledge but they include a cleanliness level of SA3 and profile characteristics with defined criteria for the distance between peaks, hook sharpness, and depth.
“We blasted with aluminium oxide. Naturally the procedure creates a large volume of aluminium dust that has to be controlled and collected. Completing the coating in the Scottish weather at the navel yard in Rosyth was certainly demanding. We used 24m by 40m enclosures to create a working environment of around 22ºC and 40% humidity, with 10 air changes per hour. Monitor coatings were responsible for the application and the blast standard, with help from other companies contracted for the actual blasting.
“All application has to be done within 24 hours of surface preparation. The coating application itself is performed by robot. The thermal metal coating is a 3 coat system. The robot applicator operates like an inkjet printer but tapers the coating out towards the edges, which are then overlapped. At the edges of the thermal coating there is another taper where regular non-skid coating is applied over the top. The nominal thickness for the thermal coating is 1200 microns.”
With any new coating and application technique there will always be questions about how it will stand up in use. Thompson and Castiglione say: “Longevity remains to be tested as the coating is new, but the normal non-skid application is expected to last 3 years and we are hoping that the new coating will have a lifetime of double that.
“Maintenance is feasible within limits. Airworthiness is the prime factor. There would normally be a daily visual inspection of the flight deck areas, and any foreign objects would be removed. If small areas of damage were discovered at sea, onboard maintenance might consist of cutting out damaged material and patching with regular non-skid coating.”
After the success of the trial area the coating has been extended to approximately 2,000 square metres of the 19,000 square metre flight deck, with the work due to be completed prior to sea trials in early 2017. The remainder of the flight deck will have a conventional non-slip coating applied.
Ian Booth, Managing Director of the Aircraft Carrier Alliance, said: “There is incredible momentum behind the programme to prepare HMS Queen Elizabeth for sea trials and integrate the F-35B Lightning II aircraft. Working with experts in the UK, we have developed a unique coating to provide the necessary protection to the flight deck of the aircraft carriers and this will ensure they can deliver the UK’s carrier strike capability for the next fifty years.”
The U.K. lost its carrier aircraft capability with the retirement of the Harrier jump jet fighters in 2010 and the scrapping of its HMS Invincible-class carriers. The two new carriers will each have a displacement of 65000 tons, three times the current Invincible class. Their range will be 8 – 10,000 nautical miles, with propulsion coming through has two propellers outputting 80MW of power (enough to run 1,000 family cards or 50 high speed trains). The propellers themselves weigh 33 tonnes each. The height of the ships from keel to masthead will be 56 metres.
At a cost of more than £6 billion each, both carriers will accommodate up to 40 rotary and fixed wing aircraft. The JSF jets will be housed below decks but can be brought to the flight deck by lifts in 60 seconds.
The first ship, HMS Queen Elizabeth, was named on 4 July 2014, with commissioning planned for later this year, and initial operating capability expected in 2020. The second, HMS Prince of Wales, is scheduled to be launched this summer and commissioned in 2020. They will be versatile enough to be used for operations ranging from supporting war efforts to providing humanitarian aid and disaster relief.
Captain Simon Petitt, Senior Naval Officer of HMS Queen Elizabeth, commented: “The new flight deck coating is one of the many 21st Century engineering innovations being incorporated in the Queen Elizabeth Class programme. As the largest warships ever built for the Royal Navy, these powerful ambassadors will protect UK interests around the globe for the next 50 years.”