Marine propulsion is the mechanism or system used to generate thrust to move a ship or boat across water. While paddles and sails are still used on some smaller boats, most modern ships are propelled by mechanical systems consisting of an electric motor or engine turning a propeller, or less frequently, in pump-jets, an impeller.
The gas turbine (GT) is rapidly becoming the standard surface combatant power plant used by navies across the world. The advantages of GTs for warship use include the capability for rapid acceleration and deceleration which is necessary in case of the ship coming under attack. In addition they have low noise, reliability, and power density. The design of GTs also facilitates the design of unmanned engine rooms, with consequent crew savings.
The current trend in Navies is moving to electric hybrid gas turbine propulsion plants, again improving efficiency and reducing the need to refuel, while increasing operational availability and performance. The German Navy’s fourth and final new F125 frigate Rheinland-Pfalz uses a reliable and powerful GE LM2500 gas turbine-based propulsion system. These gas turbines reliably operate the world over in some of the most arduous conditions in temperatures ranging from -40 to 120 degrees F/-40 to 48 degrees C. All four of the new F125 frigates built for the German Navy employ one GE LM2500 gas turbine, two electric motors and four diesel generator-sets in a combined diesel-electric and gas turbine (CODLAG) propulsion arrangement.
In Gas turbines, compressor draws in and compresses atmospheric air which is mixed with fuel injected in a combustion system and burned providing power from turbine to the shaft. In a gas turbine engine, a single turbine section is made up of a disk or hub that holds many turbine blades.
The blades are responsible for extracting energy from the high temperature, high pressure gas produced by the combustor. The gases, fresh from combustion, are at around 1,700°C; and the shaft spins at speeds in excess of 12,000rpm. The turbine blades are often the limiting component of gas turbines. To survive in this difficult environment, turbine blades often use exotic materials like superalloys and many different methods of cooling, such as internal air channels, boundary layer cooling, and thermal barrier coatings.
“Back at the birth of the jet engine, Sir Frank Whittle’s prototypes were made entirely of steel,” said Rolls-Royce chief of materials Neil Glover. “Steel is great for strength and surface hardness, but if you need high-temperature performance it isn’t actually very good; 450–500°C is about its limit.”
Its unsuitability led to a search for a more temperature-resistant material, and jet makers turned to nickel alloys. Relatively abundant, with large deposits in Australia, and low in price, nickel melts at 1,728K (1,455°C) and is resistant to corrosion – both valuable properties for components that function inside a jet engine. Even more important is its ability to form alloys, and the particular property of one of those alloys, a compound known as gamma-prime in which nickel combines with aluminium, to retain its strength at high temperatures. “In steel or even titanium, the strength rapidly drops off as you reach 40–50 per cent of the melting point,” Glover said. “Nickel alloys retain their strength up to 85 per cent of the melting point.
China makes turbine blade breakthrough that could give Type 055 guided-missile destroyers an edge. Development was a result of cooperation with Russian firm, shipbuilding source says.
China’s efforts to access critical but sensitive technologies – owned by foreign players such as General Electric in the US, Germany’s Siemens, Japan’s Mitsubishi and Italy’s Ansaldo Energia – have been turned down over the past 50 years, putting China “at risk of being choked by the neck”, especially in the field of national energy security, Science and Technology Daily said in May.
China is on way to military transformation by inducting state of art weapons and platforms including directed energy weapons and railguns. However these systems are power hungry and the high power turbine technology shall allow it to integrate such systems on its warships. China is ahead of the USA in fielding railguns and is ahead in putting integrated electric propulsion systems onto many Navy Ships. The US has integrated electric propulsion systems on the three Zumwalt destroyers.
Turbine Blade Technology
Another important turbine blade technology is the single-crystal structure that isn’t intended to cope with temperature, however; it’s to make the blades resistant to the huge mechanical loads that result from their rotational speed. “Every single blade extracts power from the gas stream equivalent to a Formula One car engine,” Glover said. “And the centrifugal force on them is equivalent to the weight of a double-decker bus.”
Normally, metals are composed of many crystals – ordered structures of atoms arranged in a regular lattice, which form naturally as the metal cools from a molten state. These crystals are typically of the order of tens of microns in size, positioned in many orientations. At high temperatures and under strain, the crystals can slide against each other, and impurities can diffuse along the boundaries between the grains. This is known as creep, and it badly affected early turbine blades, which were forged from steel and later nickel bars.
The first stage in development was to get rid of any grain boundaries at right angles to the centrifugal loading, which led to the development of blades that were cast so the metal crystals all ran from top to bottom. Later, this was optimised further by casting single crystals, with no grain boundaries at all. It’s a highly complex process: not only must the blades be cast with the internal cooling channels already in place, but the crystals are not homogeneous. Rather, zones of different composition and crystallographic structure exist within the blade
“You can think of nickel superalloys like these as being like composites,” said Rolls-Royce aerofoil turbine materials technologist Neil D’Souza. “It’s a mixture of two phases, one of which – gamma-prime – gives rise to the sustained increase in strength at high temperature.”
China makes turbine blade breakthrough that could give Type 055 guided-missile destroyers an edge
China said it has produced a blade casting that could improve the performance of the heavy duty turbines that will power the latest generation of its guided-missile destroyers.
State Power Investment Corp (SPIC), one of China’s largest state-owned power generator companies, said it could now establish its own production process for the core component of 330-megawatt gas turbines.
SPIC said on its website on Tuesday that the blade casting was “China’s first breakthrough in core components in heavy-duty gas turbines” and “the most important milestone achievement” since 2015, when China launched a task force to develop a home-grown gas turbine.
To generate electricity, air and fuel are mixed and combusted, causing the turbine blades to spin, driving the generator. The blades are typically made of superalloys designed to endure the high stresses created within the turbine.
Beijing-based naval expert Li Jie said the new component could be used on gas turbines that are designed to power later variants of China’s most advanced surface ship, the Type 055 guided-missile destroyer. China has launched four over the past 18 months and four more are under construction.
The Type 055s are designed to serve as the primary escort vessels for the People’s Liberation Army aircraft carrier strike groups. The first two Type 055s do not feature super efficient integrated electric propulsion systems (IEPS) because of their relatively low power supplies. The next six destroyers will have higher power and integrated electric propulsion systems. The power systems will enable the destroyers to operate high-energy rail guns, which can launch projectiles at two to three times the speed of regular navy guns.
“The original gas turbines used on Type 055 destroyers are not as powerful as the engines used by the Americans’ Flight III Arleigh Burke-class destroyers,” Li said. “But the gap between Chinese warships and their American counterparts will be narrowed as China overcomes the core technology [gap].”
The technical breakthrough was announced with China locked in a trade war with the US and “Made in China 2025”, a strategy aimed at transforming China into a technology and manufacturing superpower, drawing the ire of US President Donald Trump and Western governments.