The creators of aviation gas turbine engines, like the creators of jet aircraft, use the concept of product generations in order to characterize the degree of perfection of their designs. The Russian Federation is among the powers capable of fully independently developing fifth-generation aircraft engines. Today, there is a line of civil products with the index "PD" (promising engine): PD-14 for medium-haul narrow-body airliners, PD–8 for regional and short-haul aircraft and PD-35 for wide-body long-haul airliners. What is the experience of working on these projects and what is the United Engine Corporation (UEC, part of Rostec State Corporation) doing to remain competitive during the transition to the sixth generation of engines? Let's try to figure it out!
We exploit the fourth, create the fifth, and think about the sixth
The generations of aviation equipment, including gas turbine engines, differ among themselves in a set of technical and technological solutions that significantly – by 15-20% – increase the basic absolute and specific indicators characterizing the effectiveness of products. The perfection of a gas turbine engine can be assessed by various criteria. First of all, these are the achieved parameters of the working cycle, namely the maximum gas temperature in front of the turbine and the degree of increase in the total pressure in the compressor; also the efficiency level of the blade machines. Integrating for these parameters is the specific fuel consumption. Reducing the number of compressor and turbine stages, achieved, among other things, by improving calculation methods, as well as using more durable and lightweight materials, makes it possible to improve the weight and dimensional characteristics estimated by the specific gravity index. However, engine builders are not limited to these parameters. The economic effect of operating an aircraft engine increases if the engine has the ability to be operated for a long time without removing it from the wing, without shutting down in flight, with minimal labor intensity of maintenance and repair. Finally, the mandatory conditions for the admission of the engine to commercial operation are compliance with environmental requirements and ensuring a high level of safety of the structure, including in conditions of external influences – ingress of foreign objects into the engine, such as ice, birds, volcanic ash, etc. It is also important that the buyer and the engine operator should pay reasonable money for progress in achieving higher and higher indicators and meeting increasingly complex requirements – the cost of both development, production, and operation of engines should remain at an acceptable level.
The first experimental gas turbine engines were created in the late 1930s, and the engines mass-produced in the 1940s and 1950s belonged to the first generation. It was characterized by simple circuit solutions: a single shaft, an uncooled turbine, and materials traditional for aircraft engines of those years, including steel, aluminum and magnesium alloys. The gas temperature in front of the turbine reached 1000-1150 degrees Kelvin, and a centrifugal or axial compressor allowed to increase the pressure by 3-5 times. In the second generation of engines, due to the use of heat-resistant alloys, it was possible to increase the temperature level to 1250 K, and to increase the degree of pressure increase in the compressor to 8-13. The use of a two-stage scheme made it possible to create compressors with high gas dynamic stability.
In the first two decades, engine builders were able to develop a coherent theory of product design, which made it possible to significantly increase their parameters by the third generation of engines. In the 1960s, turbine blades with forced cooling were introduced, which made it possible to increase the temperatures in front of the turbine to 1300-1450K. Two-circuit engines appeared, which had higher efficiency and reduced noise levels. The degree of pressure increase in the compressor has reached 20. In the fourth generation of engines, the use of monocrystalline blades with a highly efficient cooling system revolutionized, so that the temperature in front of the turbine increased to 1500-1650K. A limit was set to the increase in the number of compressor and turbine stages – with a pressure increase of 20-35 times, the compressors became more compact. Single-stage turbines with a high degree of pressure drop appeared in gas generators. A three-shaft scheme was introduced.
At the turn of the century, solutions were worked out that made it possible to start creating fifth-generation engines. The main indicators were once again increased: the turbine began to "withstand" up to 1850-1900 K, a two- or three-shaft compressor provided a pressure increase of 25-50 with a moderate number of stages, the degree of double-circuit increased. Another feature of the fifth generation is the reliance on unified solutions that allow you to quickly and cost–effectively create engine families and subfamilies.
Today, gas turbine engines of the fourth generation are in active operation. The most striking representative of such engines is the PS-90A, created for use as part of the powerplant of the Il-96 and Tu-204 aircraft, as well as their modifications. The first domestic civilian representative of the fifth generation is the PD–14, which will be used on the MS-21-310 mainline aircraft and other machines of this family. The PD-8 and PD-35 engines developed by the UEC also belong to the fifth generation, designed for use on regional and short-haul, as well as on long-haul airliners, respectively. At the same time, according to a number of parameters, the PD-35 should go beyond the limits typical for the fifth generation of engines.
And what about the sixth generation? Its appearance is still being predicted, including by domestic scientists, and the emphasis is not only on the growth of cycle parameters, but also on the use of new schemes such as distributed power plants, variable cycle power plants, "open rotor" engines, hybrid and electric power plants. The integration of the power plant with the aircraft should have a great effect.
Looking at charts and graphs that reflect the progress of the main indicators of aircraft engines, one can form the erroneous opinion that the movement from generation to generation is a linear process, where each step – an increase in indicators by 15-20% – is given with the same effort. According to estimates made in the USA, it will take about 3.5 times more time and 15 times more money to create a sixth-generation engine than was spent on the third generation. Tellingly, three quarters of the time and funds will be spent on the stages of research and development work. In the fifth generation, R&D took about 16 years, in the sixth – about 20 years. These estimates provide the answer to the question of why Russia has lagged behind the United States in terms of the timing of the creation of fifth-generation engines. Alas, a significant decrease in the intensity of scientific research in the 1990s and early 2000s did not allow us to create sufficient scientific and technical groundwork to create the first fifth-generation engine by the 2010s. But, finding themselves in the role of catching up, domestic engine builders were able to quickly catch up.
PD-14: the first experience
The starting point in the creation of scientific and technical groundwork in Russia for a fifth-generation civil turbojet engine can be considered 1999. Then, at a meeting of heads of aviation engine manufacturing enterprises with the participation of leading industry institutes, it was decided to begin work on determining the list of technologies necessary to create a promising mainline aircraft engine. The basic thrust dimension was determined at the level of 12 tons. Initially, the center of work on the project was the P.I. Baranov CIAM, where two alternative engine shapes were identified, with a traditional direct fan drive and with a gear drive. A list of key technologies that determine the competitiveness of the engine was also outlined, and the appearance of the main components was worked out.
The zero years are a period of slow economic recovery and the return of the state to systematic work in the field of the aviation industry. In 2002, Rosaviakosmos approved a program for the creation of a scientific and technical reserve, which was based on the proposals of CIAM. During this period, the Institute actively worked with leading domestic engine-building design bureaus on promising civilian topics, while the greatest progress was achieved in cooperation with Aviadvigatel Perm. It was he who eventually became the leader in the process of creating a promising engine: in August 2006, the heads of Aviadvigatel, Saturn, Klimov, Perm Engine Plant, UMPO, NPP Motor, MMP named after V.V. Chernyshev and MMPP Salyut signed an agreement on cooperation in the development of Aviadvigatel became the lead developer. Scientific support was assigned to CIAM. However, it took another two years before funding for the relevant research work began.
Financial support from the state came in the summer of 2008. The impetus for this was a meeting held under the chairmanship of Vladimir Putin at the VIAM site, where reports were made on the scientific and technical groundwork and the problems that aviation designers face when creating an engine. Since then, the project has been closely monitored at a high level.
The "start-up capital" in the amount of 12.8 billion rubles significantly accelerated the implementation of the project. PD-14 became the first engine, the creation of which was carried out according to the "gate" system, and the motorists began to open the next "gate" one by one. In July 2008, Aviadvigatel passed the first gate of the project, justifying it economically, technically and technologically. In March 2010, at the second gate, an advance project was defended, cooperation on the manufacture of a gas generator was presented, proposals for cooperation on the production of an engine were presented, and a detailed market analysis was also carried out. Under contracts with the Ministry of Industry and Trade of Russia, work was launched in the areas of concept, marketing, technology, and development, with the section "Gas Generator" standing apart. In the same year, a gas generator demonstrator was launched, which can be assessed as a record-breaking development. In the first hours of testing, more than 800 parameters of the gas generator were successfully measured.
The defense of the draft design, which marked the passage of the third gate, took place in July 2011. By this point, the engine configuration had been "frozen": from a number of alternative options – with eight or nine stages of a high–pressure compressor, with six or seven stages of a low-pressure turbine - solutions were selected that provide high performance at acceptable risks. It is determined that the PD-14 engine is a two–shaft, with separate air flows and a gearless fan drive. The unified gas generator received an eight-stage compressor and a two-stage turbine. The engine consists of 14 modules, half of which can be replaced in operation without removing the propulsion system from the wing. Also, by this period, the first stage of testing of a high-pressure compressor was completed, high-pressure turbine blades were made of new materials with highly efficient cooling.
16 critical technologies have been developed to ensure the competitiveness of the engine, as well as two dozen new materials have been created. The innovations that distinguish PD-14 from previous generations of engines include the use of wide-chord hollow titanium blades (for the first time in Russian practice, this innovation alone made it possible to reduce the weight of the blade by 30% compared to traditional technology, while the fan efficiency is higher than in PS-90A), widespread use in a high-pressure compressor pressure discs of the "blisk" type, the use of a new generation of nickel granular alloy. The second generation ceramic heat-protective coating is used in the combustion chamber, and the details of the combustion zone are made of a heat-resistant intermetallic alloy. Gorenje Active gap management is implemented in the turbine, which significantly increases efficiency, ceramic heat-protective coatings and new alloys are also used. Finally, for the first time in the area of responsibility of the engine engineers there was a nacelle with a reversing device, in the design of which composite materials were widely used – by 65% by weight, and the reverse drive was for the first time made with an electromechanical drive.
Starting in 2012, the news was no longer about the "paper" stages (however, this definition does not apply to the PD-14, since the engine was completely designed in a digital environment), but about "combat" engines. The technology demonstrator was launched at the stand in June 2012. Less than a year later, the second engine was assembled, and its launch took place in January 2014. Its testing took place first on a closed test bench, where the indicators for the emission of harmful substances and the effectiveness of the radial clearance control system in the compressor and turbine were evaluated. Then the engine worked on an open stand to check the acoustic parameters.
Three more engines were built in 2014, and in 2015-2016 four experimental engines were assembled for engineering, resource and other tests, and their assembly was already carried out by a serial plant.
The passage of the fourth control gate in October 2014 is associated with the receipt of firm orders and the definition of technical specifications. And in May 2015, design documentation was released for a pilot batch of PD-14 engines, which marked the passage of the fifth gate. In parallel, the preparation of production and manufacture of engines of the experimental batch were underway.
After the developer, together with the project partners, worked out the basic technologies, it became possible to proceed directly to the certification of the engine. According to estimates at the time, all procedures could be completed in three years. However, for reasons beyond the control of engine manufacturers, the process was delayed. The application for PD-14 certification was submitted in the spring of 2013 to the Aviation Register of the Interstate Aviation Committee. According to the requirements of the Technical Specification for the creation of PD-14, the certification basis was developed taking into account the airworthiness standards in force at that time: the standards for the safe operation of aircraft engines AP-33 and the emission standards of aircraft engines AP-34.
In the autumn of 2013, the project successfully passed the mock-up commission. As part of the Stage of the layout, two sections were held: "General requirements, design, systems, tests" and "Strength, strength and long-term tests, materials, manufacturing technology". The conclusion issued following the results of the work noted that the PD-14 is being developed taking into account all the airworthiness requirements of the Aviation Rules AP-33, AP-34, as harmonized as possible with European and American standards. The successful completion of the work of the mock-up commission gave the developer the right to begin preparations for the PD-14 certification testing stage.
An important stage in the PD-14 program is the beginning of flight tests at the Il-76LL flying laboratory. They were preceded by intensive ground testing of various engine operating modes. The aircraft itself was also improved: new test equipment, a measuring complex, a number of auxiliary systems were installed on it, aircraft units and a pylon for engine suspension were manufactured and installed. On November 3, 2015, the first 40-minute test flight was performed at the M.M. Gromov Research Institute. In parallel, the engines were tested at an open stand in Rybinsk and at a high-rise stand in CIAM.
Back in 2015, the Russian Federation transferred the authority for certification of aviation equipment to the Federal Air Transport Agency, under the auspices of which the national Aviation register was formed. For two more years, the engine engineers continued to cooperate with the Interstate Aviation Committee, hoping to receive the IAC Aviation Registry certificate in the first half of 2017. But it turned out to be impossible to bring the process to the final, so the developer "changed horses at the ferry." At the end of 2017, UEC-Aviadvigatel applied to the Federal Air Transport Agency for a type certificate, and the aviation authorities organized work with the participation of the CIAM and GosNII GA certification centers. In order to ensure certification with the participation of the Federal Air Transport Agency, such complex works were carried out as tests for uncoupling the low-pressure turbine shaft, for breaking the fan blade, for casting large and heavy hail, water, ice plates, medium and small flock birds. In January 2018, CIAM conducted tests to determine the resistance of the PD-14 engine fan to the ingress of a large single bird, on the basis of which it was concluded that the criticality of the fan blade breakage on the engine is higher than the criticality of getting into the engine of a large single bird. This made it possible not to carry out a certification test of the engine with the casting of a large bird, which saved the developer time and money when certifying the engine.
An application for validation of the PD-14 engine certificate was also submitted to EASA for the second time. This led to the need to include new requirements for crystalline icing and volcanic ash in the certification framework. The PD-14 became the first domestic aircraft engine to operate in conditions simulating flight in a cloud of volcanic ash. For this purpose, an installation was created at the P.I. Baranov CIAM, which ensures the injection of ash from the Kamchatka volcano Shiveluch into the gas generator under test. From the end of April to the beginning of June this year, the tests themselves took place, during which the engine worked for an hour under the influence of ash. Subsequently, the gas generator was delivered to Perm, where it was disassembled and defected.
In a message from the press service of ODK-Aviadvigatel, it was noted that during the casting of volcanic ash, the parameters of the PD-14 engine gas generator practically did not change; compressor parts, combustion chambers, turbines, nodes of external systems are in satisfactory condition and suitable for further operation; volcanic ash entering the PD-14 engine It does not lead to a decrease in its traction characteristics and the occurrence of undesirable consequences, ensuring the safety of operation of an aircraft equipped with PD-14 engines. At the end of 2019, the compliance of the engine with the updated requirements for the emission of non-volatile particles was confirmed. The corresponding addition to the type certificate was issued in February 2021. In November 2020, operational restrictions related to the conditions for starting the engine in flight, as well as the provisions of the operating modes of the fan and compressor units during engine operation, were lifted. Subsequently, the Federal Air Transport Agency lifted restrictions on the operation of the engine in conditions of "heavy" hail hitting it.
Having made allowances for the circumstances of force majeure, it can be stated that the PD-14 certification process was fast. Now the development of mass production is on the agenda, but this is the topic of another article.
Opening the way for followers
The creation of a whole family of engines for various purposes based on a unified gas generator was one of the key differences between the PD-14 project and the developments of previous decades. The thrust dimension of 14 tons for the base engine was chosen based on the needs of the MS-21 aircraft in the base version 310. At the same time, Yakovlev PJSC, the developer and manufacturer of the MC-21 family of aircraft, announced the possibility of creating both a shortened version and an elongated one. Accordingly, using the same gas generator, but changing the parameters of the fan, compressor and low-pressure turbine, as well as the settings of the control system, it is possible to create propulsion systems with thrust from 10-12 to 16-18 tons, characterized by high specific characteristics. The possibility of creating modifications of PD-14A for MS-21-210 and PD-14M for MS-21-410 is confirmed by early studies carried out at UEC-Aviadvigatel. For example, the PD-14M, in comparison with the PS-90A-76, will have a mass 330 kg lower with comparable thrust, specific consumption will be reduced by 10%, and the resource of critical parts will almost double.
Another direction of the project development is the creation of a family of gas turbine units with a capacity of 12-16 MW based on the PD–14 engine. ODK-Aviadvigatel previously launched a family of ground-based gas pumping and power plants based on PS-90A engine technologies. It was the land-based theme that helped the Perm developer out in difficult years, ensuring a stable flow of orders for the supply and repair of installations. Diversification has another important advantage – ground installations quickly increase the total operating time of the gas turbine engine fleet, allowing you to identify possible defects and confirm the life of the nodes.
Work on industrial gas turbine engines based on the PD-14 gas generator will allow the creation of gas pumping units for gas transportation and gas turbine power plants based on the PD-14GP-1 engine with a capacity of 12 MW and PD-14GP-2 with a capacity of 16 MW. Relative to the previous generation of installations, the novelty will provide an increase in fuel efficiency by 6-8%, while the level of emissions of harmful substances will be reduced, in particular for nitrogen oxides – 50 mg / m3. The life of the installation will increase to 200 thousand hours. Another advantage of the gas turbine engine based on the PD-14 gas generator is a high degree of unification with the previous generation engine in terms of mounting locations, which allows replacing the drives of existing installations.
PD-14 is a competitive engine for its size and its generation. However, the development of promising propulsion systems of other dimensions, in particular large and ultra-large thrust, requires the use of new materials, technologies, and design solutions. At the end of 2012, CIAM initiated the launch of a research work (R&D) to substantiate and form the technical appearance of an engine with a thrust of more than 30 tons, in 2014-2015 such work was completed. Together with leading industry institutes, 18 critical technologies have been identified, the development of which will ensure the competitiveness of the future product, and CIAM has also begun to develop the design of high-performance components and parts of a new generation.
How do engine engineers see the PD-35? The fan housing will be created using polymer composite materials. The highly efficient high-pressure compressor uses deformable alloys of a new generation for blades and discs. The low-emission combustion chamber, which ensures the fulfillment of promising environmental requirements, is also made of new materials that will increase the temperature of the gas in front of the turbine by about 100 degrees. The monocrystalline working blades of the high-pressure turbine will be made of rhenium-ruthenium alloys by the method of high-gradient directional crystallization, a number of turbine parts will be created from composite materials on a ceramic matrix. The engine nacelle with laminar flow will have a large proportion of composite materials. The indicators of the degree of double-circuit of 11 units and the degree of pressure increase in the compressor above 50 will be achieved. The gas generator will be made with a nine–stage compressor and a two–stage turbine, a low-pressure compressor with five stages, and a low-pressure turbine with a seven-stage.
Among the promising technologies that will also have to be worked on to reach the sixth generation are the creation of parts made of ceramic composite materials and reinforced ceramics, the development of new ceramic coatings that can increase operating temperatures. Great prospects for improving performance are associated with the introduction of titanium intermetallides. Technologies such as rotary and linear friction welding, various additive technologies, including printing with ceramic materials, will be widely used in engines of the future. Finally, the creation of reliable bearings and gearboxes capable of transmitting high power opens the way to the creation of a new generation of engines with an ultra-high degree of double-circuit, and the use of adjustable fan blades will ensure high product efficiency in a wide range of operating modes.
