“Life is Still Catching Up with Nikolay Kuznetsov’s Dreams”
Interview with Igor Papsuev, Advisor to PJSC «Kuznetsov» Deputy General Director-Managing Director
Igor Papsuev – mechanical engineer, Advisor to PJSC «Kuznetsov» Deputy General Director – Managing Director. Born in 1944. Graduated from Kuibyshev Aviation Institute (now Samara National Research University named after Academician S.P. Korolev) in 1968. After the graduation he started to work as an engineer in the Department of Series Engine Flight Testing of Kuibyshev Machine Building Design Bureau (currently a structural subdivision of PJSC «Kuznetsov»). Over the period 1976-1999 he served first as a Deputy Chief and then the Chief of Maintenance and Repair Department of Kuibyshev Motor-Building Production Association named after M.V. Frunze (currently PJSC «Kuznetsov»). During that period, he was engaged in the maintenance of NK aircraft engines, including the NK-12, NK-22, NK-25, and NK-32. In the 2000s he became the Chief Technologist and was in charge of mass production of engine components and parts. Over 2012-2012 he served as the First Deputy Executive Director – Chief Engineer at the company. Since 2014 he has been acting as Advisor to PJSC “Kuznetsov” Deputy General Director – Managing Director.
Q: What were the key challenges you faced while developing the NK-12 engine?
A: There was some specific point. We had a bit of trouble with the planetary gearbox while developing the NK-12 engine. The challenge was that the light-weight and small-sized gearbox had to transfer nearly 12,000hp. Therewith we had to be sure that the forces induced inside that gearbox would not tear it apart. We came up with an ingenious solution: the gearbox was executed in such a way that individual gears did not actually pass any torque directly to the gearbox casing. They only transfer the torque to two shafts of the front and rear props, thereby balancing each other. The result of that novel design was a planetary differential gearbox. No one has come up with a similar design ever since.
Of course, certain problems arose in serial production itself. The precision and alignment of all the components had to meet the highest standards; no crossover of gear centerlines was allowed. The gears themselves had to meet very high manufacturing requirements. So during the development there were doubts as to whether we could succeed in bringing the gearbox to mass production. Some believed that we should stop the works. But Andrey Tupolev threw his weight behind the project. He was backed by Vladimir Klimov, the second chief designer – for engines. They said to the powers that be, “Give Nikolay Kuznetsov another four months and he will bring this gearbox up to scratch.” In the end, that was exactly what happened, and the gearbox is performing well to this day. That was a major challenge we managed to overcome; neither the Americans nor the British have managed to come up with anything similar. This is one of the key distinctive features of the NK-12 engine.
Our plant started serial production of these engines in 1954. Apart from the gearbox, there were other challenges we had to solve. Take, for example, the turbine blades: the blades of the first two stages were made by casting, and those of the rest stages – by pressing. That was a step forward; these days, everyone makes turbine blades by casting. That was the second challenge. The third one was to design an annular-type (rather than can-type) combustion chamber, which enabled us to achieve a more uniform temperature field at turbine inlet, thereby raising the temperature inside the turbine itself. That was also a major step forward. We made use of some new materials, such as the ZhS-6K alloy, which was invented back at that time and still remains in use.
Additionally, we were facing a major problem adjusting the performance of the propellers with the operation of the turbo compressor. The thing is, the propellers generate thrust, while the turbo compressor drives the propellers. A gearbox is needed because the speed of turbo compressor is higher than that of the propellers. Otherwise the propeller blade tips would achieve supersonic speeds. Balancing of these two speeds was not an easy task. Also, in case of engine malfunction, it should be able to stop rather than keep rotating due to the incident flow that keeps the propellers spinning. If something has gone wrong with the turbo compressor, that defect could get even worse if the propellers keep spinning the engine. So, we had to find a way of stopping the rotation of the propellers as well. That’s when we came up with the idea of propeller feathering; it means that the propeller blades are turned edge-wise to the incident air flow. That way, any mechanical failure in the engine is stopped from getting worse. Another thing to keep in mind is that propeller windmilling results in high drag. In case of one engine shutdown, a multiprop aircraft would experience such a high drag that it could cause aircraft turn sideways, which can be very dangerous. We had to fix all these issues when developing the NK-12.
Furthermore, propellers afford to create reverse thrust, which reduces the plane rolling distance at landing.
Other issues we had to deal with included burn-throughs in the combustion chamber walls. That necessitated the development of a special coating for the chamber walls. It was also important to have a sufficient gas-dynamic stability margin. But one of the advantages of the NK-12 is that it has a constant speed of 8,300 rpm at all engine modes; while thrust variation is defined by the propellers depending on the throttle level.
So, bare listing of the problems we had to solve, I believe, gives you an idea of how complex and challenging that program was, and how skillful the designers had to be to resolve all those challenges.
The Tu-95MS planes powered by NK-12 engines remain in service operation to this day. As you know, they are involved in Syria conflict operations.
Q: What was the average NK-12 production output during the Soviet period?
A: We made up to 28 engines per month.
Q: Several An-22 planes crashed in the first few years after that plane’s entry into service. Were those crashes caused by the NK-12 engines installed in those planes?
A: No. To give you an idea of that engine reliability- just think – the Tu-114 airliners powered by NK-12 engines were used on the Moscow-Tokyo, Moscow-Khabarovsk, and Moscow-Havana routes. The Tu-114 powered by NK-12 remained in service for 14 years, and during that entire period, there wasn’t a single incident caused by the engines. The Tu-114 was then retired, but it is still remembered as one of the best Soviet planes. So, to reiterate, the NK-12 was very reliable.
Q: Was the NK-12 also installed in the Orlyonok wing-in-ground effect aircraft?
A: It was used as the main propulsion engine; the lift engines were of NK-8 model. The NK-12s used in the Orlyonok were not specially modified. It was a standard maritime version, the same one that was used in the Indian Tu-142ME planes, with special corrosion-resistant coating applied on compressor blades.
Q: It was reported that the first NK-12 based gas-pumping units used aeroderivatives removed from the retired Tu-114 planes. Is that so?
A: Yes, that is correct. The reason why the Soviet Union had to develop its own gas-pumping units for compressor stations was that our foreign “colleagues” put certain obstacles for delivery of Italian and French gas turbines and pipes for our compressor stations. The Soviet government therefore tasked us with developing an indigenous engine for application in gas compressor stations. The task was given to Nikolay Kuznetsov. He quickly came up with a design based on aeroderivatives from the engines used in the Tu-114s. A large fleet of Tu-114s had just been retired since large propeller-driven airliners had become obsolete, especially abroad, due to increased noise level. Aeroflot began transition to Il-62 planes powered by jet engines. In the process of Tu-114 disposal the engines with the residual life were removed. And then foresighted designer Nikolay Kuznetsov started the project of flight engine conversion for application in gas-pumping lines. The compressor remained unchanged, the gearbox was eliminated, while the turbine was divided into two parts. The so-called “power turbine” drove the gas compressor that pumped natural gas into the pipeline. The gas turbine engine itself was modified for operation with natural gas instead of kerosene, so its combustion chamber was modified. On the whole, the resulting product was a great success. For a long period about 70% of the Soviet gas-pumping line compressor stations used NK-12 aeroderivatives.
Of course, the engine was originally designed to operate at an altitude of 8,000 to 9,000 meters, and its gas-dynamic characteristics were set accordingly. So the engine was redesigned in the years that followed, and the resulting NK-14 engine was more suitable for ground application. It was in great demand not only in the Soviet Union and Russia, but also abroad, including Uzbekistan and Argentina.
Q: You have mentioned India. Was Kuznetsov involved in the maintenance of NK-12 engines used on the Indian Tu-142ME anti-submarine planes?
A: That was our company’s first delivery abroad, and we went about it very carefully. We weren’t completely familiar with the operational environment in India (high air temperature and humidity). At high air temperature and lower air density, the engine delivers lower thrust than at sub-zero temperatures. The Indians – pilot crews as well as technicians – spent long time training at our country. We kept in close touch, and set up a working team that maintained the planes. We understood each other quite well. My first trip to India lasted 12 months because that’s how long the engine warranty lasted, and we had to resolve all the emerging issues very quickly. It was in 1988.
The location was Goa, which is a dream destination these days, but back at the time it was in the middle of nowhere. To get in touch with the head office, one had to relay the call through Delhi, then London, then Moscow and on to Kuibyshev. There were three of us there: a designer from the Special Design Bureau, a test engineer, and myself. During those 12 months, we managed to resolve many issues, and there wasn’t a single major incident with the engines. There was very strict selection of the personnel sent to India: the first selection round was at the company itself, and then the candidates were approved by the local Communist Party committee and then by the Communist Party Central Committee in Moscow.
Q: What do you think about the Indian Tu-142ME pilots and technicians in terms of their skills and training?
A: The pilots were very well trained – at least those we worked with. But they did have some quirks. They tended to be less diligent in some areas. Take, for example, the take-off run. As I already mentioned, the engine produces less thrust at increased ambient temperature. So they would start messing with the engine fuel system – what is worse, they would start doing it when we were away. Then we would say to them, “Why r did you try to change the fuel system settings? What for?” And they would reply, “Two weeks ago I reached sufficient speed for take-off near that light over there, but today it was near that other light farther away. I do not seem to get enough thrust”. So we had to convince them that the runway length would be sufficient for them to take off.
There were also other problems, for example, with rapid descent. When they hunted a submarine, they would dive to it – and then complained that the propellers started to feather. Nevertheless, they were very experienced and skillful pilots. They have a large selection of planes of different manufacturers to choose from. For example, when we were at the airfield, we saw British Harrier vertical-takeoff-and-landing aircraft. So, the Indians would always compare different hardware – but they did like our plane at the time.
Also, they have an interesting quirk in how their personnel are deployed. They have adopted the British approach, with constant personnel rotation: today you are assigned to the Air Force, tomorrow to the Navy, and the day after to the tank troops. We asked them why, and they would say that when a specialist works at the same unit all the time, he becomes less diligent in following all the requirements and instructions. So when we used to come there, we had to train a new group of technicians each time.
Q: Did you learn any lessons in India in terms of design and engineering?
A: We had to adapt to a new operational environment – namely, increased air temperature and humidity. We once had an incident when the crew reported an engine fire in mid-flight. But as the plane approached the airfield, we saw no sign of any fire from the ground. We inspected the plane after it landed, and again, no signs of fire. It turned out that the fire detectors in the engine nacelle had their electrical contacts damaged by corrosion, setting off a fire alarm. The first stage of the fire extinguishing system was automatically initiated (the second and third stages required manual input) and fed quenching fluid into the engine.
We kept struggling with corrosion. We also had to manually rinse the icing detector because it kept getting clogged with mosquitos.
Besides, the Indians used their own locally made fuel and engine oils, although that had of course been agreed with the supplier back home and approved by the chief designer. Nevertheless, it also effected the engine operation and maintenance.
Q: Am I right that the final NK-12 production run in the mid-2000s was made under an Indian contract?
A: That is correct. It was in 2008. When Indian delegations came here, and I always tried to meet them to discuss whether there had been any issues with the engines.
Q: Some reports claim that when the company installed new machine tools, there were problems with manufacturing of NK-12 parts using those new tools. Are those reports correct?
A: It’s not that the new machine tools weren’t as good as the old ones. We did receive several large batches of new tools, especially in the 1970s and 1980s. But that technology refresh was quite useful. We never buy new equipment just for the fun of doing that. Before placing the order, we formulate our technical assignment document. Then we inspect the produced hardware on site to make sure that all our requirements had been met, the process source inspection is also performed there. Whenever found that some of the items of our technical assignment document were not met, we would tell the manufacturer to fix any remaining issues. So I can’t really say that we had any major remarks in that area.
Q: During the Soviet period, there were certain limitations on the production of powerful engines as part of the rocket program, especially the huge N-1 rocket designed by Korolev. Those limitations were imposed by the level of the available machine tools. Who was your main machine tool supplier back at the time, and did your company have any Western machine tools?
A: The NK-25 and NK-32 designs included very long shafts that no-one had ever made in the Soviet Union. For example, the LP shaft was over 1.5 meters long. We were the first company to manufacture such shafts. But how were we to process them? The Soviet industry simply did not make any machine tools suitable for processing such shafts. We were therefore forced to buy the necessary tools abroad. The NK-25 and NK-32 engines had very large engine diameters; and the diameter imposes its own requirements on the machine tool base plates. So, we had to import machine tools. Back at the time, especially in the 1970s, the government allocated huge funds for the development of the aerospace industry. Our companies imported German and Swiss machine tools offered by reputable and reliable manufacturers. Our engines also required large-size blades, with large longitudinal dimensions. Yes, those imported machine tools were expensive, and we had to pay for them in hard currency, but we simply couldn’t do without them. The design of the new engines necessitated the purchase of new manufacturing equipment.
Q: But there were certain limitations imposed on us in terms of machine tool capabilities, is that right? We weren’t allowed to buy some of the tools we wanted.
A: I have to give credit to our technical managers, who always found the way to buy the hardware we needed. They did find the way, and all those problems were solved thanks to the government’s support. All we had to do was draw up our technical assignment.
Q: Did you rely on cooperation with other Soviet companies, or was basically everything done in-house?
A: As to NK-25 and NK-32 programs, we worked in cooperation with Ufa and Kazan. That was cooperation within Russia, and we still keep it. When there is a need for it now, our partners are always ready to get involved. I should give credit to chief designer Kuznetsov: he always tried to involve other companies in his projects.
Q: Did you have any competitors among the other Soviet companies, i.e. the other engine design bureaus or motor-building plants? Or did you have your own uncontested market niche?
A: Speaking of chief designer Kuznetsov’s projects, his engines were excellent, and their development time was quite short. Nevertheless, we always had someone else breathing down our necks. Take, for example, the Tu-144 plane. It was originally powered by the NK-144 engine, which was later replaced with the D-36-51 engine, produced in Rybinsk, and featuring lower specific fuel consumption. Another example is the Il-62 plane. Initially it only relied on the NK-8 engines, but later on it switched to the D-30KP. The Tu-154 was also initially powered by the NK-8, but it then switched to the D-30KU, etc.
My take on the matter is that chief designer Kuznetsov was good at creating a reliable engine very quickly. These engines were then followed by other models, developed by someone else, with better fuel efficiency. But the new aircraft would enter their trial service fitted with Kuznetsov engines, because it’s very important for aircraft manufacturers to enter the trials as early as possible.
A case in point is the Tu-144. When the Soviet government tasked the industry with developing a supersonic airliner, time was of the essence because we had to catch up with the British and the French, who were working on the Concorde program. That is why Tupolev decided to turn to Kuznetsov for an engine, because he knew that Kuznetsov already had an engine with an afterburner that provided the required performance level. The expectation was that this plane flight profile would have a considerable transonic section, and therefore minor reheat at the supersonic cruising section would not significantly affect the plane fuel efficiency. So, to enable a rapid and reliable entry of the Tu-144 into service, and to begin its trials as soon as possible, it was decided to use the NK-144 engine. In the end, the plane entered mass production, and the first Tu-144s used the NK-144 engines. The first commercial flights also relied on those engines. But the flight distance was only 3,200km; the plane was used on Moscow-Almaty route. That is not a great distance at all for such a plane. The entire flight took only two hours. But the idea was to reduce the flight time on such routes as Moscow-Khabarovsk or Moscow-Vladivostok to just a few hours, and that would require an engine with lower specific fuel consumption. Such an engine was developed in Rybinsk, but they did not manage to complete flight trials, and the entire Tu-144 program was cancelled.
Developing a supersonic plane throws up a whole host of new challenges. Suffice it to say that the plane skin reaches temperatures of 120-130C in flight, which leads to heating fuel in the tanks and thus creates fire hazard. We needed an engine to start the plane flight test development, so we had to use the NK-144. Then the new RD-36-51A engine was developed by P.A. Kolesov. It managed to provide the normal test flight from Moscow to Vladivostok, but the engine began to develop problems with the compressor disk. And, as I have already said, while they were trying to resolve that problem, the whole Tu-144 program was cancelled.
Nevertheless, aerospace engineers have not completely abandoned the idea of supersonic flight. Some day we will have supersonic airliners once again. The Americans used our Tu-144LL flying laboratory, powered by NK-32 engines, to conduct numerous tests in the field of commercial supersonic aviation, and accumulated a lot of data. They studied such issues as the impact on the ozone layer of the Earth, and the temperature of the fuel in the tanks. Of course, it’s not just the Americans who have used that laboratory – we also tested various modifications of such planes. The flying laboratory made 19 flights under its own Soviet/Russian programs and joint programs with the Americans, and eight more flights were conducted by American test pilots together with our pilots. All the information gathered during those flights is waiting to be used.
Q: Was the NK-144 designed only for the Tu-144 plane?
A: That’s right. The NK-144 and NK-22 engines (the latter installed in the Tu-22M bomber) were both bypass two-spool engines with an afterburner. The lessons learnt during the operation of the NK-144 engines installed in Tu-144 planes, and of the NK-22 in the Tu-22 bomber, boosted the development of the NK-25 engine for the Tu-22M3 heavy bomber and of the NK-32 for the Tu-160 bomber.
Q: How difficult was it for your company to launch serial production of the NK-22, NK-25, and NK-32 engines?
A: Of course, things weren’t as simple and easy as we would have liked. But we were very enthusiastic to get these new products off the ground. The workload was extreme at the time. It’s not an easy thing to move on from a turboprop to a turbojet engine with an afterburner and a variable-geometry nozzle. But our designers and production specialists were up to that challenge. We developed that new technology, and it gave rise to many other innovations, such as the heat-resistant coating of turbine blades. That coating, in its turn, necessitated the development of special machines for its applying on turbine blades using plasma spraying process.
The nozzle itself – especially the NK-32 nozzle– is of de Laval type, which features variable geometry depending on the operating mode of the engine. It provides an additional margin of gas-dynamic stability. The compressor is divided into three parts: LP, MD, and HP. The shafts are nested within each other, separated by minimum clearances. We had serious problems with vibrations. All three of these shafts meet together at the turbine support, and we needed to develop appropriate seals to separate the individual shafts from each other and all of them from the turbine support.
We used carbon seals, and we also needed to maintain minimum clearances at the blades. These were new challenges that had to be overcome before we could launch serial production of these engines.
It wasn’t easy. We worked in three shifts; the top managers did not seem to leave the plant at all. But as a result we developed a good, flight-worthy engine. There were numerous challenges, and some of them still remain, but I am always pleased when we are a success.
Q: What was the maximum output of the NK-25 and NK-32 engines during the Soviet period?
A: Up to 10 engines per month.
Q: Was there anything noteworthy about the use of these engines in the Soviet Air Force?
A: Before the Tu-22M bomber, the Air Force used the Tu-16, so that was the model the pilots transitioned from. But I believe the transition process went on smoothly. There were more problems with the Tu-22 planes powered by the RD-7 engines made in Rybinsk. The plane itself was beautiful, shaped like a bullet. But it had very high landing speeds because of its fixed-geometry wing; and the emergency ejection system ejected the pilot downwards rather than upwards. I think the pilots were quite happy to switch to the Tu-22M2 and Tu-22M3.
There were two pilot training centers back at the time (one in Ryazan for the Air Force, and another in Nikolayev for Navy pilots). From my conversations with several pilots, I got the impression that they were happy with the new planes.
Q: Were there any teething problems with the NK-32 engines at the combat units?
A: All the teething problems had been dealt with at the NK-25 development phase. There were some small difficulties with the nozzle feedback, but they weren’t critical, and we resolved them very quickly. The new engine had a more complicated nozzle design. But those difficulties were never a limiting factor, and they have never caused any major incidents.
On the whole, the Tu-160 is an excellent plane – and it probably remains one of the best in the world.
Q: What can you tell us about the NK-36 gas turbine used in gas-pumping lines?
A: It’s a 25MW engine, used to drive the compressors of the Nord Stream 1 gas pipeline. It will also be used in the Nord Stream 2. There are a total of 13 compressor stations equipped with 41 NK-36 gas turbines. The nearest station that uses the NK-36 gas turbine is the experimental station in Tolyatti.
Our NK-37 gas turbines, which are very similar to the NK-36, are used at the heat-and-power plants in Kazan and Grodno. They are switched on during the peak demand hours, and as a reserve power supply.
Q: Were you involved in the development of gas turbine locomotives?
A: We already have two of them, and their performance is worthy of the Guinness Book of World Records. They have already set a world record for the heaviest train pulled by a single locomotive: 159 carriages weighing 15,000 tonnes. The gas turbine locomotives present an excellent technology, and there is a lot of demand for it, especially in the North, where the rail network is not electrified. A gas turbine locomotive with a gas tank carriage attached to it can travel up to 1,000 km without refueling. So that is a very promising technology. It arose a keen interest not only in Russia but in India as well.
Q: Your company supplied the burner for the Olympic flame at the Sochi Games. Can you fill us in on that project?
A: The Sochi Olympic flame project had very tight schedule. The Olympic Committee was about to start sending out invitations, but the supplier of the flame burner had yet to be chosen. I hosted the project managers here at the plant, and showed them what we could do. They awarded the contract to us. And again, chief designer Kuznetsov brought up a whole generation of excellent specialists in combustion technology. We had excellent combustors, with good environmental characteristics in terms of the CO and CX emissions.
A burner is not as complex as a combustion chamber. But the burner for the Olympic flame must keep working even at hurricane-strength side winds. We had to conduct corresponding tests. We used the NK-12 engine to generate the hurricane side wind at the burner. We engineered the burner to perfection; in particular, we had to improve the fuel consumption. But in the end, we had total confidence that our burner would keep burning no matter what Mother Nature threw at it – and so it did.
However when the Olympic Games ended, the flame had to be lit up once again for the Paralympic Games. When we checked the burner, it turned out that the natural gas fed to it wasn’t clean enough. We had to do a bit of maintenance on the burner, and it lit up once again without any problems for the Paralympics.
It was an interesting project, and we were proud that it was entrusted to us. We all watched the Olympic Flame Lighting Ceremony on television.
Interview by Andrey Frolov