DEFENCE TURKEY: First of all, could you briefly introduce the TEI-TF6000 Turbofan Engine, which was designed in less than two years and exhibited for the first time at TEKNOFEST Black Sea? What can you say about the compressor and fan design and the number of turbine stages?

Prof. Dr. Mahmut F. AKŞİT: Our TEI-TF6000 engine is actually an engine that we, as the TEI engineering team, built to train our team and develop them as a talent on our way to the National Combat Aircraft (MMU/TF-X). We have brought it up to this point with our own means. It is actually the most powerful engine ever designed and developed in Türkiye. It produces 6,000 lbf of thrust. The engine's thrust capacity can be increased to 10,000 lbf if an afterburner is added to the exhaust section. So it's a pretty powerful engine. We have built our engine as a technology development project to study the technologies to be used in the National Combat Aircraft's engine. We will also use it to validate and calibrate the design analysis software developed by our engineering team and the hypothesis’ used in the design.

DEFENCE TURKEY: It's kind of like a steppingstone for the MMU/TF-X engine...

Prof. Dr. Mahmut F. AKŞİT: Yes, we aim to validate the technologies to be used in the larger engine via the TF6000 and then apply them to the larger engine...

DEFENCE TURKEY: Isn't TEI one of the three companies already bidding for the MMU/TF-X Engine?

Prof. Dr. Mahmut F. AKŞİT: As we mentioned before, TEI is involved in the national development of the MMU/TF-X engine. It is essentially a military turbofan engine configuration. There are two fan stages in the front and compressor stages behind it…

DEFENCE TURKEY: Do commercial engines have a different fan placement?

Prof. Dr. Mahmut F. AKŞİT: Commercial engines have a large single-stage fan at the front. A massive fan, you can even see the light behind it. In commercial turbofans, the actual corer engine is located in the middle. Because the fan is so large, most of the airflow bypasses the engine directly to the rear. In military turbofans, a small part of the air still bypasses the engine, while a significant amount flows through the core to produce the actual thrust. The air is sucked by the compressor and fed into the combustion chamber where it is, compressed, mixed with fuel, ignited, and ejected from the nozzle at the back. Some of this power is used to spin the fan. Military turbofan engines have a smaller fan diameter. There are different reasons for this. First, if you want to reach high speeds, it is not possible to go higher than the speed of sound with such a big fan. As the speed increases, the large cross-section creates greater drag resistance. It is more convenient to use small size fans in military turbofans. In fact, this is what military turbofans are made for. One advantage is, of course, the afterburner configuration. The bypassing stream of air around the engine core reduces fuel consumption and increases range. In this way, you can fly more economically in military aircraft, except for emergencies. A typical jet engine uses only about half the oxygen it ingests. In emergency and high-thrust situations, the afterburner combines much of the remaining oxygen with jet fuel, injected into the high-speed exhaust stream from the engine's turbine, and ignites the mixture. That's how you get to 10,000 lbf. This is an indispensable need for military engines.

DEFENCE TURKEY: You said there are compressor stages behind the fans…

Prof. Dr. Mahmut F. AKŞİT: Yes, there is a compressor, but the compressor does not ingest all the air provided by the fan. In this configuration, it uses about half of it.

DEFENCE TURKEY: Does the TEI-TF6000 have single crystal turbine blades?

Prof. Dr. Mahmut F. AKŞİT: Yes, after the air from the compressor burns in the combustion chamber, the hot exhaust gas hits the turbine blades. And since turbine blades are in direct contact with the flame and the hot exhaust gas, they must be made of heat-resistant single crystals to operate at higher temperatures.

DEFENCE TURKEY: So are the single crystal turbine blades and the IBR used in the F135 TF Engine the same thing?

Prof. Dr. Mahmut F. AKŞİT: No, IBR is used on the compressor. We call these new generation blades blisk (compressor disc and blades produced as a single piece), which means "bladed disc." Some other engine manufacturers call it IBR (Integrally Bladed Rotors). They are essentially the same thing. In other words, the blades are not manufactured separately and stacked on a disc. Discs and blades are integrated; they are designed and manufactured together.

DEFENCE TURKEY: Does the TEI-TF6000 Engine also have blisk technology?

Prof. Dr. Mahmut F. AKŞİT: Yes, our engine has that new generation blisk or IBR technology. This directly affects the efficiency of the engine.

DEFENCE TURKEY: Mr. Akşit, you mentioned that the temperatures in the combustion chamber reach very high degrees. High temperatures require cooling and special materials. Are these materials (such as titanium, nickel, steel, aluminum, and composite), especially needed for the hot section in the TEI-TF6000 Engine Project, produced domestically or outsourced? Did the Aviation Engine Materials Development Projects previously initiated by the SSB and TEI, such as CEVHER and KÜLÇE, provide the desired solutions?

Prof. Dr. Mahmut F. AKŞİT: At TEI, we saw this need even before our first helicopter engine project started, so we began working with our own means, just like the TEI-TF6000 engine. As you already know, the number of projects supported by the SSB has increased significantly. Their resources are also limited; the SSB cannot support every project at all times. So, what are we doing? We start to work on the technologies that are necessary, beforehand. When they reach a certain point, we ask for additional support from the SSB and continue our studies. For example, we started the Single Crystal Turbine Blade Casting Process Techniques studies before the TEI-TS1400 Turboshaft Engine Development Project. Afterward, we turned it into an SSB R&D Project, and thanks to their support, we successfully cast single crystal blades for the first time in Türkiye. Likewise, we started the CEVHER-1 and CEVHER-2 projects, which focus on material technologies. Now we are beginning the second phase of CEVHER-2. We carry out R&D projects to produce the strategic materials that we are dependent on abroad, step by step, in Türkiye.

DEFENCE TURKEY: In which parts of the engine are Titanium, Aluminum, Steel, and Composite materials used in the engine?

Prof. Dr. Mahmut F. AKŞİT: Generally, the compressor side on the main flow path at the front is made of titanium. It is relatively cold. The turbine stages behind the combustion chamber are made of nickel superalloys and are directly exposed to the flame. Unfortunately, titanium cannot work at those high temperatures. In other words, the front of the engine is usually titanium, and the rear side is usually nickel superalloy.

DEFENCE TURKEY: Both materials are used heavily in aviation (including the engine). So, is TEI working on producing these materials locally under the CEVHER-1 and CEVHER-2 Projects, which aim to develop materials and manufacturing processes used in aviation?

Prof. Dr. Mahmut F. AKŞİT: Yes. Of course, they have different grades. The material developed for blade casting is a nickel superalloy, but it can be used in the single-crystal form. There are also other materials we use for stator nozzles and vanes. They are fixed parts and do not spin. They are also nickel superalloy, but they can be made of other materials as well. As I said, they have different grades. Under the CEVHER-1 and CEVHER-2, we developed the most urgent and indispensable ones in the first stage. Now we have moved to the second stage with CEVHER-2. In the long term, we are following a strategy that will hopefully produce all the critical materials that our engine needs in our country. Let me give you the good news about alloys. As of today, we have developed the single crystal blade material in Türkiye.

DEFENCE TURKEY: Do you work with TÜBİTAK MAM Materials Institute?

Prof. Dr. Mahmut F. AKŞİT: Yes, TÜBİTAK is also involved as well as our subsidiary companies (GÜR METAL and Varzene). We have a large material engineering team in our R&D Department. We have a Material Process Department of its own. Under their coordination, we developed both single crystal blade material and alloys, such as the n superalloy “Inconel 718, 718 Plus, 738” in Türkiye, together with the TÜBİTAK MAM Materials Institute team and our subsidiaries. But there are more special materials with higher quality and higher resilience, such as those used in discs, and we are currently working on them.

DEFENCE TURKEY: What is the current situation in the TEI-TF6000 Turbofan Engine Project? Has prototype production started?

Prof. Dr. Mahmut F. AKŞİT: Engine design is a cyclical, iterative process. For example, we fired the core of our helicopter engine,the TEI-TS1400, the first helicopter engine manufactured in Türkiye, in November 2017. You can find its videos online. That engine, for example, was in the second design cycle. In other words, the engine was designed from the ground up once, then optimized and designed once again. We manufactured and ran that second design. That version was tested for hundreds of hours while design iterations continued. For better, results and continuous improvements.. At the moment, we will produce and deliver our first two engines, which we will hopefully approve for manned flight, before the end of this year. Actually, TUSAŞ asked us to do some of the tests at TEI before delivering the engines. Therefore, we will carry out those tests and send the engines after that. So maybe it will be delayed until mid or end of February 2023, but it doesn't matter.

The TEI-TS1400 engines, which we will deliver ready for the first manned flight, are in the 8th design cycle. Now coming to the TEI-TF6000 engine, this engine is still in its early design cycles. During the TEI-TS1400 helicopter engine process, we decided that it could be fired in the second design cycle. Of course, with the experience from the helicopter engine, we progressed faster in the design cycles of the TEI-TF6000 and started the prototype manufacturing process within 2.5 years. So the first TF6000 prototype is currently in production.

DEFENCE TURKEY: Can you give us a date for the first test run?

Prof. Dr. Mahmut F. AKŞİT: With such large engines, manufacturing the first prototype especially takes a significant amount of time because you need to write CNC codes for individual models of all parts and test sample pieces. You also prepare molds when necessary. You can proceed to manufacture actual materials after validating that the software processes everything at the desired size. It takes quite a while to do this for hundreds of parts. Manufacturing engineering is also very important here. It normally takes at least two or three years. We are currently trying to fit the whole process into one year.

DEFENCE TURKEY: Will you show the prototype of the TEI-TF6000 at the IDEF '23 Fair?

Prof. Dr. Mahmut F. AKŞİT: We are pushing the team right now, but there are also problems with the supply of materials. Supply chains are broken all over the world, especially due to COVID. So think of an engine; it has hundreds and thousands of parts and components.

DEFENCE TURKEY: Can you share the number of parts in the TEI-TF6000 engine?

Prof. Dr. Mahmut F. AKŞİT: The number of main parts that make up the actual engine is between 200-300. However, there are many additional things on it, such as more than 1,500 screws, bolts, cables, and clips, large and small.

DEFENCE TURKEY: Mr. Akşit, at the beginning of our conversation, you pointed out that TEI designs the TEI-TF6000 as a capability to train the team on the way to the MMU/TF-X. But ultimately, you will deliver a TF engine, which will incur a development cost. On which platforms do you intend to use the TEI-TF 6000? For example, KIZILELMA MIUS?

Prof. Dr. Mahmut F. AKŞİT: Let's put it this way, as we have said for our the TEI-TJ300 Turbojet Engine before, the TEI-TF6000 is also a technology acquisition and know-how build-up project. Think about it; we made the TEI-TJ300 after the TEI-TJ90 Engine. We made it to gain experience. We can say that the TEI-TJ90 was our first jet engine. A small engine that powers the ŞİMŞEK High-Speed Target Drone. The TEI-TJ300 is specially designed with an axial compressor. The reason is that all these military turbofan engines have axial compressors. The TEI-TF6000 also has an axial compressor. We are adopting the know-how from the TEI-TJ300 to a slightly larger scale to use as a steppingstone. But we pay special attention when choosing the sizes of these intermediate products to use in possible projects. Our country has to someway benefit from these efforts, right? For example, the TEI-TJ300 has been utilized an anti-ship missile engine. MIUS can use the the TEI-TF6000 engine; it is already suitable for its power class. We specifically chose that. Also, four of these engines can power an assault boat. It can also be used as a gas turbine to generate electricity... It produces enough power to supply the electricity of approximately 3,000-4,000 homes on its own.

DEFENCE TURKEY: Could you give us some information about the latest situation in the TEI-TS1400 Turboshaft Engine Program? Finally, with the use of Alp Aviation's gearbox, we believe the engine has become 100% indigenously produced. When do you plan to begin on-platform flight tests?

Prof. Dr. Mahmut F. AKŞİT: As I already mentioned, the latest versions of our TEI-TS1400 engines are currently in the assembly phase. We will finish their assembly before the end of the year. That was our goal. We will conduct some required tests per our discussions with TUSAŞ before delivering them. If nothing goes wrong, we will provide the engines to them at the end of February next year, with all the mandatory tests completed. If they can speed up the integration process into the helicopter, we hope that we can take off with our national engine in the first half of next year.

DEFENCE TURKEY: Are the 8th design cycle engines you will deliver compatible with the T625's engine bay? Will you work on adapting the engine mounts on the T625 GÖKBEY Helicopter after the delivery?

Prof. Dr. Mahmut F. AKŞİT: Of course. The first prototype we delivered at the ceremony attended by our President is 'form fit.' It has the exact dimensions of the latest mature version of the engine. It fits in the same place, the connections are the same, and everything is the same. That's why we delivered it as soon as possible so they can start the formality and integration work early. So, when we provide the final design engines, TUSAŞ can plug them in quickly and fly. Otherwise, the integration process of an engine into an aircraft takes several years.

DEFENCE TURKEY: After all, it is a brand-new engine that has never flown. Will it be airworthy immediately upon delivery, or will the TEI-TS1400 be used together with a foreign engine on the helicopter?

Prof. Dr. Mahmut F. AKŞİT: As far as I know, both engines on the helicopter will be the TEI-TS1400 on the first flight. Of course, TUSAŞ will decide this. First, they secure the helicopter to the ground and run up the main rotor to full power. If there is no problem, they untie the chains and then perform the tests 10-15 meters above the ground. Therefore, they do not immediately take off with the helicopter.

DEFENCE TURKEY: A similar procedure was followed in the maiden flight of the T625 GÖKBEY helicopter.

Prof. Dr. Mahmut F. AKŞİT: Yes, we call it "hovering." We expect the same here. In other words, if we pass the maximum performance tests on the ground without any problems towards the middle of next year, I hope we will see the helicopter take off from the ground.