The journey probably starts with a propulsion model that provides a system level look of the engine comprising of a turbo fan, which sucks in air from ambient and sends it aft, some through the core and the rest to bye pass the engine whose only purpose is to generate thrust. The passage of air ingested through a low pressure compressor, high pressure compressor, combustor, high pressure turbine and a low pressure turbine. At it’s heart is the Brayton cycle that takes in compressed high pressure adiabatically heated air from the compressor into a combustor, mixing it with fuel and burning it to generate high velocity combustion gases at temperatures that can exceed 1500C. Very close to the melting point of the alloys used. The high pressure turbine extracts part of that energy converting it to rotational energy which enables the compressor to compress ambient air to higher temperature and pressure and providing further axial thrust as the combustion airplane travels to the aft of the engine. Further energy is extracted in the low pressure turbine till almost all the useful energy is extracted and the combusted air exits. Remember the turbofan? In a commercial engine it is the biggest part of an aircraft engine. 90% of the ingested air travels aft bye passing the core generating thrust. It’s only 10% that travels through the core, getting compressed, burning the fuel to provide energy for the fan to rotate, and finally exiting as hot air and providing the thrust. In a military engine we need speed, acceleration, manoeuvrability. The turbofan is smaller. Most of the airflow goes through the core; to get the added acceleration an after burner is added at the exhaust. When fuel is introduced there with exhaust gases already at several hundred Celsius the energy released is tremendous. The exhaust gases accelerate as they exit, generating tremendous thrust and acceleration. The Mach 2 velocities come from the after burner.
Thus from a system design as described above one comes to the mechanical design. Where each sub system, the compressor, the combustor and high pressure turbine and low pressure turbine is broken down to components. Each component is reduced to a finite element mesh, where each mesh point is defined by stresses acting on it along with the prevailing temperature. The stress-temperature is in essence an ask from the selected material. Can it withstand the conditions at each mesh point taking into account failure modes, yield strength, fatigue strength and creep strength, along with oxidation. Since each subsystem, such as compressors, turbines and combustors are actually an assembly of components, some welded, some bolted, many sliding to form a fit, relative motion comes into picture and wear remains a matter of concern. In the high pressure turbine, temperature are so high that it takes a combination of thermal barrier coatings and cooling airflows to keep temperatures where known alloys can operate. Thus translating the design to practise becomes an exercise in materials selection.
The rubber starts to hit the road here. The jet engines we or our parents may have first flown in the 1970s, five decades back still in a Boeing 747 consumed twice the fuel and generated half the thrust compared to the same engines today per passenger kilometer. Technology has advanced relentlessly. And with it, so have the designs and materials used.
So this brings us to why can’t we make an aircraft engine?
Are our designs up to speed. And do we have materials that meet those needs? Can we manufacture those materials with the required integrity? Can we life those materials and predict how long will they last.
The purpose at this point is not to provide a yes or a no to these probing questions. But articulate a possible strategy to get there. A cursory review of open domain literature will give us compositions of alloys used for compressors airfoils and disks turbine airfoils and high pressure turbine disks. Our first task is to ensure that critical alloy families can be manufactured to the end geometry. By the required process; disks are forged, high temperature airfoils are cast as directionally solidified grains or as single crystals. The national labs serve as nuclei where some capabilities exist to cast airfoils as. Single crystals or directionally solidified. High temperature rotors have alloys which start as powders which are compacted and forged to provide defect free components with required composition. Can the technology be transferred to a select few private sector companies. Can private sector companies step forward do their R&D and make the first entry at the component and subassembly level.
A target for this exercise is to at least reach a point where the select world leaders were at 2000. In 2000 a twin engine aircraft could fly from London to Singapore. This could be our Maruti moment in aircraft engines. But while we are catching up in manufacturing our scientists need to design the next gen aircraft engine with new materials. The infrastructure created for manufacturing needs to be advanced to make new materials. Simultaneously we need to advance subsystem level testing capabilities so that designs and manufacturing are continuously validated.
The road ahead is long. The key cultural shift that needs to happen is that we need to be ruthlessly honest and recognize the current status and the efforts needed. But we need to be confident. Knowing that once we enter the arena we will succeed. Even if it takes 20 years we will prevail.
Dr. Anand K
GE Retiree
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