Presently jet engines are the workhorses of airplanes carrying millions of people, trillions of miles every year, at supersonic speed and with high safety; the failure rate is only once every million flight hours. NASA explains its principle, “All jet engines, which are also called gas turbines, work on the same principle. The engine sucks air in at the front with a fan. The fan is spinning in the front of the engine, which continually draws a large amount of air into the engine. The fan generates the bulk of the thrust that moves the airplane forward.
The first stop in the core is a component called the compressor. A compressor raises the pressure of the air. The compressor is made with many blades attached to a shaft. The blades spin at high speed and compress or squeeze the air.
The compressed air is fed into the combustor, also known as the combustion chamber. The compressed air is then sprayed with fuel and an electric spark lights the mixture. The burning gases expand and blast out through the nozzle, at the back of the engine. As the jets of gas shoot backward, the engine and the aircraft are thrust forward.
As the hot air is going to the nozzle, it passes through another group of blades called the turbine, located near the back of the engine. The turbine is attached to the same shaft as the compressor. Spinning the turbine causes the compressor to spin. The turbine blade extracts the energy from the high pressure and temperature air to keep the fan at the front spinning and drawing in more air. Along the way, power is generated by the spinning components in the engine’s core to charge the electrical components of the aircraft, similar to how your car’s battery is recharged while driving on the interstate.
There are two main species of jet engines for aviation: low-bypass turbofans, usually called turbojets, and high-bypass turbofans. Turbojets are optimized for high-performance, pushing fighter jets to above Mach 2 (and the SR-71 “Blackbird” to well over Mach 3), but pay for that performance with terrible fuel efficiency. The performance outcome of a conventional turbojet is dominated by the operation of the high-pressure engine core (compressor, combustion, turbine, and exhaust nozzle).
In contrast, high-bypass turbofans are the heavy lifters of commercial aviation, being optimized for subsonic thrust and fuel efficiency, but performing poorly at supersonic speeds. A conventional turbofan adds lower-pressure airflow from an oversized fan which is driven by the jet turbine. The fan airflow bypasses the combustion chamber, acting like a large propeller.
Over the years jet engine technology has developed in leaps and bounds. The target has been to enhance power with reduced size and weight, low maintenance cost and maintenance time, low noise etc. There have been developments in the STOL and VTOL capabilities also. F135-600 engine of Joint Strike Fighter is one such example.
NASA’s HyTEC, reported in May 2022
HyTEC, or Hybrid Thermally Efficient Core, is a NASA project kicked off in June 2021 seeking to make the aviation industry more sustainable by developing a small core for a turbofan jet engine that increases fuel efficiency. The project also includes work on hybridization – developing methods to pull more electrical power from this engine to power other systems aboard the aircraft, which could increase fuel efficiency in much the same way a hybrid car does.
The key to HyTEC’s work is what’s called a bypass ratio. This is the ratio of how much air the fan pushes out as thrust versus how much enters the engine’s core to power the fan itself. For example, the GE engines currently used on the Boeing 787 Dreamliner have a bypass ratio of 9. This means that nine times more air bypasses the engine’s core as thrust instead of entering the core.
HyTEC is working on drastically increasing this bypass ratio. The project aims to create an engine with a bypass ratio of 15 by keeping the fan the same size while shrinking the engine’s core within – all while maintaining the same thrust level.
In order to maintain the same level of thrust as a larger core, however, the pressure and temperature of the air that’s pushed into the core increases. This more efficient, smaller core powering an equal-size fan will deliver the same thrust while using less fuel. Because the materials currently used in engine construction cannot withstand the increased pressure and heat, the project is developing newer, more durable construction materials like ceramic matrix composites and environmental barrier coatings.
These materials must be tested and proven in the lab. Researchers are currently working in the materials laboratories at NASA Glenn and at partner facilities to make sure the potential materials can withstand the heat and pressure and are sufficiently durable.
With a smaller engine core, aspects like design ratios and dimensions or aerodynamics change, causing efficiency penalties. To maintain the performance while shrinking the size, new technology innovations and designs need to be investigated and tested.
Once these components are demonstrated, the HyTEC team will work with industry partners to construct a fully operational small core and run it through its paces. During this phase, they will be examining how much more electrical power can be drawn from the small-core engine, which also increases the engine’s fuel efficiency by making it more of a hybrid engine.
“Right now, the most electrified engine out there is on the Boeing 787 Dreamliner, and those engines give 5% of their power to electricity. We want to jump that up to 10-20%. That’s 2-4 times more electrical power,” said Nerone. This electrical power can be used in many ways – from powering electrical systems aboard the aircraft to adding hybrid technology to the jet engines themselves, augmenting power during flight.
Once these green technologies are demonstrated and proven, HyTEC’s goal is to have them readily available for adoption by the aviation industry in the next decade.“What we’re trying to do at NASA is push more next-generation sustainable propulsion technologies into jet engines using our unique expertise and funding,” said Anthony Nerone, who leads the HyTEC project from NASA’s Glenn Research Center in Cleveland. “With the small-core engine in specific, we are targeting a 5-10% fuel burn reduction.”
Next Generation Engines
United States and other countries are now developing sixth-generation fighters, a conceptualized class of fighter aircraft design more advanced than the fifth-generation jet fighters which are currently in service.
The next generation of military engines will be adaptive, moving from high bypass ratios to low bypass ratios to provide additional performance when necessary, but also optimize performance in cruising modes.
Russia started work on 6th Generation engines
Russian designers have already started work on the creation of a sixth-generation fighter jet, according to director-general of the Foundation for Advanced Research Andrei Grigoryev. Russia has planned for new aircraft that is capable of reaching speeds above 11 thousand kilometers per hour, is super-maneuverable and is capable of maneuvering into the near outer space.
Russia’s United Engine Corporation (part of the state tech corporation Rostec) is launching R&D work on the sixth-generation engine for combat aircraft, United Engine Corporation Deputy CEO for Strategy Mikhail Remizov told TASS in July 2021.
“Now work is starting in several promising areas: the technologies of the sixth-generation engine, a combined powerplant, and the more electric engine technology. The R&D effort on the sixth-generation demonstrator engine technology has been included in the long-term work schedule of the United Engine Corporation and our applications for state program financing,” he said.
“Since the arrival of the high-bypass turbofan almost 50 years ago, engineers have made huge strides in jet engine efficiency without altering the basic architecture. Two air flows of constant volume either bypass or enter a core section. The core combusts a mixture of compressed air and fuel to generate thermal energy, which is then converted to supply some of the aircraft’s thrust and most of the power,” says AFRL
For combat aircraft in the mid-2030s, that basic architecture is not going to work, according to the US Air Force Research Laboratory (AFRL). USAF has plans to insert laser weapons into future fighters and armed drones, that will overwhelm their power and thermal management capacity. Ever-growing range and endurance standards require another step-change in fuel efficiency for thrust, while advances in ground-based air defences will only increase demand for instant acceleration and nimble manoeuvring. “In some ways, you’re going to ask the engine to do things they haven’t been asked to do before,” says Chuck Cross, the AFRL’s chief of the Turbine Engine Division.
The future commercial aviation requires flying cleaner, quieter and using less fuel.
In the AFRL’s vision, the architecture of the jet engine for military aircraft will change dramatically over the next two decades. Bypass ratios will ebb and flow depending on mission need. Key elements of the compressor will change shape in mid-flight, reshaping the air flow as it is squeezed en route to the combustor. Electrical power could be extracted from low-pressure and high-pressure compressor sections, feeding energy to power-hungry lasers and advanced sensors. The heat created by that power will be stored in newly-created systems, such as electrical accumulators or wax-based heat exchangers.
Safran contracted to speed up 6th generation fighter jet engine development, reported in March 2021
The New Generation Fighter (NGF) sixth-generation fighter jet developed by France, Germany, and Spain in the framework of the Future Combat Air System (FCAS) will require more powerful engines than the ones currently equipping the Dassault Rafale (Safran M88) or the Eurofighter Typhoon (Eurojet EJ200).
Higher thrust means higher temperatures. According to the French Ministry of Armed Forces, those temperatures could reach 2100 Kelvin (1826.85 degrees Celsius) at the turbine inlet – 250 Kelvin (250 degrees Celsius) more than those of the M88. Current materials are not able to sustain such conditions.
Therefore, the French Ministry of the Armed Forces announced the launch of the ADAMANT (Acceleration of the Development of Alloys and Multilayer Systems for Application to New Turbines, quite a mouthful) research project. It brings together the Ministry’s procurement and innovation agency (DGA), the French national aerospace research center (ONERA), and the engine manufacturer Safran.
The project aims to accelerate the development of “new metal alloys and multilayer systems for high-temperature applications on turbine blades and disks.” To do so in short time constraints that traditional techniques could not meet, the program will use several innovative solutions, such as digital metallurgy, artificial intelligence, and data mining. “The success of this project will be based on the association of the scientific expertise of metallurgists, chemists and mechanics, with that of specialists in new digital methods,” explains the Ministry.
The development of the FCAS engine was entrusted to Safran and the German manufacturer MTU Aero Engines. After an initial development led by Safran, the two manufacturers will eventually set up a joint venture, aimed at achieving the certification of the new engine.
Britain and Japan to Jointly Develop Engines for F-3 Sixth Generation Fighter
Japan is one of several countries confirmed to be developing a sixth generation fighter aircraft, alongside leading contenders in the field including China, the United States and Russia.
The upcoming F-3 sixth generation fighter will replace the F-2 and much of Japan’s license built fleet of heavyweight F-15 fighters in service in the early-mid 2030s, and is being developed to bridge the performance gap with neighbouring China and Russia – the former which fields the world’s only full-strength squadrons of non-American fifth generation fighters, and the latter which is developing at least two sixth generation combat jets.
To develop engines for the sixth generation fighter, Japan’s IHI Corporation will reportedly partner with Britain’s Rolls-Royce, with the final design developed potentially also influencing Britain’s own sixth generation fighter program the Tempest. Japan has notably flown a testbed for next generation fighter technologies, the Shinshin X2, although even working with Britain the country may struggle to develop an engine on par with that expected to be used by America’s sixth generation fighter.
Advanced Turbine Technologies for Affordable Mission-Capability
The ATTAM Phase I Program is a joint DoD/NASA/DOE/FAA/Industry effort to develop revolutionary and innovative technologies by the 2026 timeframe that will provide an increase in fuel efficiency, propulsive capability and increase power and thermal management goals.
Advanced Turbine Technology for Affordable Mission Capability (ATTAM), is the third in a 29-year-old series of engine development umbrella projects. The first such initiative — Integrated High-Performance Turbine Engine Technology (IHPTET) — lasted from 1987 to 2005, yielding a step-change in thrust and power performance with the Pratt & Whitney F119 and F135 fighter engines. A second program — Versatile Affordable Advanced Turbine Engines (VAATE) — sought to deliver another step-change in power, while adding a focus on driving down development and maintenance costs.
The overall goals of the ATTAM Phase I program are to (each goal depends on engine class) are Increase fuel efficiency from 10%-30% , Increase power and thermal management goals by 2x to 20x and Increase propulsive capability by 10%-25% . ATTAM will be inter-connected with the AFRL’s aircraft based efforts, including projects called Energy Optimised Aircraft and the Megawatt Tactical Aircraft Initiative, Cross says.
Several VAATE initiatives set the stage for further advances that will be pursued under ATTAM. For example, the AFRL has funded GE Aviation and Pratt & Whitney to demonstrate separate prototypes of a jet engine with an adaptable bypass flow. These adaptive engine technology demonstrators (AETDs) will lead to a follow-on competition to develop a final prototype of an engine that can be inserted into the Lockheed Martin F-35A after 2021, allowing that short-ranged fighter a range boost of up to 25%, writes Stephen Trimble.
Another programme that will transition from the VAATE initiative is a follow-on from the highly efficient embedded turbine engine (HEETE). Originally conceived for mobility aircraft, the advanced compressor developed under the re-named ADAPT programme will feature variable stator vanes inside the compressor, shifting the direction of the airflow as the AETD technology adjusts the amount of bypass flow. Additionally, future engine cores could engage another stage of compression in certain flight modes, writes Stephen Trimble.
As the engines become more efficient, the materials used inside the core must be adapted to endure hotter temperatures, Cross says. So temperature-resistant metallics and ceramic matrix composites now used behind the combustor could migrate to the last stages of the high-pressure compressor, he says.
For ATTAM, however, the key is to integrate the aircraft’s propulsion system and onboard power systems. “The big challenge we see in the future is how do I really design and procure systems that take into account all these different variables, and it may not mean buying the best engine – the most optimised cycle. You may sub-optimise one part for the greater system,” Cross says. “As we work closer with the power and energy community, we start to really investigate those give and takes.”
Kratos Defense & Security Solutions has revealed the completion of a core engine test campaign supporting development of an affordable powerplant for future munitions and attritable unmanned aerial vehicles (UAVs).
Announcing the milestone in June 2021 the company said that its Kratos Turbine Technologies (KTT) division had “successfully demonstrated key performance and operability targets of the core engine” in conjunction with the Turbine Engine Division of the Air Force Research Laboratory (AFRL/RQT).
ADVENT (ADaptive VErsitile ENgine Technology) for future fighter
The Air Force Research Laboratory has been working with GE and P&W on adaptive, “three-stream” engine technology for several years, under a science and technology program called Adaptive Engine Technology Development (AETD). Both companies finished up design review this year, and will continue to build and test individual components under AETD. The follow-on program, AETP, will build and test full-up engines, Kenyon said.
In an ADVENT (ADaptive VErsitile ENgine Technology) engine, the high-pressure core exhaust and the low-pressure bypass streams of a conventional turbofan are joined by a third, outer flowpath that can be opened and closed in response to flight conditions. For takeoff, the third stream is closed off to reduce the bypass ratio. This sends more of the airflow through the high-pressure core to increase thrust. When cruising, the third bypass stream is opened to increase the bypass ratio and reduce fuel consumption.
The extra bypass duct can be seen running along the top and bottom of the engine. This third duct will be opened or closed as part of a variable cycle to transform it from a strike aircraft engine to a transport-type engine. If the duct is open the bypass ratio will increase, reducing fuel burn, and increasing subsonic range by up to 40 percent, leading to 60 percent longer loiter times on target. If the ducts are closed, additional air is forced through the core and high pressure compressor, enabling thrust and speed to increase and providing world-class supersonic performance.
Full-scale prototype engine testing in the AETP program is the capstone of a multi-year technology maturation and risk reduction effort to bring adaptive cycle engines to full maturity in close partnership with the U.S. Air Force. Efforts began in 2007 with the Adaptive Versatile Engine Technology (ADVENT) program, continued in 2012 with the Adaptive Engine Technology Development (AETD) program, and culminated in the launch of AETP in 2016. This development work provided GE a solid foundation of design analysis and empirical test activities to successfully achieve a full-scale prototype engine. GE is the only engine company to compete for, be awarded, and complete both the preceding ADVENT and AETD programs.
Testing on GE’s First XA100 Adaptive Cycle Engine Concludes
GE’s ADVENT designs are based on new manufacturing technologies like 3-D printing of intricate cooling components and materials such as super-strong but lightweight ceramic matrix composites and titanium aluminides, and techniques such as additive manufacturing, to make the engines lighter and more robust while running hotter and providing more power. GE’s AETD design improves fuel consumption by 25 percent, increases thrust by 20 percent, and extends aircraft operating range by 30 percent
GE’s David Tweedie, general manager for advanced combat systems, said his company’s engine also meets the Air Force’s goals and offers “a significant reduction in carbon emissions” as a byproduct. Fighter engine technology hit a wall in the early 2000s. Engineers struggled to squeeze even small improvements in thrust or range from fighter turbofan designs. Adaptive technology—which adds a third stream of airflow to the engine and the ability to adjust it—offered a way to break through that wall.
“There were three major technology efforts,” according to Tweedie. The first was adaptive technology, “the ability to reconfigure, in flight, toward either a more fuel-efficient mode or a high-thrust mode,” he said. Second was creating “the third-stream architecture for thermal management demands … unique to fifth- and sixth-generation combat aircraft.” The third was “advanced … manufacturing techniques.”
New ceramic matrix composites (CMCs) replace metal alloys in some critical components, offering “lighter weight [and] higher temperature capability,” Tweedie said. This enables the engines to run hotter and thus more efficiently, without sacrificing durability. Additive manufacturing—also called 3D printing—also“really helped engineers unlock the design space … to be able to answer questions like, ‘How do you fit all this into a real airplane, like the F-35?’”
GE has completed testing of its first XA100 adaptive cycle engine, ushering in a new era of combat propulsion. GE initiated testing at its Evendale, Ohio, altitude test facility on December 22, 2020. The engine’s performance and mechanical behavior were consistent with pre-test predictions and fully aligned with the U.S. Air Force’s Adaptive Engine Transition Program (AETP) objectives. This successful test validates the ability of GE’s XA100 engine to deliver transformational propulsion capability to fighter aircraft.
“We were exceptionally pleased with how the engine performed throughout the test,” said David Tweedie, GE Edison Works’ General Manager for Advanced Combat Engines. “Bringing a new centerline fighter engine to test for the first time is a challenging endeavor, and this success is a testament to the great team that worked so hard to get us here. We’re looking forward to working with the Air Force and other stakeholders to identify the next steps toward bringing this revolutionary capability out of the test cell and into the hands of the warfighter.”
The XA100-GE-100 engine combines three key innovations to deliver a generational change in combat propulsion performance:
- An adaptive engine cycle that provides both a high-thrust mode for maximum power and a high-efficiency mode for optimum fuel savings and loiter time
- A third-stream architecture that provides a step-change in thermal management capability, enabling future mission systems for increased combat effectiveness
- Extensive use of advanced component technologies, including ceramic matrix composites (CMC), polymer matrix composites (PMC), and additive manufacturing
These revolutionary innovations increase thrust 10%, improve fuel efficiency by 25%, and provide significantly more aircraft heat dissipation capacity, all within the same physical envelope as current propulsion systems.
Assembly of GE’s second prototype XA100 engine is well underway, with testing on that engine expected to begin later in 2021. Once complete, that will conclude the major deliverables of the AETP program
GE’s Future Affordable Turbine Engine (FATE) program
The FATE program set its goals at 35 percent reduction in specific fuel consumption, 80 percent improvement in power-to-weight, 20 percent improvement in design life and a 45 percent reduction in production and maintenance costs relative to currently fielded engines.
GE also successfully tested a FATE inlet particle separator, compressor, combustor and turbines that validated advanced technologies like 3D aero designs, ceramic matrix composites and additive manufacturing, in which the company invests $1.8 billion annually to develop.
“The FATE engine is the world’s most advanced turboshaft engine, incorporating technologies for the next generation of propulsion,” Harry Nahatis, GE Aviation’s general manager of Advanced Turboshaft Programs, said. “We’re very encouraged by the test results thus far and are incorporating the lessons learned into our ITEP offering.”
A geared design with a variable pitch system, UltraFan™ is based on technology that could be ready for service from 2025 and will offer at least 25 per cent improvement in fuel burn and emissions against the same baseline.
UltraFan will utilise the new advanced core architecture, enhanced with further technologies and the broader application of innovative high-temperature materials to push the core overall pressure ratio to more than 70:1. UltraFan features a new geared architecture to meet the challenges of the future.
A power gearbox is introduced between the fan and intermediate pressure compressor to ensure the fan runs at its optimum speed. In common with three-shaft architecture, the engine compressor and turbine continue to run at their optimum speed to deliver optimum performance. The carbon titanium fan system is further developed to allow the deletion of the thrust reverser, enabling a truly slim–line nacelle system.
Boeing patent reveals radical ‘fusion’ engine powered by lasers in 2015
Future aircraft could be powered by lasers and nuclear explosions if Boeing has its way. The aerospace firm claims a new-type of engine could produce energy-efficient thrust by firing lasers at radioactive material, such as deuterium and tritium. The technology could mean that planes and spaceships will require only a fraction of the power to operate, according to a recent patent filed by the company.
US Patent and Trademark Office approved an application from Boeing’s Robert Budica, James Herzberg, and Frank Chandler for a laser-and-nuclear driven airplane engine. According to the patent filing, the laser engine may also be used to power rockets, missiles, and even spacecraft.
‘At least one laser is positioned to vapourise the propellanet with at least one laser-beam into a thrust producing flow,’ the company wrote in the document. A report in Business Insider compared the process to a small thermonuclear explosion.
High-powered lasers will be used to vaporise the radioactive material producing fusion a reaction. The by-products of the process would be hydrogen or helium, which would leave the back-end of the plane creating thrust. Meanwhile, the inside wall of the engine’s thruster chamber will react with the neutrons created by the nuclear reaction. The resulting heat can then be harnessed by placing a coolant on the sides of the combustion chamber. The idea is to use this heat to produce electricity that can then drive the engine’s lasers. Other than the radioactive material, the engine requires very little in terms of external energy.
Boeing claims energy-efficient thrust can be produced by firing lasers at deuterium and tritium and then having the neutrons activate uranium 238 to generate more heat.
* Hot gases produced by the laser induced fusion are pushed out of a nozzle at the back of the engine, creating thrust.
* a neutrons hit a shell of uranium 238 which causes fission and generates lots of heat
* a heat exchanger uses the heat from the fission reaction to drive a turbine that generates the electricity that powers the lasers
They have different configurations
* one configuration generates ISP of about 2000 to 5000 seconds
* another configuration has an ISP of about 5000 to 25000 seconds
* another configuration an ISP of about 100,000 to 250,000.
Several energy conversion technologies have been identified or proposed that may have potential for application in commercial aircraft e.g., solar power, nuclear power, battery/fuel cell power, and hydrogen engines. However, these advanced concepts would require major innovations, development, and changes in infrastructure before they could serve as viable alternatives to hydrocarbon-powered gas turbine engines.
“The future will continue to see different and diverging requirements for the military and commercial sectors. Variable mission and adaptive capabilities will drive military programs, while time on wing and efficiency will continue to be the drivers for commercial aircraft. Material choices will differ with the technological and engineering trade-offs within each sector, and the portfolio of materials for aircraft engines will continue to grow with additional research and development, says Ernest ,” said Arvai at Pratt and Whitney
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