The key to innovation in the aerospace industry has always been the evolution of propulsion systems and engines. These mechanisms use thrust to propel an object generated through Newton’s Third Law of action and reaction. Propulsion is ultimately responsible for a flight and is the underlying physics principle for the development of modern aircraft engines. As technological possibilities expand, advanced propulsion systems are designed with increased capabilities, efficiency, and thrust. Military aircraft are commonly at the forefront of innovation due to the high-level performance requirements and large investments made into developing them. This report will investigate advanced propulsion systems in the context of the Lockheed Martin F-35 multirole fighter.
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Propulsion is a force that pushes an object forward. Commonly, an aircraft propulsion system consists of an engine that generates an excess thrust which accelerates the plane by overcoming drag. Military aircraft such as the F-35 multirole fighter needs fast acceleration and must be able to maintain high speeds (Hall, 2015). Therefore, propulsion systems in these aircraft are more advanced with afterburners and other components which create excessive thrust to overcome high drag at supersonic speeds. Also, they must meet specifications of maintaining the balance, mobility, and efficiency necessary in a fighter aircraft while being able to withstand the tremendous physical stress of combat flight patterns.
Lockheed Martin’s F-35 Lightning II multirole fighter developed for the U.S. military has one of the most advanced propulsion systems ever designed in the aerospace industry. The defense program benefited from significant investment as well as complex testing and integration of numerous large aerospace contractors cooperating on its production. The F-35 was developed in three variants meant for different branches of the military: Navy, Marines, and Air Force.
Therefore, each model has individual requirements, and designing a propulsion system with support for each one was a monumental achievement of the manufacturer Pratt & Whitney. The F-35A uses a traditional take-off and landing (CTOL) design, similar to the F-35C, which must be able to operate on a naval aircraft carrier (CV system) with its rigorous conditions. Meanwhile, the F-35B operates with a shortened take-off and vertical landing (STOVL), which remains the most challenging to implement.
CTOL and CV propulsion designs are nearly identical, but STOVL consists of a turbomachine similar to the others in combination with unique components. These components include a lift fan, roll post system, clutch driveshaft, three-bearing swivel duct, and a specialized exhaust nozzle (Wurth & Smith, 2018).
The propulsion system on F-35 needed to be designed and tested to meet the requirements of all three variant specifications, but STOVL required special attention. The complexity of this engine, along with necessary ground and in-air testing, led to the establishment of innovative flight test programs. Its design and capabilities can be attributed to high specifications, cohesive systems engineering, and the ability of Pratt & Whitney to overcome development challenges. Overall, the propulsion engine, in this case, was designed as a trivariate solution in order to achieve commonality (such as ease of interchangeable parts) and affordability.
As F-35 came into exploitation by the military, a number of significant issues arose with its advanced propulsion system, which the government considers a vital component. Investigations by the U.S. Government Accountability Office (GAO) and the Department of Defense revealed that the Pratt & Whitney designed engines continued to experience problems and failures. The F-35 aircraft has been grounded for years, and after its introduction into the armed forces, it remains not operationally suitable due to issues with its propulsion system, amongst other shortcomings.
Substantiate the Problem
These issues are worrying considering that F-35 is a vital part of the US and NATO’s latest generation military air superiority strategy. The reliability aspect can be a national security concern as well as a factor of negligence considering the economic resources that have been invested in the plane’s development and production. It is likely that the trivariate approach and complexity of the propulsion engine, which needs to combine thrust and mobility with vertical takeoff while being significantly interchangeable in parts, are contributing to mechanical problems. Nevertheless, it offers consideration that a different advanced propulsion system should be chosen as an alternative for F-35 and future military fighters.
The F-35 engine underwent a seemingly significant amount of testing during development. This includes 17,000 ground test hours and 13,000 flight test hours, along with 3,000 hours of corrosion testing using salt, sand, and rainwater (Thoreson, 2018). The propulsion system also underwent ballistic testing and fuel ingestion to fit within survivability requirements. At least 61 instances were recorded of the F-35 propulsion system failing to meet regulatory standards set by the Pentagon during the inspection.
The issues were widespread, including complications in the supply line and project management along with mechanical and software deficiencies. Aspects such as project management oversight, safety item compliance, risk management, supplier management, software quality, and continuous improvement are all part of the extensive network of problems in the development of the F-35 engine (Bender, 2015).
In its report, GAO indicated that the F-35 propulsion system is severely limited and characterized by poor engine reliability, which requires time and resources to address. Reliability is an indicator of performance for a propulsion system design over a period of time without failure, degradation, or repair. A lack of reliability strongly limits the overall program reliability of F-35 as a multirole combat fighter considered of critical importance for the propulsion system.
Problems with reliability impact safety concerns as well as flight hours since frequent maintenance is required. On average, the F-35B flew 47 hours instead of a required 90 hours before maintenance, while the other variants had 25 flight hours out of the intended 120 hours. In all cases, the fighter could no longer operate safely because of “engine design issues” (Bender, 2015).
The first alternative to the current F-35 engine is using Electrified Aircraft Propulsion (EAP) which uses electric motors to create thrust in propulsors. Distributed electric propulsion (DEP) is a concept that is meant to address environmental concerns of aviation travel as well as logical innovation efficiency and technological capabilities of flight propulsion systems. On such an aircraft, EAP systems and propulsors do not share common power sources or mechanical driveshaft with the power-producing method. The propulsors can be any combination of electrically driven mechanisms such as fans or propellers powered independently. The ability to decouple power sources allows for innovative aircraft configurations which are more efficient, compact, and employ transmission systems (Kim, Perry, & Ansell, 2018).
A highly promising aspect of EAP systems is turboelectric distributed propulsion which is being actively studied by researchers and industry leaders. It implies the use of numerous highly efficient and lightweight electric motors to power propulsors in an aircraft. The power source for these motors is generated by a separate electric generator driven by gas turbines. This arrangement and distribution allow for a highly effective bypass ratio which promotes superior efficiency and power similar to large engines.
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However, traditional engines are physically separated from propulsors. Turboelectric distributed propulsion is connected to propulsors through an efficient electric transmission system, which provides the effect of spinning the engine, a combination of a gas turbine and electric generator, at any speed. It offers a wide variety of benefits such as different speeds for the turbine and propulsors, a change of speed ratio in-flight, and higher flexibility in positioning when designing the aircraft (Kim et al., 2018).
Currently, this technology is in its infancy, and in the foreseeable future, it will remain usable at subsonic speeds. However, it can be a vital alternative to the SVTOL system to incorporate an EAP design alongside a regular jet engine without the necessity to direct the engine’s resources to both creating excessive thrust for the fighter’s acceleration and the ability to vertically land and hover. Electric propulsion systems are more advanced and stable, inherently providing pilots with enhanced vehicle control while avoiding numerous mechanical issues experienced by the current advanced propulsion system.
One of EAP’s strongly developed benefits is cruise efficient short take-off and landing (Kim et al., 2018). As a relatively rare and expensive aspect to implement in current military fighters (which makes F-35 so unique), the widespread integration of EAP could provide this ability to all types of aircraft. In the end, an advanced EAP system is both a mechanical and strategic benefit.
Another alternative is the use of Adaptive Versatile Engine Technology (ADVENT), currently in development by GE Aviation on a defense contract from the US Department of Defense. Adaptive engine technology is based on the concept of varying airflow bypassing the core, particularly opening the third stream in cruise mode. It is meant to improve fuel consumption by 25 percent while increasing range and in-flight capabilities (Trimble, 2018).
Adaptative engines will also feature ceramic matrix composites in high-pressure turbine blades, which reduce cooling loads or increase functional temperatures in the engine design. Power and thermal management allow increasing thrust substantially as well. It improves bleed and control systems. An adaptive engine is more flexible as well as integrational with current designs (Kjelgaard, 2018).
A strong advantage that ADVENT holds over traditional engines is the ability to maintain constant airflow during cruise throttling, which leads to decreased spillage drag and can improve installed performance (Chen, Zhang, & Tang, 2018). Adaptive engines use variable geometry where the fan pressure ratio and bypass ratio can be altered, both of which are the main factors impacting thrust and fuel consumption. A third stream is added outside traditional duct and core bypasses. This mechanism can either restrict core airflow, creating a high bypass engine or increase core flow and create more thrust (Patel & Wilson, 2018).
A modern military aircraft engine has multiple design objectives such as short take-off, vertical landing, longer combat radius, and faster speeds. The engine design must have turbojet abilities of high thrust for supersonic cruising but also a turbofan aspect for lower specific fuel consumption (Chen et al., 2018). An adaptive jet engine eliminates the critical tradeoff between thrust and fuel efficiency. The ADVENT propulsion system can actively switch between ‘modes’ of high performance or maximized economy. This technology has a wide range of potential applications for various military aircraft, including the three variants of the F-35 joint strike fighter, without sacrificing certain aspects of engine design or capabilities.
The current propulsion system in place in the F-35 joint strike fighter is inherently flawed and inefficient. It requires a complete redesign with the introduction of features discussed in this report and, most importantly, increased reliability. The recommendation would be to develop a new propulsion system which will be a combination of the two alternatives proposed – ADVENT and EAP engines. The ADVENT propulsion system greatly improves the thrust capabilities of the fighter as well as engine functionalities and reliability. Meanwhile, EAP addresses provide a high-bypass ratio while being extremely flexible in integration and positioning in the aircraft design.
Both propulsion systems enhance aircraft range, control, and thrust capabilities which would be viable for both long-range striker variants such as F-35A and F-35C, as well as the short-range and vertical landing functionalities of F-35B. While both propulsion systems are still in development, it would be optimal to develop a combined system, incorporating the benefits and design functionalities of the two.
The primary advantage is that a combined dual approach can effectively resolve both functional and mechanical inefficiencies in the current propulsion system. Propulsion systems are inherently complex and require years of development. Up to date, in the technological evolution of propulsion systems, there has always been a necessity to focus on a specific aspect while sacrificing elements of the other. For example, the increased thrust and fuel efficiency trade-off.
However, developing ADVENT and EPA propulsion systems offer the ability to regulate the mode of flight, which is advantageous as a fighter pilot can quickly adapt to the changing requirements of a mission or physical conditions around the plane. This provides a strategic and operational advantage. Furthermore, it would no longer require creating three different variants of a propulsion system for one fighter jet. Instead, the ability to manipulate engine parameters would allow using the same propulsion system in all variants, providing uniformity.
The primary disadvantage of this approach would be its cost. It is likely that the development of a combined electric-adaptive propulsion system would require a significant investment, R&D, and resources. It would be a long-term perspective for the military and possibly civilian aircraft, and by the time it was developed and tested, the F-35 would have been obsolete. Furthermore, it can be argued that the complexity of such a design is more likely to result in mechanical failures and would be both costly and difficult to fix or maintain.
Propulsion systems consist of engines that generate excess thrust to move an aircraft forward. The F-35 joint fighter has one of the most advanced propulsion systems in the world, coming in three variants to account for the plane’s capabilities. Unfortunately, the F-35 engine is experiencing numerous issues in functionality and mechanical failure. An investigation of possible alternatives going forward, Electrified Aircraft Propulsion (EAP) and Adaptive Versatile Engine Technology (ADEPT), seem to be promising technologies.
However, the examination of their features revealed they share similar abilities to modify the mode of propulsion mid-flight while providing numerous other benefits. It is recommended that they are combined into a unified propulsion system to complement each other as engine design flexibility can possibly allow addressing the various issues of the current F-35 systems.
Bender, J. (2015). The F-35 has run into one of its most significant problems yet. Business Insider. Web.
Chen, M., Zhang, J., & Tang, H. (2018). Performance analysis of a three-stream adaptive cycle engine during throttling. International Journal of Aerospace Engineering, 1–16. Web.
Hall, N. (2015). Welcome to the beginner’s guide to propulsion. Web.
Kim, H. D., Perry, A. T., & Ansell, P. J. (2018). A review of distributed electric propulsion concepts for air vehicle technology. In 2018 AIAA/IEEE electric aircraft technologies symposium (pp. 1-21). Cincinnati, Ohio: NASA. Web.
Kjelgaard, C. (2018). P&W outlines new plan for F-35 engine upgrades. Web.
Patel, H. R., & Wilson, D. (2018). Parametric cycle analysis of adaptive cycle engine. In 2018 joint propulsion conference (pp. 1-17). Cincinnati, Ohio: AIAA. Web.
Thoreson, H. (2018). Lessons learned from developing the F-35B’s F135 engine and lift system. Web.
Trimble, S. (2018). USAF starts work on defining adaptive engine for future fighter. Web.
Wurth, S. P., & Smith, M. S. (2018). F-35 propulsion system integration, development & verification. In 2018 aviation technology, integration, and operations conference (pp. 1-21). Atlanta, Georgia: AIAA. Web.