This chapter identifies the specifications of megawatt (MW)-scale electric machines (EMs) that facilitate electrified aircraft propulsion (EAP). Because their specific power (SP) must increase by an order of magnitude over the current state-of-the-art (SOTA), a review of conventional EMs for EAP is presented, focusing on approaches for mass reduction and SP increase. Design challenges associated with high specific power (HSP) MW-scale machines are identified, and a review of EAP powertrain architectures and how HSP EMs enable them is provided. Next follows a description focused on propulsion motors, high speed generators, and system considerations unique to the EAP powertrain. Design principles for high power, lightweight, MW-scale EM development are discussed, and a comprehensive survey of over 50 SOTA HSP machines is presented. Common HSP machine features are identified and promising options for further weight reduction are discussed. Despite their SOTA classification, the surveyed machines fall well short of the lofty requirements necessary for next generation components, and some emerging technologies that may help to remedy these shortcomings are described.

Power electronic circuits enable electrified aircraft propulsion (EAP) – from the More-Electric Aircraft (MEA) (an aircraft where the propulsion systems are still traditional, but some or all of the secondary non-propulsion-related subsystems are electrified) to the All-Electric Aircraft (AEA) (an aircraft with fully electrified propulsion and secondary subsystems) – and their importance cannot be understated. This chapter provides general power conversion concepts while fostering a solid high-level understanding of power electronic circuits, focusing on those circuits and devices that are crucial for EAP. Power system metrics, including power density and voltage, and integration techniques are presented. This is followed by a description of relevant converter topologies, including two- and multi-level inverters, direct and indirect matrix converters, rectifiers, circuits for open winding and multi-phase electric machines, and fault-tolerant topologies. A discussion of semiconductor devices and materials, including a brief discussion of silicon-carbide (SiC) devices, concludes the chapter.

The viability of electrified aircraft propulsion (EAP) architectures, from small urban air mobility vehicles to large single-aisle transport aircraft, depends almost exclusively on their energy storage requirements. Because energy storage increases with specific energy and power density, these metrics strongly influence the adoption of EAP architectures. This chapter provides an overview of electrochemical energy storage and conversion systems for EAP, including batteries, fuel cells, supercapacitors, and multifunctional structures with energy storage capability. An overview of today’s state-of-the-art battery technology and related EAP concepts is followed by a review of energy storage requirements for various classes of electrified aircraft. Recent battery technology advances are then reviewed along with their applicability and limitations for expanding the electrified aircraft market. Alternative electrochemical energy storage and conversion systems (e.g., fuel cells, flow batteries, supercapacitors, etc.) are also addressed. The chapter concludes with a review of multifunctional structures with energy storage capability and their potential application to EAP.

The electrical systems in turboelectric and hybrid-electric aircraft provide unmatched flexibility, coupling the power turbines to the fan propulsors and facilitating tight propulsion system-airframe integration. Reduced noise, emissions, and fuel burn result. However, the associated weight and efficiency penalties offset these benefits. Luckily, studies have shown significant aerodynamic improvements from electrically sourcing a small fraction of propulsive power. Partially turboelectric and hybrid-electric propulsion systems provide an intermediate step between conventional turbofan and fully turboelectric or all-electric architectures. This chapter details the benefits of electrified propulsion for large aircraft, using numerous trade studies and analyses of concept vehicles. It presents a first-order breakeven analysis that reveals key electrical power system requirements, providing a framework for comparing electric drive system performance factors, such as electrical efficiency, in the context of electrified and traditional propulsion systems. This can guide electrical system component research and provide aircraft designers with rational component expectations.

Electrified aircraft propulsion (EAP) in large commercial aircraft demands electric machines (EMs) with power densities and efficiencies far beyond those of current state-of-the-art technologies. At fixed speed, EM power density can only be increased with electromagnetic loading. Ohmic losses in conventional conductors impose fundamental limits on this, but superconducting (SC) wires eliminate Ohmic losses at cryogenic temperatures. SC machine windings enable large electromagnetic loading increases, resulting in high-power density machines without gearboxes. SC machines are an attractive long-term option for EAP, offering several advantages: high specific power and efficiency, and reduced thermal management challenges and risks of partial discharge and arcing. This chapter introduces SC physics and key EAP SC machine design considerations. It presents several SC machine topologies currently being researched, including a detailed assessment of challenges unique to EAP applications, namely, ac losses in superconducting coils and the need for compact cryocoolers. Finally, the physics and advantages of SC cables are presented, followed by a look at future SC technology trends.

Cryogenic power electronics enable the highly efficient ultra-dense power conversion systems that are critical for electrified aircraft propulsion (EAP) and have the potential to transform aircraft powertrain design. Much like superconducting electric machines, cryogenic power electronics offer benefits achieved through improved power device performance, reduced conductor electrical resistivity, and increased heat transfer temperature differential. In this chapter, key steps in the development of cryogenic power electronics are presented, from the component to the converter level. First, the characterization of critical components – including power devices and magnetics – at cryogenic temperature is introduced to establish the basic knowledge necessary for cryogenic design and optimization. Second, special considerations specific to cryogenic design, and trade and design studies for the cryogenic power stage and filter electronics are detailed. Finally, an example of a high-power cryogenically-cooled inverter system for an EAP application is illustrated, with safety considerations and the protection scheme highlighted.

This chapter details the unique challenge of managing electric drivetrain (EDT) waste heat in electrified aircraft propulsion (EAP) architectures. Hybrid-fueled and hybrid-electric propulsion systems make up the hybrid propulsion family, and these are composed of various combinations of fueled-engine and battery-electric propulsion systems. All comprise EAP systems, and they are analyzed here to assess their thermal performance and thermal management (TM) impacts on the aircraft. An introduction to the TM of HEPS is followed by a discussion of EDT heat sources and aircraft heat sinks. Next follows a summary of TM challenges specific to EDTs and thermal management system (TMS) heat acquisition and component cooling approaches. A review of TMS architectures leads to a reference TMS that is used to introduce general analysis concepts and heat transport and rejection components, including the underlying physics-based equations necessary for their analysis. The chapter concludes with a detailed step-by-step design of the reference TMS, listing constraints, imposed conditions, calculated values, and free parameters.

Most electrified aircraft propulsion (EAP) studies define a concept architecture and compute its potential benefits and key dependencies while attempting to answer the question: Can this architecture “buy its way” onto an aircraft? An important follow-on question is when will the architecture become physically and economically viable? Answering these two questions is the goal of the performance assessment process, a systematic method of analyzing the trade-offs when choosing an EAP system over a traditional one. Its methodologies and assumptions must be reasonable, and detailed comparisons to an appropriate baseline architecture must be included. This chapter outlines a systematic performance assessment process for EAP concept architectures, providing a means of deriving system-level figures of merit. Key steps in the process are identified and details are given for how they might reasonably be performed. Concepts and conclusions from earlier chapters are incorporated as needed. This serves as a guide to aircraft designers – new and old – who are beginning to delve into this exciting field and starting to explore the large design space enabled by EAP configurations.

There are numerous potential benefits associated with electrified aircraft propulsion (EAP). Achieving economical and safe EAP in transport aircraft would constitute an enormous leap forward in aviation. However, as with all potential engineering breakthroughs, the devil is in the details. This chapter begins to examine some of these details by introducing the electric power system (EPS) and summarizing its design, control, and protection functions. With the electrification of propulsion systems, EPS power levels (i.e., generation, distribution, and loads) are expected to increase by at least an order of magnitude, with far-reaching implications on the overall system design. Since all aspects of the EPS will be impacted, a thorough understanding and appreciation of the EPS and its functions is necessary to fully comprehend the challenges ahead. Several key EPS components and functions are described, and the solid foundation provided by the material in this chapter prepares the reader for the focused discussions of individual system components that follow in subsequent chapters.