NASA Electrified Aircraft Propulsion Efforts

NATO UNCLASSIFIED

NASA Electrified Aircraft Propulsion Efforts

Ralph H. Jansen, Dr. Cheryl L. Bowman NASA Glenn Research Center, 21000 Brookpark Road, Brookpark, Ohio UNITED STATES OF AMERICA ralph.h.jansen@nasa.gov, cheryl.l.bowman@nasa.gov Sean Clarke Armstrong Flight Research Center, Edwards Air Force Base, California UNITED STATES OF AMERICA sean.clarke@nasa.gov David Avanesian, Dr. Paula Dempsey, Dr. Rodger W. Dyson NASA Glenn Research C enter, 21000 Brookpark Road, Brookpark, Ohio UNITED STATES OF AMERICA da vid.avanesian@nasa.gov, paula.j.dempsey@nasa.gov, rodger.w.dyson@nasa.gov Keywords: Integrated Vehicle Design , Hybrid/Electric Propulsion Systems Architectures , Electrified Aircraft Propulsion ABSTRACT NASA’s broad investments in Electrified Aircraft Propulsion (EAP) are reviewed in this paper. NASA investments are guided by an assessment of potential market impacts, technical key performance parameters, and technology readiness attained through a combination of studies, enabling fundamental research, and flight research. NASA has determined that the impact of EAP varies by market and NASA is considering three markets: national/international, on - demand mobility, and short haul r egional air transport. Flight research is underway to demonstrate integrated solutions and inform standards and certification processes. This paper focuses on the vehicle related activities, however there are related NASA activities in air space manageme nt and vehicle autonomy activities as well as a breakthrough technology project called the Convergent Aeronautics Solutions Project. A key finding is that sufficient technical advances in key areas have been made which indicate EAP is a viable technology f or aircraft. Significant progress has been made to reduce EAP adoption barriers and further work is needed to transition the technology to a commercial product and improve the technology so it is applicable to large transonic aircraft. This paper will re view the activities of the Hybrid Gas Electric Subproject of the Advanced Air Transport Technology Project, the Revolutionary Vertical Lift Technology Project, and the X - 57 Flight Demonstration Project, and discuss the potential EAP benefits for commercial and military applications.

STO - MP - AVT - 323 PAPER NBR - 3 - 1 NATO UNCLASSIFIED NATO UNCLASSIFIED 1.0 INTRODUCTION NASA is investing in research to enable Electrified Aircraft Propulsion (EAP). EAP is the use of electric motors to drive some or all of the propulsors on an air vehicle. The energy source for the system can be e lectric (electric energy storage), hybrid (a mix of electrical and fuel based energy storage), or turboelectic (fuel based energy storage only). NASA is working across a range of markets from urban air mobility to subsonic transport; each market has diffe rences in vehicle sizes, ranges, and speeds. The overarching strategy is to create enabling technology, demonstrate this technology in flight - test vehicles, and transfer the knowledge to industry for future products. This paper focuses on vehicle related a ctivities, however there are additional ongoing NASA activities covering air space management and vehicle autonomy, as well as a breakthrough technology project called the Convergent Aeronautics Solutions Project. In this paper the following topics will be reviewed: the activities of Hybrid Gas Electric Subproject of the Advanced Air Transport Technology Project, the Revolutionary Vertical Lift Technology Project, and the X - 57 Flight Demonstration Project. Additionally, the potential EAP benefits for commer cial and military applications will be discussed.

Electrified aircraft propulsion has varying impact on air vehicle design depending on the key requirements of the market that the vehicle is intended to serve. NASA Aeronautics considers market impact in making technology investment decisions. Three markets under consideration are: the existing national and international commercial transport aircraft market, a potentially emerging market of on - demand mobility characterized by vertical take - off vehicles with relatively short range, and the short range regional transport market ( Figure 1 - 1 ).

The national / international market is typified by transonic operation with design ranges greater than 3000 miles, served by aircraft of the single aisle size and lar ger. The on demand mobility market is an emerging market where air taxi services could be used to provide transport around large urban areas, and would likely be served by vertical takeoff, short range aircraft which will be partially or fully automated. The regional market is a small existing market featuring lower passenger load and shorter routes, operating into smaller airports and usually serviced by turboprop aircraft. Studies have been conducted to determine potential benefits of EAP and in some ca ses autonomous flight operation in these markets. Benefit assessments are largely dependent on the underlying technology assumptions. NASA technology investments are guided by key performance parameters determined from these studies. Fuel burn reduction and corresponding emissions reductions are potential benefits from EAP for the national/international air transport market. The combination of EAP with autonomous flight operation has the potential to enable the on demand mobility market which would be a new paradigm for local transportation. EAP combined with autonomy to reduce pilot operations has the potential to revitalize the economic case for shorter, more lightly loaded regional routes.

Market: National/International Market: On demand mobility Market: Regional Impact: Fuel Burn/Emission Impact: New mobility capability Impact: Revitalization of smaller Reduction routes Figure 1 - 1 : Benefits of Electrified Aircraft Propulsion by Market STO - MP - AVT - 323 PAPER NBR - 3 - 2 NATO UNCLAS SIFIED NATO UNCLASSIFIED 2.0 OVERVIEW OF NASA ELE CTRIFIED AIRCRAFT PROPULSION (EAP) PROJECTS 2.1 Hybrid Gas Electric Subproject of Advanced A ir Transport Technology Project The NASA Advanced Air Transport Technology (AATT) Project continually challenges the technical community to improve the noise generation, emi ssion output, fuel burn, and overall efficiency of commercial transport aircraft.

This project traditionally invests in a broad research portfolio that includes fundamental improvements in gas turbine engines, advanced airframes including radical improveme nts in propulsion - airframe integration, and improvements in aircraft safety through icing research as well as engine and airframe acoustic remedies. This NASA project focuses on the commercial transport market with emphasis on narrow - body size aircraft. T he Hybrid Gas Electric Propulsion subproject (HGEP) was created in 2014 within the AATT Project to find a viable transport - class EAP aircraft concept, identify barrier technologies, and advance the technology readiness level of those barrier technologies. The HGEP team used a three tiered approach to 1) study aircraft concepts and identify potential aerodynamic efficiency gains, 2) investigate powertrain architectures, and 3) develop the fundamental components that will enable broad improvements in aircraft power systems. Elements of each of these three tiers will be reviewed in this paper.

Key concepts of several NASA sponsored aircraft designs are summarized here with further details available (Bradley, 2012; Lents, 2016; Perullo, 2017; Bowman, 2018). Boe ing, United Technology Research Center and Rolls - Royce North America have performed detailed designs hybrid - electric propulsion systems that added battery energy storage and incorporated minimal changes to the narrow - body aircraft outer mold - line. These s tudies showed that sufficiently advanced battery management systems (750~1000 W - hr/kg) combined with optimization of the turbine engine operation could provide narrow - body aircraft with improved total energy usage. When the Rolls - Royce North America team d id their assessment for a 90 - passenger aircraft using 1.5 MW electric machinery and 400 w - hr/kg batteries, they found that the hybrid system provided a two percent energy benefit for a 926 km mission, and fuel benefits approaching 14 percent for shorter ra nge missions (O’Brien, 2018). Significant progress in several technologies are required to realize the gains projected from these hybrid electric propulsion studies, with the most noticeable challenge being the need for improved battery management systems.

Other early EAP configurations such as the N3 - X and ECO - 150 considered fully turboelectric propulsion, in which all of the turbine shaft power is converted to electricity and distributed to numerous motor - driven fans (Felder, 2011; Schiltgen, 2016). Full y distributed concepts can take advantage of the propulsion efficiency benefits of distributed fans and advanced boundary layer ingestion, but require extremely efficient electrical machines and power distribution to handle the large electrical power loads . NASA has evaluated a tailcone thruster concept called Single aisle Turboelectric AiRCraft with Aft Boundary Layer ingestion (STARC - ABL) as one minimalist approach to partial turboelectric distribution (Welstead, 2016). The advantage of partial turboelect ric distribution is that it opens the door to new propulsion - airframe - integration efficiency while using nearer - term technologies.

The projected fuel savings of “revision B” of this aircraft were 2.7 percent for a typical economic mission (1667 km or 900 nm), and 3.4 percent savings for the full design mission of 6482 km (3500 nm) (Bowman, 2018). The fuel and total energy savings are the same in this case, because fuel is the only energy source used in a turboelectric concept. The fuel/energy savings of nominally three percent is significant because in this study the technology development assumptions required were relatively modest, and completely commensurate with the component research that will be discussed below. Figure 2.1 - 1 illustrates the STARC - A BL architecture and summarizes many of the key technology assumptions.

In addition to evaluating aircraft designs with fixed technology assumptions, it is also instructive to parametrically compare performance improvements of specific aircraft concepts as the performance of the powertrain STO - MP - AVT - 323 PAPER NBR - 3 - 3 NATO UNCLASSIFIED NATO UNCLASSIFIED Figure 2.1 - 2: Breakeven Curves for turboelectric Figure 2.1 - 1: Single Aisle Vehicle Concept propulsion components improves. NASA authors have used a series of breakeven based evaluations to trade between competing parameters such as specific power and efficiency, and to evaluate the relative value of improvements in one co mponent class versus another (Jansen, 2015; Duffy, 2018). Figure 2.1 - 2 shows the breakeven curves as a function of electrical drive system specific power and efficiency for a STARC - ABL type configuration, when considering varying electrical contribution t o the partial turboelectric propulsion. Here points of specific power and efficiency that lie on the curves represent power system architectures with equal fuel consumption to the conventional propulsion system, while points above the curves describe impr ovements beyond this breakeven point. Note that the relationship between efficiency and specific power is not flat, and that a positive benefit is achieved with less aggressive technology when a smaller fraction of the propulsion power is required.

The aircraft configuration studies and power architecture performance assessments show that aircraft fuel and energy savings potentially exist. Technology development at the subsystem, component, and material levels are required to make these designs viable. A few key efforts are summarized here. NASA has developed test beds specifically for EAP architectures (Jansen, 2017). NASA is investing in electric machine (motor/generator) development to achieve 13.2 kW/kg specific power and 96 percent efficiency (And erson, 2018; Yoon, 2018; Jansen, 2018). NASA investment has demonstrated power inverters (machine controllers) with 19 kW/kg specific power and greater than 99 percent efficiency (Zhang, 2018, Niu, 2019). New soft magnetic materials have been demonstrate d that operate with low electrical losses at frequencies up to 100 kHz and operating temperatures as high as 400°C (Leary, 2019). Furthermore, NASA is using unique modelling approaches to design motor slots as a composite system, and to tailor motor wire insulation for optimal thermal performance (Woodworth, 2018).

2 .2 X - 57 Maxwell The X - 57 “Maxwell” is a technology demonstrator aircraft supported by the NASA Flight Demonstrations and Capabilities Project. This experimental plane uses a crew - rated electric propulsion system designed to augment the aircraft performance in the high speed cruise condition (Borer, 2016) . FAA certification standards do not currently exist for EAP propulsion technologies. The project will develop best - practices knowledge for STO - MP - AVT - 323 PAPER NBR - 3 - 4 NATO UNCLAS SIFIED NATO UNCLASSIFIED passenger applications of electric propulsi on technologies , and will demonstrate the principles to achieve an 80% reduction in energy required per passenger - mile in the 150 - knot speed class. Development of new airworthiness standards in key regulator and industry forums is needed to accelerate safe adoption of EAP technologies on Urban Air Mobility and Thin Haul vehicles.

The X - 57 vehicle test program is divided into three spiral - development configurations shown in Figure 2.2 - 1 (Clarke, 2015 ). Mod I established baseline flight performance with pis ton engines. Mod II is an electrified modification which will increase the maturity of the new electric power plant and propulsion system. Mod III integrates the new, optimized wing with the propulsion system mounted at the wingtips, but without the origin al low speed flight envelope. Mod IV recovers performance across the original flight envelope with the addition of distributed folding propellers driven by electric motors that provide high lift during low speed flight. The baseline performance characteriz ation phase (Mod I) is complete. The electrified powertrain developed for Mod II is being integrated into the vehicle presently, with formal system - level tests planned for the end of 2019 followed by taxi tests and a series of flight tests in 2020. The hig h - performance Mod III/IV wing has been fabricated and will complete flight loads testing in 2019 , followed by traction and instrumentation system integration before flights in 2021. The Mod IV systems are in the preliminary design phase with critical desig n review planned for late 2019 followed by flight inverter and motor development, qualification, and acceptance testing in 2020 , and flights in 2021 and 2022 following the Mod II flight program.

Integrated design for effective interaction between the wing , the propellers, and the mission flight path is the core demonstration effort on the X - 57 aircraft. Modern advancements in high - performance electric motors, power inverters, and battery technologies enable this new design paradigm. The X - 57 is the first e lectrified X - plane and because the electric powertrain is central to the capability of this platform, the aircraft has been designated "Maxwell" in honor of James Clerk Maxwell's foundational work describing the nature of the electromagnetic forces that ar e harnessed in the electric motors, motor inverters, power buses, and batteries that comprise the X - 57 traction system.

The high lift system design, coupled with the high aspect - ratio wing design and the mission flight path planning approach (Schnulo, 2018) require an innovative , multi - disciplinary development to achieve the planned advances. The high - performance wing has been optimized to maximally improve aerodynamic efficiency at cruise flight conditions of 150 knots , and incorporate s an in novative vortex drag reduction capability through the location of the cruise propellers at the wingtip, enabled by lightweight, high - efficiency electric motors. The low - speed aerodynamic performance of the wing is ensured by incorporating 12 propellers dis tributed along the leading edge of the wing , each driven by a smaller electric motor and inverter system , which would be used only for low - speed maneuvers such as takeoff and landing. The X - 57 project team has developed advanced propeller design and optimi zation workflows that enable uniform velocity boosting when activated, while also supporting folding and low - drag stowing of the propeller blades during high speed mission segments ( Litherland , 2017).

The X - 57 avionics power, traction power, and command systems are optimized for system reliability , given the constraints of a flight research program showcasing experimental hardware in critical systems (Clarke, 2016) .

The system architecture relies on red undancy to limit the scope of failures, component testing to limit the likelihood of failures, and failure analysis and training to limit the persistence of failures. The resulting architecture may serve as a case study for development of future experiment al aircraft or commercial aircraft that rely on these technologies. Evaluation of each developmental component by way of component independent design review, endurance testing, and function validation in the integrated system are essential to ensuring reli ability. The integrated X - 57 system design will be evaluated for failure modes and will be integrated into an aircraft simulator with a flight - like cockpit that will be used for pilot training, ensuring rapid response to the STO - MP - AVT - 323 PAPER NBR - 3 - 5 NATO UNCLASSIFIED NATO UNCLASSIFIED Figure 2.2 - 1: X - 57 Spiral Development Approach Figure 2.2 - 2: - X - 57 Maxwell with Mod II systems Figure 2.2 - 3: X - 57 cockpit includes new instrument integrated including electrified powertrain. panel configured to manage the electric powertrain most severe fault cases. Figur e 2.2 - 2 and Error! Reference source not found. show the Mod II aircraft and cockpit.

The X - 57 vehicle test program is designed to solve a number of technical challenges for electrified propulsion in crew - rated aircraft. Aircraft electric propulsion requires special consideration to accommodate weight, safety, and operating environment requirements , which complicates adaptation of commercially available solutions. While electr ic propulsors (motors and inverters) are more efficient than fuel - burning engines, they still produce heat STO - MP - AVT - 323 PAPER NBR - 3 - 6 NATO UNCLAS SIFIED NATO UNCLASSIFIED during operation , requiring integrated vehicle - level thermal management designs. The common technique of solving such heat problems is to employ dedi cated heat dissipation methods such as liquid cooling or large metal heat sinks, but these can cause dramatic increase in airplane weight, reducing the benefit of the electric conversion.

In order to address these issues, research and development in the ar eas of material science, high efficiency motors and power electronics, additive manufacturing, and advanced controls is used by X - 57 project.

2.3 Revolutionary Vertical Lift Technology (RVLT) Project The overarching goal of the Revolutionary Vertical Lift Technology (RVLT) Project is to develop and validate tools, technologies, and concepts to overcome key barriers for vertical lift vehicles. The scope encompasses technologies th at address noise, speed, mobility, payload, efficiency, environment, and safety concerns for both conventional and non - conventional vehicle configurations as well as new missions and markets. One of these new markets, Urban Air Mobility (UAM), is a concept for passenger - carrying air transportation around metropolitan areas. Vehicles for these missions use all - electric and hybrid - electric propulsion concepts capable of vertical take - off and landing, and are referred to as eVTOL. A critical challenge for UAM market growth is the public acceptance of eVTOL as being as safe as, or safer than, commercial air travel and automotive transportation.

Standards are developed by significant in - service experience and analysis supported by experimental evidence. FAA cert ification standards do not currently exist for these new propulsion technologies. The certifying authorities are adapting/adopting new modified rules to better match the new electric propulsion technologies and are working closely with Industry Standards Groups (Horan, 2018). Part of the RVLT Project focus is to perform Figure 2.3 - 1: RVLT Concept Vehicles research that informs standards for electric and hybrid - electric propulsion systems of eVTOL.

To enable this research, NASA designed four UAM concept vehicles of varying payloads, range, ty pe and propulsion systems to identify crucial technologies, define research requirements, and explore a range of propulsion systems (Johnson, 2018; Silva 2018).

Figure 2.3 - 1 illustrates the concept vehicle configurations and their propulsion systems.

Figure 2.3 - 1: Magnetic Gearing STO - MP - AVT - 323 PAPER NBR - 3 - 7 NATO UNCLASSIFIED NATO UNCLASSIFIED The p ropulsion system architectures are being used to establish system safety assessments and to identify knowledge gaps in safety, durability, means of compliance, and provide insights into critical component areas.

Results can be used to inform standards for future certification needs and provide guidance to new eVTOL developers.

Under RVLT sponsorship, NASA is also developing electrical ports in Numerical Propulsion System Simulation (NPSS), a software tool used for conceptual design of propulsion systems, f or future electric propulsion capabilities (Csank, 2018). The addition of an electric port will enable electrical power systems for eVTOL to be designed and analyzed within the NPSS framework. RVLT is also leading the development of magnetic gearing for u se in the propulsion architectures of eVTOL. Magnetic gearing technology uses magnets to transfer torque through magnetic forces, eliminating the need for physical contact between gear teeth. Figure 2.3 - 1 illustrates an example of a concentric magnetic gear replacing conventional gearing in the RVLT quad rotor concept vehicle.

NASA research has resulted in lightweight, reliable powertrain designs for aerospace applications and a new facility to test magnetic gearing powertrain designs (Scheidler, 2018).

Results of RVLT electric propulsion research will contribute to improving the safety, reliability, and efficiency of eVTOL for the UAM market.

2.4 Potential EAP Benefits for Commercial and Military Applications In the commercial aviation world, hybrid/electric propulsion is considered to be a promising technology for fuel, emissions, and noise reduction in support of the challenging goals established by 2050 EU Flightpath/SRIA, NASA ARMD Strategic Implementation Plan, and the US Air Force ATTAM p rograms.

Ongoing results indicate operational benefits are possible in those three technical areas.

Considering military applications, hybrid/electric propulsion may yield further significant improvements by enabling new, unorthodox mission capabilities. Potential benefits are expected in the areas of vehicle signature reduction (lower noise, lower exhaust signature), usage in enhanced flight environments, minimized human - in - the - loop workload by offering a platform compatible with future goals of autonomou s operations facilitation, maintenance cost reductions, and performance burst/dash energy. Additional synergies are likely when used in conjunction with energy weapons.

Some potential areas of mutual interest could include, dusty operations capability, r emote supply capability, extended surveillance, dispatch able power, fuel - flexible vehicles, and autonomous rescue equipment. In order to establish a technology roadmap toward relevant systems, we must identify both mission requirements as well as potenti al future technology gap estimates. Results show that the role of the battery in hybrid electric propulsion systems regional air vehicles with 350 NM range can be competitive to conventional propulsion systems. As shown in Figure 2.4 - 1 and Figure 2.4 - 2 , most of the near - term small Unmanned Aerial Systems (SUAS) hybrid electric vehicles under development have a range of less than 1000 nm and are in Groups 1 - 3 ( Hoelzen , 2018; Rottmayer, 2017).

In addition, hybrid electric aircraft architectures enable new ways of integrating components to achieve new mission capability. Specifically, the ability to integrate atypical energy sources and the ability to place propulsors more broadly across the airframe broadens the vehicle design space. Synergistic use of v ehicle subsystems has the potential to enable quiet, long endurance, pulse - power for dash and directed energy weapons.

While the technologies required are still immature, it is prudent to consider their military applications in advance to guide future res earch thrusts.

STO - MP - AVT - 323 PAPER NBR - 3 - 8 NATO UNCLAS SIFIED NATO UNCLASSIFIED A future task is to identify key performance corner points in which hybrid/electric propulsion systems could improve tactical, logistics, and operations on the battlefield while leveraging existing commercial technology development. Many of the key technologies that have been demonstrated in ground operation must now be qualified for altitude conditions. We expect to have many flight qualified powertrain systems over the next decade for military planners to choose from for new future battlefield capability.

Figure 2.4 - 1: Unmanned Aerial System Sizes 3.0 CONCLUSION NASA is broadly investing in Electrified Aircraft Propulsion (EAP). NASA investments are guided by a combination of potential market impacts and technical key performance parameters. The impact of EAP varies by market and NASA is considering three markets: national/international, on - demand mobility, and short haul regional air transport. Technical advances Figure 2.4 - 2 : SUAS Power/Pr opulsion Goals in key are as have been made that indica te EAP is a viable technology. Flight research is underway to demonstrate integrated solutions and inform standards and certification processes. Significant progress has been made to reduce EAP adoption barriers and further w ork is needed to transition the technology to a commercial product and improve the technology so it is applicable to large transonic aircraft.

4.0 ACKNOWLEDGMENTS The activities in this paper are sponsored by the NASA Aeronautics Research Mission Directora te under the Advanced Air Vehicles Program and the Integrated Aviation Systems Program.

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