Timothy P. Dever

Glenn Research Center, Cleveland, Ohio

Kirsten P. Duffy

University of Toledo, Cleveland, Ohio

Andrew J. Provenza, Patricia L. Loyselle, Benjamin B. Choi, and Carlos R. Morrison

Glenn Research Center, Cleveland, Ohio

Angela M. Lowe

Aerospace Systems Design Laboratory (ASDL), Georgia Institute of Technology, Atlanta, Georgia

Assessment of Technologies for 

Noncryogenic Hybrid Electric Propulsion

NASA/TP—2015-216588

January 2015
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Timothy P. Dever

Glenn Research Center, Cleveland, Ohio

Kirsten P. Duffy

University of Toledo, Cleveland, Ohio

Andrew J. Provenza, Patricia L. Loyselle, Benjamin B. Choi, and Carlos R. Morrison

Glenn Research Center, Cleveland, Ohio

Angela M. Lowe

Aerospace Systems Design Laboratory (ASDL), Georgia Institute of Technology, Atlanta, Georgia

Assessment of Technologies for 

Noncryogenic Hybrid Electric Propulsion

NASA/TP—2015-216588

January 2015

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NASA/TP—2015-216588 iii 

Content 

Summary .................................................................................................................................................................... 1

1.0 Introduction ..................................................................................................................................................... 1

1.1 Background .............................................................................................................................................. 1

1.2 Power Requirements ................................................................................................................................ 3

1.3 Assumptions ............................................................................................................................................ 4

2.0 Motors ............................................................................................................................................................. 5

2.1 Conductors with High Electrical Conductivity ........................................................................................ 6

2.1.1 State of the Art of Carbon Nanotube Conductors ........................................................................... 6

2.1.2 Possible Improvements to the Electrical Conductivity of Carbon Nanotube Conductors ............... 9

2.1.3 System Benefits of Nanoconductor Improvements ......................................................................... 9

2.1.4 Path Forward for Carbon Nanotube Conductors ............................................................................. 9

2.2 Wire Insulation ........................................................................................................................................ 9

2.2.1 State of the Art of Wire Insulation ................................................................................................ 10

2.2.2 Possible Improvements to Wire Insulation ................................................................................... 10

2.2.3 Summary of Wire Insulation Findings .......................................................................................... 11

2.3 Permanent Magnets................................................................................................................................ 11

2.3.1 State of the Art of Permanent Magnets ......................................................................................... 12

2.3.2 Possible Improvements to the Properties of Permanent Magnets ................................................. 12

2.3.3 System Benefits of Motor System Power-Density Improvements ................................................ 15

2.3.4 Summary of Benefits of Permanent Magnets................................................................................ 15

2.4 Thermal Management ............................................................................................................................ 15

2.4.1 State of the Art of Thermal Management ...................................................................................... 15

2.4.2 Possible Improvements to Thermal Management ......................................................................... 17

2.4.3 System Benefits of Thermal Management Improvements ............................................................ 18

2.4.4 Benefits Summary of Thermal Management Improvements ........................................................ 19

2.4.5 Path Forward for Thermal Management ....................................................................................... 19

2.5 Structural Weight Reduction.................................................................................................................. 19

2.5.1 State of the Art of Lower Weight Structures ................................................................................. 20

2.5.2 Possible Improvements From Adding Carbon Nanotubes to Materials ........................................ 20

2.5.3 System Benefits of Low-Weight Carbon Nanotube Materials ...................................................... 21

2.5.4 Path Forward for Carbon Nanotube Materials .............................................................................. 21

2.6 Bearingless Motor Technology .............................................................................................................. 21

2.6.1 State of the Art of Bearingless Motors .......................................................................................... 22

2.6.2 Possible Improvements to Bearingless Motors ............................................................................. 22

2.6.3 System Benefits of Bearingless Motors ........................................................................................ 23

2.6.4 Summary of Bearingless Motor Improvements ............................................................................ 23

2.7 Power Electronics .................................................................................................................................. 24

2.7.1 State of the Art of Power Electronics ............................................................................................ 24

2.7.2 Possible Improvements to Power Electronics ............................................................................... 24

2.7.3 System Benefits of Improved Power Electronics .......................................................................... 26

2.7.4 Summary of Power Electronics Improvements ............................................................................. 26

2.7.5 Path Forward for Power Electronics ............................................................................................. 26

2.8 Path Forward for Cumulative Motor Improvement Benefits ................................................................. 27

3.0 Energy Storage .............................................................................................................................................. 28

3.1 Batteries ................................................................................................................................................. 30

3.1.1 State of the Art of Batteries ........................................................................................................... 30

3.1.2 Possible Improvements to Batteries .............................................................................................. 31

3.1.3 System Benefits of Improved Batteries ......................................................................................... 32

3.1.4 Summary of Battery Findings ....................................................................................................... 35

3.1.5 Path Forward for Batteries ............................................................................................................ 35

3.2 Flywheel Energy Storage ....................................................................................................................... 35

3.2.1 State of the Art of Flywheels ........................................................................................................ 36

3.2.2 Possible Improvements to Flywheels ............................................................................................ 38
NASA/TP—2015-216588 iv 

3.2.3 System Benefits of Flywheels ....................................................................................................... 38

3.2.4 Path Forward for Flywheels .......................................................................................................... 38

3.3 Supercapacitors ...................................................................................................................................... 38

3.3.1 State of the Art of Supercapacitors ............................................................................................... 39

3.3.2 Possible Improvements to Supercapacitors ................................................................................... 40

3.3.3 Benefit Summary of Supercapacitor Improvements ..................................................................... 40

4.0 Related Research on System Modeling ......................................................................................................... 41

5.0 Technology Assessment Conclusion ............................................................................................................. 42

References ................................................................................................................................................................ 43

 
NASA/TP—2015-216588 1 

Assessment of Technologies for Noncryogenic Hybrid Electric Propulsion 

Timothy P. Dever 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Kirsten P. Duffy 

University of Toledo 

Cleveland, Ohio 44135 

Andrew J. Provenza, Patricia L. Loyselle, Benjamin B. Choi, and Carlos R. Morrison 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Angela M. Lowe 

Aerospace Systems Design Laboratory (ASDL) 

Georgia Institute of Technology 

Atlanta, Georgia 30332 

Summary 

The Subsonic Fixed Wing Project of NASA’s Fundamental 

Aeronautics Program is researching aircraft propulsion 

technologies that will lower noise, emissions, and fuel burn. 

One promising technology is noncryogenic electric propulsion, 

which could be either hybrid electric propulsion or turboelectric 

propulsion. Reducing dependence on the turbine engine would 

certainly reduce emissions. However, the weight of the electricmotor-related components that would have to be added would 

adversely impact the benefits of the smaller turbine engine. 

Therefore, research needs to be done to improve component 

efficiencies and reduce component weights. This study projects 

technology improvements expected in the next 15 and 30 years, 

including motor-related technologies, power electronics, and 

energy-storage-related technologies. Motor efficiency and 

power density could be increased through the use of better 

conductors, insulators, magnets, bearings, structural materials, 

and thermal management. Energy storage could be accomplished through batteries, flywheels, or supercapacitors, all of 

which expect significant energy density growth over the next 

few decades. A first-order approximation of the cumulative 

effect of each technology improvement shows that motor power 

density could be improved from 3 hp/lb, the state of the art, to 

8 hp/lb in 15 years and 16 hp/lb in 30 years. 

1.0 Introduction 

The Subsonic Fixed Wing Project of NASA’s Fundamental 

Aeronautics Program is researching aircraft propulsion technologies that will lower noise, emissions, and fuel burn. One 

promising technology is electric propulsion, which could be 

either hybrid electric propulsion or turboelectric propulsion. In 

hybrid electric propulsion, the fan would be driven with either a 

turbine engine or an electric motor. In turboelectric propulsion, 

the turbine engine and fan would be separated; the engine would 

drive a generator, and the electric motor would directly drive the 

fan. This would allow the fans to be distributed in a way that 

would decrease aircraft noise. Reducing the dependence on the 

turbine engine would certainly reduce emissions. However, the 

additional weight of the electric-motor-related components that 

would have to be added would reduce the benefits of the smaller 

turbine engine. Before this technology can be implemented, 

research needs to be done on improving component efficiencies 

and reducing component weights. 

1.1 Background 

Hybrid electric and turboelectric propulsion concepts must 

have high efficiencies in converting electrical power into usable 

thrust, comparable to those of the gas turbine engines that they 

will replace. The most powerful and efficient electric motors 

employ superconducting elements that must be operated at 

cryogenic temperatures. Although the benefits of cryogenic 

electric motors would appear to make them ideal candidates for 

electric propulsion, the cryogenic technology exacts a steep 

price in terms of the complexity and additional weight of the 

components required to maintain the extremely low temperature of the motor’s superconductors. As an alternative, ambient 

temperature (noncryogenic) motors should become viable 

candidates for electric propulsion as expected technology 

improvements are made over the next few decades. 

Figure 1 shows a schematic of a hybrid electric propulsion 

concept for aircraft (Ref. 1). Propulsion is provided by a fan, 
NASA/TP—2015-216588 2 

which is either driven by the turbine engine or by an electric 

motor. Energy is provided to the motor through a transmission 

line from some kind of energy storage device, denoted as a 

battery pack. During cruise, the motor can drive the fan without 

the turbine engine, reducing emissions and fuel burn. The 

turbine engine also can drive the fan when extra power is 

needed, such as during takeoff. 

For successful implementation of the hybrid electric 

propulsion system, there will need to be significant increases in 

the energy density and power density of the electrical 

components. The objective of this report is to (1) assess the state 

of the art for each of the electrical components, (2) project how 

future advances in technology can enable components with high 

power density and high energy density, and (3) project 

technology capabilities for electrical components in 15 and 

30 years. This study is focused on electrical components for 

two cases: (1) eight-passenger aircraft and (2) single-aisle, 

150-passenger aircraft. 

Figure 1.—Hybrid electric propulsion (Ref. 1).

The technology improvements being investigated in this 

study (Fig. 2) include motor-related technologies, power 

electronics, and energy-storage-related technologies. Motor 

efficiency and power density could be improved through the use 

of better conductors, insulators, magnets, bearings, structural 

materials, and thermal management. Energy storage could be 

accomplished through batteries, flywheels, or supercapacitors, 

all of which expect significant energy density improvements 

over the next few decades. Table 1 describes the types of 

improvements expected in each technology area over the years. 

Improvements will come from new high-performance, lowdensity materials and technologies, allowing increases in 

efficiency, power density, and energy density. 

This report summarizes the results of a study performed at 

the NASA Glenn Research Center that assessed potential 

technology improvements for noncryogenic hybrid electric 

propulsion. The study was done under the NASA Subsonic 

Fixed Wing Project and was meant as a parallel path to 

complement the existing Cryogenic Hybrid Electric Program. 

Cryogenic hybrid electric aircraft will require the development 

of new technologies throughout the aircraft system, including 

superconducting motors, generators, and transmission lines; 

cryogenic power electronics; and cooling (e.g., cryocoolers). 

These technologies will bring great efficiency benefits, but all 

of them are at a relatively low technology readiness level (TRL). 

This study was an attempt to determine the benefits of a hybrid 

electric aircraft that would not need cryogenic components, 

which are currently at low TRLs. 

Figure 2.—Technology improvement areas for electric propulsion (based on Ref. 1).

 
NASA/TP—2015-216588 3 

TABLE 1.—MOTOR TECHNOLOGIES AND EXPECTED BENEFITS 

Technology Component Component improvement Benefits 

Motor 

Conductor material Lower resistance and lower density materials Reduced weight, reduced loss, 

and increased power 

Insulator material Higher temperature and higher voltage capability 

Higher thermal conductivity Higher power operation 

Permanent magnet 

material 

Higher coercivity and remanent flux materials 

High-temperature magnet Increased power 

Bearingless motor Bearingless motor incorporates magnetic bearing within the 

motor, reducing the number of components Reduced weight 

Structural materials Higher specific strength materials Decreased weight 

Thermal 

management Lower density and higher thermally conductive materials Increased efficiency and power density 

Power electronics Rectifiers and 

inverters 

Improved switching components with high switching speed 

and topologies, improved filter components, new materials, 

and improved packaging 

Increased efficiency and power 

density 

Energy storage 

Batteries New battery chemistries Higher energy density 

Supercapacitors New supercapacitor chemistries Higher energy density without 

sacrificing power density 

Flywheels Higher specific strength materials allowing higher speed at 

lower weight Higher energy density 

TABLE 2.—SYSTEM REQUIREMENTS 

Time from 

present, 

years 

Electric motor 

power, 

MW 

Motor 

voltage, 

V 

Number of 

passengers 

Power profile—portion of entire system power used, percent 

(Electric portion/fuel portion, percent) 

Takeoff 

(5 min) 

Climbout 

(30 min) 

Cruise Cruise duration, 

hours 

Landing 

(5 min) 

+15 1 700 8 100 (50/50) 90 (50/50) 50 (100/0) 2 100 (50/50) 

+15 10 700 130 100 (50/50) 90 (50/50) 50 (100/0) 2 100 (50/50) 

+30 1 1000 8 100 (50/50) 90 (50/50) 50 (100/0) 2 100 (50/50) 

+30 10 1000 130 100 (50/50) 90 (50/50) 50 (100/0) >2 100 (50/50) 

1.2 Power Requirements 

Table 2 summarizes the initial requirements for the study. On 

the basis of the analysis of requirements, this study considered 

improvements that were likely to be made by 15 and 30 years 

from now for 1- and 10-MW motors. 

The power profile is based on zero emissions at cruise and 

the need for extra power in the event of a touch-and-go landing. 

The 700-V voltage limit was drawn from Reference 2. Beyond 

700 V, thicker and costlier insulation is required, and insulation 

degradation due to partial discharge and corona becomes an 

issue. A system voltage of 1000 V, 30 years from now, assumes 

that technology improvements will allow the increase. The 2-h 

cruise is based on the 737 study (Ref. 3), which describes a 

900-mi range at ~600 mph. Motor sizes of 1 and 10 MW were 

chosen as appropriate for use in the aircraft of interest. These 

motor power output classes were verified as described in the 

following paragraphs. 

The Boeing 737–300 was used as an example to determine the 

required motor input power for a 150-passenger aircraft (Fig. 3). 

This was done to justify the consideration of the 10-MW motor 

as an alternative propulsor. The 737–300 was initially powered 

by two CFM56–3B–1 turbofan engines (Fig. 4) that each 

produced a maximum of 20 000 lbf of thrust. If the rule of thumb 

for turbofan engines is used that engine output horsepower is 

roughly equivalent to the thrust at rolling takeoff, and if it is 

assumed that each engine is producing maximum thrust at that 

moment, each CFM56 is thus producing 20 khp to drive its fan, 

or almost 15 MW. At takeoff, roughly 30 MW of mechanical 

power is being extracted from the fuel to accelerate this aircraft 

to flight. Under a main assumption of this study that the electric 

propulsion system must develop half of the required takeoff 

power, if two motors were used, each motor would have to 

produce 7.5 MW. Including the system losses adds to the required 

motor input power, so the total input power would be on the order 

of 10 MW. This validates the 10-MW motor output requested for 

this study for a 150-passenger vehicle. 
NASA/TP—2015-216588 4 

Figure 3.—Boeing 737–300—an example of a 150-

passenger aircraft (Ref. 4). Arpingstone grants anyone the right 

to use this work for any purpose, without any conditions, unless such 

conditions are required by law.

Figure 4.—CFM56–3 series turbofan engine found on 

the wings of the 737–300 (Ref. 5). Copyright 2005 GE 

Aviation; used with permission.

A similar study was performed for an eight-passenger aircraft, 

with the Learjet 25 used as an example (Fig. 5). The Learjet 25 

uses two GE CJ610 turbojet engines, each of which produces a 

maximum of 3000 lbf of thrust (Ref. 7). Applying the same rule 

of thumb means that each engine generates approximately 

3000 hp of output power. This is equivalent to 2.25 MW, for a 

total of 5 MW at takeoff. Half of this split between the two 

electric motors would be 1.13 MW, which is on the order of 

1 MW. Considering motor system losses, this number would 

grow to at least 1.2 MW. This validates the assumption that an 

electric motor used to power an aircraft of this size might be on 

the order of 1 MW. 

Figure 5.—Learjet 25—example of an eight-passenger aircraft 

(Ref. 6). Copyright Jetphotos.net; used with permission.

1.3 Assumptions 

The following list summarizes the assumptions that the team 

used in this study: 

(1) The aircraft is a hybrid electric or turboelectric aircraft. 

This assumption means that a portion of the propulsive power 

comes from fuel burn: that is, this is not an all-electric aircraft. 

Two basic configurations exist—the hybrid electric system and 

the turboelectric system. 

(a) The turboelectric system converts the fuel burned to 

electricity via a turboshaft engine and a generator, 

which provides electrical power for electric motors that 

drive fans to produce thrust. In this case, almost all of 

the thrust comes from electric motors. Figures 6 and 7 

show a concept for the electrical ducted fan and a 

schematic of the turboelectric propulsion system, 

respectively. 

(b) The hybrid electric system combines the turbine 

engine and electric motor on one shaft (see the 

schematic in Fig. 1). Thus, propulsion comes directly 

from fuel and electrical power. 

(2) There will be no emissions at cruise (per the 

requirements in Table 2). 

(3) The “electric motor power” is the output of one motor. 

(4) The vehicle configuration is left to the vehicle designers. 

This study addressed the motor and energy-storage aspects of 

the vehicle. The configuration of the vehicle (e.g., turboelectric 

vs. hybrid, or number of engines per plane) was left to the 

vehicle designer and was beyond the scope of this study. 

(5) The energy storage systems considered in this study were 

batteries, flywheels, and supercapacitors. 
NASA/TP—2015-216588 5 

Figure 6.—Electrical ducted fan.

2.0 Motors 

Permanent magnet (PM) motors were addressed specifically 

in this study because they are an attractive design for scaling to 

aircraft powers. The motor provides “free” flux from the PM; 

that is, unlike other motor types, PM motors do not need electric 

power to generate the flux. Also, they can be more power dense 

than their induction machine counterparts, they are quieter, and 

they include a smoother rotor, which provides windage and 

structural benefits. In addition, a bearingless configuration 

(which could be adapted to multiple electric motor types) was 

considered because of the potential benefits in terms of 

robustness, redundancy, and power density—allowing the 

lubrication system to be eliminated. However, because every 

electric motor type includes most of the key components 

considered in this study, the improved technologies identified 

would provide benefits for any motor type used in a hybrid 

electric aircraft. 

Figure 8 shows the continuous power densities of some 

selected motors. There are several ways to define power 

density, depending on the power definition and the motor mass 

used. Sometimes the “active mass”—only the rotor and stator, 

without the housing—is used; this results in a higher power 

density. For this study, the total mass—rotor, stator, and 

housing—were used for the power density calculation, along 

with the continuous power, rather than the peak power. As we 

look to increase power density, we also will be targeting 

reducing the weight of the housing and support structure. For 

industrial megawatt-size motors, the power density is 

~0.1 hp/lb. Several design studies for megawatt-class motors in 

support of U.S. Navy electric propulsion have proposed viable 

Figure 7.—Turboelectric propulsion system (Ref. 1).

Figure 8.—Continuous power density of selected motors (data 

from Refs. 8 to 12).

designs with power densities on the order of 2 hp/lb. If a power 

density of 3 hp/lb is achieved, a 1-MW (1340-hp) motor should 

weigh approximately 215 kg (470 lb). The state-of-the-art 

power density for kilowatt-size motors is increasing steadily 

because of the technology need for hybrid-electric and allelectric automobiles. Hybrid vehicle motors can achieve 

~1 hp/lb, and a new 5-kW axial-flux Halbach PM motor 

manufactured by LaunchPoint has achieved 5 hp/lb. In addition, many improvements could be made to the motor to 

improve both its power density and its efficiency, making it a 

more viable candidate for part of an aircraft propulsion system. 

Electric motor components include the copper conductor that 

carries the electric current, the wire insulation, PMs in certain 

motor types, a composite wrap containing the rotating PMs, the 

bearings for the motor rotor, and the motor housing and structural 

support. Electric motors heat up during use, degrading efficiency 

and power. Also, the power electronics needed to convert power 

and drive the motor add more weight to the system. 

The power density of the motor can be increased by 

(1) increasing power output, (2) decreasing weight, and/or 

(3) increasing the power density of the power electronics. The 

power output P of a motor is governed by the relation (Ref. 13): 

P = kKABd2LN (1) 
NASA/TP—2015-216588 6 

where k is a factor that includes geometrical and electrical information, K is the efficiency, A is surface current density, B is the 

air-gap flux density, d is the air-gap surface diameter, L is the 

stator core length, and N is the rotational speed. The surface 

current density and air-gap flux density can be increased by 

incorporating advanced electromagnetic designs, increasing the 

strength of the magnets, increasing the electrical conductivity 

of the conductors, improving thermal management techniques 

for keeping the component temperatures within their operating 

limits, and increasing the temperature capability of various 

components (e.g., magnets and electrical insulation materials). 

The weight of the motor can be decreased by introducing 

lightweight, high-strength materials and advanced structural 

concepts. 

The conductors used in motors today are typically copper. 

Although copper is highly conductive, its conductivity decreases 

with temperature and it is quite heavy. Future improvements are 

expected in new materials with high conductivity, low resistivity, 

and low density. 

In addition, the wire is surrounded by an electrically 

insulating material. This material has limits in terms of voltage 

capacity, maximum temperature, and thickness. The motors for 

electric propulsion are expected to run at very high voltages and 

higher temperatures, and it will be important to develop new 

insulation materials to handle this extreme environment. The 

insulation should also have high thermal conductivity, improving heat transfer within the motor. 

The strongest PMs today are made of neodymium-iron-boron 

(NdFeB) or samarium-cobalt (SmCo)—materials that have 

high coercivity and high remanent flux. However, new magnet 

materials are being developed with even better properties, 

which would improve the power density of the motor. 

In the future, the bearings required to suspend the motor rotor 

might be active magnetic bearings. If the magnetic bearing 

function is made part of the motor design itself, such as in a 

bearingless motor, component weight will decrease. 

During operation, the motor will heat up because of I

2

R losses 

within the conductor (where I is current and R is resistance). 

Increasing temperature has a negative effect on the conductor’s 

resistivity, the insulation properties, and the magnet strength. 

Although incorporating thermal management into the motor 

system would improve the motor’s efficiency, a heat exchanger 

would add weight to the motor system, decreasing its power 

density. New materials that are being developed with high 

thermal conductivity and low density could be used as part of a 

heat exchanger that would help improve the motor’s efficiency 

without adding significant weight. 

This type of aircraft propulsion system will require power 

electronics to convert power and to drive motors. Significant 

improvements are possible to state-of-the-art power electronics, 

including components, switch and filter materials, switching 

topologies, and packaging. 

The following sections give detailed explanations of the 

predicted improvements in these motor components and technologies along with the benefits expected. 

2.1 Conductors With High Electrical 

Conductivity 

Copper is the most widely used electrical coil material used 

today for electric motors. Superconducting materials have 

significantly higher electrical conductivity than copper. However, 

the best high-temperature superconducting materials, such as 

yttrium barium copper oxide (YBaCuO) compounds and 

magnesium diboride (MgB2), require cryogenic temperatures: 

YBaCuO requires cryogenic temperatures corresponding to liquid 

nitrogen, and MgB2 requires temperatures corresponding to liquid 

hydrogen. However, cryogenic technology exacts a steep price in 

terms of the complexity and weight of the containment vessel for 

the cryogen required to maintain the extremely low temperature 

of the motor’s superconductors. This weight/complexity problem 

must be surmounted before cryogenic propulsors can be 

integrated into future aircraft. Room temperature conductors with 

significantly higher electrical conductivity than copper would be 

preferred. One type of candidate, nanoconductors, are fabricated 

from carbon nanotubes (CNTs). CNTs hold the promise of 

creating nanoconductors that would have substantially higher 

conductivity at room temperature and lower weight than copper 

or any of its alternatives (such as aluminum). 

2.1.1 State of the Art of Carbon Nanotube Conductors 

CNTs are members of the fullerene structural family—that is, 

Buckminsterfullerene (buckyball), which is a form of carbon. 

Other forms of carbon include diamond, graphite, lonsdaleite, 

and amorphous carbon. CNTs have cylindrical structures, with 

typical examples shown in Figure 9 (Ref. 14). These structures 

Figure 9.—Single-wall carbon nanotube in 

various orientations (Ref. 14). Copyright Vincent 

Crespi, Dept. of Physics, Pennsylvania State University; 

used with permission.
NASA/TP—2015-216588 7 

usually exhibit a length-to-diameter ratio of 132 000 000:1, and 

they also have unusual properties, which make them valuable 

to the fields of optics, material science, and electronics 

(Ref. 15). CNTs have been fashioned in lengths ranging from 

the shortest, observed in the organic compound cycloparaphenylene, which was synthesized in 2009, to the longest 

(18.5 cm long), which also was produced in 2009 (Ref. 15). 

They also vary in diameter from about 3 to 30 nm (30 to 300 Å) 

(Ref. 16). These tubes exhibit very high thermal conductivity, 

comparable to that of diamond, and they are the strongest and 

stiffest materials discovered to date, with tensile strengths on 

the order of 63 GPa. This tensile property was demonstrated in 

2000, and a 2008 study revealed strengths of about 100 GPa 

(Refs. 15 and 17). 

Our interest in this material lies in its electrical properties. 

The symmetrical arrangement of the carbon atoms of graphene 

(the building blocks of CNTs) (Ref. 18), coupled with its unique 

electronic structure, have sparked interest in this material as an 

alternative to copper and aluminum with its wire variant, known 

as copper-clad aluminum (Ref. 19). Depending on their crystal 

lattice structure, CNTs can be either metallic conducting or 

semiconducting. In their conducting mode, CNTs exhibit 

current densities on the order 4×109 A/cm2: 1000 times that of 

copper (Ref. 15). Clearly, fashioning this material into a 

conductor could bring us closer to the goal of room temperature 

superconductivity, without the complexity and weight of 

cryogenic technology. The density of solid CNT material is on 

the order 1.3 to 1.4 g/cm3 (Ref. 15), compared with copper’s 

8.96 g/cm3 (Ref. 20). Using CNTs will result in an 84-percent 

reduction in weight relative to copper. In addition, CNT 

conductivity does not decrease with temperature as it does with 

copper. Obviously, the superior conductivity and weight 

savings in employing nanoconductor wiring in aircraft and in 

electric motor coils will substantially improve fuel burn 

efficiencies. The properties just described apply to random, 

nonaligned nanotubes. The goal now is to fabricate a wire 

conductor by effectively connecting individual nanotubes into 

one continuous strand that has the same desirable properties as 

its individual components. Fabricating nanoconductors is a 

rather difficult and complex task, which elevates the cost of the 

wires. Overcoming these challenges is a critical step on the path 

to replacing copper conductors with nanoconductors. 

The nanoconductors fabricated thus far have much higher 

resistivity than copper or aluminum. One theory suggests that 

electrons flow unimpeded through individual nanotubes (superconductivity on a nanoscale) but experience difficulty crossing 

the boundary between connected tubes, thus contributing to the 

higher resistivity observed in nanoconductors. Because of the 

significant hurdles presented by nanoconductor fabrication, only 

a handful of companies and universities have taken up the challenge of producing nanoconductors. A few years ago, General 

Nano, LLC (Ref. 21), in partnership with Glenn, proposed building an electric motor wherein the copper wire would be replaced 

with CNT “thread electrical fiber.” In addition, Nanocomp 

Technologies, Inc. (Ref. 22), incorporated in 2004, has a commercial production facility that manufactures CNT sheets and 

cables Figs. 10 and 11). The nanoconductor currently produced 

by Nanocomp has a resistivity 234 times that of copper. 

In 2011, Rice University (Ref. 23) reported producing doublewalled nanotubes that were doped with iodine and subsequently 

fashioned into cables having resistivity of 1.5×10–7 ȍ-m. This is 

approximately 9 times more resistive than oxygen-free copper at 

1.68×10–8 ȍ-m. The specific conductivity of this material is 

71 percent that of copper, which could make it a viable alternative 

to copper for transmission lines in the near future. However, 

motor coils have limited space, so the nanoconductor will need 

an absolute resistivity closer to that of copper to see power 

density benefits. Behabtu shows that the specific conductivity of 

the nanoconductors is about an order of magnitude higher than 

that of copper (Ref. 24). Clearly, this trend is advancing in 

the right direction, and nanoconductors significantly more conductive than copper could become a reality in 15 years and enjoy 

widespread application in 30 years. 

Figure 10.—Nanocomp large conductors (Ref. 22).

Courtesy of Nanocomp Technologies, Inc. 

Figure 11.—Nanocomp large sheet capability (Ref. 22). 

Courtesy of Nanocomp Technologies, Inc.
NASA/TP—2015-216588 8 

Figure 12.—Nanotubes embedded in metal cladding (Ref. 25). 

(Unless otherwise indicated, this information has been authored by an 

employee or employees of the Los Alamos National Security, LLC (LANS), 

operator of the Los Alamos National Laboratory under Contract No. DE-AC52-

06NA25396 with the U.S. Department of Energy. The U.S. Government has 

rights to use, reproduce, and distribute this information. The public may copy 

and use this information without charge, provided that this Notice and any 

statement of authorship are reproduced on all copies. Neither the Government 

nor LANS makes any warranty, express or implied, or assumes any liability or 

responsibility for the use of this information.)

Figure 13.—Carbon nanotube microstructure using the 

“Ultraconductus” process (Ref. 25). (Unless otherwise 

indicated, this information has been authored by an employee or 

employees of the Los Alamos National Security, LLC (LANS), 

operator of the Los Alamos National Laboratory under Contract No. 

DE-AC52-06NA25396 with the U.S. Department of Energy. The U.S. 

Government has rights to use, reproduce, and distribute this information. The public may copy and use this information without charge, 

provided that this Notice and any statement of authorship are reproduced on all copies. Neither the Government nor LANS makes any 

warranty, express or implied, or assumes any liability or responsibility 

for the use of this information.)

Another promising conductor technology uses the 

Ultraconductus process developed at Los Alamos National 

Laboratory (Figs. 12 to 14 and Ref. 25). Aligned CNTs, 

approximately 10 nm in diameter, are embedded in a metal 

cladding (a process involving laser ablation). This ensures 

conduction horizontally between tubes. There is very little 

resistance along the length of the nanotubes (i.e., they are 

ballistic or quantum conductors). Unlike copper, the 

ultraconductor electrical conductivity does not decrease with 

temperature.

Figure 14.—Electrical conductivity of ultraconductor 

versus copper (Ref. 25). (Unless otherwise indicated, 

this information has been authored by an employee or 

employees of the Los Alamos National Security, LLC (LANS), 

operator of the Los Alamos National Laboratory under Contract 

No. DE-AC52-06NA25396 with the U.S. Department of Energy. 

The U.S. Government has rights to use, reproduce, and 

distribute this information. The public may copy and use this 

information without charge, provided that this Notice and any 

statement of authorship are reproduced on all copies. Neither 

the Government nor LANS makes any warranty, express or 

implied, or assumes any liability or responsibility for the use of 

this information.)
NASA/TP—2015-216588 9 

TABLE 3.—CONDUCTOR BENEFITS SUMMARY 

Nanoconductor characteristics State of the art +15 years +30 years 

Reduction in weight (CNT/copper), percent ~16 ~16 ~16 

Increase in resistivity (CNT/copper) 9 × Cu 1 × Cu 0.5 × Cu 

Increase in power loss (CNT/copper), percent 900 100 50 

Improvement in horsepower per pound (CNT/copper), percent –87 to –81 20 to 72 140 to 245 

2.1.2 Possible Improvements to the Electrical 

Conductivity of Carbon Nanotube Conductors 

To improve the electrical conductivity of CNTs, researchers 

may target lengthening the nanotubes (reducing the number of 

boundaries between nanotubes), improving the conductivity 

among shorter nanotubes, or finding a way to separate the 

conductive and nonconductive nanotubes. In light of the technical challenges, Table 3 shows a projection of the advancement of CNT conductor characteristics 15 and 30 years from 

now. By 15 years, one can expect an 84-percent reduction in 

conductor weight, with a simultaneous decrease in resistivity to 

where it is on par with that of copper, and by 30 years, a 

decrease in resistivity to half that of copper conductor. A 

50-percent further decrease in resistivity would mean that the 

conductor could carry twice the current load at a given voltage 

while simultaneously cutting power losses in half. With significant increases in the electrical conductivity of CNTs, CNTreinforced copper composites will also be potential candidates 

to replace copper for the conducting coil in motors. 

2.1.3 System Benefits of Nanoconductor Improvements 

The power density benefits shown in Table 3 assume a copper 

weight of 20 to 50 percent of the motor’s active weight. The 

actual amount will depend on the motor type. 

2.1.4 Path Forward for Carbon Nanotube Conductors 

Our projection is that CNT-based fibers will have electrical 

conductivity equal to or better than that of copper. However, 

breakthroughs in CNT synthesis and fiber fabrication processes 

could result in a significant increase in electrical conductivity 

much earlier than 30 years. In the meantime, research effort 

should be focused on building electrical motors with available 

CNT fibers so that challenges related to the development of 

nanotube-conductor-based electric motors can be identified 

much earlier. 

2.2 Wire Insulation

The power of a given motor configuration is limited by the 

allowable current within the conductor. This current is, in turn, 

limited by the temperature capability of the wire insulation. 

Motor wire insulation is specified by maximum temperature, 

for example, 105 qC for plain enamel and 240 qC for polyimide 

insulation (Ref. 26). Increasing the high-temperature capability

 

Figure 15.—Effect of high-temperature-conducting (HTC) 

insulation on power density of a turbine generator 

(Ref. 27). Copyright Institute of Electrical Engineers of Japan; used 

with permission. 

will enable operation with a higher current, increasing the 

power density of the motor. 

A stator coil will have insulation surrounding each wire 

strand and insulation around each turn, in addition to 

groundwall insulation. This buildup of insulation typically 

allows for very low thermal conduction from the wires to the 

surroundings. Increasing the thermal conductivity of the insulation materials will decrease the temperature within the 

conductor, allowing for operation with a higher current. Again, 

this will enable an increase in the power density of the motor. 

Figure 15 illustrates the effect of temperature rise on the 

power density of an electric machine. This particular case is a 

600-MVA-class turbine generator that is indirectly cooled by 

hydrogen. It shows the impact of doubling the thermal 

conductivity of the stator coil insulation, allowing a 10 percent 

increase in power density or increased efficiency for the same 

power density (Ref. 27). 

Another challenge for the wire insulation used in hybrid 

electric propulsion is the expected higher voltage: 700 to 

1000 V (see Table 2). This has implications for both the motor 

wire and the transmission cables. Paschen’s Law states that the 

breakdown voltage of air is much lower at the low pressure 

levels at aircraft cruise altitudes, approaching a minimum at less 

than 400 V. Transmission cables especially need to use highvoltage insulation because they can have the full high-voltage 

difference between cables. Motor coils will also see higher 
NASA/TP—2015-216588 10 

voltage between turns or windings, although not the full 700 to 

1000 V. In addition, electrical surges may occur, for example 

from the inverter (Fig. 16, from Kikuchi and Hanawa, Ref. 28). 

To accommodate high voltage within an aircraft, the insulation 

material must have high dielectric strength. Corona discharge 

often produces pin holes, degrading the insulation. Employing 

a proper insulation material will minimize not only corona 

discharge but also arcing between wiring or adjacent components. The dielectric breakdown strength is typically given in 

units like kilovolts per millimeter; hence, better insulating 

materials might also result in thinner insulation. This would 

reduce the size of the motor windings and potentially the motor 

weight, thereby increasing the power density. 

In addition to improved thermal and voltage properties, the wire 

insulation must have the strength and flexibility to enable it to be 

wound into a coil, must be thin enough to make the coil compact, 

and must be compatible with the substances in the environment 

and other substances used in the motor coil (e.g., varnish). 

Figure 16.—Typical terminal voltage waveform (Ref. 28), 

where Vd is the drive voltage and Vp is the peak voltage. (a) Voltage at inverter terminal. (b) Voltage at 

motor terminal. © 2012 IEEE. Reprinted, with permission, Kikuchi, 

Hideyuki; and Hanawa, Hidehito: Inverter Surge Resistant Enameled 

Wire With Nanocomposite Insulating Material. IEEE Trans. Dielec. 

Elect. Insul., vol. 19, no 1, 2012, pp. 99–106. 

Figure 17.—Mechanism of erosion suppression (Ref. 28). 

© 2012 IEEE. Reprinted, with permission, Kikuchi, Hideyuki; and Hanawa, 

Hidehito: Inverter Surge Resistant Enameled Wire With Nanocomposite 

Insulating Material. IEEE Trans. Dielec. Elect. Insul., vol. 19, no 1, 2012, 

pp. 99–106.

2.2.1 State of the Art of Wire Insulation 

Motor wire (also known as magnet wire) is defined by 

diameter, temperature class, and breakdown voltage. The 

insulation defines the temperature class and breakdown voltage, 

in addition to the application environment. The insulation can 

be built of several layers, depending on the required properties. 

A typical high-temperature wire insulation is polyimide, 

which has a 240 qC maximum temperature. This material also 

has a relatively high dielectric breakdown strength, on the order 

of 200 kV/mm (Ref. 29). However, the thermal conductivity of 

wire insulation is fairly low, for example on the order of 0.01 to 

0.1 W/mK (Refs. 29 and 30) compared with 400 W/mK for the 

copper conductor. 

Polyetheretherketone (PEEK), another high-temperature 

wire insulator, has a maximum operating temperature of 260 qC

(Ref. 31). Its dielectric strength—about 20 kV/mm—is lower 

than that of polyimide, and its thermal conductivity—

0.25 W/mK—is higher (Ref. 32). 

To improve the thermal and voltage capability of magnet wires, 

extra layers of insulation can be added with special materials, or 

fillers can be added to the insulation. For example, a heat-resistant 

wire has been developed that uses nanosized silica particles 

dispersed within the polyimide insulation layer over a nickelplated copper conductor (Ref. 33). This material increases the 

temperature capability from 240 to 280 qC and reduces the 

breakdown voltage of the wire from 12.0 kV to about 9.8 kV. 

However, it is better able to sustain the breakdown voltage over a 

longer period of time. Another wire insulation configuration was 

constructed specifically to improve discharge resistance. The 

researchers suggest that the added silica nanoparticles extend the 

path through which the charged particles must travel, which 

suppresses breakdown (see Fig. 17 from Ref. 28). 

2.2.2 Possible Improvements to Wire Insulation 

Boron nitride (BN) is one compound that potentially could be 

used as an insulator. BN has good thermal stability and conductivity and good electrical resistance properties. However, 

there are some immediate challenges that must be overcome 

before BN can be a viable insulator. One key challenge is 

effectively dispersing BN nanotubes (BNNTs) in an appropriate polymer, which would then be used to coat the conductor. 

This technique has not yet been perfected. Apparently it is 

difficult to disperse BNNTs within polymers without affecting 

mechanical properties. 

BNNTs that were synthesized and tested at Glenn showed 

excellent thermal stability (Fig. 18) and had a high electrical 

bandgap of 5.5 eV. In theory, single-walled BNNTs can have a 

thermal conductivity of 3000 W/mK; however, in practice this 

value is lower. Currently, a thermal conductivity of 200 to 

300 W/mK can be expected from a BNNT with an approximate 

diameter of 40 nm (Ref. 34). 
NASA/TP—2015-216588 11 

Figure 18.—Weight change of carbon nanotubes 

(CNTs) and boron nitride nanotubes (BNNTs) 

in air. 

Figure 19.—Thermal conductivity of polyimide/boron 

nitride (Pl/BN) composite films with different BN 

contents (Ref. 35), where m and n refer to different BN 

sizes and ratios. Reprinted with permission from Li, Tung-L.; and 

Hsu, Steve Lien-Chung: Enhanced Thermal Conductivity of Polyimide 

Films Via a Hybrid of Micro- and Nano-Sized Boron Nitride. J. Phys. 

Chem. B, vol. 114, 2010, pp. 6825–6829. Copyright 2010 American 

Chemical Society.

TABLE 4.—INSULATION IMPROVEMENT 

FORECAST SUMMARY 

Insulation characteristics +15 year +30 year 

Thermal conductivity increase 10 times 20 times 

Insulation thickness decrease, percent 50 75 

Power density increase, percent 25 40 

Before BNNTs can be utilized in magnet wire insulation, 

research needs to be performed in the manufacture of the 

composite insulation. Zhi et al. (Ref. 34) show that dispersing 

BNNTs within certain polymers can increase their thermal 

conductivity to as high as 3.6 W/mK, a twentyfold increase over 

the baseline. Coil insulation will more likely be fabricated of 

something like polyimide. Li and Hsu (Ref. 35) fabricated 

thermally conductive polyimide film with up to 30 wt% 

microsized and nanosized BN. Figure 19 shows their results for 

thermal conductivity of the composite material as a function of 

BN content. The highest thermal conductivity resulted from a 

combination of 70 wt% microsized BN and 30 wt% nanosized 

BN, yielding almost an order of magnitude increase in thermal 

conductivity versus the baseline polyimide material. 

2.2.3 Summary of Wire Insulation Findings 

Although researchers are exploring the use of BNNTs as part 

of electrical insulation, challenges remain in the implementation. 

It is quite possible that in 15 years BNNT composite insulation 

that has 10 times greater thermal conductivity and 50 percent 

thinner insulation will be widely available. We hope that, in 30 

years, thermal conductivity will increase 20 times and insulation 

thickness will decrease by 75 percent. These advances should 

allow for a 25-percent increase in power density in 15 years and 

a 40-percent increase in 30 years (see Table 4). 

2.3 Permanent Magnets 

The noncryogenic motors used to drive fans in commercial 

transport aircraft will likely contain PMs. Brushless, 

synchronous, and toothless PM motors can have smooth rotors, 

potentially larger air gaps, and lower losses than other motor 

types. NdFeB magnets will be used unless temperature requirements call for SmCo magnets, which maintain their magnetization at higher temperatures but are not as strong as NdFeB 

magnets (Ref. 36). 

PMs can be purchased in different shapes and sizes. Their 

cost depends on their rare earth material content, fabrication 

method, and value of (BH)max, the maximum product of the 

magnet’s remanence Br and its coercivity Hc. Essentially, 

(BH)max determines a magnet’s strength: the higher the number, 

the stronger the magnet. Figure 20 compares the sizes of 

magnets with different energy products required to achieve a 

flux density of 1000 G at 5 mm from the pole surface (Ref. 37). 

The number in the name of each magnet is its energy product 

value. As BH increases, the required size of the magnet 

decreases. Benefits of higher BH include lighter weight 

actuators and lower circuit leakage, which lead to higher power 

density and greater efficiency, respectively. 

Reference 38 details the historical development of several 

types of PMs, showing that NdFeB, despite its relatively recent 

discovery in 1982, is much stronger than its predecessors and is 

the state of the art. 
NASA/TP—2015-216588 12 

Figure 20.—Relative magnet size and shape to generate 

1000 G at 5 mm from the pole surface (Ref. 37). Alnico is an 

alloy primarily consisting of Al, Ni, and Co; SmCo, samarium 

cobalt; V, volume; BH, energy product. Copyright 2010 Arnold 

Magnetic Technologies Corp.; used with permission.

Figure 21.—History and predictions of energy product 

improvements for neodymium iron boride (NdFeB) sintered 

permanent magnets.

2.3.1 State of the Art of Permanent Magnets 

Currently, NdFeB magnets can be purchased with energy 

products up to 55 MGOe (Ref. 39). NEOmax offers a PM with 

this capability. The theoretical limit for the energy product of 

an NdFeB magnet is 64 MGOe, and it is likely that a magnet 

with this energy product will be available by 2042. In 2005, 

NEOmax achieved, in its laboratory, the current record for 

energy product—59.5 MGOe (Ref. 40). However, magnets 

with this much energy are not yet available commercially. 

Figure 21 presents the NdFeB PM data graphically. The 

demand for these magnets has increased significantly since 

their discovery. In 1991, 1200 metric tons of NdFeB were 

produced in Japan. In 2004, 7200 tons were produced (Ref. 40). 

As demand has increased, so too has the level of research 

toward improving the capabilities of these magnets, which will 

facilitate continued improvements in this class of PM. 

2.3.2 Possible Improvements to the Properties of 

Permanent Magnets 

Figure 21 shows a history of NdFeB laboratory energy 

product advances as well as predictions for commercially 

available product. It is reasonable to anticipate that NdFeB 

magnets with an energy product greater than 58 MGOe will be 

available by 2027. By 2042, NdFeB magnets likely will 

approach their full potential of 64 MGOe. Figure 22 shows 

predicted future values of remanence Br and coercivity Hc for 

NdFeB at 15 and 30 years out. The upper bound of 1.65 T for 

Br by 2042 is 10 percent higher than that for magnets available 

today. 

Different processing techniques and chemical constructs will 

facilitate the tailoring of the remanence and coercivity for 

applications where one or the other is more important. For 

noncryogenic motors designed for hybrid aircraft propulsion, 

both remanence and coercivity will be important. For candidate 

motors like LaunchPoint’s Halbach motor (Ref. 41), the higher 

the remanence, the better. Motor air-gap flux is related to Br. 

The output power of motors of this type are directly proportional to Br, at least in simple one-dimensional analyses. The 

coercivity of the magnet is important because, when placed 

in close proximity to motor coils, these magnets could 

demagnetize if the local coil’s magnetic field ever spiked high 

enough. Higher coercivity means a higher energy product, 

which in turn means that the magnet will be less susceptible to 

demagnetization. 
NASA/TP—2015-216588 13 

Figure 22.—NdFeB permanent magnet Br and Hc versus 

energy product; data extracted from product sheets in 

Reference 39. 

Figure 23.—GE construction of finely tuned molecularscale permanent magnets (Ref. 38). Copyright 2013 GE 

Global Research; used with permission.

In Reference 42, an energy product of 137 MGOe is claimed 

to be achievable in a PM when an approach is used that involves 

clever exchange hardening in nanoscale combinations of 

alternating soft and oriented hard magnetic material phases. The 

137-MGOe energy product is more than double what is 

theoretically possible with NdFeB magnets. PMs of this type 

will contain a much smaller percentage of rare earth elements 

and more iron (Refs. 42 to 44). 

Coey and Skomski (Ref. 42) also suggest that a PM with a 

remanence of over 2.0 T should be achievable. In addition, it 

may not be outside the realm of possibility that a PM could be 

constructed that can maintain a remanence of 2.4 T, which is 

the saturation level of soft magnetic alloys like Hyperco 50. 

The Department of Energy has funded academia and industry 

to develop new improved magnets. The main drive of the effort 

is to build magnets up on a molecular level using much less 

rare-earth-element content. Rare earth elements are expensive 

and are not readily found domestically. The United States 

depends exclusively on foreign sources of these materials. This 

puts the U.S. economy and energy security at risk. Because 

these new magnets will contain up to 80 percent less rare earths 

(like neodymium and samarium) and more readily available 

iron, they should be cheaper. 

Both GE Global Research (Ref. 43) and the University of 

Delaware (Ref. 44) were recently funded to develop the 

techniques to fabricate such magnets. Their predicted performance numbers are eye opening. These magnets would 

revolutionize more than just motors. GE and the University of 

Delaware are working under Department of Energy funding to 

develop new magnets that would be more powerful than NdFeB 

magnets and commercially available by 2027. Clever pinning of 

magnetic moments using nanoscale material particles and threedimensional arrangement techniques could potentially take the 

value of remanent flux density to levels typically found in soft 

magnetic materials such as Hyperco. Hyperco (not a PM) has a 

saturation flux density of 2.4 T. This is 63 percent higher than the 

remanent flux density of NdFeB magnets currently available 

today. For a machine such as LaunchPoint’s ironless Halbach 

motor (Ref. 41), this could result in an increase in power density 

of at least 63 percent (based on a simple one-dimensional 

analysis) because of PM improvements alone. There could be 

additional benefits due to an increase in coercivity. Most 

increases in magnet energy product today are due to higher 

coercivity Hc and not remanent flux density Br. This is because, 

in today’s PMs like NdFeB, Br is already close to theoretical 

spontaneous magnetization limits (Ref. 42). Higher coercivity 

will allow for the consideration of windings that produce higher 

alternating current (AC) in a given motor design. Higher 

coercivity, coupled with a higher magnet Curie temperature, also 

makes a PM less susceptible to demagnetization at elevated 

temperatures. Coupled with new conductors, insulators, and 

advanced thermal management techniques, improved NdFeB 

magnets and future nanocomposite magnets will undoubtedly 

help push motor power densities higher. 

The devil, of course, lies in the details of just how to fabricate 

these new magnets from molecular-sized pieces. In both 

projects, soft and hard magnet nanosized building blocks are to 

be arranged in optimum three-dimensional ways to orient and 

pin magnetic moments to achieve very high remanent flux 

densities as well as more coercivity than is typical. 

Figure 23 provides GE’s basic composite magnet layout with 

alternating soft and hard magnetic nanoparticles (Ref. 38). The 

hard-phase material orients the field in the soft material and 

holds it. 
NASA/TP—2015-216588 14 

Researchers at the University of Delaware also are utilizing 

both soft and hard materials (Ref. 44). The university plans to 

use material preparation techniques such as wet chemistry or 

ball milling to make the nanoparticles prior to using an 

unmentioned technique to assemble them into a magnet. A 

doubling of energy product is predicted. 

Figure 24 graphically shows why using smaller building 

blocks of hard and soft material creates a better magnet (Ref. 45). 

In Figure 24(a), the lower blue material is hard (NdFeB) and the 

upper white material is soft (iron). The arrows represent the 

direction of the magnetic field within the material. Notice that the 

arrows in the hard material stay fixed, pointing to the left over 

the span. In the soft material, the arrows are aligned with those in 

the hard material at the boundary between materials. But as the 

distance from the boundary increases, the arrows become less 

oriented. The hard material aligns the direction of the field within 

the soft material at the boundary, but the influence decays. This 

shows that the smaller the vertical span of the soft layer is, the 

better aligned the field will be throughout it. This gives credence 

to assembly of these materials with small particles. 

Figure 25 shows the predicted improvements expected from 

these new nanocomposite magnets (Ref. 46). Use of nanocomposites can almost double the maximum energy product 

(BH)max.

Figure 24.—Graphical explanation of exchange coupling showing improved PM energy 

product through the combination of hard and soft materials (Ref. 45). (a) Magnetic field in 

hard (lower portion) and soft (upper portion) materials. (b) Exchange coupling of hard 

and soft materials. H, magnetic field; M, magnetization. Copyright 2008 George Hadjipanayis, 

University of Delaware; used with permission.

Figure 25.—PM operating temperature versus energy product for existing and 

future permanent magnets (Ref. 46). Alnico is an alloy primarily consisting of 

Al, Ni, and Co; SmCo, samarium cobalt magnet; PtCo, platinum cobalt 

magnet; Nd2Fe14B, neodymium iron boride magnet. Copyright Electron Energy 

Corporation (EEC); used with permission.
NASA/TP—2015-216588 15 

TABLE 5.—PERMANENT MAGNET BENEFITS SUMMARY 

Magnet characteristics State of the art +15 years +30 years 

NdFeB permanent magnet (PM) remanence, Br, T 1.51 1.58 1.65 

Nanocomposite PM remanence, Br, T Unknown 1.75 2.0 

NdFeB-only motor (increase in hp/lb), percent ------------ 4.6 9.3 

Nanoscale PM considered (increase in hp/lb), percent ------------ 16 32.4 

2.3.3 System Benefits of Motor System Power-Density 

Improvements 

For a one-dimensional model of a Halbach-type motor, 

torque is directly proportional to the air-gap field, and 

horsepower is directly proportional to torque. Assuming that the 

motor design is such that the negative slope of the circuit load 

line keeps it above the knee of the demagnetization curve, airgap flux is proportional to Br. Predicted future benefits of higher 

Br are listed in Section 2.3.4. 

2.3.4 Summary of Benefits of Permanent Magnets 

Table 5 summarizes the estimated magnet properties and 

resulting system benefits expected in 15 and 30 years. 

2.4 Thermal Management 

Thermal management provides benefits to motors in terms of 

both efficiency and specific power. For example, if a motor is 

cooled, the copper conductor will have lower resistance, which 

lowers the I

2

R heat loss and improves efficiency. Cooling 

provides benefits to other materials as well. For example, wire 

insulation has a maximum temperature limit, and PM strength 

decreases with temperature. With cooling, the motor could be 

run at a higher power without impacting component temperatures, thereby increasing the specific power of the motor. 

Thermal management can also improve the efficiency of the 

associated motor electronics. 

Figures 26 and 27 (from a Boeing contractor report for 

NASA on Subsonic Ultra Green Aircraft Research (SUGAR), 

(Ref. 3)) show potential configurations for hybrid electric and 

turboelectric configurations. In both cases, the motor turns the 

fan rotor shaft. The heat produced by the motor will need 

to be moved to a heat sink, which could be the bypass air 

stream, the airframe, the aircraft fuel, batteries, or even a 

Figure 26.—SUGAR Volt hybrid electric concept (Ref. 3). 

Figure 27.—SUGAR Volt turboelectric concept (Ref. 3).

phase-change material that stores the heat temporarily. For the 

turboelectric configuration (Fig. 27), a likely heat sink is the 

bypass air, which might even confer an efficiency benefit as a 

low-grade afterburner. 

2.4.1 State of the Art of Thermal Management 

Lightweight thermal management could be accomplished by 

(1) cooling within the motor itself, (2) transferring heat away 

from the motor to a heat sink, (3) incorporating cooling into 

existing structures (multifunctional structures), or (4) using new 

lightweight, highly conductive materials (Fig. 28). The type of 

cooling used has a direct impact on the current density 

allowable in the conductor. The higher the current density is, 
NASA/TP—2015-216588 16 

Figure 28.—Advanced thermal management concepts (Refs. 47 to 49). Graphite foam photograph reprinted from Carbon, vol. 41, 

Gallego, Nidia C.; and Klett, James W., Carbon Foams for Thermal Management, pp. 1461–1466. Copyright 2003, with permission from Elsevier.
NASA/TP—2015-216588 17 

the tighter the conductor can be packed into the stator slot. This 

in turn affects the size of the stator, so a higher current density 

will result in a smaller motor. Figure 29, based on information 

from Gieras (Ref. 50) shows typical current densities for 

various cooling techniques. 

The I

2

R losses from the end turns of motor windings account 

for a significant portion of the total copper loss. Ways to 

conduct heat from these end turns include using potting 

compounds with heat conductive particles, direct stator end turn 

cooling, or hollow conductors. 

Conduction through the structure could provide a path for the 

heat to travel to the heat sink. Both single-phase and two-phase 

convection are also available, with two-phase convection (e.g., 

evaporative cooling) giving the highest heat transfer rate. An 

active system will yield higher cooling than a passive system, 

but it will also add weight. Lawson and Pointon (Ref. 51), Lowe 

(Ref. 52), and Ohadi (Ref. 53) discuss many different means of 

providing heat transfer from motors used for electromechanical 

actuators in aircraft and/or harsh environments. Ultimately, the 

optimal type will depend on the motor size, location, 

configuration, and required heat transfer rate. 

For the aircraft application, Filburn, Kloter, and Cloud 

(Ref. 54) state that compact heat exchangers are often used in 

aircraft engines, specifically for cooling engine fluids. They are 

Figure 29.—Current densities for various cooling 

concepts (Ref. 50).

typically heavy, for example, over 23 kg (50 lb) for a 35-kW 

(2000-Btu/min) heat exchanger. Filburn et al. showed that 

simple changes in design and materials can improve the weight 

significantly, with a concept using bypass air as the heat sink. 

This type of heat exchanger could fit nicely within the hybrid 

electric or turboelectric concept. 

2.4.2 Possible Improvements to Thermal Management 

New materials are expected to improve the thermal 

management of motors considerably. One of the biggest 

impacts could be through the use of thermally conductive 

electrical insulators for the motor conductor. One such material 

might include nanofibers—such as the polyethylene nanofibers 

with high thermal conductivity and low electrical conductivity 

being investigated at the Massachusetts Institute of Technology 

(Refs. 55 and 56). Table 6 shows the thermal conductivity of 

individual chains of these nanofibers. Another material is the 

electrical insulator with BNNTs discussed in Sections 2.2.2 

and 2.2.3. 

We expect that, in the future, carbon-fiber-composite 

materials will compose significantly more of the aircraft 

structure, reducing the total weight. Conduction is expected to 

be lower because of the reduced thermal conductivity of 

carbon-fiber-composite materials in comparison to metals 

(Ref. 51). However, research is being done to develop new 

thermally conductive composite materials. Oak Ridge National 

Lab has come up with high-thermal-conductivity graphite 

foams that have a specific thermal conductivity that is 

significantly higher than that of copper or aluminum (Ref. 47 

and Fig. 28). This kind of material, which was used at Glenn to 

develop a lightweight heat-exchanger prototype for space 

applications (Ref. 48 and Fig. 28), could be used in conjunction 

with a convective heat-transfer system to provide thermal 

management with lower weight than today’s systems. Table 6 

shows some values for the thermal conductivity of various 

materials as given in References 56 and 57. Clearly copper has 

the highest thermal conductivity of all the materials shown. 

However, its specific thermal conductivity is well below 

aluminum, graphite/epoxy composite, and carbon foam. 

TABLE 6.—THERMAL CONDUCTIVITY OF CANDIDATE MATERIALS 

Material Density, 

g/cm3 

Thermal conductivity 

(in-plane), 

 W/m-K 

Specific thermal conductivity 

(in-plane), 

 W/m-K/(g/cm3) 

Copper (Ref. 57) 8.9 398 44.7

Aluminum (Ref. 57) 2.7 247 91.5 

E-Glass/epoxy composite (Ref. 57) 2.1 0.16 to 0.26 0.1 to 0.2 

Discontinuous carbon/polymer composite (Ref. 57) 1.7 10 to 100 6 to 59 

Natural graphite/epoxy composite (Ref. 57) 1.94 370 190 

Carbon foam (Ref. 57) 0.6 to 0.9 135 to 245 220 to 270 

Low-electrical-conductivity polyethylene nanofiber (Ref. 56) ---------- 104 ------------ 
NASA/TP—2015-216588 18 

Cooling could be incorporated into the motor structure itself, 

as is currently done with liquid-cooled casings. Glenn 

researchers developed a new coil design for cryogenic 

switched-reluctance motors, which could also be cooled 

noncryogenically. These “self-finned coils” were designed to 

improve heat transfer from the end turns by allowing the 

cooling fluid to pass through the end turns, making higher 

current possible (Ref. 49 and Fig. 28). This design could be 

combined with high-thermal-conductivity nanofiber insulation. 

Another self-cooling concept is one incorporated within 

LaunchPoint’s Halbach motor, which is air cooled by a fan 

integrated into the rotor itself (Ref. 10). 

The thermal management solution should be optimally 

incorporated within the aircraft structure design itself. A 

thoughtful solution will include thermal paths in addition to the 

other critical parameters, including strength, weight, and aerodynamics. For example, by using new thermally conductive 

composite materials in conjunction with the bypass air as the 

heat sink, the finned surface heat exchanger in Reference 54, 

used to cool engine fluids, achieved a weight reduction of 

73 percent versus the baseline heat exchanger. An additional 

weight savings was expected through incorporating the heat 

exchanger within the engine case directly adjacent to the bypass 

stream. This unit was designed to have a heat transfer rate of 

35 kW (2000 Btu/min). The baseline unit mass was approximately 23 kg (50 lb), and the lightweight designs were 11.5 kg 

(25.4 lb) for an aluminum unit and 6.1 kg (13.5 lb) for a carboncarbon unit. All of this was accomplished through thoughtful 

design, using technology and materials available today. 

Benefits will increase with the advanced materials being 

developed for future use. 

2.4.3 System Benefits of Thermal Management 

Improvements 

To get a sense of the amount of heat loss expected in the 

current study, we plotted the heat produced by the motors per 

megawatt of power (Fig. 30) for 95- to 98-percent motor 

efficiency. Each percent increase in efficiency over 95 percent 

decreased the heat loss by about 5 kW. Clearly, motor design 

and material improvements (e.g., conductor material and 

magnet material) that increase efficiency can significantly 

improve the thermal management problem. 

Decreasing the winding temperature will improve the motor 

efficiency. The efficiency of a Halbach motor depends on the 

resistance of the motor windings. Merritt et al. (Ref. 58) show 

that the equation for the efficiency is K = RL/(RO + RL), where 

RL is the load resistance and RO is the winding resistance. 

Reducing the winding resistance has a clear impact on the 

efficiency. The temperature coefficient D for the resistance of 

copper is 3.9u10–3 qC–1, meaning that each degree centigrade of 

increase in temperature over room temperature results in a 0.39-

percent increase in resistance and therefore, loss of efficiency. 

The efficiency equation shows that for an RO/RL ratio of 

approximately 0.04, a 70 qC decrease in copper conductor temperature can increase efficiency from 95 to 96 percent, reducing 

heat loss by about 5 kW for the 1-MW motor. 

Note that the maximum heat shown of 26.3 kW for 

95-percent efficiency is comparable to that of the heat 

exchanger in Reference 54 (35 kW). Assuming a specific power 

of 4.9 kW/kg (3 hp/lb) and 95-percent efficiency, the mass of 

the 1-MW motor using today’s technology would be around 

215 kg (470 lb), and the carbon-carbon heat exchanger unit 

from Reference 54, sized for a 26.3-kW heat load, would add 

less than 3 percent to the mass. 

Figure 30.—Heat produced during flight for a 1-MW motor.
NASA/TP—2015-216588 19 

Benefits based on making material changes to this heat 

exchanger were predicted for 15 and 30 years into the future. In 

15 years, we expect to use a carbon-fiber-composite material 

with relatively high thermal conductivity and lower density. 

The 15-year estimate is based on a carbon-fiber-composite 

material with a motor efficiency of 95 percent for sametemperature operation. Within 30 years, new nanomaterials will 

be available with even higher thermal conductivity, leading to 

lower temperature motors with lighter heat exchangers. The 

30-year estimate is based on materials with high specific 

thermal conductivity and a motor efficiency of 97 percent for 

lower temperature operation. 

To see how the thermal management system could affect the 

motor power density, a simple analysis was done to predict how 

increased current density could decrease motor size for the 

same power level. Assuming that the state-of-the-art stator 

conductor slot would be 25 percent of the motor cross-sectional 

area and that the back-iron thickness and rotor size would stay 

the same, doubling the current density would reduce the motor 

cross-sectional area (and therefore mass) by 22 percent, 

increasing the motor power density by 28 percent. If the current 

density quadrupled, the motor cross-sectional area would 

decrease by 33 percent, increasing the power density by 

49 percent. 

2.4.4 Benefits Summary of Thermal Management 

Improvements 

Table 7 summarizes the expected benefits in heat exchanger 

mass and power density due to thermal management 

improvements. 

2.4.5 Path Forward for Thermal Management 

To realize the potential improvements described here, 

research must be conducted in several areas. First, new 

materials with high thermal conductivity must be developed. 

These include materials for the heat exchanger as well as for 

wire insulation within the motor itself. Oak Ridge National 

Laboratory has already developed a graphite foam with very 

high specific conductivity, and the addition of nanofibers could 

increase conductivity even further. Nanomaterials are also 

being investigated for high-thermal-conductivity materials that 

also have low electrical conductivity. These materials could be 

used as part of or around the winding insulation, providing the 

greatest impact on heat transfer. 

As motor concepts and materials are developed, a thermal 

analysis must be performed for each configuration to determine 

the heat transfer requirements. This analysis should include any 

new materials (wire, insulation, magnets, and/or thermal conductors) in addition to any heat exchanger concept. Testing will 

be required to evaluate the effectiveness of these new materials 

and concepts. 

As the hybrid electric or turboelectric application becomes 

better defined, a systems study should be performed that 

encompasses all aspects of thermal management, including the 

weight benefit and the efficiency benefit of operating at a lower 

temperature. 

2.5 Structural Weight Reduction 

Motor structures include the rotor, stator, back iron, and 

related components, such as the motor frame, brackets, 

supports, rotor containment, and housing. For future electric 

aircraft, the motor structures need specific strength improvements to allow for higher loads at lighter weight when the rotor 

is operating at high speed, resulting in power density improvement. Thus, a new material development incorporating CNTs 

is discussed in Sections 2.5.1 to 2.5.4 to identify potential 

improvements in material properties—such as tensile strength, 

high-cycle fatigue, and stress rupture. In particular, a highspeed rotor and/or a rotor retention ring that contains and 

positions the high-speed rotating magnets would be crucial 

technology developments. Trends in improving composite 

filament strength are presented, and reasonable estimates of 

further improvement in 15 and 30 years are projected. 

Table 8 shows the approximate distribution of motor 

structures to the total motor weight for the 60-kW interior PM 

motor for the 2010 Prius automobile (Ref. 8). The dominant 

weight components of motors are the stator core, rotor core, and 

casing. The weight of these components could be reduced 

dramatically if advanced material and fabrication processes are 

developed, the use of polymer composites and composites 

reinforced with nanofibers is increased, and multifunctional 

structures with thermal management capability are utilized. In 

addition, the thickness of the electrical insulation of the copper 

windings could be decreased if advanced materials are used. 

TABLE 7.—PREDICTED BENEFITS FROM IMPROVEMENTS IN 

MATERIAL SPECIFIC CONDUCTIVITY 

Improvement State of the art +15 years +30 years 

Reduction in heat exchanger mass/MW, kg 17 4.6 1.5 

Increase in current density, percent --- 200 400 

Decrease in motor weight, percent --- 22 33 

Increase in motor power density (hp/lb), percent --- 28 49 
NASA/TP—2015-216588 20 

2.5.1 State of the Art of Lower Weight Structures 

Polymer matrix composites have a very high specific strength 

(~1000 kN-m/kg for carbon fiber composites compared with 

~100 kN-m/kg for stainless steel). This makes them ideal 

candidates for use in aircraft, where reducing weight is critical. 

As shown in Table 9, candidate fibers for composite structures 

include glass fibers, aramid fibers, and carbon fibers. Currently, 

carbon fiber has the highest tensile strength. 

CNTs are the strongest and stiffest materials yet discovered. 

This strength results from the covalent bonds formed between the 

individual carbon atoms. As stated in Section 2.1.1, in 2000, tests 

showed that a multiwalled CNT had a tensile strength of 63 GPa 

(Ref. 59). Further studies, conducted in 2008, revealed that 

individual CNT shells have strengths of up to ~100 GPa (Ref. 

60). Because of the extraordinary mechanical properties of 

CNTs—high Young’s modulus, high tensile strength, and high 

thermal conductivity, researchers are studying the use of CNTs 

to improve the strength of composite materials. We are proposing 

that CNTs be incorporated into currently existing materials to 

improve the material properties of high-speed motor structures. 

TABLE 8.—MOTOR COMPONENT 

MASSES FOR 2010 PRIUS MOTOR 

[From Ref. 8.] 

Component Mass, 

kg 

Motor 22.7 

Stator 16.0 

Casing 14.1 

Rotor 6.7 

Copper 4.9 

Permanent magnets .8 

Total mass (motor and casing) 36.8 

The literature survey shows that incorporating small volume 

fractions of multiwalled CNTs in a matrix significantly 

increases the tensile strength, high-cycle fatigue strength, and 

durability. Thus, this work aims to investigate and predict the 

optimal material improvement from utilizing CNTs, on the 

basis of assumed CNT material properties in 15 and 30 years. 

Currently, no commercially available materials are reinforced 

with CNTs. 

2.5.2 Possible Improvements From Adding Carbon 

Nanotubes to Materials 

Carbon fiber composites provide the baseline for the 

candidate material strength. Improvements in strength through 

the addition of CNTs were projected using assumed fiber 

strengths of 50 GPa in 15 years and 200 GPa in 30 years. 

Adding today’s CNTs to existing composite materials could 

nearly double their tensile and fatigue strengths (Refs. 61 

and 62). 

For calculating the projected values, we assumed that 

composite strength is a function of fiber strength. The nanofiber 

composites were assumed to have the same ratio of composite 

to fiber strength as for Toray T1000 (carbon fiber). For 

example, T1000 composite has a tensile strength of 3.04 GPa, 

and the fiber has a tensile strength of 6.37 GPa. Therefore, the 

composite strength to fiber strength is (3.04/6.37)100 percent = 

47.7 percent. We expect that stress rupture and fatigue will 

reduce the tensile strength and that the derated value will 

depend on how the material is used—total length of time (stress 

rupture) and number of cycles (fatigue). In this study, a derating 

of 50 percent was assumed for the composite materials, and 

the specific tensile stress (stress/density) was calculated 

accordingly. Table 9 shows the resulting tensile strengths for 

proposed composites with nanofibers in comparison to existing 

materials. 

TABLE 9.—TENSILE STRENGTHS OF CURRENT MATERIALS 

Material Tensile strength, 

GPa 

Derated tensile strength, 

GPa 

Typical density, 

kg/m3

Specific tensile strength, 

kN-m/kg 

Composite with 200-GPa nanofiber (predicted) 95.4 47.7 1570 30 400 

Composite with 100-GPa nanofiber (predicted) 47.7 23.9 1570 15 200 

Composite with 50-GPa nanofiber (predicted) 23.9 11.9 1570 7 600 

Carbon fiber composite with nanotubes (predicted) 6.08 3.04 1570 1 940 

Carbon fiber composite 3.04 1.52 1570 970 

Aramid composite 1.10 .55 1400 390 

E-Glass composite with nanotubes 2.40 1.20 2000 600 

E-Glass composite 1.20 .60 2000 300 

Titanium alloy .95 .95 4430 210 

Stainless steel 1.0 1.0 7800 130 

Aluminum alloy .31 .31 2700 110 
NASA/TP—2015-216588 21 

 For advanced motor development, new high-strength 

materials also could be used to improve primary and secondary 

motor structures, such as the high-speed rotor and shaft, rotor 

containment sleeve, motor core support, and electric cable core. 

For example, for a PM retention ring (Fig. 31), the ring hoop 

stress is proportional to the square of the motor rotor speed; 

thus, a higher strength retention ring would allow the rotor to 

spin faster, leading to high power density. In other words, we 

could increase power by increasing the material strength. For 

flywheel energy storage, carbon nanofiber rotors, which can 

have very high strength-to-weight ratios, would give flywheels 

the potential for high specific energy. 

Another approach to reducing motor weight is the development of advanced manufacturing techniques. For example, 

using screen printing to manufacture windings for motors 

could reduce motor weight. One company uses an etched 

copper sheet to fabricate the coil, allowing for a higher packing 

ratio (Ref. 64). 

2.5.3 System Benefits of Low-Weight Carbon Nanotube 

Materials 

Table 10 quantifies and summarizes, for 15 and 30 years into 

the future, the projected advancements of material characteristics based on anticipated CNT tensile stress estimates, carbon 

nanofiber stress improvements, and the advanced manufacturing 

Figure 31.—Ironless Halbach permanent 

magnet motor (Ref. 63).

techniques mentioned in Section 2.5.2. The predictions assume 

the use of high-strength, lightweight materials throughout the 

motor and supporting structures, with benefits similar to those 

for aircraft structures that use nanotube- or nanofiber-reinforced 

composites. 

2.5.4 Path Forward for Carbon Nanotube Materials 

Numerous efforts in developing nanofibers are ongoing, but 

the fabrication process is very limited and in its infancy. Glenn 

has begun investigating composite materials for high strength 

and low cost, but additional work needs to be done to mature 

carbon-fiber and CNT material technologies. Small batches of 

fiber need to be produced with consistent properties. Strength, 

fatigue, creep, and environmental effects need to be characterized. In addition, the best material system needs to be brought 

up to production levels. This work needs to be continued for 

several decades to achieve stronger and stronger composite 

materials. This would ensure that, 30 years from now, commercial products utilizing nanofibers are available for motor 

structures. 

2.6 Bearingless Motor Technology 

One possible configuration to improve the power density and 

system reliability of motors is to employ a bearingless motor, 

which can eliminate conventional mechanical bearings and the 

associated complex bearing lubrication system. This would 

significantly increase power density and extend life. 

The bearingless motor concept stems from active magnetic 

bearing technology, which uses the principle of electromagnetic 

suspension. As shown in Figure 32 (Ref. 65), it consists of an 

electromagnetic assembly, a set of power amplifiers that supply 

current to the electromagnets, a controller, and gap sensors with 

associated power electronics to provide the feedback required 

to control the position of the rotor within the gap. Numerous 

efforts have been made to develop a variety of magnetic 

bearings for turbomachinery applications, accomplishing tangible technical milestones. However, the practical application of 

magnetic bearings to turbomachinery may be limited because 

of the additional subsystem weights necessary to improve 

system reliability, redundancy, and performance. 

TABLE 10.—STRUCTURAL MATERIALS BENEFITS SUMMARY 

Improvement State of the art +15 years +30 years 

Structural weight reduction, percent --- 25 50

Power density, hp/lb 3 4 6 
NASA/TP—2015-216588 22 

Figure 32.—Active magnetic bearing components. (a) Schematic of magnetic bearing 

and control system (Ref. 65). (b) Active magnetic bearing stator.

However, the motor drive power electronics and control 

systems that are associated with electric motors are key 

components to the implementation of a magnetically suspended 

rotor system. An electric bearingless motor (also called an 

“integrated motor-bearing” or “self-bearing motor”) combines 

the magnetic bearing and motoring functions into magnetic 

actuators that produce both radial force and torque. Such a 

design can eliminate conventional mechanical bearings and 

their complex lubrication systems, resulting in weight reduction, maintenance cost savings, extended life, and improved 

reliability. Furthermore, vibration isolation of the electric motor 

drive is possible through proper feedback control of the radial 

(or magnetic bearing) force generation (Ref. 66). 

Thus, the bearingless motor is a strong candidate for a highpower-density motor for future electric propulsion systems. In 

the sections that follow, the bearingless motor was investigated 

to identify and characterize variables that could improve power 

density, system reliability, and redundancy for fail-safe operation. Also, reasonable power-density improvements resulting 

from the bearingless motor technology were evaluated and 

projected for 15 and 30 years into the future. 

2.6.1 State of the Art of Bearingless Motors 

The bearingless configuration superimposes the magnetic 

bearing function and motoring function by adding a separate 

magnetic bearing coil winding to each stator pole motor 

winding for rotor levitation (two coils per stator pole). This 

feature looks like a bearingless motor, but it requires additional 

magnetic bearing coils and an electrical subsystem for the extra 

coil windings. During the last decade, this approach has been 

used by many researchers for a variety of motors, including 

PM (Ref. 67), induction (Ref. 68), synchronizing-reluctance 

(Ref. 69), and switched-reluctance types (Ref. 70). 

Figure 33.—Eurocopter equipped with bearingless 

rotor hub (Ref. 71). © Eurocopter, now Airbus Helicopters; 

used with permission.

Unfortunately, a bearingless configuration also decreases the 

power density by adding the levitation coil windings and 

associated extra power electronics into the existing motor 

configuration. Furthermore, it produces coupled magnetic flux 

on each stator pole, resulting in inefficient control efforts. 

State-of-the-art bearingless motors have been demonstrated 

in research laboratories, and in recent progress on rotorcraft, a 

bearingless rotor hub was demonstrated by Eurocopter 

(Fig. 33). Key characteristics of the new bearingless rotor 

(Ref. 71) are its extremely compact and lightweight rotor hub 

and its improved flight characteristics. 

2.6.2 Possible Improvements to Bearingless Motors 

Here we propose a novel bearingless motor system, where 

only one coil winding per stator pole can perform both radial 

force (magnetic bearing) and torque production (motoring). 

This feature would play an essential role in developing a highpower-density motor because it can eliminate additional 
NASA/TP—2015-216588 23 

levitation coil windings and the associated power electronics, 

resulting in an authentic bearingless motor. We selected a 

switched-reluctance motor (SRM) to explain the working 

principle of this bearingless motor system because the SRM’s 

inherent fault tolerance, robustness under high temperatures, 

and rotor reliability at high rotational speeds (no coil windings 

on the rotor) make it a potential candidate for future airborne 

systems. However, compared with standard rotor-type motors, 

this SRM has a salient rotor structure, so it requires a highly 

nonlinear dynamics characterization process and analysis. 

Because of this system complexity, the SRM has historically 

been omitted for consideration as a candidate motor type for 

future electric propulsion. 

As shown in Figure 34, only one coil winding is wound per 

stator pole. For room temperature operation, the coil currents 

must be minimized so that core flux production is limited to the 

linear region of the core material BH curve to avoid magnetic 

saturation. The required motoring and levitating currents are 

summed and go into a single motor coil per pole. This enables 

the highest power output of the motor because it allows more 

space for motor coil windings (high-power-density feature). 

From a controls perspective, a practical observation-based 

controller using a proportional-derivative control algorithm can 

be developed to avoid mathematical complexity, thereby 

providing faster real-time calculation and allowing a much 

higher rotor speed. This simpler algorithm could significantly 

reduce controller development time and cost. 

Further power-density improvements could be achieved with 

CNT conductors, which can produce higher current density and 

significantly reduce weight. However, such conductors were 

out of scope for this section, and only copper windings were 

considered for the bearingless motor study. Further advances in 

the material characteristics of copper were assumed to be 

negligible 15 and 30 years in the future. 

2.6.3 System Benefits of Bearingless Motors 

This novel bearingless configuration would significantly 

contribute to developing a higher power-density motor. It also 

would extend life, eliminate the contact of moving parts, extend 

the operating temperature range, and improve rotordynamic 

control. The following list summarizes the innovative features 

and associated benefits in terms of hardware and software: 

(1) Significant system-level weight reduction would be 

achieved by eliminating mechanical bearings and the associated 

lubrication system. According to an estimate of the weight for 

a cryogenic electric system for the turboelectric propulsion 

system that is being modeled for a Boeing 777 class aircraft, the 

weight savings for the propulsor would be on the order of 

5.3 percent and the weight savings for the generator would be 

on the order of 5.4 percent. However, the weight savings for the 

noncryogenic copper windings for a Boeing 737–400 class 

aircraft would be much higher because of the large bearing size 

and lubrication system. Thus, a 10-percent weight reduction is 

the best estimate (in the case of copper windings). 

Figure 34.—Phase A motor winding configuration 

and levitation currents polarity of the 12–8 SRM 

(Ref. 72).

(2) Additional motor-level weight reduction could be 

achieved by eliminating separate levitation coils and the 

associated power electronics. An additional 10-percent weight 

savings would be a reasonable estimate. 

(3) System reliability would be improved. The inherent fault 

tolerance and redundancy feature would enable continuous 

operation even after partial failures of the electronics or coil 

wirings. 

(4) Maintenance costs would be reduced. Cost benefits 

include improving motor efficiency by 0.5 to 1.0 percent, 

eliminating oil consumption, reducing costs for rotor support 

system maintenance, and reducing acquisition costs. 

(5) Design time and costs for developing the bearingless 

motor and controller would be reduced significantly. The 

implementation process does not require a new motor design 

but uses motor winding coils in a conventional motor configuration. Also, the adaptive controller, which uses a gain 

schedule scheme, does not require a complicated mathematical 

plant model. Thus, it can be implemented in real time at much 

higher rotor speeds. 

(6) However, because only one motoring coil winding is 

used for each stator pole to produce the levitation force, a 

fraction of the motoring coil current might be used for the 

magnetic bearing force. Consequently, 5-percent motoring 

power loss would be a reasonable estimate. 

2.6.4 Summary of Bearingless Motor Improvements 

On the basis of the anticipated weight reductions and power 

loss described in Section 2.6.3, the projected improvements of 

power density and characteristics for 15 and 30 years into the 

future were quantified and summarized in Table 11. It was 

assumed that noncryogenic copper windings would be used, 

and no hardware modification was done for this study. We 

concluded that employing the proposed bearingless technology 

would improve power density by more than 20 percent. 
NASA/TP—2015-216588 24 

TABLE 11.—PROJECTED VALUES OF COMPONENT AND MOTOR LEVEL IMPROVEMENT.

Improvement +15 years +30 years 

Motor-level efficiency, percent 1.0 1.5 

Increase in motor-level power density (hp/lb), percent 20 22 

Maintenance cost savings (Ref. 73), million $/plane/year ~1 ~2 

Durability increase because of elimination of additional coils and power electronics, percent 80 (durable probes) 90 (no probes) 

2.7 Power Electronics 

Two types of components are considered in this section: 

rectifiers and inverters: 

(1) In the turboelectric system, power is provided at the 

generator output as AC. This power must be converted to direct 

current (DC) and then made available to other systems on the 

DC bus. Rectifiers are used to convert AC power to DC power. 

Rectification can be performed using several approaches. 

Passive rectification, using semiconductor diodes, is a simple 

approach: the bridge rectifier is one example. However, the use 

of diodes is inefficient because of the inherent voltage drop 

across the diodes. 

We considered active rectification, or synchronous rectification, instead. This technique improves the rectification 

efficiency by replacing the higher loss diodes with actively 

controlled switches. It also allows active power factor 

correction and other improvements. 

(2) Inverters are used to change the DC from the bus 

(provided by generator and/or energy storage) to AC for the 

variable-speed motor drive for propulsion. Inverters enable a 

difference between the fan speed and the engine’s power shaft 

speed, effectively acting like a gear box with a variable ratio. 

Figure 35 shows a typical inverter configuration using six 

switches. The DC voltage is converted to AC in motor phases 

A, B, and C. 

Typically, for both inverters and active rectifiers, filtering is 

used at the input and the output: a DC bus capacitor on the DC 

side, and an output filter on the AC side. Both bus caps and 

output filters can add considerable mass to the power 

electronics system. 

2.7.1 State of the Art of Power Electronics 

 In this application, the active rectifier and the inverter are 

similar in structure; they are both composed of packaging, sets 

of power switches, and input and output filtering. Thus, the 

state-of-the-art performance of both systems is similar, and 

improvements would benefit both equally. Therefore, to simplify the discussion, only the present and potential future 

performance of inverters is discussed, with the understanding 

that inverter advances will similarly benefit active rectifiers.

Figure 35.—Inverter schematic. Vdc, direct-current voltage.

Power density and efficiency are the key performance 

metrics for inverters. Although greater performance is available 

under laboratory conditions, the state-of-the-art power density 

for inverters is 5 hp/lb (Ref. 74), and state-of-the-art efficiencies 

are in the 93- to 95-percent range (Ref. 75). 

The configuration shown in Figure 35 is a common topology 

for state-of-the-art inverters, but improved topologies exist. The 

filtering at the input and output of the inverter, required on the 

DC and the AC side for many applications, contributes considerable mass and volume, affording another improvement 

opportunity. In addition, silicon is the semiconductor used in 

the majority of devices today, but improved materials are being 

developed. These potential improvements are discussed in 

Section 2.7.2. 

2.7.2 Possible Improvements to Power Electronics 

A number of possible approaches are available to improve 

the power density and efficiency of inverters, including 

improved switching components, new material types, advanced 

switching topologies, improved filtering components, 

improved packaging, and improved thermal management. 

These possible improvements are discussed in Sections 2.7.2.1 

to 2.7.2.6. 

2.7.2.1 Improved Switching Components 

The two main types of losses in inverters are switching losses 

and conduction losses. In both cases, the power is lost in 

the switching devices (typically transistors: metal-oxidesemiconductor field-effect transistors, MOSFETS, or insulatedgate bipolar transistors, IGBTS). 
NASA/TP—2015-216588 25 

Switching losses occur during a transition, as the switch is 

being turned on or off. While the switch is off, there is no current 

flow through the switch; and while the switch is on, ideally, there 

is no voltage drop across the switch. Because power dropped 

across the switch is equal to the product of voltage and current, 

there is little power lost under either of these conditions (the “on” 

or “off” state). However, during switching, there is a finite time 

when both the voltage across the switch and the current through 

the switch are simultaneously nonzero, resulting in a net power 

loss in the switch. This is called switching loss. Decreasing this 

overlap time, and thus reducing the power lost during switching, 

is one improvement opportunity. Also more rapid switching can 

be achieved via the development of improved components, 

materials, manufacturing processes, and design approaches. 

The ideal switch has zero resistance while it is in the “off” 

state. However, the semiconductor switching devices used in 

inverters are not ideal. Their resistance while in the “on” state, 

for example, is not zero. This nonideal performance leads to 

conduction losses. Improvements in component materials (the 

development of materials with lower “on-state” resistance) will 

certainly decrease these losses as well. Also, paralleling devices 

to lower overall resistance could reduce conduction losses, 

although this approach might incur a weight penalty. 

2.7.2.2 New Materials 

Another approach for improving inverter performance is the 

development of new materials. One very promising material is 

silicon carbide (SiC). SiC-based power electronics could enable 

power systems that are significantly more efficient, lighter, and 

smaller than systems based on silicon electronics (Ref. 76). 

Benefits from SiC-based components include higher breakdown 

voltage, lower “on” resistance, higher thermal conductivity, 

higher operating temperature, higher reliability, and radiation 

hardness (Ref. 77). 

Some SiC-based devices are available now: for example, 

Schottky diodes and field-effect transistors. However, performance, reliability, and device cost issues persist because of 

defects in commercially available SiC semiconductor wafers 

(Ref. 76). Considerable progress can be expected over the next 

15 and 30 years, and many more SiC-based power electronic devices should become available in that timeframe. 

Reference 78 shows that using SiC in an inverter design enabled 

a 50-percent reduction in module size and a 70-percent reduction 

in power loss. 

2.7.2.3 Advanced Switching Topologies 

The six-switch topology (Fig. 35) is very commonly used in 

inverters. However, advanced topologies can be used to 

improve performance. Two such advanced topologies are the 

multilevel inverter and the “soft-switching” inverter. 

To generate the output waveform, multilevel inverters use 

extra sets of switches and an advanced control algorithm. This 

approach can be used to improve performance in several ways. 

One way is to generate a precise voltage waveform, with 

minimum harmonic distortion. This could also be used to 

provide active power quality control, eliminating the need for 

output filtering and, thus, saving weight. In addition, the ability 

to utilize switches of different power levels within the inverter 

to generate different portions of the output waveform may 

provide benefits. However, one potential drawback to the 

multilevel inverter approach is the need for isolated power 

supplies for the stages, which would have to be traded for 

weight savings. 

Typical inverters are “hard-switching” types, meaning that 

during switching, nonzero currents flowing through the device 

and nonzero voltages across the device exist simultaneously, 

resulting in power loss. Soft-switching inverters are configured 

so that switching transitions are made while voltage across, or 

current through, the device is zero. In addition to lowering 

switching loss, soft switching lowers switch stress and 

electromagnetic interference. 

2.7.2.4 Improved Filter Components 

Filter components are typically used on the input (DC bus 

caps) and output (AC filter) of the inverter. Because these 

components can contribute significant weight to the inverter, 

reducing their size would benefit power density. 

Current DC bus capacitors occupy a significant fraction of 

the volume and weight of inverter modules. In addition, they 

cannot tolerate high temperatures (>120 qC) and they have poor 

packaging and inadequate reliability (Ref. 76). Figure 36 shows 

an example volume breakdown for a 10-kW inverter. 

Research is being conducted on dielectric improvements, 

including polymer and glass materials. Any improvement in 

capacitor performance could be used to decrease component 

weight. 

Inductors and capacitors are two key components of AC 

filters. The capacitor improvements discussed in this section 

could be applied to the AC side as well. Also, any advancement 

in inductors (magnetic material or conductor improvements) 

would benefit inductor weights as well, by enabling reduced 

filter size and weight. 

2.7.2.5 Improved Packaging 

Improving packaging could improve the performance of 

power inverters. New packaging concepts could be developed 

to provide improved electrical performance, reliability, and 

manufacturability. Other potential benefits include improved 

ruggedness and, of course, increased power density (Ref. 76). 

2.7.2.6 Improved Thermal Management 

As devices with higher current-density capability are 

developed, cooling must also be provided to maintain them at 

below their maximum allowable temperature. One improved 

thermal management technique is double-sided cooling, as 

opposed to single-sided cooling (Ref. 79), allowing multiple 
NASA/TP—2015-216588 26 

Figure 36.—10-kW Inverter part volume breakdown by liter and percentage (Ref. 76). 

TABLE 12.—POWER ELECTRONICS AND SYSTEM IMPROVEMENTS 

Power electronics characteristic State of the art +15 years +30 years 

Power electronics efficiency, percent 93 to 95 98 99 

Power electronics power density, hp/lb 5 10 15

System power density (hp/lb) improvement, percent 0 23 33 

cooling paths, thus improving thermal management. Additional 

advanced cooling techniques are also possible, including direct 

cooling of the chip itself. 

Thermal management is required for other components in the 

vehicle. Another opportunity is to incorporate power electronics cooling with existing cooling systems. Improved (higher 

temperature) materials may open up opportunities for part 

placement to take advantage of this synergy. 

2.7.3 System Benefits of Improved Power Electronics 

In order to capture benefits, we will consider the system to be 

the combined inverter and motor. Total system weight for a 

given power level is then 

Ws = Ps/Ds (2) 

where Ws is the system weight, Ps is the system power, and Ds

is the system power density. In this case, the system power 

density is composed of the motor and inverter power densities: 

1 1 1 

 ̧

1

·  ̈

©

§  m i

s D D

D (3)

where Dm is the motor power density and Di is the inverter 

power density. 

The goal is to drive up inverter power densities with the 

techniques discussed. The potential system benefits because of 

improved inverter power density can then be described as 

 Power density percent improvement = s

s s

D

Dc  D (4) 

where Ds

c is the improved system power density, driven by 

higher Di. 

Estimates of future power electronics power densities were 

made on the basis of the potential improvement opportunities 

discussed in Section 2.7.2. Equation (4) and the assumed motor 

power density of 3 hp/lb were used to calculate system-level 

improvements. This information is captured in Table 12. 

2.7.4 Summary of Power Electronics Improvements 

Table 12 summarizes the performance improvement. 

2.7.5 Path Forward for Power Electronics 

The recommended path forward is to address any or all of the 

potential improvement areas: 

(1) Improved switching components could be pursued, 

either by funding the production of new components externally 
NASA/TP—2015-216588 27 

and/or by performing lab tests at Glenn to quantify the benefits 

of the improvements in actual power electronics systems. 

(2) New material technologies could be pursued in a similar 

fashion. The development of SiC components is ongoing, and 

funding further efforts and/or integrating components into 

power electronics systems, as they become available, would 

further this technology. 

(3) Advanced topologies could be developed through 

research into multilevel and/or soft-switching inverters for 

hybrid aircraft use. Building, testing, and evaluation of the 

performance of these inverters would further this technology. 

(4) Improved filter components could be approached by 

funding external research or by performing internal research. 

New filter dielectrics could be discovered through materials 

research. Also new glass and new polymer dielectrics could be 

studied at Glenn, and improved inductors based on new 

available materials could be developed at Glenn. 

2.8 Path Forward for Cumulative Motor 

Improvement Benefits 

Figures 37 and 38 summarize the projected benefits to the 

motor power density due to each technology in 15 and 30 years, 

assuming a current state-of-the-art power density of 3 hp/lb. 

However, a subsequent effort should be performed to evaluate 

the cumulative effects of motor component improvements on the 

specific power and efficiency of a representative motor. This 

effort would predict improvements while considering that each 

motor component’s benefits may interact with other components. 

This analysis could be done using motor modeling software. A 

state-of-the-art baseline motor would be chosen, then the 

expected improved components would be substituted in the 

model to determine the new specific power and efficiency levels. 

In addition, this model could be used in conjunction with a 

thermal analysis to predict the benefits of thermal management.

Figure 37.—Projected power density benefits at 15 years for select motor technologies. SOA, state of the art.

Figure 38.—Projected power density benefits at 30 years for select motor technologies. SOA, state of the art.
NASA/TP—2015-216588 28 

3.0 Energy Storage 

Hybrid propulsion systems are under consideration for future 

aircraft, with the power being provided by both an electric 

energy storage system and a fuel-burning system. Chemical 

batteries, flywheels, and supercapacitors were considered for 

energy storage devices. Figure 39, from Kötz (Ref. 80), shows 

a Ragone plot of the energy density versus the power density of 

various energy storage devices. Flywheels are not shown on this 

figure, but they could improve on their energy density in 

comparison to supercapacitors and on their power density 

in comparison to batteries. Depending on the demands of the 

hybrid electric propulsion system, one or more of these devices 

will be required. 

In this study, it was assumed that the propulsive power 

required during takeoff, climbout, descent, and landing would 

be shared equally by the electric storage system and the fuelburning system. The power required during the cruise and taxi 

phases would be provided solely by the electrical energy 

storage system. 

Two power classes of electric motors were considered in this 

study: 1 and 10 MW. It was assumed that the motors were 

95-percent efficient and that the power inverter was 97-percent 

efficient. Also, two system bus voltages were assumed: 700 and 

1000 V. Table 13 shows the appropriate powers and energies 

required during each mission segment. The powers listed in 

Table 13 are average powers for each flight phase; however, the 

powers required during an actual flight will likely contain many 

short-term power “spikes” throughout the flight. The energy 

storage system size must not only provide adequate overall 

energy and average power but be able to respond to intermittent 

power spikes. Rather than designing a single-energy storage 

system (batteries) to accommodate these power spikes, incorporating supercapacitors within the design might enable a smaller 

overall energy storage system. The supercapacitors would 

provide power for the spikes and then be recharged via the main 

energy storage system to provide power for the next intermittent 

spike. Flywheels, which have a higher discharge rate, may also 

be employed to handle the power spikes that occur. 

The corresponding energy required from the energy storage 

device for each flight segment and the cumulative energy is 

shown in Figure 40 for the 1-MW motor and in Figure 41 for 

the 10-MW motor. 

When an energy storage system is sized for this scenario, the 

cruise portion of the flight has the largest impact. Not only is 

the motor working at its maximum capability and thus drawing 

the maximum power from the energy storage system, but the 

cruise phase is 2 h and makes the largest impact on the overall 

capacity needed from the system. 

Figure 39.—Ragone plot of energy density versus power density for various energy 

storage devices (approximate values). Adapted from Reference 80: Electrochimica Acta, vol. 

45, no. 15, Kötz, R.; and Carlen, M., Principles and Applications of Electrochemical Capacitors, pp. 

2483–2498, Copyright 2000, with permission from Elsevier. 
NASA/TP—2015-216588 29 

TABLE 13.—ASSUMED MISSION DURATIONS, POWER, CURRENTS, AND ENERGY REQUIREMENTS 

FOR 1- AND 10-MW ELECTRIC MOTORS 

Phase of flight Duration, 

min 

Power, 

kW 

Current 

(700 V), 

A 

Current 

(1000 V), 

A 

Electrical 

output, 

kW-h 

Cumulative 

electrical output, 

kW-h 

1-MW-class motor 

Taxi 5 54 77 54 5 5 

Takeoff 5 1 082 1 545 1 082 90 95 

Climbout 30 973 1 391 973 487 581 

Cruise 120 1 082 1 545 1 082 2 163 2 744 

Descent 30 216 309 216 108 2 852 

Landing 5 1 082 1 545 1 082 90 2 943 

Taxi after landing 5 54 77 54 5 2 947 

10-MW-class motor 

Taxi 5 541 773 541 45 45 

Takeoff 5 10 815 15 450 10 815 901 946 

Climbout 30 9 734 13 905 9 734 4 867 5 813 

Cruise 120 10 815 15 450 10 815 21 630 27 443 

Descent 30 2 163 3 090 2 163 1 082 28 525 

Landing 5 10 815 15 450 10 815 901 29 426 

Taxi after landing 5 541 773 541 45 29 471 

Figure 40.—Energy required for all phases of the flight and the cumulative 

energy for a 1-MW motor (95-percent-efficient motor and 97-percentefficient inverter).

Figure 41.—Energy required for all phases of the flight and the cumulative 

energy for a 10-MW motor (95-percent-efficient motor and 97-percentefficient inverter).
NASA/TP—2015-216588 30 

3.1 Batteries 

In this section, three potential battery chemistries are 

discussed as candidates for future energy storage for hybrid 

electric aircraft. A battery is a device that converts chemical 

energy to electrical energy. Batteries are generally made up of 

a number of cells. The power, voltage, current, and duration of 

the application determines how many cells are included in a 

battery and how they are arranged—for example, the series/ 

parallel arrangement. Battery cells are designed using different 

chemistries in different sizes, shapes, energy content, voltages, 

and discharge rates. The specific cell chosen for use in a battery 

not only depends on the application but on what characteristics 

are most important: for example, cycle life, weight and volume 

restrictions, operational temperatures, and charge/discharge 

time. The complete battery contains the cells or modules, 

battery management system, thermal management interface, 

and power takeoffs. 

In the quest for higher specific-energy batteries, a number of 

avenues can be approached. Lighter materials, both active and 

inactive to the reaction in the battery, can help lower the overall 

weight of the battery. Although the type and quantity of the anode 

and cathode material within a cell determine the amount of 

energy stored within a cell, a mass breakdown of the components 

within a typical lithium-ion battery shows that the anode and 

cathode within the cell only make up 57 percent of the overall 

cell mass (Ref. 81). The remaining cell mass is made up of the 

electrolyte, current collectors, separators, wiring, and case. 

Other methods to improve specific energy include increasing 

the number of electrons moving through the circuit. This can be 

done in a number of ways: 

(1) Increase the charge carried by the moving ion. However, 

an ion capable of carrying a higher charge is generally heavier. 

(2) Increase the amount of active material within the cell. 

This not only adds additional mass to the cell but can make it a 

challenge to fully utilize all the material in the cell. 

(3) Lower the resistive losses within a cell via improved 

conductivity of the battery electrode material, improved current 

collection design, and improved electrolytes. 

(4) Increase the voltage between the cathode and anode, 

given the same number of electrons passing during the reaction, 

resulting in a higher power. This approach not only requires 

electrode materials with higher voltages but an electrolyte that 

is stable over time at those voltages. 

(5) Reduce the weight of battery and cell case structures, 

current collectors, and wiring without sacrificing performance. 

(6) Increase the surface area at the electrode/electrolyte 

interface to increase the discharge/charge rates and the utilization of the battery material. 

3.1.1 State of the Art of Batteries 

Currently, the most promising batteries for electric aircraft 

are based on lithium, which is the lightest metal and will likely 

lead to the highest specific energy (energy stored per unit mass). 

Lithium-ion, lithium-sulfur, and lithium-air/oxygen batteries 

are the most likely candidates. Currently available specific 

energies, at the cell level, are 80 to 200 Wh/kg (depending on 

chemistry) for lithium-ion batteries, 250 to 300 Wh/kg for 

lithium-sulfur batteries, and 300 to 350 Wh/kg for lithiumair/oxygen batteries (Refs. 82 to 84). Lithium-ion batteries are 

the most mature technology of the three candidates. Although 

lithium-sulfur and lithium-air/oxygen batteries currently have 

higher specific energies, they face significant barriers to 

commercial use, including very limited cycle life. 

Figure 42 shows an example of a state-of-the-art lithium-ion 

battery—the battery used on the Mars Surveyor Lander. It 

consists of two 25-Ah batteries and has a voltage operating 

range of 24 to 32.8 V. 

In a lithium-ion battery, the lithium shuttles between the 

anode and cathode electrodes. The materials making up the 

electrodes are typically layered compounds that allow lithium 

ions to be inserted between the layers. Improvements in both 

the cathode and anode could yield significant improvements to 

the specific energy of a cell. Several options for cathode 

materials have been included in cells, each with their own pros 

and cons. However, the cathode is generally made up of 

transition metal oxides with a layered structure, and these 

materials have a relatively low capacity for lithium-ion 

insertion. Recent improvements in the material structure and 

composition (including dopants) have increased electrode 

voltage and capacity for lithium ions, but significant work 

remains (Ref. 81). 

Figure 42.—Mars Surveyor Lander battery (two 25-Ah 

batteries).
NASA/TP—2015-216588 31 

Lithium-sulfur batteries have generated a lot of interest for 

potential inclusion in electric vehicles because of the low-cost 

materials involved in the construction of the cells and, most 

importantly, the high theoretical specific energy—greater than 

2000 Wh/kg. Current lithium-sulfur batteries contain a lithium 

metal anode and a sulfur cathode, the sulfur being contained in 

a carbon electrode matrix. Initial cells produced with this 

chemistry have been promising, with specific energies of 

approximately 300 Wh/kg on the cell level (Ref. 85). However, 

these cells tolerate only a few charge/discharge cycles before 

significant capacity loss occurs. Several issues are thought to 

contribute to the loss of capacity over time with cycling 

(Ref. 86): (1) dissolution of a lithium polysulfide reaction 

product into the electrolyte, which results in a loss of lithium 

and sulfur from the anodes and cathodes; (2) morphology 

changes to the lithium itself, including the formation of 

dendrites and voids, which lead to short circuits and loss of 

electrical contact; and (3) dimensional changes in the cathode 

and anode upon cycling, leading to loss of electrical contact 

with the active material. Work is ongoing to address these and 

other problems with rechargeable lithium-sulfur batteries 

(Refs. 87 and 88). 

Lithium-air batteries also are a promising technology, with 

theoretical specific energies in excess of 3000 Wh/kg (Ref. 89). 

However, the technology still has a number of technical hurdles 

to be overcome before it is ready for commercial use. The 

highest specific energy reported so far has been 344 Wh/kg for 

a battery-level assembly (Ref. 90), but this was based on a labscale cell. Currently, the capacity of lithium-air cells cannot be 

maintained over multiple charge/discharge cycles; they rapidly 

lose capacity after only 50 cycles. Several areas will need to be 

addressed in order to make rechargeable lithium-air/oxygen 

batteries a reality. The air cathode design needs to be optimized 

to facilitate the flow of air to the electrode surface. Also, the 

cathode must effectively handle the lithium-oxygen product. 

Currently this reaction product blocks the pores of the cathode 

at the reaction sites and prematurely stops the reaction, limiting 

battery capacity (Ref. 91). Lithium-metal anodes are prone to 

dendrite formation upon cycling, which in turn leads to shortcircuiting. In addition, resistive barriers are formed on the 

lithium surface from reaction with the electrolytes and 

impurities within (Refs. 92 and 93). Stable electrolytes are 

necessary to facilitate the overall reaction while also limiting or 

controlling dendrite formation or morphology changes. 

From a system-level perspective, substantial work remains in 

the design of a working battery. Not only must the standard 

battery construction restraints be included in the design, but 

provisions must be made to provide adequate air or oxygen to 

the overall battery, and all the cells within, to maintain the 

reaction rate without incurring a significant mass or parasitic 

power penalty. In this endeavor, lessons can be taken from the 

design of fuel cells to properly design the surrounding balance 

of plant as well as the design of the air cathode. Oxygen for the 

reaction can be obtained from either stored oxygen or ambient 

air. If the battery is lithium air, provision must be included in 

the design of the aircraft to gather sufficient air from the outside 

to feed the lithium-air reaction. This air may need to be 

pressurized, which will cost additional mass and parasitic 

power. It may also need to be dehumidified before it is 

introduced to the battery. This option may be the lightest system 

upon takeoff because no cathode reactants will have to be stored 

on the aircraft; however, as the flight continues, the battery will 

continually increase in weight, until it is recharged, at which 

point oxygen will be released. If the lithium-oxygen option is 

pursued, sufficient oxygen to maintain the reaction will need to 

be stored onboard the aircraft. This will include not only the 

oxygen but the tank(s) it will be stored in. 

3.1.2 Possible Improvements to Batteries 

The following subsections describe possible improvements 

for each type of lithium battery.

3.1.2.1 Improvements to Lithium-Ion Batteries 

Anodes have generally been composed of graphite; however, 

work has begun to look at replacing graphite with lithium alloys 

of tin or silicon. These have significantly higher capacities than 

graphite, but they cannot as yet maintain their capacity during 

cycling. Ideally, pure lithium would be used for the anode; 

however, significant obstacles to cycling lithium anodes exist, 

including maintaining contact with the electrolyte surface, 

eliminating voids, and preventing the growth of lithium 

dendrites. Electrolyte chemistry also is being investigated to 

improve the performance and safety of lithium-ion batteries. 

Efforts continue to maximize the ionic conductivity of the 

electrolyte, minimize the mass impact of the electrolyte within 

the cell, improve the stability of the electrolyte at higher 

voltages, and continue to improve the overall safety of the 

electrolyte. In addition, stable cathode materials with higher 

lithium ion storage capacities and higher voltages are needed to 

advance the overall performance of lithium-ion cells. As work 

continues in all of these areas, it can be expected that the celllevel performance will continue to improve from today’s 

average of 150 Wh/kg, to approximately 400 Wh/kg within 

15 years and to 450 Wh/kg within 30 years. 

Batteries are made up of a number of individual cells, and the 

number and arrangement depend on the overall battery voltage 

and capacity required. The battery case encloses the cells, 

provides structure, thermal interface, and at times, compression 

of the cells. Integrating the battery cells within a battery 

requires electrical wiring and a battery controller within the 

case. Currently, the battery case, the wiring, and other 

associated components compose approximately 30 percent of 

the overall battery mass. As improvements in lightweight 

materials and alternative conductors (e.g., nanotubes) become 
NASA/TP—2015-216588 32 

available, it is reasonable to expect that, 15 years out, the mass 

of the case and associated hardware could be reduced to 20 to 

25 percent of the overall battery mass and that, within 30 years, 

the mass of the case and associated hardware could be further 

reduced to 15 to 20 percent of the overall battery mass. 

3.1.2.2 Improvements to Lithium-Sulfur Batteries 

As work continues in the development of electrolytes, 

anodes, and cathodes, we can expect that cell-level performance 

will improve to where it will enable a commercial product with 

a cell-level specific energy of approximately 500 Wh/kg within 

15 years and 800 Wh/kg within 30 years. As the cells are 

incorporated within a battery, the battery structure may need to 

accommodate the dimension changes experienced within the 

cell as a result of changes in the anode and cathode during 

cycling. As a result, the battery structure may contribute more 

to the overall weight of the battery than is seen with lithium-ion 

batteries. Within 15 years, the battery structural mass could 

contribute an additional 30 percent of the overall battery mass. 

Thirty years out, that may drop to 20 to 25 percent of the overall 

battery mass. 

3.1.2.3 Improvements to Lithium-Air Batteries 

As with lithium-sulfur batteries, a considerable amount of 

work remains to develop stable electrolytes, anodes, and 

cathode structure for lithium-air batteries. As work continues in 

all of these areas, we can expect cell-level performance to 

continue to improve and enable a commercial product with a 

cell-level specific energy of approximately 600 Wh/kg within 

15 years and 1200 Wh/kg within 30 years. 

Incorporating lithium-air cells within a battery may be a more 

complicated endeavor. In addition to the normal design 

problems with incorporating the cells into a battery, the design 

must provide air or oxygen to each cell of the battery and must 

maintain adequate and equal air or oxygen to each cell to 

maintain balance across the battery. Because of this design 

constraint, we expect that the mass penalty of moving from a 

cell to battery will be larger than for the other systems. Also, 

the air-handling system and the compressor system required at 

high altitude might add more weight to the system. 

Lithium-air batteries designed within 15 years might have a 

battery-integration mass penalty of approximately 40 percent. 

Within 30 years, as materials continue to improve, the mass 

penalty could drop to 25 to 30 percent. 

3.1.3 System Benefits of Improved Batteries 

The following subsections describe the system benefits for 

each type of lithium battery.

3.1.3.1 System Benefits of Lithium-Ion Batteries 

With potential future improvements in cell performance and 

materials in mind, a predicted battery mass sufficient to provide 

power for all phases of the reference mission was calculated for 

both the 1- and 10-MW electric motors using lithium-ion 

batteries (Figs. 43 and 44). 

Figure 43.—Lithium-ion battery mass to provide power 

to the 1-MW electric motor model flight; 95-percentefficient motor, 97-percent-efficient inverter, and 90-

percent battery depth of discharge.

Figure 44.—Lithium-ion battery mass to provide power 

to the 10-MW electric motor model flight; 95-percentefficient motor, 97-percent-efficient inverter, and 90-

percent battery depth of discharge.
NASA/TP—2015-216588 33 

Figure 45.—Lithium-sulfur battery mass to provide 

power to a 1-MW electric motor; 95-percent-efficient 

motor, 97-percent-efficient inverter, and 90-percent 

battery depth of discharge. Lithium-sulfur cells are not 

currently available commercially.

Figure 46.—Lithium-sulfur battery mass to provide 

power to a 10-MW electric motor; 95-percent-efficient 

motor, 97-percent-efficient inverter, and 90-percent 

battery depth of discharge. Lithium-sulfur cells are not 

currently available commercially.

As shown in the figures, the potential improvement within 

the next 15 to 30 years will enable substantial weight savings 

for the battery system to meet the power and energy needed by 

aircraft. Battery masses for both the 700- and 1000-V electrical 

bus cases also were calculated for each power level. As can be 

seen, the mass differences of the two cases are relatively small. 

Although the overall power and capacity needed is the same for 

both electrical buses, the nominal voltage that the battery is 

expected to deliver is different. Individual cells are arranged in 

a series configuration to meet the voltage required from the 

battery, and additional strings of series-wired cells are arranged 

in parallel within the battery to meet the current draw 

requirement and capacity. For a higher voltage to be delivered, 

more cells must be wired in series, and those series banks must 

be added to meet power and capacity demands. As additional 

strings containing large numbers of cells are added, they may 

Figure 47.—Lithium air/oxygen battery mass to provide 

power to the 1-MW electric motor model flight; 95-

percent-efficient motor, 97-percent-efficient inverter, 

and 90-percent battery depth of discharge. Lithium-air 

cells are not currently available commercially.

overshoot power and capacity needs and result in a heavier 

battery. 

3.1.3.2 System Benefits of Lithium-Sulfur Batteries 

Lithium-sulfur batteries are still a relatively immature 

technology. Assuming that the current barriers to commercially 

available lithium-sulfur are adequately addressed with continued improvements in cell performance and materials, predicted 

battery masses sufficient to provide power for all phases of the 

reference mission were calculated for both 1- and 10-MW 

electric motors (see Figs. 45 and 46). 

A battery weight was not included for the current year, 

because no commercial lithium-sulfur cell has been developed 

that can sustain multiple charge/discharge cycles. As seen for 

the lithium-ion case, substantial weight savings can be expected 

with the improvement of the battery’s specific capacity. 

3.1.3.3 System Benefits of Lithium-Air Batteries 

Lithium-air batteries, like lithium-sulfur batteries, are still at 

a low level of development for commercial use. Again, no 

battery weights were included for the current year’s technology 

because a commercial product capable of multiple cycles is not 

yet available. With the potential future improvements in cell 

performance and materials in mind, a predicted battery mass 

sufficient to provide power for all phases of the reference 

mission was calculated for both 1- and 10-MW electric motors 

(see Figs. 47 and 48). 

The battery masses for both the 700- and 1000-V electrical 

bus cases were calculated for each power level. As for the 

lithium-ion and lithium-sulfur battery projections, there are 

relatively small mass differences between the two cases. The 

overall arrangement of cells to meet both system bus voltage 
NASA/TP—2015-216588 34 

Figure 48.—Lithium-air/oxygen battery mass to provide power to 

the 10-MW electric motor model flight; 95-percent-efficient 

motor, 97-percent-efficient inverter, and 90-percent battery 

depth of discharge. Lithium-air cells are not currently available 

commercially.

Figure 49.—Comparison of the three lithium battery chemistries 

under consideration to provide power to a 1-MW motor 

operating at 700 V; 95-percent-efficient motor, 97-percentefficient inverter, and 90-percent battery depth of discharge.

and capacity can result in differences in mass for the different 

bus voltages. 

3.1.3.4 Technology Comparison of Battery Types 

Figure 49 shows the three battery technologies under 

consideration for the 1-MW electric motor with a 700-V system 

bus. As can be seen, with all three technologies, substantially 

lower battery masses are expected in the coming years. 

However, at 15 years out, the lithium-sulfur system is predicted 

to be 1000-kg lighter than either the lithium-ion or lithium-air 

battery system. After 30 years of development, the lithiumair/oxygen battery is expected to have the lowest battery mass. 

It is anticipated to be 1000-kg lighter than its nearest 

competitor, the lithium-sulfur battery. 

Figure 50.—Effect of system efficiency on the lithium-ion 

battery mass for a 10-MW electric motor operating at 700 V.

3.1.3.5 Effect of System Efficiency 

Up to this point in the report, the battery weight assessments 

have assumed that the power demand of the overall propulsion 

system would remain the same within the projected 30-year 

period. However, in reality, it is likely that, with improved 

designs and materials, the power demands of an aircraft could 

drop substantially. A lower power demand will allow a smaller, 

lighter battery to be used. Another factor that is likely to change 

in the future is the overall system efficiency of the electric 

propulsion system. Specific system efficiencies were assumed 

for the calculation of the future battery mass for the reference 

flight. The two major players in system efficiency were 

assumed to be the electric motor and the inverter. It was 

assumed that the motor would be 95-percent efficient and that 

the inverter would be 97-percent efficient. Each percentage 

efficiency loss requires more power to be supplied from the 

source, that is, the battery. A larger power demand translates to 

a larger, heavier battery. Figure 50 shows the effect of improvements in the overall system efficiency on the battery weight. 

Higher efficiencies will translate to a lighter battery. For each 

percent improvement in efficiency, hundreds to thousands of 

kilograms of battery mass could be saved. 

3.1.3.6 Rechargeability of Batteries 

Just as a fuel tank must be refilled between flights, an energy 

storage system must be recharged. The choice and design of an 

energy storage system must take into account the time available 

before an aircraft must be used again, and the capability of the 

airport to provide the recharge power. Shorter recharge times 

would require a larger amount of power from the airport during 

recharging. The overall energy content provided to the energy 

storage system would be similar whether the system was 

recharged for 2 h or for 30 min, but the magnitude of the power 

necessary during the recharge would differ. One potential 

option for reducing recharge time without requiring large 
NASA/TP—2015-216588 35 

quantities of power from an airport would be to swap out the 

energy conversion unit between flights. The “recharge time” 

with this process would be the time necessary to pull the spent 

system out, replace it with a new system, and perform whatever 

system checkouts are necessary. The spent system could be 

slowly recharged during off-peak electrical-use hours, saving 

the costs associated with electric power and improving charging 

efficiency by minimizing the waste heat produced when high 

recharge rates are used. 

It also may be possible to provide some charging during the 

actual flight. Significant breakthroughs in the development of 

thin-film solar arrays have occurred in recent years. Solar array 

cells have been developed in the laboratory with a thickness on 

the order of a human hair. Also, work is underway to develop 

solar cell materials that could be painted onto a surface 

(Ref. 94). In the future, it may be possible to replace the paint 

on an aircraft with paint-on solar arrays. Power could be 

collected from both the top and bottom of the aircraft—the top 

collecting power from the incident sunlight, and the bottom 

collecting power from the light reflected off the ground. 

Approximately 1 kW/m2

 of sunlight reaches the ground under 

ideal conditions, and it has been projected that solar array 

efficiency could reach 30 to 40 percent in the future. With those 

developments, a considerable amount of extra power could be 

collected during and between flights. This power could be used 

to recharge the energy storage system to supply power during 

the flight rather than power from the energy storage system. An 

additional way to gather energy during flight could be to trade 

altitude for power during descent. 

3.1.4 Summary of Battery Findings 

Table 14 summarizes the potential benefits on a cell level. 

Although the battery-level improvements are ultimately what 

will be significant for the vehicle as a whole, the battery mass 

is a function of the mass of the cells and of all the other pieces 

making up the battery. As our understanding of batteries, 

designs, and materials continues to improve, it may be possible 

to move away from a conventional battery design and toward a 

battery more integral to the overall vehicle, allowing additional 

weight savings. 

TABLE 14.—POTENTIAL IMPROVEMENTS IN CELL-LEVEL 

BATTERY PERFORMANCE 

Battery Cell-level specific energy, 

Wh/kg 

State of the art +15 years +30 years 

Lithium-ion 80 to 200 400 450 

Lithium-sulfur ---------- 500 to 650 800 to 950 

Lithium-air ---------- 600 to 750 1200 to 1400 

3.1.5 Path Forward for Batteries 

The predicted improvements in battery technology described 

here are contingent on continued research and development. 

The three battery technologies discussed hold considerable 

promise for future inclusion in hybrid electric aircraft systems. 

The cell-level projections for 15 years out have already been 

demonstrated in small laboratory cells, but significant work is 

still necessary to bring them to a practical working size. 

Continued development of anodes, cathodes, and electrolytes is 

necessary to reduce weight and improve performance, safety, 

and cycle life. The research and development currently 

underway in an effort to develop batteries for terrestrial electric 

vehicles could be leveraged, but the safety, performance, and 

weight goals for electric aircraft are likely to be beyond those 

for other electric vehicles. As materials and cell and battery 

designs continue to evolve, it may be possible to use some of 

the battery structure as part of the vehicle structure. Advances 

in the individual components of cells and overriding battery 

design may enable battery cells and batteries that can be 

conformed to fit the available space in an aircraft while being 

strong enough to become part of the aircraft support structure. 

3.2 Flywheel Energy Storage 

An energy source will be required to power the motor for 

either the hybrid electric or the turboelectric concept. This 

energy source is typically envisioned as a chemical battery; 

however, flywheels have many benefits for use in aircraft. 

These benefits include power densities that are an order of 

magnitude higher than those of chemical batteries with resultant 

fast charge and discharge times, low temperature sensitivity, 

long life, low maintenance, high depth of discharge, and low 

toxicity. They also have the potential added benefit of providing 

stabilized control for aircraft steering. 

Flywheels store energy mechanically, in a spinning rotor. 

The flywheel is charged by speeding up the rotor and is 

discharged by slowing down the rotor, all through an integral 

motor/generator. These rotors achieve a high specific energy by 

using high-strength, low-density materials spinning at very high 

speeds. Figures 51 and 52 show a composite rim flywheel rotor 

built and tested at Glenn. The carbon fiber composite used in 

the rim has exceptionally high tensile strength, allowing the 

rotor to spin at up to 60 000 rpm. The rotor is enclosed in a 

vacuum tank and is levitated on magnetic bearings, resulting in 

very low losses and high efficiency. 

Systems studies performed as part of the SUGAR Volt 

concept (Ref. 3) for hybrid electric and turboelectric aircraft 

have shown that the energy storage device needs to have a very 

high specific-energy density, on the order of 600 to 800 Wh/kg. 
NASA/TP—2015-216588 36 

Figure 51.—Glenn’s G2 flywheel rotor.

Figure 52.—Schematic of Glenn’s flywheel.

Although battery technology is evolving rapidly, achieving 

such high energy densities will require considerable 

development. In addition, there are problems with temperature 

sensitivity in batteries, impacting efficiency, reliability, and 

life. High-specific-energy flywheels have the potential to 

achieve very high energy densities (Ref. 95) with low 

temperature sensitivity. 

3.2.1 State of the Art of Flywheels 

In 2008, Koh and Magee (Ref. 96) studied emerging energystorage technologies, including various chemical battery types, 

capacitors, and flywheels. Although chemical batteries have the 

highest specific energies today, flywheels have a much higher 

gain in specific energy trend over time, and they look to 

overtake batteries in the coming years (Fig. 53). Currently, the 

specific energy of a flywheel rotor is about 100 Wh/kg. 

Assuming the same rate of increase, forecasting ahead 15 and 

30 years indicates flywheel rotor specific energies approaching 

1000 and 5000 Wh/kg, respectively. 

The key parameter in determining the specific energy of a 

flywheel rim is the material’s strength-to-density ratio, or S/U. 

The specific energy of the rim is directly proportional to this 

ratio and is defined for an isotropic material as 

 

    »

»

1⁄4

o

«

«

¬

a

 Q   Q



 ̧

 ̧

1

·

 ̈

 ̈

©

§

U 2

2

3 1

1

i o

i o

r r

S r r

m

KE (5) 

where KE is the kinetic energy, m is the mass, ri and ro are the 

inner and outer radii of the cylindrical rim, and Q is Poisson’s 

ratio. From Equation (5), we see that maximizing the strengthto-density ratio (specific strength) of the rim material will 

maximize the specific energy. Table 15 shows some specific 

Figure 53.—Energy storage trends (Ref. 96). Reprinted from Technol. Forecast. Soc. Change, vol. 75, 

Koh, Heebyung; and Magee, Christopher L., A Functional Approach for Studying Technological Progress: 

Extension to Energy Technology, pp. 735–758, Copyright 2008, with permission from Elsevier.
NASA/TP—2015-216588 37 

TABLE 15.—SPECIFIC TENSILE STRENGTH AND RIM SPECIFIC ENERGY 

[ri/ro = 0.7.] 

Material Derated specific tensile strength, 

kN-m/kg 

Rim specific energy, 

Wh/kg 

200-GPa nanofiber (predicted) 30 400 3430 

100-GPa nanofiber (predicted) 15 200 1720 

50-GPa nanofiber (predicted) 7 600 860 

Carbon fiber with nanotubes (predicted) 1 940 220 

Carbon fiber 970 110 

Aramid 390 44 

E-Glass with nanotubes 600 68 

E-Glass 300 34 

TABLE 16.—POWER AND ENERGY REQUIREMENTS FOR FLYWHEEL ENERGY STORAGE 

Motor power class, 

MW 

Flywheel power required, 

kW 

Flywheel energy required, 

kWh 

Charge/discharge time, 

h 

1 1 082 2 947 2.7 

10 10 815 29 471 2.7 

strength values for various composite materials. Today, the 

highest strength material used in flywheels is carbon fiber 

composite, with a derated S/U of nearly 1000 kN-m/kg. For 

ri/ro = 0.7, the specific energy is 110 Wh/kg. As new highstrength materials are developed, specifically nanomaterials, 

this value will increase. A composite with 50-GPa nanofibers 

will yield a specific energy of 860 Wh/kg, similar to the 

projected value for 15 years from Figure 53. A composite 

incorporating 200-GPa nanofibers will yield a specific energy 

of 3430 Wh/kg, expected in about 30 years. 

The specific energy of the flywheel module will be lower 

than that of the rim itself because of the added weights of other 

components, including the hub, flywheel motor, magnetic 

bearings, touchdown bearings, and housing. The weights of 

these components will depend on the power and energy, which 

are shown in Table 16 for the 1- and 10-MW systems (adapted 

from Table 13). The charge/discharge time is the ratio of the 

energy to the power. 

As an example, the flywheel described by Strasik (Ref. 95) 

has a module specific energy of 38 Wh/kg, with an energy of 

5 kWh, power of 3 kW, and a total weight of 132 kg. This 

flywheel utilizes a carbon-fiber-composite rim, which from 

Table 15, should have an approximate specific energy of 

110 Wh/kg. Thus the rim mass should be about 46 kg, and other 

component masses should be about 86 kg. This particular 

flywheel is ground based, not optimized for low weight, and 

there will be opportunities to remove weight from the flywheel 

module. For example, using Kascak’s equations (Ref. 97) for 

today’s technology indicates that the specific energy of a 

5-kWh, 3-kW flywheel module could be as high as 50 Wh/kg. 

On the basis of those equations and today’s carbon-fibercomposite rotor, the 1-MW flywheel module investigated here 

should approach 80 Wh/kg. Kascak also shows that, as the 

flywheel energy gets larger, the flywheel module specific 

energy approaches the rotor specific energy. 

The rotor size will clearly depend on the flywheel energy 

required. For example, for today’s carbon-fiber-composite 

material, the flywheel rotor mass would be approximately 

33 300 kg/MW. This mass would be reduced to 970 kg/MW in 

30 years, assuming a composite material with 200-GPa fibers. 

The motor size depends on the power and the discharge time. 

A very fast discharge time requires a much larger motor. For 

example, the Strasik flywheel has a discharge time of 1.7 h, 

which is lower than for the aircraft application (which has a 

relatively larger motor). For the 2.7-h discharge time, the motor 

mass would be less than 5 percent of the rotor weight for the 

carbon-fiber-composite rotor. For the same power level, this 

motor size would stay the same regardless of the rotor size. 

The touchdown bearings would be sized to take the radial and 

axial loads of the flywheel in case of a power failure or sudden 

extreme load. Axially, the loads would be smaller for the lighter 

flywheel. However, the radial loads would be proportional to 

the rotor energy. Each rotor would have a slight unbalance 

proportional to the rotor weight, and the unbalance excitation 

force would be proportional to the square of the rotor speed. 

However, the touchdown bearing mass would be just a small 

percentage of the total flywheel module mass. 

The majority of the added mass would come from the 

housing. The housing size depends on the size of the flywheel, 

scaling by area rather than volume. Therefore, the ratio of the 
NASA/TP—2015-216588 38 

housing mass to the flywheel weight decreases with increasing 

rotor size. The power and energy required for aircraft is much 

higher than for the flywheels built and tested to date. Groundbased flywheels typically use a metal housing, often steel, so 

the mass is very high. Using high-specific-strength composites 

for the vacuum housing would significantly reduce the housing 

mass in comparison to today’s technology. 

For component and housing mass predictions, we assumed, 

for simplicity, that 10 percent of the component and housing 

mass would remain the same over the next 30 years and that the 

rest would scale with the rotor mass. 

3.2.2 Possible Improvements to Flywheels 

For a carbon-fiber-composite rim, today’s expected flywheel 

specific energy is 110 Wh/kg. Assuming 95-percent efficiency 

and 90-percent depth of discharge, the flywheel module specific 

energy is 80 Wh/kg. For a 1-MW system, the rotor mass is 

approximately 33 300 kg, and the mass of the other components 

(motor, housing, etc.) is about 12 100 kg, which is an additional 

36 percent over the rotor weight. 

In 15 years, we expect that the use of 50-GPa fibers in 

composites will reduce the weight of the rotor. The expected 

flywheel specific energy will be 860 Wh/kg, giving a 3970-kg 

flywheel with 98-percent efficiency and 90-percent depth of 

discharge. The component and housing mass is assumed to be 

2500 kg, giving a flywheel module specific energy of 

450 Wh/kg. 

Finally, in 30 years, we expect that 200-GPa fibers will be 

used in the composite rim, giving a flywheel specific energy of 

3430 Wh/kg. For 99-percent efficiency and 90-percent depth of 

discharge, the rotor mass will be 970 kg, and the expected 

component mass will be 1520 kg, giving a flywheel module 

specific energy of 1180 Wh/kg. 

3.2.3 System Benefits of Flywheels 

Reducing the weight of the energy storage system will 

directly increase the horsepower per pound achieved by the 

entire propulsion system. 

Table 17 summarizes the future specific energy benefits 

expected for the flywheel rim only as well as for the full 

flywheel module both now and in 15 and 30 years. 

TABLE 17.—BENEFIT SUMMARY FOR FLYWHEEL 

ENERGY STORAGE 

Component Specific energy, 

Wh/kg 

State of the art +15 years +30 years 

Flywheel rim 110 860 3430 

Flywheel module 80 450 1180 

3.2.4 Path Forward for Flywheels 

Several technologies need to be investigated to achieve the 

high specific-energy densities required for this application, 

including high-specific-strength rotor materials, flywheel motor 

technology, and reducing component masses. The first-order 

factor in flywheel specific energy is the rotor material strengthto-density ratio. Glenn has been investigating composite materials for high strength and low cost. Additional work needs to be 

done to mature carbon-fiber and nanotube material technology 

such that it can be utilized within a flywheel rotor system. Small 

batches of fiber need to be produced with consistent properties. 

Strength, fatigue, creep, and environmental effects need to be 

characterized. The best material system needs to be brought up to 

production levels, prototype flywheel rotors need to be made 

using the new fiber, and this work needs to be continued over 

time to achieve stronger and stronger rotor materials. 

Safety is an important consideration for any type of energy 

storage device. NASA was involved for several years with a 

rotor safe-life program in cooperation with the University of 

Texas Center for Electromechanics. Composite flywheel rotors 

were tested to make sure that they had a safe failure mode, in 

which the rim underwent a noncatastrophic failure. The 

Defense Advanced Research Projects Agency (DARPA) also 

has a flywheel safety program that is looking at failure modes 

and detection. Proper flywheel design in coordination with 

monitoring will enhance the safety of flywheels. It is advisable 

to continue research and testing to ensure the safety of 

flywheels with reasonably sized containment for use in aircraft. 

The most efficient flywheel would be one in which all the 

energy and mass were in the rotating mass. However, there are 

weights associated with the motor, magnetic bearings, and 

housing, in addition to other components. One means of 

reducing motor and magnetic bearing mass is a high-speed 

inside-out bearingless motor currently being developed at 

Glenn. This combination motor/magnetic bearing reduces 

weight in comparison to motors with individual components. 

The fact that it is inside out reduces the rotor hub size, 

concentrating more of the rotating energy out on the rim, 

increasing efficiency, and reducing weight. However, this 

technology is relatively new and has only been tested at low 

speed. Continued research in this area would go toward 

improving the specific energy of the flywheel, making it a 

viable candidate for energy storage in more-electric aircraft. 

3.3 Supercapacitors 

A third type of energy storage device that might be 

incorporated into hybrid electric propulsion is the 

supercapacitor, also known as the ultracapacitor. Like 
NASA/TP—2015-216588 39 

traditional capacitors, the supercapacitor has very high power 

density, so it can charge and discharge rapidly. This may be of 

use when short-term high power is required, such as during 

takeoff, or for other power requirements such as providing 

power to electromechanical actuators. Currently, supercapacitors are used as backup power for Airbus 380 emergency 

exits. A disadvantage of traditional capacitors is their low 

energy density. However, supercapacitors have orders of magnitude higher energy density than traditional capacitors, 

bridging the gap between capacitors and batteries, as shown 

Figure 39. Other advantages of supercapacitors include a long 

cycle life (millions of charge/discharge cycles), good performance over a wide temperature range, high efficiency, and 

low toxicity. It may be that supercapacitors could be used along 

with another type of energy storage to optimally handle the 

requirements of hybrid electric propulsion, for example to 

decrease battery size and increase battery life. 

A traditional capacitor consists of a pair of electrodes 

separated by an insulating dielectric material. When a voltage 

is applied to the capacitor, positive charge accumulates on one 

electrode, and negative charge accumulates on the other 

electrode. Energy is stored within the electric field between the 

electrodes. The capacitance is the ratio of the stored positive 

charge to the applied voltage. The energy E stored in a capacitor 

is directly proportional to the capacitance and is given by 

2

2

1 E CV (6) 

where C is the capacitance and V is the applied voltage. 

Therefore, increasing the capacitance or the voltage will 

increase the stored energy. The capacitance is related to the 

capacitor geometry and material properties through the relation 

x

s C HoHr (7) 

where Ho is the permittivity of free space, Hr is the dielectric 

constant of the insulating material, s is the surface area of the 

electrode, and x is the distance between electrodes. Clearly, the 

capacitance can be maximized by increasing the dielectric 

constant, increasing the electrode area, or decreasing the 

distance between electrodes. Supercapacitors achieve higher 

capacitance by doing just that. 

The power calculation comes from assuming that the capacitor 

is in series with an external load resistance. The maximum power 

is then inversely proportional to the equivalent series resistance 

(ESR) of the capacitor, which includes the effects of components 

such as the electrodes and dielectric material. Keeping the ESR 

low ensures high power capability. 

3.3.1 State of the Art of Supercapacitors 

There are three kinds of supercapacitors: electrochemical 

double-layer capacitors (EDLCs), pseudocapacitors, and hybrid 

capacitors. Table 18, based on Burke (Ref. 98), shows the cell 

voltage, power density, and energy density of each type, 

assuming active mass (not total mass). 

The EDLC consists of two electrodes in contact with an 

electrolyte solution, separated by a semipermeable membrane. 

Like the traditional capacitor, the charge storage is physical, not 

chemical. The membrane allows the ions to pass through while 

preventing shorting across the electrodes. The storage of charge 

is reversible, and the cycle life can be as high as 106

 cycles 

(Ref. 99). The electrodes are made of advanced materials with 

very high surface area, for example a porous material such as 

activated carbon. New electrode materials are being investigated, including single-walled CNTs, which have high 

conductivity and high molecular surface area, and graphene 

sheets, where all the atoms are at the surface, maximizing the 

molecular surface area. 

In a pseudocapacitor, energy storage occurs with charge 

transfer between the electrode and the electrolyte, where the 

ions in the solution combine with the electrode surface atoms 

(chemical charge storage). The electrode material is typically a 

metal oxide or a conducting polymer, which have high conductance, low ESR, and high capacitance. One metal oxide 

material being studied is ruthenium oxide (RuO2), but its cost is 

relatively high (Ref. 99). Another is manganese dioxide 

(MnO2), which is cheaper but has lower electrical conductivity 

(Ref. 100). A drawback of conducting polymers is lower cycle 

life. These devices are not yet commercially available. 

TABLE 18.—PROPERTIES OF SUPERCAPACITORS 

[Adapted from Ref. 98.] 

Supercapacitor type Cell voltages, 

V 

Power density, 

kW/kg 

Energy density, 

Wh/kg 

Electrochemical double-layer 2.5 to 3.5 1 to 3 5 to 12 

Pseudocapacitor 2 to 3.5 1 to 2 10 to 15 

Hybrid 1.5 to 3.3 1 to 2 10 to 15 
NASA/TP—2015-216588 40 

A hybrid capacitor is a combination of an EDLC and a 

pseudocapacitor. The three types of hybrid supercapacitors 

being studied are composite, symmetric, and battery supercapacitors (Ref. 99). Composite hybrid supercapacitors 

combine both chemical and physical charge storage in each 

electrode. Asymmetric supercapacitors include an EDLC 

electrode and a pseudocapacitor electrode. Finally, battery 

supercapacitors combine a supercapacitor electrode with a 

battery electrode. Like pseudocapacitors, hybrid capacitors 

are not yet available commercially. 

3.3.2 Possible Improvements to Supercapacitors 

Halper and Ellenbogen (Ref. 99) discuss several areas of 

research for supercapacitors. One area is the hybrid supercapacitor, which incorporates the benefits of both EDLCs 

and pseudocapacitors and allows the hybrid supercapacitor to 

outperform both. Other research areas include decreasing 

the ESR of supercapacitors (increasing power density), optimizing the electrolyte or dielectric material (increasing power 

density and energy density), and reducing the tendency of 

supercapacitors to self-discharge (affecting performance). 

Chen et al. (Ref. 100) report on a hybrid supercapacitor 

using nanostructured MnO2 and CNTs with commercially 

available sponge as the substrate (Fig. 54). The sponge 

naturally has very high porosity, is lightweight, and allows for 

continuous coating of CNTs. This device achieved a power 

density of 63 kW/kg and an energy density of 31 Wh/kg, 

based on active mass. 

Figure 54.—MnO2-carbon nanotube sponge supercapacitor 

(Ref. 100). Reprinted with permission from Chen, Wei, et al.: HighPerformance Nanostructured Supercapacitors on a Sponge. Nano Lett., vol. 

11, 2011, pp. 5165–5172. Copyright 2011, American Chemical Society.

To improve the surface area of EDLC electrodes, Liu et al. 

(Ref. 101) used curved graphene sheets in the electrode. 

Graphene sheets have high conductivity and are a single atom 

thick so the surface area is already quite high. Adding folds to 

these sheets increases the surface area considerably and also 

keeps the sheets from restacking, which can decrease the 

effective surface area. This configuration reached an energy 

density of 85.6 Wh/kg at room temperature with a power 

density of approximately 0.6 kW/kg and a discharge time of 

580 s. An energy density of 53.1 Wh/kg was achieved with a 

power density of 9.8 kW/kg and a discharge time of 20 s. The 

energy and power density values were based on active mass. 

Lin et al. (Ref. 102) describe another method of optimizing 

the surface area of graphene sheets for EDLC electrodes. 

They deposited silver nanoparticles onto the graphene sheets, 

then heated those graphene sheets in air. The silver nanoparticles were oxidized into gaseous byproducts, and then the 

silver was removed. This left holes in the graphene surface 

that were on the order of about five to tens of nanometers in 

diameter. 

Wang et al. (Ref. 103) discuss supercapacitor electrode 

materials with tuned porosity that are fabricated from paperpulp-mill sludge biowaste—a low-cost waste material that 

would otherwise be incinerated or put in landfills. With this 

material, they achieved 51 Wh/kg at 375 W/kg, and 26 to 

31 Wh/kg at 6760 to 7000 W/kg. It is important to note that, 

like other energy storage devices, the energy density and 

power density can be defined in different ways. These 

reported values include only the active material weight. Wang 

states that carbon weight accounts for about 30 percent of the 

total mass of packaged superconductors. They expect to 

achieve a practical energy density of 15 Wh/kg for a packaged 

device, an improvement of 3 times over the state of the art. 

3.3.3 Benefit Summary of Supercapacitor 

Improvements 

Both the power density and energy density of 

supercapacitors should increase with new material 

improvements in electrode and dielectric materials and with 

the development of new hybrid supercapacitors. Table 19 

details the benefits expected in 15 and 30 years. 

TABLE 19.—SUPERCAPACITOR BENEFIT SUMMARY 

Property State of the art +15 years +30 years 

Energy density (active mass), Wh/kg 5 to 15 100 to 150 200 to 300

Power density (active mass), kW/kg 1 to 3 2 to 3 4 to 6 
NASA/TP—2015-216588 41 

4.0 Related Research on System 

Modeling 

Research is ongoing to determine the benefits of hybrid 

electric and turboelectric aircraft from a systems perspective as 

well as to investigate the electrical distribution system. The 

systems-level research will help to define the motor requirements (e.g., efficiency and power density) for a particular 

aircraft and configuration and also will estimate the decreases 

in noise, emissions, and fuel burn. Glenn is involved with the 

SUGAR program as part of a collaborative effort among several 

companies and institutions, including Boeing Research and 

Technology, Georgia Institute of Technology, and Virginia 

Polytechnic Institute and State University. As an example, the 

systems analysis of the N+3 SUGAR Volt concept, a ~150-

passenger aircraft with hybrid-electric propulsion, incorporated 

various levels of energy density to predict fuel-burn benefits. 

A 8000-hp motor/generator at 2 to 3 hp/lb coupled with a 

750-Wh/kg battery gave a reduction in fuel burn of about 

60 percent (Ref. 3). 

Several studies are also ongoing with regard to the electrical 

distribution system. The use of hybrid electric or turboelectric 

propulsion will create electrical loads that are unprecedented in 

commercial aircraft. A traditional electrical distribution system 

will not be able to supply the large power loads created by the 

motors; therefore, a new system must be contrived. Because of 

the highly integrated nature of electrical systems, their design 

is highly dependent on the performance of each component. 

Because the motors will be the largest load on the system, the 

motor type and performance will have a discernible impact on 

the electrical distribution system. In order to study the effects 

of advancements in motor technologies, a modeling and simulation environment is being created that can optimize the 

electrical distribution system for a given motor design. This 

optimization aims to find the lightest and most efficient system 

possible while maintaining stability and reliability. Studies 

have shown that, using a system-level optimization routing, a 

5-percent weight savings can be achieved (Ref. 104). 

An example of the electric distribution system modeling is 

the sizing of the transmission cables performed by Angela 

Lowe at Georgia Tech as part of NASA’s Graduate Student 

Research Program. In this case study, the cable configuration 

used included a cylindrical copper core, dielectric layer, 

magnetic shield, and cooling sleeve. Calculation of the size of 

cable needed was based on the power demand of the load, 

power losses through the cable, system nominal voltage, and 

allowable heat rejection from the cable. As technologies change 

Figure 55.—Room temperature cable diameter.

Figure 56.—Room temperature cable weight.

and loads vary, the model could be updated with new 

parameters to show how a technology impacts the size of the 

cable. Figures 55 and 56 show an example of the results 

produced by the model. For the model to produce these results, 

the power load was set to 40 MW so that it was representative 

of the loads found on the NASA N3–X (Ref. 74). The allowable 

heat rejection from the cable was set to 0.2 percent of the power 

load based on the study in Reference 105. Figure 55 shows the 

cable diameter as a function of system nominal voltage and 

cable length, and Figure 56 shows the cable weight. The data in 

the curves can be regressed to provide a function that can 

predict the size of the cables based on voltage and length. Later 

this information can be used in the optimization routine to 

determine what system nominal voltages are appropriate for the 

system and what technology advances are needed to reach a 

reasonable cable size. For the study in this report, we can see 

the difference in the weight of cable for today’s 120-V system 

versus a 700- or 1000-V system in the future. 
NASA/TP—2015-216588 42 

Another study was completed by Rolls Royce under contract 

to NASA to model and characterize high-power electrical 

systems for turboelectric aircraft. This system included two 

turboshaft engines, which provided power for a bank of fans 

located at the trailing edge of the aircraft. Superconducting 

generators and motors were included, along with flywheels as 

energy storage. This study generated system architecture 

recommendations and produced a dynamic system model, the 

results of which are included in the contractor report, “Stability, 

Transient Response, Control, and Safety of a High-Power 

Electric Grid for Turboelectric Propulsion of Aircraft” 

(Ref. 106). 

Another study of the electrical distribution system was done 

by Georgia Tech as part of the SUGAR Volt project. Using the 

Numerical Propulsion System Simulation (NPSS) system 

modeling code that was originally designed at NASA, Georgia 

Tech is developing the electrical transmission and distribution 

system, including the inverter, rectifier, DC transformer, 

generator, motor, bus, and energy storage device. 

Glenn has also built a Propulsion Electric Grid Simulator, 

which represents a turboelectric propulsion system through the 

use of motors, generators, electric bus, and control system. This 

test setup enables rapid analysis and demonstration of a systemlevel proof-of-concept for the electrical distribution system. 

The information gained using aircraft systems analysis and 

electrical distribution system optimization could aid in the 

realization of a room temperature hybrid or turboelectric propulsion system by producing a lightweight, stable, and reliable 

design. The system-level optimization process could produce 

significant weight savings and increase efficiency. As the 

technologies for room temperature motors and power 

electronics progress, the models could be updated with new 

performance parameters. Lastly, this information could be used 

to determine which technology improvements have the greatest 

impact on the system. 

5.0 Technology Assessment Conclusion 

The Subsonic Fixed Wing Project of the Fundamental 

Aeronautics Program is researching aircraft propulsion 

technologies that will lower noise, emissions, and fuel burn. 

This study was initiated to evaluate the possibility of using 

noncryogenic electric motors as part of an electric propulsion 

system that would provide benefits in all of these categories 

without incurring the added risk of developing flightworthy 

cryogenic systems. 

The technology improvements that were investigated in this 

study include motor-related technologies, energy-storagerelated technologies, and system modeling approaches. Motor 

efficiency and power density could be improved through the use 

of better conductors, insulators, magnets, bearings, structural 

materials, and thermal management. Energy storage could be 

improved through batteries, flywheels, or supercapacitors, all 

of which expect significant energy density improvements over 

the next few decades. 

The benefits shown in this study are quite significant. The 

technology improvements described may bridge the gap in 

power density and efficiency between currently available offthe-shelf motors, electronics, and energy storage components 

and the projected cryogenically cooled superconducting motor 

technology. 

Table 20 summarizes the projected benefits due to each 

technology in 15 and 30 years. These improvements will only 

be realized if there is continued research and support coupled 

with a consistent desire to achieve NASA’s Fundamental 

Aeronautics goals of reducing fuel burn, emissions, and noise, 

thereby minimizing the environmental impact of aviation. 

Glenn Research Center 

National Aeronautics and Space Administration 

Cleveland, Ohio, January 20, 2015 

TABLE 20.—SUMMARY OF EXPECTED TECHNOLOGY IMPROVEMENTS 

Technology or electrical component State of the art +15 years +30 years 

Electric motor—all technologies 

Energy density, hp/lb 3 8 16 

Power electronics 

Energy density, hp/lb 5 6 7 

Efficiency, percent 93 to 95 98 99 

Energy storage (based on active mass only)—specific energy, Wh/kg 

Lithium-ion batteries (cell only) 80 to 200 400 450 

Lithium-sulfur batteries (cell only) ---------- 500 to 650 800 to 950 

Lithium-air batteries (cell only) ---------- 600 to 750 1200 to 1400 

Flywheels (rim only) 110 860 3430 

Supercapacitors (active mass only) 5 to 15 100 to 150 200 to 300 

 
NASA/TP—2015-216588 43 

References 

1. Brown, Gerald V.: Efficient Flight-Weight Electric 

Systems. Presented at the Fifth Fundamental Aeronautics 

Program Technical Conference, Cleveland, OH, 2012. 

2. N+2 Electric Distributed Propulsion Aircraft for Terminal 

Area Operations. ESAero, Pismo Beach, CA. 

http://esaero.com/ Accessed Dec. 2, 2013. 

3. Bradley, Marty K.; and Droney, Christopher K.: Subsonic 

Ultra Green Aircraft Research: Phase I Final Report. 

NASA/CR—2011-216847, 2011. http://ntrs.nasa.gov/

4. Arpingstone: Lufthansa. http://en.wikipedia.org/wiki/ 

File:Lufthansa-1.jpg Accessed Dec. 2, 2013. 

5. &)0௅ $HUR (QJLQH 0DLQWHQDQFH 7UDLQLQJ &HQWHU

2005. http://218.6.160.223/eng/prod01.htm Accessed 

Dec. 2, 2013. 

6. 37௅/8= http://www.jetphotos.net/viewphoto.php?id= 

6807140 Accessed Dec. 2, 2013. 

7. CJ610. U.S. Turbine & Accessory, LLC, 2013. 

http://www.usturbine.us/cj610.html Accessed Nov. 12, 2014. 

8. Buress, T.A., et al.: Evaluation of the 2010 Toyota Prius 

Hybrid Synergy Drive System. ORNL/TM–2010/253, 

2010. 

9. YASA Motors—Powering the Future. YASA Motors, 

Milton Park, Abingdon, 2013. http://www.yasamotors.com

Accessed Dec. 2, 2013. 

10. Launchpoint Technologies 6” Dual Halbach Air-Core 

Motor. Rev. 1.0, Launchpoint Technologies Inc. 

http://www.launchpnt.com/Portals/53140/docs/dualhalbach-motor-data-sheet_r1.pdf Accessed Dec. 2, 2013. 

11. Kuznetsov, Stephen: Machine Design and Configuration 

of a 7000 HP Hybrid Electric Drive for Naval Ship 

Propulsion. 2011 IEEE International Electric Machines 

and Drives Conference (IEMDC), 2011, p. 1625. 

12. Proven Power Dense Solution for Advanced Naval 

Propulsion. DRS Technologies, Arlington, VA, 2013. 

http://www.drs.com Accessed Dec. 2, 2013. 

13. Huang, Surong, et al.: A General Approach to Sizing and 

Power Density Equations for Comparison of Electrical 

Machines. Research Report 96– ,(((௅,$6 &RQI

RHFSS௅

14. Nanotubes and Buckyball. Nanotechnology Now, 2012. 

http://www.nanotech-now.com/nanotube-buckyballsites.htm Accessed July 7, 2012. 

15. Carbon Nanotube. Wikipedia Foundation, San Francisco, 

CA, 2013. http://en.wikipedia.org/wiki/Carbon_nanotube

Accessed July 7, 2012. 

16. Carbon Nanotube Science and Technology. A Carbon 

Nanotube Page. University of Reading, UK, 2012. 

http://www.personal.reading.ac.uk/~scsharip/tubes.htm

Accessed July 7, 2012. 

17. Peng, Bei, et al.: Measurements of Near-Ultimate 

Strength for Multiwalled Carbon Nanotubes and 

Irradiation-Induced Crosslinking Improvements. Nature 

1DQRWHFKQRORJ\ YRO   SS ௅ $FFHVVHG

July 7, 2012. 

18. Graphene. Wikipedia Foundation, San Francisco, CA, 

2013. http://en.wikipedia.org/wiki/Graphene Accessed 

July 7, 2012. 

19. Copper-Clad Aluminum. International Cablemakers 

Federation, Vienna, Austria. http://www.icf.at/en/6049/ 

copper_clad_aluminium.html Accessed July 7, 2012. 

20. Experiments With Magnets and Conductors. Magnet 

Man, 2013. http://www.coolmagnetman.com/magconda.htm

Accessed July 7, 2012. 

21. General Nano LLC. SBR/STTR, U.S. Government, 

Washington, DC. http://www.sbir.gov/sbirsearch/detail/ 

172986 Accessed July 7, 2012. 

22. The Strongest, Lightest and Most Conductive Material 

Known. Nanocomp Technologies, Inc., Merrimack, NH, 

2012. http://www.nanocomptech.com/what-are-carbonnanotubes.html Accessed July 7, 2012. 

23. Zhao, Yao, et al.: Iodine Doped Carbon Nanotube Cables 

Exceeding Specific Electrical Conductivity of Metals. 

Sci. Reports, vol. 1, no. 83, 2011, pp. 1–5. 

24. Behabtu, Natnael, et al.: Strong, Light, Multifunctional 

Fibers of Carbon Nanotubes With Ultrahigh 

Conductivity. Sci., vol. 339, no. 182, 2013, pp. 182–186. 

25. Maxwell, James L.; Rose, Chris R.; and Ramos, Octavio, 

Jr.: Ultraconductus: Innovative Electrical Conductors. 

Los Alamos National Laboratory, Los Alamos, 

NM, 2011. http://www.lanl.gov/science/NSS/issue2_2011/ 

story5full.shtml Accessed Dec. 2, 2013. 

26. Magnet Wire Insulation Guide. MWS Wire Industries, 

Westlake Village, CA, 2011. http://www.mwswire.com/ 

pdf_files/mws_tech_book/page2_3.pdf Accessed Dec. 2, 

2013. 

27. Hatano, Hiroshi: Development of High Thermal 

Conducting Insulation for Turbine Generator Stator Coil. 

EINA Magazine No. 13, 2006. http://eina.ws/no13/ 

EINA_No13p41-42.pdf Accessed Dec. 2, 2013. 

28. Kikuchi, Hideyuki; and Hanawa, Hidehito: Inverter Surge 

Resistant Enameled Wire With Nanocomposite Insulating 

Material. IEEE Trans. Dielec. Elect. Insul., vol. 19, no. 1, 

2012, pp. 99–106. 

29. DuPont Kapton Polyimide Film. DuPont publication H–

38479–9, 2012. 

30. PYRE-M.L. Wire Enamel. Industrial Summit Technology, 

Technical Bulletin #ML–1–95. http://www.istusa.com/ 

pdf/Pyre-ML-Technical-Bulletin.pdf Accessed Jan. 14, 

2015. 

31. PEEK Insulated Wire. Zeus PEEK datasheet, Zeus, Inc., 

Orangeburg, SC. http://www.zeusinc.com/UserFiles/ 

zeusinc/Documents/technical_salessheets/ 

PEEK_InsulatedWire_ZEUS.pdf Accessed Dec. 2, 2013. 

32. Poly(etheretherketone) (PEEK). ASM International, 

Materials Park, OH, 2013. http://www.asminternational.org/ 

content/docs/PEEK.pdf Accessed Dec. 2, 2013. 

33. Suzuki, Kazunori; Murouchi, Kengo; and Magari, 

Yukinobu: New Heat Resistant Magnet Wire: 

“Polyimide-Silica Hybrid Enameled Wire.” Hitachi Cable 

Review, No. 20, 2001, pp. 91–96. 
NASA/TP—2015-216588 44 

34. Zhi, Chunyi, et al.: Towards Thermoconductive, 

Electrically Insulating Polymeric Composites With Boron 

Nitride Nanotubes as Fillers. Adv. Func. Mater., vol. 19, 

2009, pp. 1857–1862. 

35. Li, Tung-L.; and Hsu, Steve Lien-Chung: Enhanced 

Thermal Conductivity of Polyimide Films Via a Hybrid 

of Micro- and Nano-Sized Boron Nitride. J. Phys. Chem. 

B, vol. 114, 2010, pp. 6825–6829. 

36. Thompson, Marc T.: Practical Issues in the Use of NdFeB 

Permanent Magnets in Maglev, Motors, Bearings, and 

Eddy Current Brakes. Proc. IEEE, vol. 97, no. 11, 2009, 

p. 1758. 

37. Constantinides, S.: The Magnetic Material Challenge. 

$53$௅(:RUNVKRS5DUH (DUWK DQG &ULWLFDO0DWHULDOV

Arlington, VA, 2010. http://www.arnoldmagnetics.com/ 

Rare_Earth_Information.aspx Accessed Dec. 3, 2013. 

38. Nanocomposite Magnets. GE Global Research, 

Department of Energy, Washington, DC, 2013. 

http://arpa-e.energy.gov/?q=arpa-e-projects/ 

nanocomposite-magnets Accessed Dec. 3, 2013. 

39. NEOMAX Magnet Product Catalog, Hitachi Metals, Ltd., 

Tokyo, Japan, 2013. http://www.hitachi-metals.co.jp/e/ 

eh2009/data/NEOMAXCatalog2.pdf Accessed Dec. 2, 

2013. 

40. Matsuura, Yutaka: Recent Development of Nd-Fe-B 

Sintered Magnets and Their Applications. J. Magnetism 

Magnetic Mater., vol. 303, no. 2, 2006, pp. 344–347. 

41. Electric Vehicle Propulsion. Launchpoint Technologies 

Website, Goleta, CA. http://www.launchpnt.com/portfolio/ 

aerospace/uav-electric-propulsion/ Accessed Dec. 9, 2014 

42. Coey, J.M.D.; and Skomski, Ralph: New Magnets From 

Interstitial Intermetallics. Research Papers in Physics and 

Astronomy (Physics Scripta, vol. T49, 1993, pp. 315–321), 

Paper 14, Ralph Skomski Publications, 1993. 

http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article

=1013&context=physicsskomski Accessed June 18, 2014. 

43. Johnson, Francis: Transformational Nanostructured 

Permanent Magnets. Advanced Research Projects 

Agency, DOE. http://www.arpae-summit.com/em_reporting/ 

exhibitor_detail?exhibitor_id=4497 Accessed Dec. 9, 2014 

44. Hadjipanayis, G.C.: High-Energy Permanent Magnets for 

Hybrid Vehicles and Alternative Energy. University of 

Delaware, Newark, DE. http://www.arpae-summit.com/ 

em_reporting/exhibitor_detail?exhibitor_id=4487 

Accessed Aug. 12, 2014. 

45. Hadjipanayis, George C.; and Xiao, John Q.: Novel 

Materials for High Energy and Power Density. 

University of Delaware Energy Institute Symposium, 

Newark, DE, 2008. http://www.energy.udel.edu/pdf/ 

Hadjipanayis_UDEI_Symposium.pdf Accessed Dec. 3, 

2013. 

46. Liu, Jinfang: Sm-Co Magnets and Applications. 

Electron Energy Corporation, Landisville, PA. 

http://www.electronenergy.com/media/ 

SmCo%20Magnets%20and%20Applications.pdf Accessed 

Dec. 3, 2013. 

47. Gallego, Nidia C.; and Klett, James W.: Carbon Foams 

for Thermal Management. Carbon, vol. 41, 2003, 

pp. 1461–1466. 

48. Shin, E. Eugene; Johnston, J. Chris; and Haas, Daniel: 

Advances in the Lightweight Air-Liquid Composite 

Heat Exchanger Development for Space Exploration 

$SSOLFDWLRQV$,$$௅

49. Brown, Gerald V., et al.: Ultra-High-Power, Lightweight 

Cryogenic Motor Developed and Operated in 

Liquid Nitrogen. NASA/TM—2008-215054, 2008. 

http://ntrs.nasa.gov

50. Gieras, J.F.: Advancements in Electric Machines. 

Springer, New York, NY, 2008, p. 102. 

51. Lawson, Craig P.; and Pointon, James M.: Thermal 

Management of Electromechanical Actuation on an AllElectric Aircraft. Proceedings of the 26th International 

Congress of the Aeronautical Sciences, Anchorage, AK, 

2008. 

52. Lowe, Angela: Aircraft Energy Architecture Modeling 

Evaluation: Thermal Management System. Presentation 

for the Georgia Institute of Technology, Aerospace 

Systems Design Laboratory (ASDL), Jan. 31, 2011. 

53. Ohadi, M.; and Qi, J.: Thermal Management of HarshEnvironment Electronics. 20th IEE SEMI–THERM 

Symposium, 2004. 

54. Filburn, Tom; Kloter, Amanda; and Cloud, Dave: Design 

of a Carbon-Carbon Finned Surface Heat Exchanger for a 

High-Bypass Ratio, High Speed Gas Turbine Engine. 

ASME GT2006–90480, 2006. 

55. Chandler, David L.: Insulators Made Into Conductors. 

MIT News, 2010. http://web.mit.edu/newsoffice/2010/ 

heat-nanofibers-0308.html Accessed Dec. 3, 2013. 

56. Shen, Sheng, et al.: Polyethylene Nanofibers With Very 

High Thermal Conductivities. Nature Nanotech., vol. 5, 

2010, pp. 251–255. 

57. Jacobi, Anthony M., et al.: Novel Materials for Heat 

Exchangers. ARTI Report No. 06030–01, 2008. 

58. Merritt, Bernard T., et al.: Halbach Array Motor/ 

Generators—A Novel Generalized Electric Machine. 

Lawrence Livermore National Laboratory, UCRL–JC–

119050, 1994. 

59. Yu, Min-Feng, et al.: Strength and Breaking Mechanism 

of Multiwalled Carbon Nanotubes Under Tensile Load. 

Sci., vol. 287, 2000, pp. 637–640. 

60. Collins, Philip G.: Nanotubes for Electronics. Sci. Amer., 

vol. 283, 2000, pp. 62–69. 

61. Abu-Zahra, Esam H.: High Strength E-Glass/CNF Fibers 

Nanocomposite, Ph.D. Thesis, Cleveland State Univ., 

2007. 

62. Grimmer, Christopher S.; and Dharan, C.K.H.: HighCycle Fatigue of Hybrid Carbon Nanotube/Glass 

Fiber/Polymer Composites. J. Mater. Sci., vol. 43, no. 13, 

SS௅

63. Provenza, Andrew J.: Stator and Rotor Designed and 

Manufactured for an Ironless High-Power-Density 

Permanent Magnet Electric Motor for Pollution-Free 
NASA/TP—2015-216588 45 

Aircraft. Research and Technology 2005, NASA/TM—

2006-  SS ௅163. Available from the 

NASA Center for Aerospace Information. 

64. ThinGap Motor Technologies, Ventura, CA, 2008. 

http://www.thingap.com/ironless-composite-stator/

Accessed Dec. 4, 2013. 

65. Kascak, Albert F., et al.: Stability Limits of a PD 

Controller for a Flywheel Supported on Rigid Rotor and 

Magnetic Bearings. NASA/TM—2006-213979 (AIAA–

2005–5956), 2006. http://ntrs.nasa.gov

66. Choi, Benjamin; and Siebert, Mark: A Cryogenic 

High-Power-Density Bearingless Motor for Future 

Electric Propulsion. NASA/TM—2008-215211, 2008. 

http://ntrs.nasa.gov

67. Ooshima, Masahide, et al.: Design and Analysis of 

Permanent Magnet-Type Bearingless Motors. IEEE 

Trans. Indus. Electr., vol. 43, no. 2, 1996, pp. 292–299. 

68. Chiba, Akira, et al.: Stable Operation of Induction-Type 

Bearingless Motors Under Loaded Conditions. IEEE 

Trans. Ind. App., vol. 33, no. 4, 1997, pp. 919–924. 

69. Ichikawa, O., et al.: A Decoupling Control Method of 

Radial Rotor Positions in Synchronous Reluctance Type 

Bearingless Motors. Proceedings of the 1995 

International Power Electrical Conference. Vol. 1, 1995, 

pp. 346–351. 

70. Takemoto, Masatsugu; Chiba, Akira; and Fukao, Tadashi: 

A Method of Determining Advanced Angle of SquareWave Currents in Bearingless Switched Reluctance 

Motors. IEEE Trans. Ind. App., vol. 37, no. 6, 2001, 

pp. 1702–1709. 

71. Gotzhein, Christina: Successful Maiden Flight of a 

Bearingless Advanced Technology Rotor Innovative 

Rotor Technology for Increased Comfort and Reduced 

Costs. Press Release, 2006. http://www.eurocopter.com/ 

site/en/press/Successful-maiden-flight-of-a-bearinglessAdvanced-Technology-Rotor-Innovative-rotortechnology-for-increased-comfort-and-reducedcosts_390.html?iframe=true&width=700 Accessed 

Dec. 4, 2013. 

72. Choi, Benjamin; and Siebert, Mark: A Bearingless 

Switched-Reluctance Motor for High Specific Power 

Applications. NASA/TM—2006-214486 (AIAA–2006–

4804), 2006. http://ntrs.nasa.gov

73. Technical presentation, NAVY MAG BEARING 

PAYOFFS. Naval Aviation Systems TEAM, Propulsion 

& Power Engineering. 

74. Brown, Gerald V.: Weights and Efficiencies of Electric 

Components of a Turboelectric Aircraft Propulsion 

System. AIAA–2011–225, 2011. 

75. Lightweight, Efficient Power Converters for Advanced 

Turboelectric Aircraft Propulsion Systems. Final Report, 

NASA 2010 Phase 1 SBIR, NNX10CC71P, MTECH 

Laboratories, LLC, Ballston Spa, NY, 2010. 

76. 2011 Annual Progress Report: Advanced Power 

Electronics and Electric Motors. U.S. Department of 

Energy, Energy Efficiency and Renewable Energy, 

Vehicle Technologies Program, DOE/EE–0676, 2011. 

https://www1.eere.energy.gov/vehiclesandfuels/pdfs/pro

gram/2011_apeem_report.pdf. Accessed June 18, 2014. 

77. Tolbert, Leon M., et al.: Impact of SiC Power Electronic 

Devices for Hybrid Electric Vehicles. SAE Technical 

Paper 2002–01–1904, 2002. 

78. Full SiC Intelligent Power Module. Mitsubishi Electric, 

Tokyo, 2013. http://www.mitsubishielectric.com/company/ 

environment/report/products/randd/sic/index.html Accessed 

Dec. 4, 2013. 

79. Momota, Seiji; Hitoshi, Abe; and Horasawa, Takayasu: 

Plated Chip for Hybrid Vehicles. Fuji Electric Review, 

vol. 54, no. 2, 2008. http://www.fujielectric.com/company/ 

tech/pdf/54-02/FER-54-02-049-2008.pdf

80. Kötz, R.; and Carlen, M.: Principles and Applications of 

Electrochemical Capacitors. Electrochimica Acta, vol. 45, 

no. 15, 2000, pp. 2483–2498. 

81. A Technical Assessment of High-Energy Batteries for 

Light-Duty Electric Vehicles. Global Climate & Energy 

Project, Stanford University, Stanford, CA, 2006. 

http://gcep.stanford.edu/pdfs/assessments/ev_battery_ass

essment.pdf Accessed Dec. 5, 2013. 

82. AA Portable Power Corp. www.batteryspace.com

Accessed Dec. 5, 2013. 

83. Duleep, Gopalakrishnan, et al.: Assessment of Electric 

Vehicle and Battery Technology. Delft, 2011. 

84. Scrosati, Bruno; and Garche, Jurgen: Lithium Batteries: 

Status, Prospects and Future. J. Power Sources, vol. 195, 

2010, pp. 2419–2430. 

85. Sion Power: The Rechargeable Battery Company. 

http://sionpower.com/ Accessed Dec. 5, 2013. 

86. Kolosnitsyn, V.S.; and Karaseva, E.V.: Lithium-Sulfur 

Batteries: Problems and Solutions. Russian J. Electrochem., 

vol. 44, no. 5, 2008. 

87. Liang, C., et al.: Challenges and Solutions for LithiumSulfur Batteries. Presented at the 2011 U.S.-China Electric 

Vehicle and Battery Technology Workshop, 2011. 

88. Zhang, Y., et al.: Development in Lithium/Sulfur 

Secondary Batteries. Open Mater. Sci. J., vol. 5 (Suppl. 1: 

M3), pp. 215–221, 2011. http://Benthamopen.com/ 

tomsj/articles/V005/SI0203TOMSJ/215TOMSJ.pdf

89. Albertus, Paul, et al.: Identifying Capacity Limitations in the 

Li/Oxygen Battery Using Experiments and Modeling. J. 

(OHFWURFKHP6RFYROQRSS$௅$

90. Wang, Deyu, et al.: High Capacity Pouch-Type Li-Air 

%DWWHULHV-(OHFWURFKHP6RFYROQRSS$௅

A764, 2010. 

91. Padbury, Richard; and Zhang, Xiangwu: Lithium—Oxygen 

Batteries—Limiting Factors That Affect Performance. 

J. Power Sources, vol. 196, 2011, SS௅

92. Kraytsberg, Alexander; and Ein-Eli, Yair: Review on Li-Air 

Batteries—Opportunities, Limitations and Perspective. 

J. 3RZHU6RXUFHVYROSS௅

93. Christensen, Jake, et al.: A Critical Review of Li/Air 

Batteries. J. Electrochem. Soc., vol. 159, no. 2, 2012, 

pp. 5௅5
NASA/TP—2015-216588 46 

94. Williams, Mike: Rice Researchers Develop Paintable 

Battery: Technique Could Turn Any Surface Into a 

Lithium-Ion Battery; May Be Combined With Solar Cells. 

http://news.rice.edu/2012/06/28/rice-researchers-developpaintable-battery-2/ Accessed Aug. 5, 2014. 

95. Strasik, M., et al.: Design, Fabrication, and Test of a 

5-kWh/100-kW Flywheel Energy Storage Utilizing a HighTemperature Superconducting Bearing. IEEE Trans. Appl. 

Superconductivity, vol. 17, no. 2, 2007, p. 2133. 

96. Koh, Heebyung; and Magee, Christopher L.: A Functional 

Approach for Studying Technological Progress: 

Extension to Energy Technology. Technol. Forecast. 

Soc. Change, vol. 75, 2008, pp. 735–758. 

97. Kascak, P., et al.: Modeling and Optimization of a MXER 

Tether Power System. Proceedings of the 2005 JANNAF 

Conference, Monterey, CA, 2005. 

98. Burke, A.: Ultracapacitor Technologies and Application 

in Hybrid and Electric Vehicles. Int. J. Energy Res., 

vol. 34, 2010, pp. 133–151. 

99. Halper, Marin S.; and Ellenbogen, J.C.: Supercapacitors: 

A Brief Overview. MITRE Nanosystems Group No. 

MPe05W0000272, 2006. 

100. Chen, Wei, et al.: High-Performance Nanostructured 

Supercapacitors on a Sponge. Nano Lett., vol. 11, 2011, 

pp. 5165–5172. 

101. Liu, C, et al.: Graphene-Based Supercapacitor With an 

Ultrahigh Energy Density. Nano Lett., vol. 10, 2010, pp. 

4863–4868. 

102. Lin, Y., et al.: Bulk Preparation of Holey Graphene Via 

Controlled Catalytic Oxidation. Nanoscale, vol. 5, no. 17, 

SS௅

103. Wang, Huanlei, et al.: Supercapacitors Based on Carbons 

With Tuned Porosity Derived From Paper Pulp Mill 

Sludge Biowaste. Carbon, vol. 57, 2013, pp. 317–328. 

104. Chandrasekaran, S., et al.: Optimization of an Aircraft 

Power Distribution Subsystem. Virginia Polytechnic 

Institute and State University, Blacksburg, VA, 2002. 

105. de Tenorio, Cyril: Methods for Collaborative Conceptual 

Design of Aircraft Power Architectures. Ph.D. 

Dissertation, Georgia Institute of Technology, 2010. 

106. Armstrong, Michael, et al.: Stability, Transient Response, 

Control, and Safety of a High-Power Electric Grid for 

Turboelectric Propulsion of Aircraft. NASA/CR—2013-

217865, 2013.