Ralph H. Jansen, Robert C. Wagner, and Kirsten P. Duffy

University of Toledo, Toledo, Ohio

David S. Hervol and Ronald J. Storozuk

Analex Corporation, Brook Park, Ohio

Timothy P. Dever

QSS Group, Inc., Cleveland, Ohio

Salvatore M. Anzalone

Science Applications International Corporation, Cleveland, Ohio

Jeffrey J. Trudell and Kevin E. Konno

Glenn Research Center, Cleveland, Ohio

Andrew Kenny

Texas A&M University, College Station, Texas

Redesign of Glenn Research Center

D1 Flywheel Module

NASA/TM—2002-211788

August 2002

IECEC–2002–20075
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Ralph H. Jansen, Robert C. Wagner, and Kirsten P. Duffy

University of Toledo, Toledo, Ohio

David S. Hervol and Ronald J. Storozuk

Analex Corporation, Brook Park, Ohio

Timothy P. Dever

QSS Group, Inc., Cleveland, Ohio

Salvatore M. Anzalone

Science Applications International Corporation, Cleveland, Ohio

Jeffrey J. Trudell and Kevin E. Konno

Glenn Research Center, Cleveland, Ohio

Andrew Kenny

Texas A&M University, College Station, Texas

Redesign of Glenn Research Center

D1 Flywheel Module

NASA/TM—2002-211788

August 2002

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

37th Intersociety Energy Conversion Engineering Conference

sponsored by the Institute of Electrical and Electronics Engineers,

Electron Devices Society

Washington, DC, July 28–August 2, 2002

IECEC–2002–20075
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NASA Center for Aerospace Information

7121 Standard Drive

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National Technical Information Service

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Available electronically at http://gltrs.grc.nasa.gov
NASA/TM—2002-211788 1 

ABSTRACT 

 Glenn Research Center has completed the 

redesign of the D1 flywheel module. The redesign 

includes a new rotor with a composite rim, 

motor/generator, touchdown bearings, sensors, and a 

magnetic actuator. The purpose of the relatively low 

cost module upgrade is to enable it to continuously 

operate throughout its speed range of 0-60,000 RPM. 

The module will be used as part of a combined 

attitude control and bus regulation experiment. 

INTRODUCTION 

 NASA Glenn Research Center (GRC) has an 

ongoing effort in flywheel technology development 

and deployment. Flywheels are being evaluated for 

energy storage applications as well as combined 

systems for energy storage and attitude control. 

NASA and the USAF have a collaborative program 

consisting of government, industry, and academic 

partners working to advance the state of the art in 

aerospace flywheel systems and migrate the 

technology to the end users. NASA GRC funds its 

flywheel technology development through the 

Aerospace Flywheel Technology Program (AFTP). 

McLallin (2001) provides an overview of the 

interrelation of the programs. 

 One of the AFTP program components being 

conducted at GRC is the Attitude Control and Energy 

Storage Experiment (ACESE). The first phase of this 

program is a demonstration of a two flywheel system 

which provides DC bus regulation and attitude control 

capability. Two technology demonstrator flywheel 

modules that were built under a previous task will be 

used in this phase. 

 Both of the flywheel modules required substantial 

upgrades to provide sufficient performance levels for 

incorporation into a multiple flywheel demonstration. 

This paper describes the upgrade of one of those 

modules. The unit was delivered by a contractor and, 

after the upgrade, will be referred to as the D1 

flywheel module. 

 

 This paper describes a design methodology. 

Requirements derived using that methodology are 

used to generate a final design. Analytical models 

are used to verify that the design meets the 

requirements and results of the analyses are 

presented. 

DESIGN METHODOGY 

Overview

 The D1 Flywheel Module redesign addressed 

programmatic and technical goals. The programmatic 

goal was to establish and capture a design process 

and identify areas where that process could be 

improved. The technical goal was to build a module 

that was suitable for continuous reliable operation for 

a minimum period of 12 hours through the entire 

operating speed range.

 The D1 flywheel module is a magnetically 

suspended energy storage rotor with a vacuum 

housing (Figure 1). The rotor consists of a carbon 

composite rim with a metallic hub. It is suspended by 

magnetic bearings. A motor/generator is used to 

transfer energy to and from the rotor. Mechanical 

touchdown bearings are used to capture the rotor in 

FIGURE 1 – D1 FLYWHEEL MODULE

REDESIGN OF GLENN RESEARCH CENTER D1 FLYWHEEL MODULE 

Ralph H. Jansen

Robert C. Wagner

Dr. Kirsten Duffy

University of Toledo

Toledo, Ohio 43606

David S. Hervol

Ronald J. Storozuk

Analex Corporation 

Brook Park, Ohio 44142 

Timothy P. Dever 

QSS Group, Inc. 

Cleveland, Ohio 44135

Salvatore M. Anzalone 

Science Applications International 

Corporation 

Cleveland, Ohio 44135

 

Jeffrey J. Trudell 

Kevin E. Konno 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135

Andrew Kenny 

Texas A&M University 

College Station, Texas 77843
NASA/TM—2002-211788 2 

case of magnetic bearing failure. They are not 

engaged during normal operation. The housing 

structure acts as a vacuum enclosure and provides 

mechanical support to the other components in the 

system. The flywheel module is used in conjunction 

with an avionics package of electronics and control 

software which operates the magnetic bearing and 

motor/generator. 

 As the design process was developed it was 

captured using a system modeling tool. The intention 

was to capture lessons learned during the upgrade, 

develop an approach for streamlining the engineering 

process, and establish requirements for an integrated 

design environment to be used on subsequent 

projects. The main components of the system model 

are a process model and a system architecture. 

Design Process Model

 The process model was used to capture the design 

processes and how they related to each other. The 

data inputs and outputs required for each step and 

tools used in each step were captured in the process 

model database. A system engineering tool was used 

to generate functional flow diagrams. The rotor 

design process (Figure 2) is a typical example. The 

design functions are captured along with the criteria 

for completion. The design process can also be 

viewed in a hierarchical form.

FIGURE 2 – ROTOR DESIGN PROCESS 

Flywheel Module Architecture

 A flywheel module architecture was laid out which 

divided the module into rotating and stationary 

components. The rotor and stator were further 

subdivided into components related to the 

motor/generator, touchdown bearings, magnetic 

bearings, and housing. Each component continued to 

be subdivided to whatever level was useful. For 

example, the magnetic bearing system was divided 

into the actuator, sensors, and housing. The sensors 

were further divided into the signal conditioner, 

cables, and sensor head. The highest level system 

architecture diagram is shown in Figure 3. 

REQUIREMENTS 

 The first step in the design process was to 

establish a set of requirements. They could be 

grouped into reutilization, performance, rotor and 

avionics requirements. For cost efficiency, the 

flywheel housing, one touchdown bearing, the cable 

harnesses, and the magnetic bearing stators were 

reutilized. The system performance criteria were: 

minimum of 300 w-hr usable energy, a 3 kW 

charge/discharge power rating, and a maximum 

operating speed of 60,000 RPM. The rotor was to 

have a first bending mode > 110% of maximum 

operating speed, the ratio between polar moment of 

inertia and transverse moment of inertia (Ip/It) <0.8 or 

>1.2, and an operating temperature range between 

70-170°F. The avionics requirement was to simplify 

the position and tachometer sensor system in the 

flywheel module to reduce the signal conditioning and 

control complexity of the avionics. 

 A verification matrix was established to show how 

the analysis and tests on the D1 flywheel module 

related to the requirements. In the matrix each design 

choice was related back to a requirement and the 

verification method was listed. The verification 

method shows how the design choice fulfills the 

requirement. Three verification methods were 

available: inspection, analysis, and test. During the 

design phase only the analysis verification method is 

available. After the hardware is manufactured it will be 

inspected or tested as defined by the verification plan. 

ROTOR DESIGN 

 The rotor components include the rim, hub, 

motor/generator rotor, and a number of small target 

pieces for the magnetic bearing actuator, position 

sensors, and touchdown bearings. The rotor is 

shown in Figure 4. 

 The rotor design meets the requirements above 

except for the temperature range. The rotor energy is 

364 W-hr at the maximum normal operating speed of 

60,000 RPM. With 90% depth of discharge there is 

328 W-hr of useable energy. The rotor meets all 

FIGURE 3 – SYSTEM ARCHITECTURE FIGURE 4 – ROTOR DESIGN
NASA/TM—2002-211788 3 

safety margins at the certification speed of 66,000 

RPM. It has a first bending mode 117% of operating 

speed and an Ip/It ratio of 0.80. The motor power 

rating is 3kW. Due to rim material constraints, the 

upper rotor temperature limit was reduced from 170°F 

to 150°F. The minimum operating temperature is 

70°F. 

Rim

 The rim is the main energy storage component. A 

multiring carbon fiber design was purchased from a 

commercial source. The risk, cost, and development 

time were the driving factors in this choice. The rim 

mounts directly onto the hub with an interference fit. 

There are no hub geometry changes under the rotor 

that would induce unnecessary stress concentrations 

in the rim. 

 The rim met all requirements except the 

temperature ratings. The composite rim temperature 

limit is 150°F, chosen by derating the 170°F glass 

transition temperature (Tg) of the epoxy by 20°F. 

Hub

 All of the rotor components are mounted on a 

central monolithic steel hub. A monolithic hub was 

chosen for simplicity. Alternative designs with 

multiple hub parts require verification of joints. The 

disadvantage of this decision was that it required the 

redesign of the rotor parts of one of the magnetic 

bearings. 

 Material selection was driven by strength, density, 

and magnetic properties. The minimum needed 

strength was approximately 100 ksi yield and was 

driven by rotational, thermal, and assembly stresses. 

Density affected the Ip/It ratio. This ratio needed to be 

<0.8 or >1.2 to simplify magnetic bearing control. A 

non-magnetic hub was required in order for both 

magnetic bearings and the motor/generator to 

function as originally designed. The material selected 

based on these constraints was A286 stainless steel. 

Other desirable properties were corrosion resistance, 

coefficient of thermal expansion near 6μin/in°F, and 

good fatigue characteristics. 

Motor Rotor

 A two pole permanent magnet synchronous motor 

purchased from a commercial vender was used in D1. 

The motor is interference fit onto the hub. 

Magnetic Bearings

 Each of the magnetic bearings requires a target 

area for the sensor and a set of laminated steel 

targets for the actuator. The radial magnetic bearing 

actuator target had to be redesigned due to the 

selection of a non-magnetic hub material. 

 Eddy current position sensors were chosen which 

were compatible with the hub material. The radial 

end sensors look directly at the hub while at the 

combo end the sensors look at a sensor target which 

is also A286 steel. Tight control of runout and surface 

finish is maintained on these surfaces to minimize 

sensor noise. 

 The magnetic bearing lamination assemblies are 

stacks of thin steel sheets with thicker end plates for 

assembly and axial thrust loading. Laminations of 

6 mil Hiperco 50HS hardened to 99 ksi were selected 

because they provided the best combination of 

magnetic properties and strength. The end plates for 

each lamination stack were fabricated from 4340 

because Hiperco 50HS was not available in the 

required thickness. The lamination stacks are oxided 

and bonded into an assembly which is interference fit 

onto the hub. 

 In addition to the lamination stacks each magnetic 

bearing required additional components on the hub. 

The radial magnetic bearing utilizes a magnetic ring to 

provide an axial return flux path. The combo 

magnetic bearing has a non-magnetic spacer 

between the lamination stack and the sensor target 

ring. Both of these components are interference fit 

onto the hub. 

Touchdown Bearing Surface

 The rotor has surfaces at each end which provide a 

sliding interface with the stator components of the 

touchdown bearing. These areas have high wear and 

galling potential during a touchdown. A bearing steel, 

440C, hardened to 58 Rc was chosen for these parts. 

This selection was based on its suitability in other 

tribological applications and a series of rub tests 

conducted at GRC with a several other candidate 

materials. The radial end touchdown surface only 

provides radial restraint. The combo end has two 

parts which provided radial and axial restraint. 

ROTOR ANALYSIS 

Electromagnetic Analysis

 Full magnetic analysis was performed on the radial 

magnetic bearing to size the lamination stack and the 

return flux path ring. Rough sizing was done with a 

1-D magnetic circuit analysis. Flux levels throughout 

the actuator and air gaps were determined and 

compared to saturation levels. Components were 

sized to operate near saturation. Finite element 

analysis was used to examine flux leakage areas and 

look for localized saturation. The inner diameter of 

the lamination stack and return flux path ring were 

reduced 0.25” from the original pieces. The new 

pieces have an axial cross section area increase of 

38% and a radial cross section area increase of 50% 

which carries flux previously carried in the hub. 

Rotordynamic Analysis

 Rotordynamic analysis was used to determine the 

bending modes of the rotor and estimate the loads 

during a touchdown event. The rotordynamics model 

is shown in Figure 5. The area above the horizontal 

axis is proportional to the mass distribution, while the 

area below is proportional to stiffness. Springs are 

used represent the magnetic bearing stiffness. The 

arrows show the unbalance locations. The first 

bending mode shape is superimposed on top of the 

model. The free-free 1st bending mode critical speed 
NASA/TM—2002-211788 4 

is expected to be 1167 – 1238 Hz (70.0 kRPM - 74.3 

kRPM) which is 17% - 24% above the maximum 

operating speed (60 kRPM = 1000 Hz). 

 Transient analysis was done to predict the loads 

during a touchdown event. This is a difficult condition 

to model. Key variables are the mounting stiffness 

and damping, the bearing stiffness and damping, the 

net friction, and the radial gap. First, predicted 

transient loads are plotted vs. the key variables to 

determine sensitivities. After the design was finalized, 

the maximum transient and steady state 

displacements and loads during a spin down from 

60,000 RPM at the two touchdown bearings were 

calculated. The estimate of maximum transient 

displacement and load was 11 mils and 700 lbs, and 

the estimate of maximum steady state displacement 

and load was 2.5 mils and 460 lbs. These estimated 

forces are substantially lower than the load ratings of 

the bearings. 

FIGURE 5 – ROTORDYNAMICS MODEL 

Stress Analysis

 Stress analysis was performed on the composite 

rim and all the metallic rotor components. Composite 

rim analysis verified the manufacturer’s design 

margins. Analysis of the metallic components 

determined material, heat treat selection, and preload 

levels. 

 The required environmental limits and 

appropriate safety and knockdown factors were 

applied when performing the stress analysis. The 

analysis included the appropriate interference 

tolerances, speed range from 0 to the certification 

speed of 66 kRPM, and temperature range of 70°F 

(room) to 170°F (original temperature requirement). 

Depending on the material, margins were calculated 

using either von Mises or component stresses, and 

adequate margin was maintained to prevent 

separation at speed. A knockdown factor of 0.8 was 

applied to the nominal yield strength for materials 

without statistical yield strength data available. The 

margin of safety (MOS) is: 

−1 ⋅ = FOS stress

allowable stress MOS

All components have positive margin of safety and 

maintain preload throughout the operating speed and 

temperature ranges and the certification conditions. 

Thermal Analysis

 An extremely limited thermal analysis was 

conducted to determine whether a rotor growth 

problem existed. Tests of the as-delivered hardware 

had shown that the rotor could axially expand during 

operation to a point where it made contact with the 

radial end touchdown bearing. The eddy current and 

hysteretic losses on the rotor were not well 

characterized, so the thermal analysis was done 

based on the limit temperature of the rotor. The 

purpose of the analysis was to find the relative growth 

of the rotor compared to the stator. 

 For this analysis, the stator temperature was 

assumed to be room temperature (75°F), and the 

rotor was varied from 75F to 150F. The expansion 

between the combo magnetic bearing and the combo 

touchdown bearing was 0.0023”. The expansion 

between the combo magnetic bearing and the radial 

touchdown bearing was 0.0098”. Since the radial end 

axial expansion was greater than the clearance at the 

radial end touchdown bearing, the touchdown bearing 

system needed to be redesigned. 

STATOR DESIGN 

 The stator redesign included changes to the 

touchdown bearings, the motor/generator, and the 

sensors. The combo end touchdown bearing system 

was changed to provide axial restraint in both 

directions. The motor/generator was switched to a 

different model which required redesign of the cooling 

jacket and mounting. New sensors for the magnetic 

bearing and motor generator were added. 

Touchdown Bearings

 The touchdown bearing system consists of two 

pairs of angular contact ball bearings which capture 

the rotor in the case of a magnetic bearing failure. 

The bearings are not normally engaged and have a 

radial and axial clearance of approximately 10 mils. 

In the original design each set of touchdown bearings 

captured the shaft radially and in one axial direction 

(Figure 6a). In the D1 design the shaft is captured 

radially and in both axial directions at the combo end, 

and captured only radially at the radial end. 

 Changing the touchdown bearing configuration 

FIGURE 6 – TOUCHDOWN CONFIGURATION
NASA/TM—2002-211788 5 

required redesign of the combo end touchdown 

bearing assembly. The radial end assembly could be 

used as is. Both ends used commercial 25mm bore 

high speed outer race clamped angular contact 

bearings with silicone nitride balls. The contact 

surfaces were 440C hardened to 56-58 Rc on the 

rotor and SAE52100 hardened to 62 Rc in the bearing 

bore. Vacuum grease was used in the bearings. The 

sliding surfaces were not lubricated. The combo end 

bearing housing was threaded into the touchdown 

assembly to provide axial adjustment capability at 

assembly. 

Motor/Generator

 The D1 module has a 3kW permanent magnet 

synchronous motor/generator. The motor voltage and 

number of poles were changed. The line-to-line 

voltage was reduced from 220V to 80V. The new 

motor can be operated to 60,000 RPM using a 130V 

DC bus. This is compatible with the International 

Space Station bus voltage as well as newer satellite 

buses. The number of poles was reduced from 4 to 2. 

Consequently the synchronous drive frequency at full 

speed dropped from 2kHz to 1kHz. 

 The mechanical mounting considerations were 

fairly straightforward. The motor requires a water 

cooling jacket for full power operation. The cooling 

jacket is also used to position the stator relative to the 

rotor. Axial tolerances are loose. The radial 

mechanical gap in the motor is 0.025”. The radial 

concentricity tolerance needs to be held tightly due to 

stack up error in the other stator components. The 

operating temperature range is 75-300°F. 

Sensors

 The rotor position sensors and the tachometer 

sensors were replaced in the redesign. The position 

sensors are used in the magnetic bearing feedback 

control loop. Reduction in noise and runout at the 

sensor eliminates the need for complex filtering 

operations in the controller. Some of the 

motor/generator and magnetic bearing controllers 

need a tachometer signal. The tachometer was 

changed because the target surface used in the 

original design had been eliminated. 

 The D1 redesign will result in the second position 

sensor upgrade for the module. The as-delivered 

optical sensors had been upgraded to eddy-current 

sensors. Spatially related noise was greatly reduced, 

however electromagnetic effects were increased. 

Overall magnetic bearing control effort was reduced 

substantially. Details can be found in the work by 

Dever (2001). The sensor suite will be upgraded 

again to a new model of eddy current sensor. This 

model utilizes a single signal conditioning box to drive 

four sensor heads. Sensors are used differentially to 

measure shaft position. The benefits of this upgrade 

are that sensor to sensor crosstalk is eliminated and 

the commercial electronics provided a signal range 

that is compatible with the magnetic bearing digital 

controller. 

 Some of the control algorithms developed at GRC 

for motor/generator (Kenny et al 2001) or magnetic 

bearing control (Dever et al 2002) require rotor 

angular position feedback. The as-delivered 

hardware used an optical tachometer sensor viewing 

a surface on the hub. The angle was estimated 

based on the tachometer sensor. The D1 unit will use 

an eddy-current tachometer sensor viewing a target at 

the end of the shaft. The change was required 

because the original viewing area was eliminated. 

CONCLUSION 

 The D1 flywheel module design was completed at 

GRC. The module combines a new rotor design, a 

new motor/generator, a new combo touchdown 

bearing, new sensors, and a redesigned radial 

magnetic bearing with an existing housing, magnetic 

bearing stators, radial end touchdown bearing, and 

wiring harnesses. The module redesign enables 

continuous operation throughout the design speed 

range of 0-60,000 RPM. Energy storage capacity is 

similar to the original hardware, however the 

efficiencies of the magnetic bearing and 

motor/generator systems have been increased 

significantly. Sensor changes have also simplified the 

controllers. 

 The D1 flywheel module will be used by the 

Aerospace Flywheel Technology Program this year in 

the Attitude Control and Energy Storage Experiment. 

A single axis moment control and bus regulation 

system will be demonstrated [Kascak, 2002]. 

REFERENCES

 T.P. Dever et al., 2001, “Evaluation and 

Improvement of Eddy Current Position Sensors In 

Magnetically Suspended Flywheel Systems,”

NASA/CR—2001-211137, NASA Glenn Research 

Center, Cleveland, OH. 

 T.P. Dever et al., 2002, “Estimator Based 

Controller for High Speed Flywheel Magnetic Bearing 

System”, Proceedings, IECEC 2002, Washington, 

D.C. 

 P.E. Kascak et al., 2002, “Single Axis Attitude 

Control and DC Bus Regulation with Two Flywheels”, 

Proceedings, IECEC 2002, Washington, D.C. 

 B.H. Kenny et al., 2001, “Advanced Motor Control 

Test Facility for NASA GRC Flywheel Energy Storage 

System Technology Development Unit,” NASA/ 

TM—2001-210986, NASA Glenn Research Center, 

Cleveland, OH. 

 K.L. McLallin et al., 2001, “Aerospace Flywheel 

Technology Development for IPACS Applications,”

NASA/TM—2001-211093, NASA Glenn Research 

Center, Cleveland, OH. 
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

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Available electronically at http://gltrs.grc.nasa.gov

August 2002

NASA TM—2002-211788

IECEC–2002–20075

E–13483

WU–755–1A–09–00

11

Redesign of Glenn Research Center D1 Flywheel Module

Ralph H. Jansen, Robert C. Wagner, Kirsten P. Duffy, David S. Hervol,

Ronald J. Storozuk, Timothy P. Dever, Salvatore M. Anzalone, Jeffrey J. Trudell,

Kevin E. Konno, and Andrew Kenny

Flywheels; Reaction wheels; Counter-rotating wheels; Electric motors;

Electromechanical devices

Unclassified -Unlimited

Subject Categories: 20, 33, 37, and 44 Distribution: Nonstandard

Prepared for the 37th Intersociety Energy Conversion Engineering Conference sponsored by the Institute of Electrical and

Electronics Engineers, Electron Devices Society, Washington, DC, July 28–August 2, 2002. Ralph H. Jansen, Robert C.

Wagner, and Kirsten P. Duffy, University of Toledo, Toledo, Ohio 43606; David S. Hervol and Ronald J. Storozuk,

Analex Corporation, Brook Park, Ohio 44142; Timothy P. Dever, QSS Group, Inc., Cleveland, Ohio 44135; Salvatore M.

Anzalone, Science Applications International Corporation, Cleveland, Ohio 44135; Jeffrey J. Trudell and Kevin E. Konno,

NASA Glenn Research Center; Andrew Kenny, Texas A&M University, Mechanical Engineering Department, College

Station, Texas 77843–3123. Responsible person, Ralph H. Jansen, organization code 5930, 216–433–6038.

Glenn Research Center has completed the redesign of the D1 flywheel module. The redesign includes a new rotor with a

composite rim, motor/generator, touchdown bearings, sensors, and a magnetic actuator. The purpose of the relatively low

cost module upgrade is to enable it to continuously operate throughout its speed range of 0 to 60,000 RPM. The module

will be used as part of a combined attitude control and bus regulation experiment.