Robert L. Fuller 

Barber-Nichols Inc., Arvada, Colorado 

Closed Brayton Cycle Power Conversion Unit for 

Fission Surface Power Phase I Final Report

NASA/CR—2010-215673

June 2010
NASA STI Program . . . in Profi le

Since its founding, NASA has been dedicated to the 

advancement of aeronautics and space science. The 

NASA Scientifi c and Technical Information (STI) 

program plays a key part in helping NASA maintain 

this important role.

The NASA STI Program operates under the auspices 

of the Agency Chief Information Offi cer. It collects, 

organizes, provides for archiving, and disseminates 

NASA’s STI. The NASA STI program provides access 

to the NASA Aeronautics and Space Database and 

its public interface, the NASA Technical Reports 

Server, thus providing one of the largest collections 

of aeronautical and space science STI in the world. 

Results are published in both non-NASA channels 

and by NASA in the NASA STI Report Series, which 

includes the following report types:

• TECHNICAL PUBLICATION. Reports of 

completed research or a major signifi cant phase 

of research that present the results of NASA 

programs and include extensive data or theoretical 

analysis. Includes compilations of signifi cant 

scientifi c and technical data and information 

deemed to be of continuing reference value. 

NASA counterpart of peer-reviewed formal 

professional papers but has less stringent 

limitations on manuscript length and extent of 

graphic presentations.

• TECHNICAL MEMORANDUM. Scientifi c 

and technical fi ndings that are preliminary or 

of specialized interest, e.g., quick release 

reports, working papers, and bibliographies that 

contain minimal annotation. Does not contain 

extensive analysis.

• CONTRACTOR REPORT. Scientifi c and 

technical fi ndings by NASA-sponsored 

contractors and grantees.

• CONFERENCE PUBLICATION. Collected 

papers from scientifi c and technical 

conferences, symposia, seminars, or other 

meetings sponsored or cosponsored by NASA.

• SPECIAL PUBLICATION. Scientifi c, 

technical, or historical information from 

NASA programs, projects, and missions, often 

concerned with subjects having substantial 

public interest.

• TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientifi c and 

technical material pertinent to NASA’s mission.

Specialized services also include creating custom 

thesauri, building customized databases, organizing 

and publishing research results.

For more information about the NASA STI 

program, see the following:

• Access the NASA STI program home page at 

http://www.sti.nasa.gov

• E-mail your question via the Internet to help@

sti.nasa.gov

• Fax your question to the NASA STI Help Desk 

at 443–757–5803

• Telephone the NASA STI Help Desk at

 443–757–5802

• Write to:

 NASA Center for AeroSpace Information (CASI)

 7115 Standard Drive

 Hanover, MD 21076–1320
Robert L. Fuller 

Barber-Nichols Inc., Arvada, Colorado 

Closed Brayton Cycle Power Conversion Unit for 

Fission Surface Power Phase I Final Report

NASA/CR—2010-215673

June 2010

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared under Contract NNC08CA64C
Acknowledgments

The author would like to acknowledge the support of the NASA Glenn Research Center. 

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

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

Trade names and trademarks are used in this report for identifi cation 

only. Their usage does not constitute an offi cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by expert reviewer(s).

This report contains preliminary fi ndings, 

subject to revision as analysis proceeds.
Table of Contents

1. SUMMARY AND INTRODUCTION......................................................................... 1

1.1 Summary..................................................................................................................... 1 

1.2 Introduction ................................................................................................................ 2 

2. FLUID AND SYSTEM TRADE STUDY ................................................................... 3

2.1 Fluid Study ................................................................................................................. 3 

2.2 System Trade Study.................................................................................................. 15 

3. HEAT EXCHANGER DESIGN................................................................................ 17

3.1 Recuperator Design .................................................................................................. 17 

3.2 Water/Gas Plate Fin Cooler...................................................................................... 20 

3.3 NaK Heater Design................................................................................................... 23 

3.3.1 Shell and Tube NaK Heater Design.................................................................... 23 

3.3.2 Plate/Fin NaK Heater.......................................................................................... 25 

4. CYCLE AND PERFORMANCE ANALYSIS ......................................................... 25

4.1 Nominal Temperature Difference............................................................................. 25 

4.2 High Temperature Difference................................................................................... 27 

4.3 Low Temperature Difference ................................................................................... 27 

4.4 Off-Design Performance .......................................................................................... 27 

5. COMPRESSOR DESIGN.......................................................................................... 31

6. TURBINE DESIGN.................................................................................................... 34

7. ALTERNATOR DESIGN.......................................................................................... 37

8. TURBINE ALTERNATOR COMPRESSOR (TAC) DESIGN ............................. 39

8.1 Overall Layout Section View ................................................................................... 39 

8.2 Foil Bearings ............................................................................................................ 42 

8.3 Cooling Flow............................................................................................................ 44 

8.4 Rotor Dynamic Assessment ..................................................................................... 46 

9. POWER CONTROLLER.......................................................................................... 47

9.1 Power Controller Packaging..................................................................................... 47 

9.2 Power Controller Schematic and Simulation ........................................................... 49 

10. POWER CONVERTER........................................................................................... 53

10.1 Power Converter with Shell/Tube NaK Heater...................................................... 54 

10.2 Power Converter with Plate/Fin NaK Heater......................................................... 57 

11. CONTROL CONSOLE AND DATA ACQUISITION, AND FILL SYSTEM... 61

11.1 Control Console and Data Acquisition................................................................... 61 

11.2 Fill System.............................................................................................................. 63 

12. SUMMARY ............................................................................................................... 63

NASA/CR—2010-215673 iii
Table of Figures

Figure 1: Cycle Diagram in Graphical Format Example (English Units Version)..... 4

Figure 2: Cycle Spreadsheet Input Screen Example (English Units Version) ............ 5

Figure 3: Cycle Efficiency vs Compressor Exit Pressure for Various Gases.............. 7

Figure 4: Gas Cooler Design Spreadsheet...................................................................... 8

Figure 5: Recuperator Design Sheet ............................................................................... 9

Figure 6: NaK Heater Design Sheet (Shell and Tube)................................................. 10

Figure 7: Heat Exchanger Volume vs. Compressor Exit Pressure at Various 

Temperature Ratios............................................................................................ 11

Figure 8: Optimum Shaft Speed versus Compressor Exit Pressure for Various 

Gases at low, average, and high temperature ratios........................................ 12

Figure 9: Turbine Wheel Diameter for various gases versus Compressor Exit 

Pressure for low, average, and high temperature ratios. ................................ 12

Figure 10: Compressor Wheel Diameter for various gases versus Compressor Exit 

Pressure for low, average, and high temperature ratios. ................................ 13

Figure 11: Cycle Efficiency vs. Compressor Exit Pressure for CO2, N2, Ar, and 

HeXe at 6 kW, 12 kW, and 24 kW..................................................................... 14

Figure 12: Heat Exchanger Volume vs. Compressor Exit Pressure for CO2, N2, Ar, 

and HeXe at 6 kW, 12 kW, and 24 kW. ............................................................ 14

Figure 13: Cycle Efficiency vs. Pressure ratio and Heat Exchanger Volume ........... 16

Figure 14: Cycle Efficiency vs. Recuperator Effectiveness and Specific Weight for 6 

kW, 12 kW, and 24 kW Converters .................................................................. 16

Figure 15: Cycle Efficiency vs. NaK Flow Rate and Heat Exchanger Volume......... 17

Figure 16: Fin Type/Pitch versus Recuperator Effectiveness..................................... 18

Figure 17: Fin Type/Pitch versus Pressure Drop......................................................... 18

Figure 18: Recuperator Performance ........................................................................... 19

Figure 19: Overall Recuperator Size............................................................................. 20

Figure 20: Water to CO2 Gas Cooler Dimensional Data............................................ 21

Figure 21: Water to CO2 Gas Cooler Data Sheet........................................................ 22

Figure 22: Water to CO2 Gas Cooler Performance Data........................................... 23

Figure 23: Heat Exchanger Specification Sheet for NaK to CO2 Gas....................... 24

Figure 24: Thermodynamic Cycle State Points for Nominal Design Condition ....... 26

Figure 25: High Temperature Difference Performance Analysis .............................. 28

Figure 26: Low Temperature Difference Converter Performance ............................ 29

Figure 27: Cycle Power Output versus Shaft Speed for Specified Temperature 

Differences........................................................................................................... 30

Figure 28: Off Design Performance Pressures versus Shaft Speed............................ 30

Figure 29: Front View Compressor and Vane Island Diffuser................................... 31

Figure 30: Compressor Performance Map with Thrust Calculations....................... 32

Figure 31: Compressor Wheel Stress Analysis at 10% Over-speed........................... 33

Figure 32: Compressor Wheel Campbell Diagram ..................................................... 33

Figure 33: Turbine Exit View with Nozzles, Shroud Removed.................................. 34

Figure 34: Turbine Performance and Thrust Map ..................................................... 35

Figure 35: Turbine Stress Analysis............................................................................... 36

Figure 36: Turbine Wheel Interference Diagram........................................................ 36

Figure 37: Permanent Magnet Section of Rotor .......................................................... 37

NASA/CR—2010-215673 iv
Figure 38: Alternator Performance Map ..................................................................... 38

Figure 39: Insulation Life vs. Operating Temperature............................................... 38

Figure 40: Overall TAC Layout .................................................................................... 39

Figure 41: TAC Turbine End Close Up........................................................................ 40

Figure 42: TAC Compressor End Close Up................................................................. 41

Figure 43: Combined Turbine and Compressor Axial Thrust Map.......................... 41

Figure 44: Foil Bearing Experimental Hardware........................................................ 43

Figure 45: Foil Bearing Power vs. Speed (Power is from DC Supply)....................... 43

Figure 46: Water Cooling Passages............................................................................... 44

Figure 47: Predicted Cavity Pressures at Operating Speed........................................ 45

Figure 48: Power Loss and Gas Cooling Flow Passages ............................................. 45

Figure 49: Cooling Mass Flows, Temperatures, Heat Loss ........................................ 46

Figure 50: Un-dampened Critical Speed Map ............................................................. 47

Figure 51: Power Controller (Isometric View) ............................................................ 48

Figure 52: Component Locations for Power Controller (Top View)......................... 48

Figure 53: Top Level Control Schematic...................................................................... 49

Figure 54: Motor/Generator Controller Simulation Circuit ...................................... 49

Figure 55: Boost/Buck Converter Controller Simulation Circuit.............................. 50

Figure 56: Motoring Mode Simulation ......................................................................... 50

Figure 57: Generating Mode Simulation...................................................................... 51

Figure 58: Boost Mode Simulation................................................................................ 52

Figure 59: Buck Mode Simulation................................................................................. 53

Figure 60: Power Converter with Shell/Tube NaK Exchanger .................................. 54

Figure 61: Power Converter for Shell/Tube, Three Views ......................................... 55

Figure 62: Static Thermal Conditions for Shell/Tube Power Converter System ..... 56

Figure 63: Shell Tube Stress Regions............................................................................ 56

Figure 64: Piping Interface Loads on TAC .................................................................. 57

Figure 65: Power Converter with Plate/Fin NaK Heater............................................ 58

Figure 66: Power Converter Three View with Plate/Fin NaK Heat Exchanger....... 58

Figure 67: Plate/Fin Power Converter Thermal Analysis........................................... 59

Figure 68: Plate/Fin Power Converter Stress Results................................................. 60

Figure 69: Plate/Fin Power Converter TAC Resultant Stress Results ...................... 60

Figure 70: Insulation Analysis....................................................................................... 61

Figure 71: Control Console Screen Example ............................................................... 62

Figure 72: Strip Chart Recording Example ................................................................. 62

Figure 73: Fill System Diagram and Equipment List.................................................. 63

Table of Tables

Table 1: Compressor Design Table ............................................................................... 31

Table 2: Turbine Design Dimensions............................................................................ 34

Table 3: Magnet Stress Values....................................................................................... 37

Table 4: TAC Weight Breakdown................................................................................. 42

NASA/CR—2010-215673 v

1. Summary and Introduction

1.1 Summary 

A Closed Brayton Cycle power conversion system has been developed to support the 

NASA fission surface power program. The goal is to provide electricity from a small 

nuclear reactor heat source for surface power production for lunar and Mars 

environments. The selected media for a heat source is NaK 78 with water as a cooling 

source. 

The Closed Brayton Cycle power was selected to be 12 kWe output from the generator 

terminals. A heat source NaK temperature of 850 K +/-25 K was selected. The cold 

source water was selected at 375 K +/-25 K. A vacuum radiation environment of 200 K 

is specified for environmental operation. 

The prioritized goals for the power conversion system are contained in the NASA 

specification as follows: 

• Close relevance to future flight designs, including the potential for 8 year service 

life 

• Credible development path and maturation approach exist to achieve TRL 6 by 

FY 2012 with a reasonable cost 

• Low development cost and risk 

• Design extensible to Mars (materials and design strategies compatible with mars 

environment) 

• High thermal-to-electric efficiency 

• Minimum complexity 

• Low mass and volume 

The major components of the system are the power converter, the power controller, and 

the top level data acquisition and control unit. The power converter with associated 

sensors resides in the vacuum radiation environment. The power controller and data 

acquisition system reside in an ambient laboratory environment. Signals and power are 

supplied across the pressure boundary electrically with hermetic connectors installed on 

the vacuum vessel. 

System level analyses were performed on working fluids, cycle design parameters, heater 

and cooling temperatures, and heat exchanger options that best meet the needs of the 

power converter specification. The goal is to provide a cost effective system that has 

high thermal-to-electric efficiency in a compact, lightweight package. A system level 

trade study was performed to optimize these competing factors in order to specify the 

various system components. 

The power converter is a rotating turbine-alternator-compressor (TAC), NaK heater, gasto-gas recuperator, and gas-to-water cooler. 

NASA/CR—2010-215673 1
The TAC consists of a centrifugal compressor, a two-pole permanent magnet alternator, 

and a radial in-flow turbine on a single shaft. The rotor is supported by gas foil journal 

and thrust bearings. The gas foils are coated to allow numerous startup and shutdown 

operations. 

Labyrinth seals are utilized to limit turbine and compressor leakage into the generator 

cavity. The foil bearings are cooled by a combination of the labyrinth leakage flow from 

the compressor and also a secondary pumped loop that utilizes compressor inlet gas. The 

test unit would utilize o-rings and metallic c-seals for laboratory testing and be fully 

welded for the space-based version. The generator is cooled by the pumped secondary 

flow loop and also liquid cooling with a water jacket. 

The NaK heater is a plate-fin type heat exchanger with NaK 78 heating the CO2 gas on 

the Brayton loop. The recuperator is a CO2/CO2 gas heat exchanger which preheats the 

gas from the compressor exit by utilizing waste heat from the turbine exit. The preheated 

compressor air enters the NaK heater to increase the gas energy content prior to entering 

the turbine. The CO2 to water cooler is used to expel the residual heat from the 

recuperator to cool the compressor inlet gas. The water is pumped to radiation cooler 

panels that expel the heat to the atmosphere. The system is piped and insulated with 

multi-layer insulation (MLI) to minimize heat loss. 

The power controller utilizes a full-bridge interface to the alternator to allow motoring for 

startup and active rectification for power production. A buck/boost converter is utilized 

for conversion from the specified alternator output of 400 VAC to 120 VDC. The 

controller is liquid cooled and packaged in a 19-inch rack mount chassis. 

Design decisions that resulted from the trade studies: 

• Working fluid, carbon dioxide 

• Maximum working pressure 689.5 kPa 

• Pressure ratio 2:1 

• Generator Efficiency 98% 

• Recuperator Effectiveness 90% 

• Turbine Efficiency 87% 

• Compressor Efficiency 84% 

• Shaft Speed 65 krpm 

1.2 Introduction 

The closed Brayton power converter was developed to support National Aeronautics and 

Space Administration goals of generating electricity using a nuclear heat source. While 

the use of closed Brayton cycle technology for space power is not new, the system 

proposed herein reflects modern technological improvements to satisfy a new 

specification and power goal. The system is packaged to be as compact is practical while 

insuring material stress limitations are not exceeded due to gravity, pressure, and thermal 

loading and cycling. 

NASA/CR—2010-215673 2
Closed Brayton cycle machinery has the ability to provide high reliability, good 

efficiency, and can operate for long periods as self-contained power sources for moon or 

Mars based missions. Development of the power conversion system focused on a 

compact system with a rotating turbine-alternator-compressor (TAC) power production. 

The TAC can operate in a 0 to 2 g environment in a variety of attitudes from horizontal to 

vertical and therefore could be used for not only terrestrial power, but space based power 

as well. 

The power conversion system development includes analysis that simulates full power as 

well as partially power operation. The power controller for the TAC can provide rotating 

machine control during load steps and also operates as a state machine for startup, 

shutdown, normal operation, and fault handling modes. 

2. Fluid and System Trade Study 

2.1 Fluid Study 

A study was performed to identify the best working fluid for use in the closed Brayton 

cycle power converter. The fluid selection was limited to those fluids known to be stable 

and remain a gas at working temperatures of 200 K to 875 K. Good thermodynamic and 

transport properties enable efficiency aerodynamic performance for the turbine and 

compressor as well as a direct impact on heat exchanger size and weight. Additionally, 

due to the long life, material compatibility is important. A non-reactive, inert gas is 

beneficial for material selection criteria. The fluids considered for this study were carbon 

dioxide, argon, nitrogen, and helium/xenon mixture. Helium was not considered due the 

low molecular weight causing extreme turbomachinery design challenges. 

For this trade study, a simple recuperated Brayton cycle was used. The recuperator 

effectiveness is directly tied to the size of the recuperator heat exchanger and also dictates 

the optimum pressure ratio for a specific working fluid. A spreadsheet was utilized to 

perform the cycle analysis for the trade study. The spreadsheet is linked to Refprop 

(National Institute of Standards and Technology Property Database) to calculate the 

thermal and transport properties automatically. Figure 1 shows the cycle in graphical 

form and the cycle input sheet is shown in Figure 2. This is an example only and is not 

the final cycle design. 

NASA/CR—2010-215673 3
Figure 1: Cycle Diagram in Graphical Format Example (English Units Version) Fluid: Shaft Power: 16.06 B/sec Cycle Efficiency: Alternator Windage: 0.17 B/sec Percent of Cycle Carnot: Bearing Losses: 0.83 B/sec System Efficiency: Core Losses: 0.30 B/sec Percent of System Carnot: 170 F 194 F Converter Losses: 0.738 B/sec 150.0 psi 149.5 psi Net Power Out: 14.79 kW 0.02 lbm/s 0.02 lbm/s 1115 F 170 F 21.8 psi 150.0 psi 2.83 lbm/s Water Jacket 0.49 lbm/s 1060 F 1060 F 190 F 190 F 99.0 psi 99.0 psi 48.0 psi 47.6 psi 1.18 lbm/s 1.18 lbm/s 1.19 lbm/s 1.19 lbm/s NaK e= 78.8% e= 86.1% e= 98.0% e= 84.2% e= 90.3% Water Heating Q= 67.9 B/s PR= 2.07 N= 65694 RPM PR= 2.07 Q= 50.8 B/s Cooling Loop Loop 853 F 915 F 311 F 377 F 99.2 psi 47.9 psi 99.4 psi 47.6 psi 1.18 lbm/s 1.18 lbm/s 1.19 lbm/s 1.19 lbm/s 1000 F 272 F 21.7 psi 149.9 psi 2.83 lbm/s 0.49 lbm/s 269 F 190 F 47.9 psi 48.4 psi 0.012 lbm/s 0.012 lbm/s 853 F 311 F 99.3 psi 99.4 psi 1.18 lbm/s 1.18 lbm/s e= 90.0% Q= 164.1 B/s 915 F 377 F 47.9 psi 47.7 psi 1.19 lbm/s 1.19 lbm/s Compressor Cooler Recuperator CO2 Heater Alternator 21.7% 38.0% 20.6% 34.4% Turbine

NASA/CR—2010-215673 4
Figure 2: Cycle Spreadsheet Input Screen Example (English Units Version) Temperature Pressure Density Enthalpy Entropy Cycle Generator F Psia lbm/ft^3 B/lbm B/ft-F Fluid CO2 gen efficiency 98.00% Turbine In 1060.000 98.956 0.267 463.162 0.821 Units English conv efficiency 95.00% Turbine Out 915.227 47.890 0.143 422.771 0.826 net turbine power 16.061 B/sec Cycle Efficiency 21.73% windage loss 0.171 B/sec Recup Hot In 914.722 47.879 0.143 422.632 0.826 System Efficiency 20.64% bearing loss 0.833 B/sec Recup Hot Out 377.144 47.720 0.235 283.585 0.698 Cycle Carnot 57.23% core loss 0.301 B/sec System Carnot 60.00% converter loss 0.738 B/sec Cooler In 377.140 47.637 0.234 283.585 0.698 Percent Cycle Carnot 37.97% gen power 14.756 B/sec Cooler Out 190.294 47.604 0.303 240.850 0.641 Percent System Carnot 34.40% gen power 15.565 kW conv power 14.018 B/sec Comp In 190.330 48.032 0.306 240.850 0.640 delta P hot 0.524 psi conv power 14.787 kW Comp Out 311.291 99.405 0.534 267.457 0.645 delta P cold 0.283 psi full power 12 kW percent power 123.23% Recup Cold In 311.289 99.372 0.534 267.457 0.645 Recup Cold Out 853.475 99.306 0.310 405.609 0.780 Turbine Fluid Mass Heater In 853.474 99.227 0.310 405.609 0.780 speed 65694 rpm Pipe 2 0.007 lbm Heater Out 1060.000 99.032 0.267 463.162 0.821 mdot 1.180 lbm/sec Pipe 3 0.053 lbm pressure ratio 2.066 Pipe 4 0.096 lbm efficiency 86.07% Pipe 5 0.077 lbm Heater In 1115.330 21.800 45.978 power 47.661 B/sec Pipe 6 0.113 lbm Heater Out 1000.000 21.656 46.978 Pipe 1 0.060 lbm Heater 0.297 lbm Cooler In 170.330 150.000 60.814 Compressor Cooler 0.026 lbm Cooler Out 272.100 149.946 58.203 speed 65694 rpm Recuperator-Hot 0.240 lbm mdot 1.1876497 lbm/sec Recuperator-Cold 0.530 lbm Jacket In 170.330 150.000 60.814 pressure ratio 2.070 Hot Side 0.658 lbm Jacket Out 193.867 149.500 60.292 efficiency 84.23% Cold Side 0.842 lbm power 31.600 B/sec Total 1.500 lbm Design Point 1.482 lbm Pressure Balance 0.500 psid Mass Balance 0.018 lbm Heater Cooler Recuperator Jacket Fluid NaK Fluid Water Heat Transfer 164.076 B/s Fluid Water mdot 2.833 lbm/s mdot 0.494 lbm/s LMTD 63.523 F mdot 0.02 lbm/sec Heat Transfer 67.913 B/s Heat Transfer 50.754 B/s Pressure Drop - Hot 0.159 psi Delta P 0.5 psia LMTD 55.330 F LMTD 60.230 F Pressure Drop - Cold 0.066 psi heat transfer 0.472 B/sec Pressure Drop 0.195 psi Pressure Drop 0.033 psi effectiveness 90.00% effectiveness 78.85% effectiveness 90.35% efficiency 90.00% efficiency 90.75% Pipe Losses Pipe 2 0.011 psid Bearings Pipe 3 0.083 psid Bearing delta P 0.5 psid Pipe 4 0.072 psid Bearing Lube Percent 1.00% Pipe 5 0.033 psid Bearing lube 0.012 lbm/s Pipe 6 0.079 psid

Enthalpy Out 258.389 B/lbm Pipe 1 0.076 psid

NASA/CR—2010-215673 5
Several caveats were placed on the fluid design trade study. All fluids were treated 

equally with fixed expected turbomachinery efficiency, recuperator effectiveness, 

generator efficiency, converter efficiency, and bearing losses. Windage losses were 

calculated as part of the analysis since it was easy to perform with this calculation 

imbedded in the worksheet and the variable windage that is inherent in gas selection. 

The operating pressure was optimized for each fluid using an Excel based routine. This 

was done for all the working fluids under consideration for the three temperature ratios. 

The high temperature ratio is the highest NaK temperature and the lowest cooling water 

temperature. This results in the highest power output condition. The medium 

temperature ratio is the nominal NaK and cooling water temperature. This is the design 

point and per the specification should obtain 12 kWe of power from the generator. The 

low temperature ratio is the lowest NaK temperature coupled with the highest cooling 

water temperature. The recuperator effectiveness was set high with the knowledge that 

the actual recuperator sizing trade study would happen after the working fluid selection 

was performed. Therefore the overall cycle efficiency was optimistic relative to 

size/volume constraints placed in the specification after the project was initiated. 

An initial survey of candidate operating gases was done to ascertain cycle efficiency and 

maximum operating pressure. The cycle spreadsheet used an internal optimization 

routing to find the maximum efficiency and operating pressure for a fixed recuperator 

effectiveness and temperature ratio. The gases considered were nitrogen, carbon dioxide, 

argon, and HeXe mixture. The data are shown in Figure 3. It is clear that the HeXe 

mixture allows the highest cycle efficiency. The optimum system operating pressure is 

similar for all gases except carbon dioxide, which is lower. The lower operating pressure 

should yield lighter pressure containment structure and therefore the power per unit 

weight for the system operating on carbon dioxide may be higher. 

NASA/CR—2010-215673 6
Cycle Efficiency vs. Compressor Exit Pressure (12kW)

22

24

26

28

30

32

34

36

500 750 1,000 1,250 1,500 1,750 2,000 2,250 2,500 2,750 3,000 3,250 3,500

Compressor Exit Pressure (kPa)

Cycle Efficiency (%)

CO2high

CO2avg

CO2low

N2high

N2avg

N2low

Arhigh

Aravg

Arlow

HeXehigh

HeXeavg

HeXelow

Figure 3: Cycle Efficiency vs Compressor Exit Pressure for Various Gases 

The results of the fluid study at 12 kWe point to an obvious fluid choice, based on 

efficiency alone, of HeXe mixture. However given the cost of the HeXe fluid, it would 

probably be necessary to initially design the system for Argon use to test the TAC 

components due to the similar molecular weight of Argon and HeXe mixture. The heat 

exchangers would need to be larger to accommodate the Argon properties and therefore 

the test relevance to future flight needs would be compromised. Certainly the cost of 

Xenon may violate the need for a reasonable cost program. 

With HeXe not a practical option for the goals of this program, Argon and Carbon 

Dioxide would be the next logical choices based on cycle efficiency alone. Given that 

Carbon Dioxide would operate at a lower system pressure it was concluded that carbon 

dioxide would be a better choice than either Argon or Nitrogen based on cycle efficiency 

and operating pressure, neglecting system level trade parameters such as heat exchanger 

size, turbine size, compressor size, operating speed, system weight, and other parameters. 

An additional interest in fluid selection is the size and therefore the weight of the heat 

exchangers. The analysis was done utilizing the same spreadsheet that was formulated to 

yield an estimated size and weight for the heater, recuperator, and gas cooler. Figure 4

shows the gas cooler design sheet that was used for the analysis. Figure 5 shows the 

recuperator design sheet, and Figure 6 the gas cooler design sheet. 

NASA/CR—2010-215673 7
Temperature Pressure Density Enthalpy Entropy Viscosity Prandtl

Thermal 

Conductivity

Specific 

Heat Units English

F Psia lbm/ft^3 B/lbm B/ft*F lbm/ft*s - B/hr*ft*F B/lbm*F

Shell Side In 215.000 100.000 59.770 183.542 0.317 0.000186 1.718 0.393 1.008 Reset Loops (0 then 1)

Shell Side Out 227.706 98.600 59.442 196.357 0.336 0.000173 1.599 0.394 1.010 Shell 1

Shell Average 221.353 99.300 59.606 189.949 0.326 0.000180 1.659 0.394 1.009 Tube 1

Tube Side In 395.494 39.486 0.190 288.050 0.712 0.000015 0.731 0.018 0.239

Tube Side Out 225.000 39.474 0.238 248.655 0.661 0.000013 0.744 0.014 0.222

Tube Average 310.247 39.480 0.214 268.353 0.686 0.000014 0.737 0.016 0.231

Tube Side Shell Side Units

Fluid CO2 Water Heat Transfer 44.851 B/s

Mass Flow Rate 1.139 3.500 lbm/s

Pinch 1 167.787 F

Velocity In 21.837 ft/s Pinch 2 10.000 F

Velocity Out 17.429 ft/s LMTD 55.95076673 F

Average Velocity 19.38562181 0.977337065 ft/s UA (LMTD) 0.801615719 B/s*F

Reynolds Number 9200 7849 - (10,000) UA (design) 0.829434947 B/s*F

Viscosity Ratio 1.032291085 - efficiency 94.67% -

Heat Capacity 0.262812721 3.530762691 B/s*F effectiveness 94.46% -

Nusselt Number 31.12 61.02 -

Convection Coefficient 15.88 991.71 B/hr*ft^2*F Tube Thermal Conductivity 100 B/hr*ft*F Aluminum alloy

wt kg

Pressure Drop Initial Estimate Shell Internal Diameter 8.000 in 7.7 shell 20.26704

delta P/P in 0.03% 1.40% Tube Length 64.000 in 61.9 tubes 94.03908

delta P 0.012 1.400 psi Baffles 10 - 10 total 114.3061 51.95733

Resulting Pressure 39.486 98.600 psia Tube Internal Diameter 0.374 in

Tube Wall Thickness 0.032 in

Pressure Drop Final Estimate Clearance 0.100 in

Roughness 0.000005 ft Percent usable cross-section 90%

Friction Factor 0.032 0.0024 - / ft^2/in^2 Tube Outside Diameter 0.438 in

Major Pressure Drop 0.047 0.6228 psi Tube Pitch 0.538 in (Triangle Pitch)

Minor Pressure Drop 0.013 psi Pitch Area 0.050 in^2

delta P/P in 0.15% 0.62% Number of Tubes 360

Resulting Pressure 39.53 99.37721861 psia Tube Flow Area 39.549 in^2

Baffle Space 5.818 in

Shell Cross Flow Area 8.652 in^2

Shell Hydraulic Diameter 0.291 in

Re > 10,000 Update f from Fig 29, Kern Tube Heat Transfer Area 27070.978 in^2

Shell Heat Transfer Area 31703.445 in^2

Volume 3485.073 in^3

2.017 ft^3

Figure 4: Gas Cooler Design Spreadsheet 

The gas cooler design sheet utilizes a tube shell heat exchanger calculation. The 

eventual heat exchanger technology chosen is plate/fin, but for a fluid trade study, the 

defining metric is the relative size for the various gases and therefore the tube/shell 

calculation is valid and is more widely represented in literature and therefore more 

reliable from the analysis perspective. 

NASA/CR—2010-215673 8
Temperature Pressure Density Enthalpy Entropy Viscosity Prandtl

Thermal 

Conductivity

Specific 

Heat Units English

F Psia lbm/ft^3 B/lbm B/lbm*F lbm/ft*s - B/hr*ft*F B/lbm*F

Hot Side In 929.421 39.584 0.117 426.727 0.837170628 0.000023 0.724 0.032 0.277 Reset Loops (0 then 1)

Hot Side Out 395.494 39.486 0.190 288.050 0.712024156 0.000015 0.731 0.018 0.239 Hot 1

Hot Side Average 662.457 39.535 0.153 357.389 0.775 0.000 0.727 0.025 0.258 Cold 1

Cold Side In 335.494 75.000 0.389 273.429 0.665449319 0.000014 0.736 0.017 0.235

Cold Side Out 871.796 74.873 0.231 410.720 0.796617567 0.000022 0.724 0.030 0.274

Cold Side Average 603.645 74.936 0.310 342.074 0.731 0.000 0.730 0.023 0.254

Hot Side Cold Side Units 442.208

Fluid CO2 CO2 Q 157.8838719 B/s 157.8839

Mass Flow Rate 1.1385 1.15 lbm/s

Pinch Hot 57.625 F

Inlet Velocity 12.55266473 3.805549785 ft/s Pinch Cold 60 F

Exit Velocity 7.726347251 6.425215413 ft/s LMTD 58.80429358 F

Reynolds Number 564 601 - (600) UA (LMTD) 2.684903811 B/s*F

UA (Heat Exchanger) 3.024949401 B/s*F

Pressure Drop Initial Estimate

delta P/P in 0.25% 0.17% efficiency 90.72% -

delta P 0.099 0.128 psi effectiveness 90.30% -

Resulting Pressure 39.584 74.873 psia

Fin Thermal Conductivity 12.3 12.3 B/hr*ft*F Stainless Steel

Velocity Term 5.0194E-05 8.11264E-06 - Plate Thermal Conductivity 12.3 B/hr*ft*F

Entrance Effect 1.979364037 1.979364037 -

Flow Acceleration Effect -0.768970985 1.376760666 - Table 9.3 (Kays & London)

Core Friction Effect 45.24729031 74.58383663 - Surface Designation 17.8-3/8W 17.8-3/8W

Exit Effect 0.492020162 1.349630519 - Plate Spacing 0.0345 0.0345 ft

Delta P/P1 0.23% 0.06% - Hydraulic Diameter 0.00696 0.00696 ft

Pressure Drop 0.09 0.05 psi Fin Thickness 0.006 0.006 in

Resulting Pressure 39.58 74.95 psia Area/Volume Ratio 514 514 ft^2/ft^3

Fin Area Ratio 0.892 0.892 -

Nusselt Number 8.0 8.6 -

Convection Coefficient 28.7 28.8 B/hr*ft^2*F Frontal Area 2 2 ft^2 1.9

m 96.5 96.8 ? Flow Length 1.4 1.4 ft 1.3

Fin Efficiency 0.559 0.558 - Layers 1 1

Fin Effectiveness 0.607 0.606 - Plates per layer 10 10

Width 0.796140493 0.796140493 ft

Plate thickness 0.063 in

Volume 2.8 ft^3

Heat Transfer Area to Volume 223.0566038 223.0566038 ft^2/ft^3

Heat Transfer Area 624.5584906 624.5584906 ft^2

Free to Frontal Area Ratio 0.388118491 0.388118491 -

Free Flow Area 0.776236981 0.776236981 ft^2

Table 10.6 (Kays & London)

St*Pr^2/3 0.0158 0.0158 - Update for Reynolds Number

Friction Factor 0.0738 0.0738 -

Figure 5.4 (Kays & London)

Entrance Loss Coefficient 1.13 1.13 - Adjust for Reynolds Number and Free to Fron

Exit Loss Coefficient 0.05 0.05 -

8000 lb Weight 89.70553592 kg

75 ft^3

106.6666667 lb/ft^3

64.0753828

29.125174 kg/ft^3

Figure 5: Recuperator Design Sheet 

The recuperator design is a plate fin type that utilizes correlations from “Compact Heat 

Exchangers”, W.M. Kays and A.L. London, ISBN-10: 1575240602. The method utilized 

includes the proper heat transfer parameters, but does not include any structural analysis 

regarding the pressure containment boundary. Therefore the analysis is useful for 

relative size and weight from gas to gas, but not necessarily accurate in an absolute sense. 

NASA/CR—2010-215673 9
Temperature Pressure Density Enthalpy Entropy Viscosity Prandtl

Thermal 

Conductivity

Specific 

Heat Units English

F Psia lbm/ft^3 B/lbm B/ft*F lbm/ft*s - B/hr*ft*F B/lbm*F

Shell Side In 1070.000 20.000 46.371 0.000118 0.006 15.057 0.208 Reset Loops (0 then 1)

Shell Side Out 987.835 18.952 47.084 0.000125 0.006 15.152 0.208 Shell 1

Shell Average 1028.917 19.476 46.727 0.000122 0.006 15.105 0.208 Tube 1

Tube Side In 871.796 74.873 0.231 410.720 0.797 0.000022 0.724 0.030 0.274

Tube Side Out 1060.000 74.753 0.202 463.230 0.834 0.000025 0.723 0.035 0.284

Tube Average 965.898 74.813 0.216 436.975 0.815 0.000023 0.724 0.032 0.279

Tube Side Shell Side Units

Fluid CO2 NaK Heat Transfer 59.783 B/s

Mass Flow Rate 1.139 3.500 lbm/s

Pinch 1 116.038 F

Velocity In 30.103 ft/s Pinch 2 10.000 F

Velocity Out 34.431 ft/s LMTD 43.25740042 F

Average Velocity 32.12174774 1.579635275 ft/s UA (LMTD) 1.382020938 B/s*F

Reynolds Number 9250 14875 - (10,000) UA (design) 1.45124174 B/s*F

Viscosity Ratio 0.996948943 -

Heat Capacity 0.317478429 0.727495432 B/s*F effectiveness 94.95% -

Nusselt Number 30.09 12.87 -

Convection Coefficient 31.35 7939.34 B/hr*ft^2*F Tube Thermal Conductivity 12.3 B/hr*ft*F Stainless steel

Pressure Drop Initial Estimate Shell Internal Diameter 6.600 in 6.6

delta P/P in 0.16% 5.24% Tube Length 95.000 in 90.5

delta P 0.120 1.048 psi Baffles 15 - 15

Resulting Pressure 74.753 18.952 psia Tube Internal Diameter 0.374 in

Tube Wall Thickness 0.063 in

Pressure Drop Final Estimate Clearance 0.100 in

Roughness 0.000005 ft Percent usable cross-section 90%

Friction Factor 0.032 0.0024 - / ft^2/in^2 Tube Outside Diameter 0.500 in

Major Pressure Drop 0.195091 1.6385 psi Tube Pitch 0.600 in (Triangle Pitch)

Minor Pressure Drop 0.037066 psi Pitch Area 0.058

delta P/P in 0.31% 8.19% Number of Tubes 197 in

Resulting Pressure 74.64 18.36152981 psia Tube Flow Area 21.642 in^2

Baffle Space 5.938 in

Shell Cross Flow Area 6.531 in^2

Shell Hydraulic Diameter 0.294 in

Re > 10,000 Update f from Fig 29, Kern Tube Heat Transfer Area 21989.295 in^2

Shell Heat Transfer Area 29397.453 in^2

Volume 3400.667 in^3

1.968 ft^3

Figure 6: NaK Heater Design Sheet (Shell and Tube) 

The results of this study, as seen in Figure 7, indicates that the HeXe heat exchanger 

sizing is the smallest. This is due to the excellent transport properties of the mixture. 

Since this gas mixture does not suit the cost of this program, the next choice is Carbon 

Dioxide. The Carbon Dioxide heat exchangers will be approximately 50% larger, but 

due to the lower pressure requirement, the anticipated weight will not scale with the same 

percentage. Clearly the Argon and Nitrogen heat exchangers are quite a bit larger and 

therefore Argon and Nitrogen are inferior relative to system weight and size. The data is 

presented at the low, medium, and high temperature ratios. 

NASA/CR—2010-215673 10
Figure 7: Heat Exchanger Volume vs. Compressor Exit Pressure at Various 

Temperature Ratios 

Other figures of merit are the compressor and turbine efficiency. At the 12 kWe power 

range, the compressor and turbine wheel diameters become relatively small. This results 

in larger clearance losses due to practical machining tolerances and wheel clearances. 

Secondarily is the operating speed, slower speeds would mean heavier and larger 

equipment, but higher speeds may mean higher windage, bearings, and generator core 

losses. The shaft speed for the various gases and compressor exit pressures are shown in 

Figure 8. The turbine wheel diameter for various gases and operating pressures are 

shown in Figure 9. The compressor wheel diameters for various gases and operating 

pressures are shown in Figure 10. It is clear that Carbon Dioxide yields a higher shaft 

speed with similar turbine and compressor wheels sizes. With anticipated turbine and 

compressor efficiencies the same regardless of operating gas, then Carbon Dioxide would 

yield a much smaller alternator and therefore a much lower weight and volume TAC. 

NASA/CR—2010-215673 11
Figure 8: Optimum Shaft Speed versus Compressor Exit Pressure for Various 

Gases at low, average, and high temperature ratios. 

Figure 9: Turbine Wheel Diameter for various gases versus Compressor Exit 

Pressure for low, average, and high temperature ratios. 

NASA/CR—2010-215673 12
Figure 10: Compressor Wheel Diameter for various gases versus Compressor Exit 

Pressure for low, average, and high temperature ratios. 

Additional analysis was performed for 6 kW, 12 kW, and 24 kW power converters. The 

Brayton cycle TAC has significant challenges to produce efficient power at lower power 

levels. This is primarily due to the small turbine and compressor wheel sizes and 

associated clearance losses. This lowers the overall efficiency for rotating machinery of 

this type. The anticipated cycle efficiencies with a larger 98% effective recuperator for 

various gases are seen in Figure 11. 

The heat exchanger volume is also presented for the 6 kW, 12 kW, and 24 kW power 

levels for CO2, N2, Ar, and HeXe mixture, Figure 12. The heat exchanger sizes 

generally scale in a linear fashion with power level. Larger power levels could anticipate 

slightly lower weight per kW thermal transfer due to efficiency in pressure boundary 

construction. 

NASA/CR—2010-215673 13
Figure 11: Cycle Efficiency vs. Compressor Exit Pressure for CO2, N2, Ar, and 

HeXe at 6 kW, 12 kW, and 24 kW. 

Figure 12: Heat Exchanger Volume vs. Compressor Exit Pressure for CO2, N2, Ar, 

and HeXe at 6 kW, 12 kW, and 24 kW. 

NASA/CR—2010-215673 14
Summary of Fluid Trade Study Information: 

• Helium Xenon Mix 

– Best Efficiency 

– Lowest Heat Exchanger Volume 

– Low Weight 

– Most Expensive Fluid 

• Argon 

– Comparable Efficiency to CO2 

– High Heat Exchanger Volume 

– High Weight 

– Reasonable Working Fluid Cost 

• Carbon Dioxide 

– Good Efficiency 

– Moderate Heat Exchanger Volume 

– Low Weight (Lower Pressure and Smaller turbo-generator) 

– Least Expensive Fluid 

– Available on Mars at Low Pressure 

The helium xenon system would provide the most efficient power conversion system, but 

on a kW/kg or kW/m^3 evaluation, it is not as clear for a 12 kWe power system. The 

TAC generator on HeXe would be approximately twice the size and roughly twice the 

weight primarily due to the lower shaft speed and larger alternator size. Secondarily the 

higher optimum operating pressure would lead to heavier pressure boundary containment. 

The prohibitive cost of the Xenon at approximately $1 US per gram (2008 price) does not 

fit the program goals as stated by NASA 

The CO2 system is slightly heavier, but given the higher speed alternator, and relatively 

small heat exchanger sizes, it a good match for the program goals. It was selected 

primarily due to the advantageous cost of the working fluid 

2.2 System Trade Study 

The system trade study is used to identify the sensitivity of the various system 

components and parameters. The heat exchangers comprise the majority of the overall 

system volume and weight. The goal is to optimize the cycle efficiency relative to 

system specific weight and specific volume. Figure 13 identifies the influence of 

pressure ratio and heat exchanger volume on the overall cycle efficiency. It can be seen 

that a cycle efficiency reduction of 0.5-1% can reduce the heat exchanger size by 10-20% 

near the optimum point. This is an important consideration for a flight weight system. 

Figure 14 identifies the influence of recuperator effectiveness versus the overall cycle 

efficiency. Overlaid on this figure is the specific weight of the heat exchanger relative to 

recuperator effectiveness. An inflection point is obvious for the considered power levels 

of 6 kW, 12 kW, and 24 kW. For the 12 kW system, the inflection point is between 88-

90% effectiveness. Larger recuperator effectiveness values can easily increase the heat 

exchanger weights by 50%. 

NASA/CR—2010-215673 15
Figure 13: Cycle Efficiency vs. Pressure ratio and Heat Exchanger Volume 

Cycle Eff v Rec % and Sp Wt

87.0

88.0

89.0

90.0

91.0

92.0

93.0

94.0

95.0

96.0

97.0

98.0

15 17 19 21 23 25 27

Cycle Efficiency %

Recuperator Effectiviness %

0.0

10.0

20.0

30.0

40.0

50.0

60.0

Specific Weight kg/kW

Specific Weight 12kWe

Recuperator

Effectiveness

12 kWe Only

Specific Weight 24We

Specific Weight 6kWe

Figure 14: Cycle Efficiency vs. Recuperator Effectiveness and Specific Weight for 6 

kW, 12 kW, and 24 kW Converters 

NASA/CR—2010-215673 16
Another sizing consideration is relative to the NaK to CO2 heat exchanger. NaK flow 

rate was not initially provided in the specification and was varied from 1.5 to 3.5 kg/s for 

the analysis to evaluate sensitivity. It is important to keep the CO2 temperature as close 

to the NaK temperature as possible without increasing the heat exchanger volume to an 

unacceptably large size. Figure 15 shows the effect of NaK Flow on cycle efficiency and 

NaK heater volume with a 5 deg K approach temperature. 

Cycle Eff v NaK Flow v HX Volume

12 kWe 

0

0.5

1

1.5

2

2.5

3

3.5

4

19.2 19.3 19.4 19.5 19.6 19.7 19.8 19.9 20 20.1 20.2 20.3

Cycle Efficiency %

NaK Flow lb/sec

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

HX Volume m^3

Cycle Eff v NaK Flow

Cycle Eff v HX Volume

Figure 15: Cycle Efficiency vs. NaK Flow Rate and Heat Exchanger Volume 

The system trade study identifies the important parameters regarding the effect of heat 

exchanger sizes and flow rates on cycle efficiency. At this point heat exchanger vendors 

were contacted regarding refining the designs relative to performance and fabrication. 

3. Heat Exchanger Design 

3.1 Recuperator Design 

The recuperator is used to preheat the compressor exit gas prior to entering the NaK to 

gas heater. It utilizes the residual heat from the turbine exit gas. Niagara Thermal 

Products was contacted regarding the refinement of the recuperator design and ultimately 

provide a manufacturing quotation regarding this heat exchanger. The initial task was to 

confirm the Barber-Nichols recuperator design relative to performance, size, weight, and 

pressure drop estimates. Niagara Thermal Products identified several fin types that were 

tooled and available for use with this heat exchanger. The fin types offer different 

pressure drop and effectiveness options that are presented in Figure 16 and Figure 17. 

NASA/CR—2010-215673 17
Barber‐Nichols Recuperator

Effectiveness vs Fin Type and Fins per Inch

0.8

0.82

0.84

0.86

0.88

0.9

0.92

0 5 10 15 20 25

Fins per inch

Effectiveness

.15 Wavy Fin .15 Plain Fin .19 Wavy Fin .19 Plain Fin

Figure 16: Fin Type/Pitch versus Recuperator Effectiveness 

Barber‐Nichols Recuperator

Hot Side Pressure Drop vs Fin Type and Fins per Inch

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 5 10 15 20 25

Fins per inch

Pressure Drop (in H2O)

.15 Wavy Fin .15 Plain Fin .19 Wavy Fin .19 Plain Fin

Figure 17: Fin Type/Pitch versus Pressure Drop 

NASA/CR—2010-215673 18
An effectiveness of 89-90% is required for high cycle efficiency so the choice is between 

the .19 wavy fin and the .15 plain fin. The .19 wavy fin was chosen based on lower 

pressure drop. 

After fin selection, the recuperator design was finalized and the performance chart was 

constructed as seen in Figure 18. 

CO2 Recuperator

Hot Side In @ 909.0°F

Cold Side In @ 355.2°F

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4

CO2 Flow Rate lb/sec

Effectiveness

0.000

0.020

0.040

0.060

0.080

0.100

0.120

0.140

0.160

0.180

0.200

Pressure Drop psid

Effectiveness Pressure Drop Cold Side Pressure Drop Hot Side

Figure 18: Recuperator Performance 

The recuperator is a brazed construction with welded tank heads and manifolds. A 

preliminary design was provided by the manufacturer that indicated the approximate size 

and weight of the heat exchanger. Since the raised face flanges are not necessary in the 

final design, they were eliminated and the tank heads were modified to lower the 

estimated weight. The overall recuperator size is shown in Figure 19. 

NASA/CR—2010-215673 19
Figure 19: Overall Recuperator Size 

3.2 Water/Gas Plate Fin Cooler 

Niagara Thermal Products generated a compact water-to-CO2 gas cooler design. The 

heat exchanger is a plate/fin type much like the recuperator. The construction material is 

316L stainless steel with AMS 4777 braze filler. The overall design is shown as given by 

the supplier in Figure 20. The ASME flanges will not be used and the weight is 

subtracted for those to come to a lower weight estimate. The supplier data sheet is shown 

in Figure 21 and the off design performance is shown in Figure 22. 

NASA/CR—2010-215673 20
Figure 20: Water to CO2 Gas Cooler Dimensional Data 

NASA/CR—2010-215673 21
Inquiry: 28112

Rev: 0

Date: 11/18/08

Customer:

Customer P/N:

Description:

Application:

Design File: 28112C30 Prepared by:

Function & Design:

Construction:

Core:

Tanks

Dimensions:

Core ( W x H x D ):

Cold Side Fin:

Hot Side Fin: 

Estimated Weight:

Testing:

Cold Side Hot Side

Leak Test Pressure: 100 psig 50 psig

Proof Test Pressure: 150 psig 75 psig

Maximum Leak Rate Allowed : 10-3 sccs helium

.240" Louver Fin- 13 fpi

.060" Lance and Offset Fin- 20 fpi

 76 lbs(including tanks)

Doug Turner

316L Core Vacuum Brazed with AMS 4777 braze

316L Fabricated and welded with ER 308L filler

12.0" Wide x 11.1" High x 7.0" Deep (see sketch page 4)

NIAGARA THERMAL PRODUCTS LLC

The heat exchanger is designed to cool 1.1 lb/sec of CO2 Vapor at 415 oF using .825 

lb/sec Water at 215.33 o

F. 

Page 1 of 6 NIAGARA THERMAL PRODUCTS LLC

3315 Haseley Drive, Niagara Falls, NY 14304 

Heat Exchanger Data Sheet

Barber-Nichols

CO2 / Water Cooler

Figure 21: Water to CO2 Gas Cooler Data Sheet 

NASA/CR—2010-215673 22
CO2/Water Cooler

CO2 In @ 415.0°F

Water In @ 215.33°F

and .375 kg/sec

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Air Flow Rate lb/sec

Effectiveness

0.000

0.020

0.040

0.060

0.080

0.100

0.120

0.140

0.160

0.180

0.200

Pressure Drop psid

Effectiveness Pressure Drop Cold Side

Figure 22: Water to CO2 Gas Cooler Performance Data 

3.3 NaK Heater Design 

3.3.1 Shell and Tube NaK Heater Design 

The NaK heater operates at a temperature range of 825 K to 875 K. The flow rate was set 

at 1.75 kg/sec of NaK 78. Holtec International was selected to design the shell and tube 

heat exchanger. It was decided that a vendor with molten salt, and preferably NaK 

experience, be utilized. This limited the number of vendors that were available to consult 

on this heat exchanger. Holtec International was deemed the most capable supplier and 

NASA/CR—2010-215673 23
capable of only shell/tube type designs. Several iterations were done to trade pressure 

drop and heat exchanger size. The final shell/tube design sheet is shown in Figure 23. 

Figure 23: Heat Exchanger Specification Sheet for NaK to CO2 Gas 

The primary material of construction is 316 stainless steel. It contains the NaK on the 

shell side and the CO2 on the tube side. The heat exchanger is approximately 0.3 m 

diameter x 1.2 m long. The tube bundle contains 676 x 6mm tubes with a wall thickness 

0.45mm. 

This heat exchanger was deemed to be too heavy and not flight worthy. A search of an 

alternative vendor for a plate fin heat exchanger with NaK experience was not successful. 

NASA/CR—2010-215673 24
Niagara Thermal Products was capable of providing the heat exchanger, but had no 

experience in handling NaK. 

3.3.2 Plate/Fin NaK Heater 

Researching literature provided a basis for past molten salt heat exchanger designs for 

space applications. A previous NASA program for the SNAP reactor (1970’s vintage) 

provided valuable information regarding proposed heat exchanger design types and 

material compatibility. A liquid metal design manual, “SNAP Technology Handbook”, 

G.F. Burdi, NAA-SR-86 17, VOLUME I, REACTOR TECHNOLOGY, SNAP 

REACTORS, SNAP PROGRAM, TID-4500 (29th Ed.) M-3679 (34th Ed.) was utilized 

to provide material data and ultimately to design a plate fin heat exchanger that is much 

lighter weight than the shell/tube design. 

Essentially, the thermal transfer of the NaK heater and the gas cooler are approximately 

equal. With the gas cooler design was used as a basis for scaling the NaK heater with a 

slight change in the braze material from AMS 4777 to AMS 4778 to remove the iron in 

the braze filler. All other parameters for pressure drop, thermal transfer showed this heat 

exchanger exceeded the specifications for the NaK heater. This drops the weight of the 

heat exchanger to below 30 kg and reduces the size to approximately 0.3m x 0.2m x 

0.3m. 

4. Cycle and Performance Analysis 

After sizing the heat exchangers, an estimate of turbine and compressor performance can 

be made from a thermodynamic cycle simulation of the complete converter. Typically 

the cycle design will change slightly once the turbine and compressor performance is 

estimated. This design cycle also must include turbine and compressor stress estimates. 

This design cycle utilizes many areas of expertise from aerodynamics to stress to 

thermodynamics. Once the turbine and compressor designs are finalized, off 

performance maps are generated and implemented into the cycle design. The cycle 

design spreadsheet can then be used to estimate the performance for on and off design 

conditions. 

The cycle spreadsheet calculates all state point data and displays losses and efficiencies 

for major components. The spreadsheet also calculates cycle efficiency as well as system 

efficiency. Cycle efficiency is defined as operating gas side efficiency. System 

efficiency includes the NaK and water side energy. The efficiency relative to Carnot is 

also provided. 

4.1 Nominal Temperature Difference 

The nominal temperature difference is specified at 850 K NaK inlet temperature to the 

heat exchanger at a 1.75 kg/sec flow rate (Figure 24). The water temperature for the 

nominal case is 375 K with a .8 lb/sec flow rate. The heat exchangers are designed for 

the nominal condition as is the turbomachinery. The spreadsheet is linked to both the 

heat exchanger and turbomachinery dynamically. This allows off design calculations to 

be performed in a relatively quick manner. It also balances the mass and pressure against 

NASA/CR—2010-215673 25
the turbine, compressor, and system design volumes to simulate actual steady state 

running conditions. 

Figure 24: Thermodynamic Cycle State Points for Nominal Design Condition Fluid: Shaft Power: 13.88 kW Cycle Efficiency: Alternator Windage: 0.18 kW Percent of Cycle Carnot: Bearing Losses: 0.69 kW System Efficiency: Core Losses: 0.26 kW Percent of System Carnot: 375 K 380 K Converter Losses: 0.64 kW 1034 kPa 1031 kPa Net Power Out: 12.11 kW 0.02 kg/s 0.02 kg/s 396 K 850 K 339 kPa 375 K 150 kPa 0.005 kg/s 1034 kPa 1.75 kg/s Water Jacket 0.36 kg/s 839 K 839 K 386 K 386 K 673 kPa 673 kPa 339 kPa 339 kPa 0.52 kg/s 0.52 kg/s 0.53 kg/s 0.53 kg/s NaK e= 90.8% e= 86.8% e= 98.0% e= 83.8% e= 89.9% Water Heating Q= 67.0 kW PR= 1.98 N= 65694 rpm PR= 1.96 Q= 50.2 kW Cooling Loop Loop 728 K 728 K 451 K 486 K 675 kPa 341 kPa 676 kPa 341 kPa 0.52 kg/s 0.52 kg/s 0.53 kg/s 0.51 kg/s 809 K 408 K 149 kPa 1034 kPa 1.75 kg/s 0.36 kg/s 498 K 341 kPa 0.005 kg/s 728 K 451 K 675 kPa 676 kPa 0.52 kg/s 0.52 kg/s e= 90.1% Q= 156.2 kW 760 K 484 K 341 kPa 341 kPa 0.53 kg/s 0.53 kg/s Compressor Cooler Recuperator CO2 Heater Alternator 19.0% 35.3% 18.1% 32.3% Turbine

NASA/CR—2010-215673 26
The nominal condition operation results in a predicted system efficiency of 18.1%, with a 

cycle efficiency of 19.0%. 

4.2 High Temperature Difference 

The high temperature difference is specified at 875 K NaK inlet temperature to the heat 

exchanger at a 1.75 kg/sec flow rate. The water temperature for the nominal case is 325 

K with a .375 kg/sec flow rate. The heat exchangers are designed for the nominal 

condition, as is the TAC, however the goal is to predict the performance of the power 

conversion system with the excess available heat. Again, the spreadsheet method is 

utilized so that the TAC and heat exchanger off-design characteristics can be met as a 

system with the proper heat and mass balance. Figure 25 shows the state points of the 

high temperature difference analysis. 

The high temperature difference analysis results in a maximum power of 16.24 kWe from 

the power converter. The system efficiency is calculated at 21.4% and a cycle efficiency 

of 22.5%. 

4.3 Low Temperature Difference 

The low temperature difference is specified at 825 K NaK inlet temperature to the heat 

exchanger at a 1.75 kg/sec flow rate. The water temperature for the nominal case is 375 

K with a .375 kg/sec flow rate. The heat exchangers are designed for the nominal 

condition, as is the TAC, however the goal is to predict the performance of the power 

conversion system with the minimum available heat. Again, the spreadsheet method is 

utilized so that the TAC and heat exchanger off-design characteristics can be met as a 

system with the proper heat and mass balance. Figure 26 shows the state points of the 

low temperature difference analysis. 

The high temperature difference analysis results in a maximum power of 9.55 kWe from 

the power converter. The system efficiency is calculated at 16% and a cycle efficiency of 

16.8%. 

4.4 Off-Design Performance 

The spreadsheet can also predict performance over a range of speeds. This analysis is 

useful to predict the initial charge pressure for the system and also the startup 

characteristics. The analysis also predicts the power produced as a function of TAC shaft 

speed. This allows the operator to select a speed and heat amount for operating offdesign. The other control strategy is to operate the TAC at maximum speed and raise and 

lower the heat amount to effect part-load operation. 

The maximum power produces versus shaft speed is presented in Figure 27. The analysis 

is done for the low, medium, and high temperature ratios. The startup power and 

pressure conditions are presented in Figure 28. 

NASA/CR—2010-215673 27
Figure 25: High Temperature Difference Performance Analysis 

NASA/CR—2010-215673 28
Figure 26: Low Temperature Difference Converter Performance 

NASA/CR—2010-215673 29
Off Design Performance Estimate

0

5

10

15

20

25

0 10000 20000 30000 40000 50000 60000 70000 80000

Shaft Speed (RPM)

Cycle Net Power Out (kWe)

High Temp Ratio

Nominal Temp Ratio

Low Temp Ratio

Nominal Speed

Nominal Power

14.3%

18.4%

21.8%

System Efficiency 

noted for each 

temperature ratio at 

nominal speed

Figure 27: Cycle Power Output versus Shaft Speed for Specified Temperature 

Differences 

Off Design Performance Estimate

0

100

200

300

400

500

600

0 10000 20000 30000 40000 50000 60000 70000 80000

Shaft Speed (RPM)

Compressor Inlet Pressure (kPa)

0

1

2

3

4

5

6

Compressor Pressure Ratio

High Temp Ratio

Nominal Temp Ratio

Low Temp Ratio

High Temp Ratio

Nominal Temp Ratio

Low Temp Ratio

Figure 28: Off Design Performance Pressures versus Shaft Speed 

NASA/CR—2010-215673 30
5. Compressor Design 

A centrifugal compressor is selected for high efficiency and a compact design. The 

centrifugal compressor operates as a single stage with a set of diffuser vanes to maximize 

pressure recovery and efficiency. The final design is shown in Table 1. A compressor 

intake side view with the shroud removed is shown in Figure 29. 

Parameter Design Value 

Vaned-Island Diffuser [# vanes] 22 

Impeller w/ Splitter Blades 7+7 

Diameter Impeller-Exit mm 84. 78 

Diameter-eye mm 37.11 

Diameter-hub mm 9.75 

Impeller Exit Blade Height mm 4 

Exit Blade Angle [deg-from tang] 45 

Impeller Clearance mm .203 

Table 1: Compressor Design Table 

Figure 29: Front View Compressor and Vane Island Diffuser 

The compressor was optimized for maximum efficiency by reducing operating clearances 

and specifying a high degree of surface finish on the compressor wheel and flow 

passages. The compressor wheel is manufactured from Inconel 718, enabling thin blade 

sections for increased performance. 

NASA/CR—2010-215673 31
A performance map of the compressor with thrust calculations is combined in Figure 30. 

Figure 30: Compressor Performance Map with Thrust Calculations 

The compressor wheel structural analysis is shown in Figure 31. The maximum blades 

are designed so that centrifugal effects do not produce blade bending. The aerodynamic 

loading is included, but is a very small value. The compressor wheel is manufactured of 

Inconel 718 with a yield strength of 986 MPa. The stress analysis is performed at an over 

speed of approximately 10%. This results in a large safety margin with a calculated burst 

speed of 150,000 rpm, over 2X operating speed. 

Figure 32 is the blade interference Campbell diagram for the compressor wheel. The 

diagram shows all them modes generated by FEA analysis. Without presenting all the 

details, the wheel has not modes generated by the 1X, 2X, 3X, and 4X vibration 

signatures. The nozzle and blade passing frequency is quite high and no blade pass 

excitable modes are shown at operating speed. For this reason it is recommended to 

operate part load performance at a constant speed and not vary the speed. This will 

prevent wheel vibrations occurring at off-design speeds. The specification states a 

constant generator output voltage of 400 VAC and therefore the speed will need to be 

fixed. 

NASA/CR—2010-215673 32
igure 31: Compressor Wheel Stress Analysis at 10% Over-speed 

igure 32: Compressor Wheel Campbell Diagram 

F

F

0

5000

10000

15000

20000

25000

30000

35000

0 10000 20000 30000 40000 50000 60000 70000

Speed (RPM)

Frequency (Hz)

± 20% of 

Operating Speed

1/R

2/R

3/R

6/R

22/

R

4/R

Rotating - Compressor Wheel Structural Analysis 

276 MPa 

Stress

(Pa) • Inconel 718 at 449.8 K 

– Ultimate Strength = 1179 MPa 

a 

6 MPa 

30 

a 

29 

ge tangential stress = 207 

MPa 

– Yield Strength = 986 MP

• 10 % overspeed: 72,000 RPM 

• Max Net Section Stress = 27

– Ultimate SF = 4.

– Yield SF = 3.58 

• Max Local Stress = 358 MP

– Ultimate SF = 3.

– Yield SF = 2.75 

• Burst Speed = 150,000 rpm 

– Avera

Max 

Local 

Stress: 

358 MPa 

Max NetSection 

Stress: 

NASA/CR—2010-215673 33
6. Turbine Design 

The turbine wheel is a radial inflow type. It utilizes nozzles to direct the flow at the 

ptimum angle for high efficiency. The design dimensions and details are shown in 

 front view with shroud r in Figure 33. 

Design Value

o

Table 2. The emoved is shown

Parameter

# Nozzles 13

# Blades 11

Diameter Turbine Inlet mm 89.6

Diameter Turbine Exit mm 61.1 

Diameter Turbine Hub mm 23.2 

Turbine Inlet Blade Height mm 18.92

Impeller Clearance mm .203 

Table 2: Turbine Design Dimensions 

Figure 33: Turbine Exit View with Nozzles, Shroud Removed 

NASA/CR—2010-215673 34
igure 34: Turbine Performance and Thrust Map 

n 

n 

 compressor wheel thrust to 

eutralize the thrust load on the rotor support system. 

x, 3x, and 4x modes are not excited, 

or is the blade pass modes at the operating speed. 

F

The turbine performance indicates an efficiency of slightly higher than 87% (Figure 34). 

The turbine and compressor are on the same shaft and a compromise must be reached o

overall performance. Slightly higher turbine efficiency could be reached at a different 

operating speed, but not enough to warrant two individual shafts. The thrust map is a

important consideration and ideally is balanced with the

n

The turbine wheel is made from Inconel 718 with yield strength at temperature of 931 

MPa. This provides a 2:1 margin on burst speed. The analysis was performed at 10% 

over speed. The details of the stress analysis are shown in Figure 35. The interference 

Campbell diagram is shown in Figure 36. The 1x, 2

n

NASA/CR—2010-215673 35
Rotating - Turbine Wheel Structural Analysis 

Figure 35: Turbine Stress Analysis 

• Inconel 718 at 810.9 K 

– Ultimate Strength = 1117 MPa 

– Yield Strength = 931 MPa 

• 10 % over-speed: 72,000 RPM 

• Max Net Section Stress = 207 MPa 

– Ultimate SF = 5.4 

– Yield SF = 4.5 

• Max Local Stress = 400 MPa 

– Ultimate SF = 2.70 

– Yield SF = 2.25 

• Burst Speed = 160,000 rpm 

– Average tangential stress = 173 

MPa 

400 

MPa 

400 MPa 

Stress

(Pa) 

0

5000

10000

15000

20000

25000

0 10000 20000 30000 40000 50000 60000 70000

RPM

Freq (Hz)

1/

2/R

3/R

4/R

6/R

13/

± 20% of 

Operating Speed

Figure 36: Turbine Wheel Interference Diagram 

NASA/CR—2010-215673 36
7. Alternator Design 

The alternator is a permanent magnet design. It is a two pole (one pole pair) design using 

Samarium Cobalt magnets material shown in Figure 37. The magnets are cylindrical 

with a hollow center to provide a passage for a tie bolt. The magnets are kept in 

compression over the entire speed range by shrink fitting an Inconel 718 sleeve over the 

rotor structure. The magnet stresses are listed in Table 3. The stress analysis includes 

startup conditions at 200 K at results in 450 K at operating temperature. 

Permanent Magnet Section

Figure 37: Permanent Magnet Section of Rotor 

 TANGENTIAL 

STRESS 

 

 

CYLINDER TOTAL TOTAL Due to Due to Due to 

NAME Cold Hot Press. - Cold Press. - Hot Speed 

 Mpa Mpa Mpa Mpa Mpa 

INNER-ID -46.9193 -107.3 -133.3473 -193.728448 86.428 

INNER-OD -45.9428 -82.962 -79.2773646 -115.175041 33.3346 

OUTER-ID 414.493 540.559 278.4071578 404.473031 136.086 

OUTER-OD 329.916 431.498 224.3372224 325.919624 105.578 

Inner SmCo -550 Crush 

Outer Inconel 

718 

986 Yield 

Table 3: Magnet Stress Values 

NASA/CR—2010-215673 37
The alternator must be capable of high efficiency over the anticipated nominal and high 

temperature difference range. High efficiency of the alternator is important for overall 

cycle efficiency, but also to minimize the stator operating temperature. The performance 

map for the alternator is shown in Figure 38. The stator operating temperature defines 

the insulation method to meet the goal of 8 year life, or approximately 70,000 hours. 

Figure 39 indicates that the alternator will have sufficient life if the operating temperature 

is below 190 deg C with a goal of 180 deg C and is insulated to UL type N insulation. 

Power and Efficiency vs Speed

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 10000 20000 30000 40000 50000 60000 70000 80000 90000

Speed (rpm)

Power (watts)

80

82

84

86

88

90

92

94

96

98

100

power14amp

power28amp

power40amp

Eff14amp

Eff28amp

Eff40amp

Figure 38: Alternator Performance Map 

 

1

10

100

1000

10000

100000

0 50 100 150 200 250 300

Temperature deg C

Life (Hours)

Class A

Class B

Class F

Class H

Class N

Over 100k hrs life with 

Class N Insulation or 

better at 180 deg C

Insula tion

L ife vs

M otor Temp

b ased o n'

A rrh en ius Da ta

Figure 39: Insulation Life vs. Operating Temperature 

NASA/CR—2010-215673 38
The alternator stator is made from Hiperco 50 laminations, .006” thick. The properties of 

this material allow very low core losses in a small, lightweight package. The stator itself 

is wound with UL Class “N” materials. The stator has 18 slots in is wound in a 

sinusoidal fashion. The alternator must also act as a motor for power converter startup. 

8. Turbine Alternator Compressor (TAC) Design 

8.1 Overall Layout Section View 

The overall layout is shown in Figure 40. The compressor is shown on the right end. 

The compressor inlet routes the flow into the compressor eye. The pressure is increased 

by the rotational speed and the flow exits the compressor wheel and diffuser vanes into a 

volute. The volute discharges the flow to the recuperator inlet on the power converter 

system. The heated gas then enters the turbine inlet plenum. The flow then travels 

through the turbine nozzles, into the turbine, and exits axially through the conical 

diffuser. 

640 mm

Turbine 

Compressor

Alternator

Foil Journal Bearings

Foil Thrust Bearing

Figure 40: Overall TAC Layout 

NASA/CR—2010-215673 39
A closer view of the turbine end of the TAC is shown in Figure 41. The materials of 

construction are annotated and are primarily stainless steel and Inconel 625 or 718. 

Inconel 625 is utilized for welded construction to avoid the heat treatment and potential 

warping of components when using Inconel 718. 

Inconel 718 

Exit Diffuser 

Inlet Plenum 

Turbine Wheel 

Bearing Journal 

Bearing Holder 

316 Stainless 

Endplate 

Labyrinth Seals

Generator 

Housing 

Figure 41: TAC Turbine End Close Up 

The compressor end close up of the TAC is shown in Figure 42. Again the materials of 

construction are primarily stainless steel and Inconel alloys. 

The TAC utilizes foil journal bearings and bi-directional thrust bearing. The overall 

thrust profile is shown in Figure 43. As the machine operates at higher pressures, the 

thrust profile may vary. The foil thrust bearing must compensate for the variable thrust 

profile. 

The overall TAC weight breakdown is shown in Table 4. The major component weights 

are the alternator, inlet and exhaust volutes/plenums, and the main alternator housing. 

Smaller items are the generator rotor, bearings, end plates, seals, and the turbine and 

compressor wheels. 

NASA/CR—2010-215673 40
Stainless 316 

Stainless 15-5PH 

Inconel 625 Volute Cast 

Inconel 625 Inlet 

Inconel 718 Compressor

Figure 42: TAC Compressor End Close Up 

Figure 43: Combined Turbine and Compressor Axial Thrust Map 

NASA/CR—2010-215673 41
Compressor 0.5 kg 

Turbine 0.46 kg 

Main Housing 3.45 kg 

Compressor Volute 3.3 kg 

Alternator Stator 4.45 kg 

Turbine Plenum 4.85 kg 

Alternator Rotor/Bearings 0.919 kg 

Endplates/Seals 5.5 kg 

Guide vanes, Bolts, Piping 6.67 kg 

Total Weight 30.1 kg 

Table 4: TAC Weight Breakdown 

8.2 Foil Bearings 

The TAC is supported on foil bearings. It is the intent to utilize foil bearings from a 

Capstone C30 Microturbine. The CO2 environment for these bearings is considerably 

different than that of the microturbine so bearing loss and cooling data is difficult to 

verify. For this reason a small scale experiment was performed to quantify the foil 

bearing losses on air and CO2. 

A Capstone natural gas compressor was modified by removing the pumping elements and 

adding a flywheel mass. This mass simulates the weight of the TAC and should more 

closely approximate the bearing losses. A pressurized piston arrangement was used to 

load the thrust bearing. This was limited to approximately 50 N of force. A photograph 

of the test rig is shown in Figure 44. 

The test rig was operated to 30 krpm on air and 35 krpm on CO2 at 20 psi pressure. The 

results are shown in Figure 45. The measured power was from the DC supply feeding the 

controller. Efficiency estimates for the controller and the motor are approximately 85%, 

so the foil bearing power loss is approximately 85% of the value in the plot. The data 

repeated nicely and had a power law profile that was curve fit. This data was 

extrapolated to provide an estimated bearing loss for the TAC thermal analysis and 

cooling flow. 

NASA/CR—2010-215673 42
88mm Thrust Bearings

3 kg Test Rotor 39mm Journal Bearings 

Added 

Flywheel

Mass 

Figure 44: Foil Bearing Experimental Hardware 

Speed vs Power

50 N thrust

Air is slightly lower than CO2 y = 0.2166x2 - 1.3126x + 50.209

0

50

100

150

200

250

300

0 5 10 15 20 25 30 35 40

Speed (krpm)

Power (watts)

Figure 45: Foil Bearing Power vs. Speed (Power is from DC Supply) 

NASA/CR—2010-215673 43
8.3 Cooling Flow 

The TAC is cooled by two primary flows, the compressor inlet flow and a cooling water 

jacket. The cooling flows cool the bearings and alternator cavity. The cooling jacket is 

designed to cool the stator core and copper losses, with the auxiliary compressor flow 

removing heat from the bearings and windage. The auxiliary compressor flow is 

primarily from a hollow compressor wheel inlet that is pumped by radial passages in the 

generator cavity on both ends of the stator. This raises the generator cavity pressure to 

direct flow to the generator and also through the bearings. The water cooling passages 

are shown in Figure 46. The predicted pressures in the TAC at full operating pressure are 

shown in Figure 47. The secondary flow passages are shown in Figure 48 with the 

predicted flow rates for proper cooling shown in Figure 49. 

Figure 46: Water Cooling Passages 

NASA/CR—2010-215673 44
 

 Figure 47: Predicted Cavity Pressures at Operating Speed 

P = 48 psia 

P = 82 psia 

P = 71 psia 

P = 49 psia 

P = 73 psia P = 68 psia 

P = 54 psia P = 50 psia 

P = 50 psia 

200 W 100 W 200 W 300 W 

80 W 80 W 

Figure 48: Power Loss and Gas Cooling Flow Passages 

NASA/CR—2010-215673 45
Figure 49: Cooling Mass Flows, Temperatures, Heat Loss 

The mass flows are controlled by the labyrinth seals. It is recommended that the final 

labyrinth and cavity flow be adjusted to provide adequate cooling when the unit is 

initially tested. Minimizing the mass flow, which is a parasitic loss, will maximize the 

system efficiency. Controlling the generator temperature is the primary criteria for 

proper mass flow through the compressor bypass routes. 

8.4 Rotor Dynamic Assessment 

The rotor with foil bearings is very stiff and resistant to bending. The foil bearings are 

compliant. This combination results in rigid body modes that are at very low shaft 

speeds. As seen in Figure 50, the 1st two rigid body modes are below 15,000 rpm. The 

bearing liftoff speed has been identified at about 9,000 rpm. This dictates that the 

minimum operating speed should be above 15,000 rpm and ideally would be 20-25 krpm. 

The 1st bending mode is calculated conservatively at 81,000 rpm. The bending mode can 

be modified by increasing the retention tube that surrounds the magnets. A thicker tube 

will increase the weight, but also increase the bending mode frequency. Since the 

operating frequency is approximately 65,000 rpm, 81 krpm is adequate for operation. 

NASA/CR—2010-215673 46
1000

10000

100000

1000. 10000. 100000. 1000000. 10000000.

Bearing Stiffness, lb/in

Critical Speed, cpm

Figure 50: Un-dampened Critical Speed Map 

9. Power Controller 

The power controller serves several functions. It provides a boost converter function to 

increase the 120 VDC load supply voltage as specified by NASA to 450 VDC (selected 

by Barber-Nichols Inc.) to operate the generator at 400 VAC as specified by NASA. 

Another function is to use the alternator as a motor to start the Brayton cycle power 

converter. The power controller also converts any power available from the alternator by 

active rectification to the 450 VDC bus and the buck regulate the power to 120 VDC for 

the load. The power controller also has fault handling capability. It engages a set of 

temporary resistors in the event of shaft over speed. 

9.1 Power Controller Packaging 

The power controller is packaged in a 19 inch rack mount. It is a 4U size and can be air 

or water cooled, water cooling is preferred. The control rack layout is seen in Figure 51

and Figure 52. 

NASA/CR—2010-215673 47
Figure 51: Power Controller (Isometric View) 

Figure 52: Component Locations for Power Controller (Top View) 

Buck / Boost 

Half-Bridge 

Buck / Boost 

Inductor 

3 phase

Inverter

DC 

Fuses

3 phase

Fuses 

LEM 

I feedback

X 5 

Top Level 

Controller 

Brake Chopper

Half-Bridge 

NASA/CR—2010-215673 48
9.2 Power Controller Schematic and Simulation 

The power controller utilizes a full bridge IGBT package that contains a full-bridge threephase drive and an auxiliary switch placed across the DC bus that is used as a brake. The 

top level schematic with function blocks is shown in Figure 53. 

Buck / Boost IGBT 

Module 

Low-side IGBT 

operated during boost 

mode to deliver 

energy from 120 Vdc 

bus to 450 Vdc bus for 

motoring. 

High-side IGBT 

operated during buck 

mode to deliver 

energy from 760 Vdc 

bus to 120 Vdc bus for 

generator operation. 

Chopper IGBT 

Module 

Low-side only 

IGBT module. 

Operated to 

deliver excess 

energy from 120 

Vdc bus to dump 

resistor during 

generator mode 

operation. 

Three Phase IGBT Module 

w/ Brake 

Bi-directional converter 

used to motor during 

startup and rectify during 

generator mode operation. 

On board Brake provides 

redundant over speed 

protection in the event of a 

Buck failure 

DC+

Figure 53: Top Level Control Schematic 

The power converter simulation models can be broken down into the motor generator 

operational section and the boost converter section. The motor generator controller 

section is shown in Figure 54. The boost converter section is shown in Figure 55. 

Figure 54: Motor/Generator Controller Simulation Circuit 

32

11

Vdc

12

X5

12 12

12

12

12

VFBA

IA 44

X2

44 44

44

44

44

43

46

X4

46 46

46

46

46

A B C

BHpwm CHpwm

44

VFBB

IB

46

VFBC

IC

CLpwm

X6 46

35

BLpwm

X3 44

ALpwm

X1 12

VDCFB

32

Idc

11

R41

100e-3

AHpwm

12

D1

12

D2

44

D3

44

D4

46

D5

46

D6

32

30

C1

2000e-6

32

30

R21

250e-3

30

DChi

11

11

32

R33

1e6

32

43

V16

ICHswitch

VDChi

35

V20

ICLswitch

35

32 32 32

M A

B

C

DCBrake

Input / Output 

Filter Inductor 

Provides energy 

storage during 

boost mode 

operation and 

provides output 

filtering during 

buck mode 

operation. 

NASA/CR—2010-215673 49
VDC_hi

Figure 55: Boost/Buck Converter Controller Simulation Circuit 

The motor controller simulation circuit is used to identify waveforms and predict 

performance for the motoring and generating modes of operation. Figure 56 shows the 

motoring mode simulation results and waveform. Figure 57 shows generating mode 

simulation results and waveforms. 

Figure 56: Motoring Mode Simulation 

1

2

X1

2 2

2

2

2

Hipwm

2 3

I_bridge

V_bridge_fb

Lopwm

2

4

X2

2

D1

2

D2

1

I_hi_sw

V_hi_sw_fb

20

VDC_lo

4

V_lo_sw_fb

4

17

18

V_dc_lo

20 17

I_dc_lo

V_dc_fb_lo

6

R41

100e-3

17

19

C1

4000e-6

8

R21

25e-3

8

6

6

5

R33

1e6

5 5

5

5 5

I_lo_sw

2 2

5

5 5

3 20

L4

{FilterL}

3

5

6

V_dc_hi

5

I_dc_hi

V_dc_fb_hi

18

R4

500e-3

5

8

C6

2000e-6

19

R10

25e-3

19

18

18

17

R11

1.2

17 17

17

17

17

20 20

20 20

3⁄4Motor Mode Simulation @ 600 Hz 

3⁄45A +/- 2.5A Switch Points on Phase IGBTs 

3⁄4I leg = 2.0 Arms, 1.55 kW In 

3⁄4Phase-Phase Vpk = 333V 

3⁄4450 Vdc source on Hi DC bus 

1 ia 2 vemfa

1.80m 2.20m 2.60m 3.00m 3.40m

time in seconds

-200

-100

0

100

200

vemfa in volts

-8.00

-4.00

0

4.00

8.00

ia in amperes

Plot1

12

NASA/CR—2010-215673 50
3⁄4Generator Mode Simulation @ 1050 Hz 

3⁄430A +/- 5A Switch Points on Phase IGBTs 

3⁄4I leg = 22.9 Arms, 15.85 kW out 

3⁄4Phase-Phase Vpk = 583V 

3⁄4760 Vdc sink on Hi DC bus 

3⁄4Sensorless Generator operation viable 

3⁄4“speed-loop” winding added for redundancy 1 ia 2 vemfa 8 vswa

1.00m 1.20m 1.40m 1.60m 1.80m

time in seconds

-40.0

-20.0

0

20.0

40.0

-400

-200

0

200

400

vemfa in volts

-40.0

-20.0

0

20.0

40.0

ia in amperes

Plot1

Figure 57: Generating Mode Simulation 

The buck-boost converter simulation is used to predict operating waveforms and also size 

components. The converter uses an inductor to store energy for the boost operation. The 

simulation also identifies the proper DC bus capacitor size for load step simulation. The 

boost mode is used at low power to motor the shaft to approximately half speed. The 

simulation is performed at the predicted power and speed level for cold startup operation. 

After the unit is motored to half speed, heat can be applied and the machine will operate 

from a power consumer to a power supplier. 

The buck simulation is performed at full power. The buck simulator takes the rectified 

high voltage and converts it to regulated 120 VDC low-voltage. The specification 

dictates 6% DC bus ripple maximum (120 +/- 6 VDC). The simulation identified the 

minimum bus capacitance to satisfy the specification. No load step specification was 

supplied, however, a larger than necessary capacitor was added to allow 50% load drop 

testing to be performed. Figure 58 shows the boost mode simulation while Figure 59

shows the buck mode simulation. vswa in volts

12

8

NASA/CR—2010-215673 51
3⁄4Boost Mode Simulation 

3⁄412.5 +/- 7.5 Switch Points on Lo IGBT 

3⁄4125 Vdc source on Lo DC bus 

3⁄4135 ohm load on Hi DC bus @ 450 Vdc 

3⁄4I Hi = 3.5 A, I Lo = 12.5 A 

3⁄41.58 kW in, 1.56 kW out, Total loss = 1.27% 

Figure 58: Boost Mode Simulation 

1 i_dc_hi 2 i_dc_lo 11 vdc_hi 12 vdc_lo

1.60m 1.80m 2.00m 2.20m 2.40m

time in seconds

100

200

300

400

500

vdc_hi in volts

-30.0

-20.0

-10.0

0

10.0

i_dc_lo, i_dc_hi in amperes

Plot1

11

1

2

vdc_lo, 

12

NASA/CR—2010-215673 52
3⁄4Buck Mode Simulation 

3⁄4100A +/- 10A Switch Points on Hi IGBT 

3⁄4760 Vdc source on Hi DC bus 

3⁄41.2 ohm load on low DC bus @ 120.3 Vdc 

3⁄4I Hi = 16.6 A, I Lo = 100 A 

3⁄412.55 kW in, 12.03 kW out, Total loss = 4.1%

Figure 59: Buck Mode Simulation 

10. Power Converter 

The power converter consists of the TAC, NaK Heater, Recuperator, and a Gas Cooler. 

Two types of NaK heaters were assembled into a converter. A shell tube NaK heater and 

a plate fin NaK heater. The shell tube NaK heater power converter was larger and 

heavier than the plate fin exchanger. The shell tube NaK heater design was supplied by a 

company with molten salt heat exchanger experience, while the plate fin was design was 

supplied by a company without NaK experience. 

The plate fin NaK exchanger construction is much more compact and lightweight and 

was also identified and proposed during in the 1960’s and 1970’s SNAP reactor program. 

Regardless of the pedigree of the heat exchanger manufacturer, the design of the NaK 

exchanger should use the knowledge base generated from the SNAP reactor program 

relative to materials of construction. The performance and construction risk is regarded 

as less important than the risk of a structural failure and subsequent leak. From this 

standpoint, the fixed tube sheet exchanger with multiple tube welds is more risky than a 

brazed and welded plate-fin heat exchanger. 

1 i_dc_hi 2 i_dc_lo 3 vdc_hi 4 vdc_lo

4.10m 4.30m 4.50m 4.70m 4.90m

time in seconds

-100

0

100

200

300

i_dc_hi, i_dc_lo in amperes

100

300

500

700

900

vdc_hi in volts

Plot1

3

21

vdc_lo, 

4

NASA/CR—2010-215673 53
10.1 Power Converter with Shell/Tube NaK Heater 

The power converter layout with the shell/tube heat exchanger is shown in Figure 60. 

Figure 60: Power Converter with Shell/Tube NaK Exchanger 

The power converter with the shell/tube exchanger has an estimated weight of 780 kg. 

The volume identified in the system layout is approximately 1500mm x 1550mm x 1442 

mm. The layout in three views is shown in Figure 61. 

The power converter stress analysis is done in a manner to simulate the system from a 

cold 200 K startup to operating temperature. The operating temperature differs by 

component. The stress analysis is done with the goal of matching thermal growth by 

matching thermal growth areas. This enables the loop to be welded pipe construction and 

eliminate any bellows. Bellows would complicate the system and have piping pressure 

drop implications reducing performance. The thermal boundary conditions and steady 

state operating thermal condition is shown in Figure 62. The system primary stress 

regions are shown in Figure 63. As the piping undergoes thermal growth, the TAC 

piping interfaces must be analyzed for stress. The results are shown in Figure 64. 

NASA/CR—2010-215673 54
1500mm x 1550mm 

Complete System Layout

Tube Shell Nak Heater

1422mm height

Figure 61: Power Converter for Shell/Tube, Three Views 

NASA/CR—2010-215673 55
Figure 62: Static Thermal Conditions for Shell/Tube Power Converter System 

• Analyzed via steady-state thermal analysis 

(assumed slow thermal ramp from cold initial 

condition) 

• Forced Convection (5887 W/m2C) applied to 

piping (simulates internal flow) 

Convection 

(W/m2

-C) 

Static - Stress in Piping due to Weight & Thermal Growth 

Shell/Tube Heat Exchanger System 

145 MPa 

152 

MPa 

IN625 at 841 K:

Fty = 283 MPa 

• Structural Analysis (Thermal 

Loads + Weight) of 3D Shell 

Model 

– Materials, thicknesses, 

component stiffness’ 

representative of full 3D 

model 

– Max piping load = 152 

MPa 

 (FSy = 1.86) 

• Forces & Moments 

calculated at 

Turbomachinery nozzle 

interfaces 

Figure 63: Shell Tube Stress Regions 

NASA/CR—2010-215673 56
Figure 64: Piping Interface Loads on TAC 

Max Stress: 159 MPa 

Stress 

(Pa) 

Turbine Housing

Turbine 

Inlet 

IN718 at 839 K

• Su = 1089 MPa 

• Sy = 910 MPa 

FS = 5.7 

Compressor 

Inlet 

Compressor Housing

Max Stress: (55 MPa) 

IN625 at 436 K

• Su = 772 MPa 

• Sy = 338 MPa 

FS = 6.1 

10.2 Power Converter with Plate/Fin NaK Heater 

The power converter designed with the plate fin heat exchanger is more compact and 

more importantly has considerably less weight. The overall dimensions are 

approximately 1200mm x 1200mm x 1200mm with an overall weight estimate of 260 kg. 

The isometric solid model of the power converter is shown in Figure 65. A three view of 

the same configuration is shown in Figure 66. 

NASA/CR—2010-215673 57
Recuperator 

Nak Exchanger 

Turbo-Alternator

Compressor 

Gas Cooler 

Figure 65: Power Converter with Plate/Fin NaK Heater 

Complete System Layout

Plate Fin NaK Exchanger

~260 kg estimated weight

Inconel 625 for Hot and Cold Side Piping

Water In/Out

NaK In/Out 

Top View

1200mm tall Side View Back View

1200mm x1200mm top view envelope

Figure 66: Power Converter Three View with Plate/Fin NaK Heat Exchanger 

NASA/CR—2010-215673 58
The thermal model is constructed in the same manner as the shell/tube heat exchanger. 

The boundary conditions are supplied to distribute the temperature in a manner that 

reflects the actual operation, Figure 67. The thermal information is used to find the 

thermal stresses for the power converter, Figure 68. The thermal stresses for the 

converter focused on the TAC result in Inconel 625 being used for the piping to avoid 

bellows. The TAC compressor and turbine also use Inconel to allow high stress safety 

factor, Figure 69. 

Figure 67: Plate/Fin Power Converter Thermal Analysis 

Convection 

(W/m2

-C) 

T (o

C) 

• Analyzed via steady-state thermal analysis 

(assumed slow thermal ramp from cold initial 

condition) 

• Forced Convection (5887 W/m2C) applied to piping 

(simulates internal flow) 

• Assumed perfectly insulated exterior 

= 841 K 

= 386 K 

NASA/CR—2010-215673 59
• Structural Analysis (Thermal 

Loads + Weight) of 3D Shell 

Model 

Figure 68: Plate/Fin Power Converter Stress Results 

Figure 69: Plate/Fin Power Converter TAC Resultant Stress Results 

– Materials, thicknesses, 

component stiffness’ 

representative of full 3D model 

– Max piping load = 103 MPa 

 (FS = 2.7) 

– Max (Compressor Exit) Nozzle 

load = 207 MPa 

• Forces & Moments calculated at 

Turbomachinery nozzle

interfaces 

– Applied to 3D Compressor / 

Turbine Models 

207 MPa 

41 MPa 

103 MPa 

41 

MPa 

IN625 at 841 K: Stress

Fty = 283 MPa (Pa) 

Compressor Housing Turbine Housing

Stress 

(Pa) 

In625 at 436 K 

• Su = 772 MPa 

• Sy = 338 MPa 

Max stress = 172 MPa 

IN718 at 839 K 

• Su = 1089 MPa 

• Sy = 910 MPa

Max stress = 103 MPa 

FSyt = 1.96 FSyt = 8.8 

Stress 

(Pa) 

Max Stress: 103 MPa 

Max Stress: 172 MPa 

NASA/CR—2010-215673 60
The power converter ideally is insulated for the thermal vacuum environment. The 

insulation considered is multi-layer insulation. A very conservative method of 

calculation and the results are outlined in Figure 70. It indicates that 30-50 layers of MLI 

would be sufficient to minimize the radiated heat loss to a low level. The analysis 

considers the entire surface of the hot side of the loop as a black body radiating to the 

environment and does not consider the benefit of the radiation effect from the cold side of 

the equipment. 

Radiation loss from NAK HX

0

2

4

6

8

10

12

14

16

0 10 20 30 40 50

MLI layers

Power Loss (kW)

0

10

20

30

40

50

60

70

80

90

100

Percent of 12 kW electrical output

Heat Loss

(kW)

Percent of

Electric

Output (12

kW)

Assumptions:

1) NAK HX is a blackbody 

2) MLI Layers are blackbodies

3) Each layer of MLI reflects half of

its incident radiation back at the 

NAK HX

4) 200 K Surroundings

Figure 70: Insulation Analysis 

11. Control Console and Data Acquisition, and Fill System 

11.1 Control Console and Data Acquisition 

The data control console operates the power converter and also functions as a data 

acquisition system. The control is set up on a programmable logic controller that is 

FPGA based and manufactured by National Instruments. The data control console is 

programmed using National Instruments LabVIEW software. The control console is 

programmed as a state machine. It has a startup mode, operating mode, shutdown mode, 

and fault handling mode. The control console has a graphical interface as shown in 

Figure 71. All of the pertinent pressure and temperature measurements are acquired, 

displayed and stored. The NaK pump and water pump could also be controlled using this 

as a master system controller. The control console communicates and controls the TAC 

power controller and monitors the health and fault status of all aspects of the power 

converter. Strip chart displays, both fixed and customizable are supplied, Figure 72. 

NASA/CR—2010-215673 61
Figure 71: Control Console Screen Example 

Figure 72: Strip Chart Recording Example 

NASA/CR—2010-215673 62
11.2 Fill System 

The fill system is used to control the gas fill for the Brayton system. The diagram (Figure 

73) shows the major components for this system. A vacuum pump is supplied to evacuate 

the power converter prior to gas fill. The system is filled with a standard CO2 gas bottle. 

It is supplied with a regulator and valves for flexible operation, either manual evacuation 

and fill, or automated through the control console. 

Figure 73: Fill System Diagram and Equipment List 

12. Summary 

A set of prioritized goals were given in the specification. The design as presented is 

reviewed against these published goals. 

• Close relevance to future flight designs, including the potential for 8 year 

service life 

The power converter, as designed, is very similar to other existing power generation 

systems that have been designed by Barber-Nichols Inc. and others. The foil bearing 

NASA/CR—2010-215673 63
TAC type systems have shown to have very high reliability and life in excess of 

80,000 hours. 

• Credible development path and maturation approach exist to achieve TRL 6 

by FY 2012 with a reasonable cost 

• Low development cost and risk 

The technology is mature and can be applied to the unique specification for the power 

converter. The temperatures and power level have led to a very conservative design 

that can be achieved with an easy development path with reasonable cost. No distinct 

technology challenges exist that need to be developed. 

• Design extensible to Mars (materials and design strategies compatible with 

mars environment) 

The equipment and materials used in this equipment are all stainless steel 

construction. The compatibility with the Mars environment should be robust. 

• High thermal-to-electric efficiency 

The power converter system strives to achieve over 18% thermal to electric 

efficiency. This is a high efficiency for a 12 kWe Brayton Cycle power converter 

with the specified high and low temperature range. 

• Minimum complexity 

The power converter complexity is low. Only a single moving part is used. The 

design is derived from highly reliable air-cycle type machinery used in commercial 

aviation. 

• Low mass and volume 

The system at 260 kg is within the boundaries of prior space based power systems in 

terms of kg/kWe. The system was designed to be compact and fit in a cube of 1.2 m 

size 

• Close relevance to future flight designs, including the potential for 8 year 

service life 

The design is similar to other Brayton Cycle power conversion systems that have 

been proposed in the past. The 8 year life has been considered in this design and is 

considered low-risk. 

NASA/CR—2010-215673 64
REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 0704-0188 

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the 

data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this 

burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. 

Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB 

control number. 

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 

1. REPORT DATE (DD-MM-YYYY)

01-06-2010 

2. REPORT TYPE

Final Contractor Report 

3. DATES COVERED (From - To)

4. TITLE AND SUBTITLE

Closed Brayton Cycle Power Conversion Unit for Fission Surface Power Phase I Final Report 

5a. CONTRACT NUMBER

NNC08CA64C 

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Fuller, Robert, L. 

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

WBS 463169.04.03.01.02 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

Barber-Nichols Inc. 

6325 West 55th Avenue 

Arvada, Colorado 80002 

8. PERFORMING ORGANIZATION

 REPORT NUMBER

E-17020 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration 

Washington, DC 20546-0001 

10. SPONSORING/MONITOR'S

 ACRONYM(S)

NASA 

11. SPONSORING/MONITORING

 REPORT NUMBER

NASA/CR-2010-215673 

12. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified-Unlimited 

Subject Category: 20 

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

This publication is available from the NASA Center for AeroSpace Information, 443-757-5802

13. SUPPLEMENTARY NOTES

14. ABSTRACT

A Closed Brayton cycle power conversion system has been developed to support the NASA fission surface power program. The goal is to 

provide electricity from a small nuclear reactor heat source for surface power production for lunar and Mars environments. The selected 

media for a heat source is NaK 78 with water as a cooling source. The closed Brayton cycle power was selected to be 12 kWe output from 

the generator terminals. A heat source NaK temperature of 850 K ± 25 K was selected. The cold source water was selected at 375 K ± 25 K. 

A vacuum radiation environment of 200 K is specified for environmental operation. The major components of the system are the power 

converter, the power controller, and the top level data acquisition and control unit. The power converter with associated sensors resides in 

the vacuum radiation environment. The power controller and data acquisition system reside in an ambient laboratory environment. Signals 

and power are supplied across the pressure boundary electrically with hermetic connectors installed on the vacuum vessel. System level 

analyses were performed on working fluids, cycle design parameters, heater and cooling temperatures, and heat exchanger options that best 

meet the needs of the power converter specification. The goal is to provide a cost effective system that has high thermal-to-electric 

efficiency in a compact, lightweight package. 

15. SUBJECT TERMS

Closed Brayton cycle power conversion; Fission surface power; Power conversion systems 

16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF

 ABSTRACT

UU 

18. NUMBER

 OF 

 PAGES

73 

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email:help@sti.nasa.gov) 

a. REPORT

U 

b. ABSTRACT

U 

c. THIS 

PAGE

U 

19b. TELEPHONE NUMBER (include area code)

443-757-5802 

Standard Form 298 (Rev. 8-98)

Prescribed by ANSI Std. Z39-18