John Siamidis

Analex Corporation, Brook Park, Ohio

Lee S. Mason

Glenn Research Center, Cleveland, Ohio

A Comparison of Coolant Options for Brayton

Power Conversion Heat Rejection Systems

NASA/TM—2006-214121

February 2006
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John Siamidis

Analex Corporation, Brook Park, Ohio

Lee S. Mason

Glenn Research Center, Cleveland, Ohio

A Comparison of Coolant Options for Brayton

Power Conversion Heat Rejection Systems

NASA/TM—2006-214121

February 2006

National Aeronautics and

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NASA/TM—2006-214121 1

A Comparison of Coolant Options for 

Brayton Power Conversion Heat Rejection Systems 

John Siamidis 

Analex Corporation 

Cleveland, Ohio 44135 

Lee Mason 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

This paper describes potential heat rejection design concepts for Brayton power conversion systems. 

Brayton conversion systems are currently under study by NASA for Nuclear Electric Propulsion (NEP) 

and surface power applications. The Brayton Heat Rejection Subsystem (HRS) must dissipate waste heat 

generated by the power conversion system due to inefficiencies in the thermal-to-electric conversion 

process. Sodium potassium (NaK) and H2O are two coolant working fluids that have been investigated 

in the design of a pumped loop and heat pipe space HRS. In general NaK systems are high temperature 

(300 to 1000 K) low pressure systems, and H2O systems are low temperature (300 to 600 K) high 

pressure systems. NaK is an alkali metal with health and safety hazards that require special handling 

procedures. On the other hand, H2O is a common fluid, with no health hazards and no special handling 

procedures. This paper compares NaK and H2O for the HRS pumped loop coolant working fluid. A 

detailed Microsoft® Excel (Microsoft Corporation, Redmond, WA) analytical model, HRS_Opt, was 

developed to evaluate the various HRS design parameters. It is capable of analyzing NaK or H2O coolant, 

parallel or series flow configurations, and numerous combinations of other key parameters (heat pipe 

spacing, diameter and radial flux, radiator facesheet thickness, fluid duct system pressure drop, system 

rejected power, etc.) of the HRS. This paper compares NaK against water for the HRS coolant working 

fluid with respect to the relative mass, performance, design and implementation issues between the two 

fluids. 

Introduction 

Nuclear Electric Propulsion (NEP) is a technology of current interest because it has the potential to 

provide many benefits for deep space science missions including maneuverability to multiple mission 

targets, extended duration science, increased instrument power, and high data rate communications. 

Surface reactors may be used for the moon or Mars to power human outposts enabling extended stays, insitu resource utilization, and closed loop life support. In either case, the reactor power system (reactor, 

power conversion, and heat rejection), is a critical element. Closed Brayton Cycle (CBC) converters are 

one of several promising options for power conversion within a reactor system. The Heat Rejection 

Subsystem (HRS) must dissipate waste heat generated by the Power Conversion Subsystem (PCS) due to 

inefficiencies in the thermal-to-electric conversion process. Brayton systems tend to optimize at 

efficiencies of about 20 to 25 percent with radiator temperatures in the 400 to 600 K range. 

Two previous design studies examined a possible heat rejection concept for a 100 kWe Brayton PCS 

for the proposed Jupiter Icy Moons Orbiter (JIMO) mission (Mason, 2003; Siamidis, 2005). 

In the first design study (Mason, 2003) the PCS included two 100 kWe Brayton converters for a 

100 kWe net output power. The HRS consisted of a pumped sodium-potassium (NaK) heat transport loop 

coupled to a water heat pipe radiator as shown in figure 1. The total radiator area was 170 m2

 configured 

in two separate wings that extended radially from a central truss structure. The radiator panels provided 
NASA/TM—2006-214121 2

Figure 1.—Two Brayton heat rejection subsystem schematic. 

two-sided heat rejection and were arranged in a “stair-case” configuration contained within a 10° halfangle as dictated by the conical reactor shield. Each radiator wing was dedicated to a single Brayton 

converter operating at 50 percent of rated power. Cross-strap NaK piping and a dual passage waste 

heat exchanger, allowed the full radiator (both wings) to serve a single Brayton converter operating at 

100 percent power should a failure occur. The total mass of the HRS including radiator panels, pumps, 

plumbing, and deployment structure was reported as 854 kg, or 5 kg/m2

 (based on total surface area). 

In the second design study (Siamidis, 2005) the PCS included four 50 kWe Brayton converters for a 

100 kWe system. The HRS consisted of a pumped NaK heat transport loop coupled to a water heat pipe 

radiator as shown in figure 2. The radiator area was configured in two separate wings that extended 

radially from a central truss structure. The radiator panels provided two-sided heat rejection and were 

arranged in a “two-fold” configuration contained within a 10° half-angle as dictated by the conical reactor 

shield. Each radiator wing was dedicated to a single Brayton converter operating at 100 percent of rated 

power with the capability to service either or two Brayton units associated with that wing. The paper 

discussed the interplay between heat pipe spacing and heat pipe diameter and their effect on heat pipe 

maximum heat flux, maximum heat pipe power, heat pipe area exposed to micrometeoroid and orbital 

debris (MMOD), system pressure drop and pump power for a fixed geometry radiator. 

This paper uses the previous design concept (Siamidis, 2005) as a starting point for more rigorous 

definition of the Brayton HRS. Specifically, the paper evaluates two heat transport working fluids 

(NaK-55 and H2O) for several system pressure drops and for several radiator inlet outlet temperatures. 

NaK and H2O are two coolant working fluids that have been investigated in the design of a pumped 

loop and heat pipe space HRS. In general NaK systems are high temperature (300 to 1000 K) low 

pressure systems, and H2O systems are low temperature (300 to 600 K) high pressure systems. NaK is 

an alkali metal with health and safety hazards that require special handling procedures. On the other 

hand, H2O is a common fluid, with no health hazards and no special handling procedures. Historically 

the design of pumped loop and heat pipe space HRS used NaK-78 as the coolant fluid. For the present 

JIMO application NaK-55 was chosen over NaK-78 for its lower freeze-temperature (280 K) and higher 

specific heat. 
NASA/TM—2006-214121 3

Figure 2.—Four Brayton heat rejection subsystem schematic. 

The previous design concept (Siamidis, 2005) for JIMO has been updated. The PCS includes four 

100 kWe Brayton converters to produce 200 kWe net output. The HRS consists of a pumped NaK or H2O 

heat transport loop coupled to a water heat pipe radiator as shown in figure 3. The radiator area is 

configured in two separate wings that extend radially from a central truss structure. The radiator panels 

provide two-sided heat rejection and are arranged in a four boom/segment configuration contained within 

a 12° half-angle as dictated by the conical reactor shield. Under nominal operations one pumped-fluid 

loop is used to transport the waste heat from the gas-cooler of each of the two operating PCS units to a 

series of radiator panels containing water heat pipes and white-painted carbon-carbon (C-C) radiator fins. 

A separate, isolated pumped-fluid loop is provided for each of the two non-operating PCS units should a 

PCS-string switchover become necessary through a failure or anomaly condition. The HRS fluid loops of 

one operating and one non-operating PCS unit share the same radiator area for single-fault tolerance 

against failure of either the PCS unit or HRS fluid loop without loss of power generating capabilities. 
NASA/TM—2006-214121 4

Figure 3.—Updated Brayton heat rejection subsystem schematic. 

HRS Design Basis 

The HRS accepts heat from the Brayton power converters and rejects it into space through the 

radiator panels. A NaK-55 or H2O cooling loop connects the Brayton converters to the radiator panels. 

The Brayton gas coolers serve as the thermal interface to the coolant loops. The design was modified 

from four 50 kWe Brayton converters to four 100 kWe Brayton converters, each having its own dedicated 

cooling loop. During nominal operation, only two of the four converters are used to produce the required 

200 kWe. The radiator panels use a construction consisting of regularly-spaced circular heat pipes 

contained within two composite facesheets as shown in figure 4. 
NASA/TM—2006-214121 5

 (a) Radiator details 

(b) Radiator/cooling loop details 

Figure 4.—Radiator detail and heat pipe integration with cooling loop. 

The heat pipes use water as the working fluid and titanium containment. Heat pipes provide an 

efficient means of spreading the heat across the radiator surface with minimal temperature drop. The high 

conductivity composite facesheets serve as the radiator fin. The heat pipes are thermally connected to the 

facesheets through a PocoTM (Poco Graphite, Inc., Decatur, TX) foam saddle extending along the entire 

axial length of the heat pipe. The saddle provides compliance to address fin-heat pipe thermal expansion 

and a degree of micrometeoroid shielding. The heat pipe-to-saddle and saddle-to-facesheet bond is 

accomplished through brazing or high temperature thermal adhesive. One of the key advantages to this 

type of radiator is its ability to withstand damage from micrometeoroid and orbital debris (MMOD). A 

fatal MMOD impact to a single heat pipe, even though it will result in the failure of that heat pipe, would 

have minimal system performance impact. 

The water heat pipes interface to the coolant through curved sections partially contained within the 

cooling loop as shown in figure 5. The heat pipe evaporators are “sandwiched” between two cooling 

loops. One loop is active and the second is the backup. A Poco foam saddle is introduced between the 
NASA/TM—2006-214121 6

Figure 5.—Heat pipe integration with cooling loop. 

heat pipe evaporators and the cooling loop ducts to improve heat transfer. The two HRS fluid loops 

provide fault tolerance against loss of either a PCS unit or HRS fluid loop without affecting power 

generating capabilities. The fluid ducting is made of titanium. The duct cross-section is oval for NaK-55 

and split circular for H2O. Each cooling loop includes a pump and a bellows accumulator. 

Analytical Model 

The design of the Brayton HRS depends on many parameters. An Excel spreadsheet model, called 

HRS-Opt, was developed to evaluate the design trade space described in the previous design study 

(Siamidis, 2005). Parameters were varied to compare the design options on the basis of pump system 

pressure drop and required pump power, heat pipe unit power and radial flux, radiator panel areal mass, 

and overall HRS mass. For the analysis presented in this paper, the HRS-Opt spreadsheet model was 

modified to accommodate the updated HRS design. Additional capabilities, including NaK-55 and H2O 

coolant properties, were also added. 

The fin efficiency is a critical part of this analysis since it varies widely with heat pipe spacing and 

facesheet thickness. A closed-form equation for fin efficiency (Gilmore, 1994) was adopted in HRS-Opt, 

as discussed in the previous design study (Siamidis, 2005). 

The calculation of fluid loop system pressure drop was simplified from that required in the 

previous design (Siamidis, 2005). The previous design used heat pipe evaporators immersed within the 

fluid loop, oriented normal to the fluid flow. A computational fluid dynamics code (CFD AceTM, CFD 

Research Corporation, Huntsville, AL) was necessary to generate pressure drop parameters for use in 

HRS-Opt. The updated concept removes the heat pipes from the flow path, simplifying the flow 

geometry. Thus, a closed form fluid loop pressure drop calculation is possible. A simple equation, 

[∆P = f *(L/d) * r * (u

2

/2)], (Holman, 1981) was used to calculate the system pressure drop in the 

coolant duct. 
NASA/TM—2006-214121 7

Figure 6.—Updated HRS radiator geometry. 

HRS Design Parameters 

The Brayton HRS described in the previous design study (Siamidis, 2005) specified the primary 

thermal design requirements and configuration. These parameters were updated according to the latest 

HRS design. Table 1 includes the major parameter changes from the previous design study to the updated 

HRS design, one-half of which is shown in figure 6. 

Additional design details were needed before a substantive evaluation of the HRS trade space could 

be performed. This included a preliminary definition of construction materials and some of the 

dimensional parameters. For this study, these parameters were held constant, although the model allows 

them to be varied. The heat pipes were assumed to use water with a 10 percent liquid fill fraction. The 

assumed heat pipe containment was titanium (4.5 g/cm3

). The thermal saddles were assumed to be made 

of 0.54 g/cm3

 Poco graphite. The facesheets were carbon-carbon with a density of 1.92 g/cm3 and thermal 

conductivity (normal to the direction of the heat pipes) varying as a function of temperature. The coolant 

duct was titanium with wall-thickness and cross-section given in table 1. The duct supply and return 

length for the H2O (~ 365 m) is greater than that of NaK (~ 200 m) due to its split design as seen in 

figure 5. 

TABLE 1.—DESIGN PARAMETER CHANGES 

Parameter Previous design study 

(Siamidis, 2005) HRS 

Updated HRS 

Radiator heat load (kWt) 364 590 + 5 percent margin 

Radiator inlet temperature (K) 556 507 

Radiator exit temperature (K) 399 387 

Radiator area (m2

) 170 (includes 10% margin) 422 (includes 10% margin) 

Duct wall-thickness (cm) 0.05 cm (NaK-55) 0.075 (NaK-55) and 0.15 (H2O) 

Duct cross section (cm2

) Square (NaK-55) Oval (NaK-55) and Circular (H2O) 

Duct supply and return length (m) ~50 ~200 (NaK-55) and ~365 (H2O) 

HP Tube wall thickness (cm) 0.05 0.07 

Carbon-carbon facesheet in-plane 

thermal conductivity (W/m-K) 600 Function of temperature 

Heat pipe saddle min. thickness (cm) 0.10 0.375 

Pump efficiency 15% (NaK-55) 20% (NaK-55) and 30% (H2O) 
NASA/TM—2006-214121 8

Figure 7.—NaK-55 and H2O fluid property ratios for H2O/NaK-55. 

The primary emphasis of this study is coolant fluid selection and its effect on overall HRS 

performance. The coolant used in the previous design study analysis was NaK-78. This analysis compares 

NaK-55 with H2O. NaK-55 was chosen over NaK-78 for its lower freeze-temperature (280 K) and higher 

specific heat. Higher specific heat reduces fluid mass flow rate and higher liquid density reduces fluid 

volumetric flow-rate, which in turn reduce fluid loop mass and pumping power. A comparison of the 

thermal properties of NaK-55 and H2O are shown in figure 7 where the ratios of H2O/NaK-55 properties 

are plotted versus fluid temperature. Other factors to consider in evaluating the two fluids are listed in 

table 2. 

TABLE 2.—NaK-55 AND H2O DIFFERENCES 

NaK-55 H2O 

Alkali metal, health and safety hazard 

• Requires non-standard handling procedures in crewed 

environments 

• Requires careful cleaning procedures prior to assembly 

and test 

Common material, no health hazards 

• No special handling procedures required 

Moderate specific-heat 

• Efficient single-phase pumped fluid option 

Extremely high specific-heat 

• Very efficient single-phase fluid option 

• Specific-heat approximately four (4) times that of NaK-55 

High thermal conductivity 

• Very small (~1K) fluid to wall temperature difference 

Low thermal conductivity 

• Small (~6K) fluid to wall temperature difference 

Low vapor pressure 

• Allows thinner and lighter fluid loop components 

High vapor pressure 

• Requires thicker and more robust fluid loop components 

High electrical conductivity 

• Eddy current losses reduce mechanical pump efficiency to 

~20% net 

• Well suited for Electromagnetic Magnetic (EM) pumping 

options, pumping efficiency typically <15% net 

Low electrical conductivity 

• Highly efficient (>30%) mechanical-pumps 

• Electromagnetic Magnetic (EM) pumping not an option 

Other design variables considered in this study were heat pipe spacing and system pressure drop. The 

range of parameters considered is provided in table 3. Given the seven different spacing options and three 

system pressure drops for each fluid, a total of forty-two individual design cases were examined. The 
NASA/TM—2006-214121 9

primary output parameter of interest was the total HRS mass. The total HRS mass includes the radiator 

panel mass, and the fluid loop mass with ducts, pumps, accumulators, and other miscellaneous 

components such as flexible fluid joints. 

TABLE 3.—DESIGN VARIABLES USED IN STUDY 

Parameter Value Basis

Coolant fluid Nak-55, H2O Input 

Heat pipe spacing (cm) 7, 8, 9, 10, 11, 12, 13 Input 

Heat pipe inner diameter (cm) 1.25 Input 

Fluid duct system pressure drop (kPa) 100, 200, 300 Input 

Pump efficiency (%) 20% NaK, 30% H2O Input 

Radiator area (m2) 422 (Includes 10% margin) Input 

Facesheet thickness (mm) Varied Input 

Duct size (NaK: W x H, H2O: Dia) (cm x cm, cm) Varied Input 

HRS mass (kg) Calculated 

Total pump power (W) Calculated 

Once the forty-two individual design cases were analyzed, the minimum mass design point was used 

to further investigate the effect of different radiator inlet and outlet temperatures on the HRS mass for a 

fixed system pressure drop of 200 kPa. Table 4 provides the additional cases analyzed. 

TABLE 4.—ADDITIONAL DESIGN VARIABLES USED IN STUDY 

Parameter Design Point Design Point –25 K Design Point +25 K

HRS coolant inlet temperature (K) 507 482 532 

HRS coolant outlet temperature (K) 387 362 412 

HRS coolant DT (K) 120 120 120 

Analytical Results 

The process used to analyze each configuration was as follows. A heat pipe spacing and heat pipe 

inner diameter was selected. An iterative process of varying three parameters then followed. The 

facesheet thickness was adjusted until the radiator coolant exit temperature reached the required value of 

387 K and the radiator heat load matched the required value of 590 kWt. At the same time, the duct size 

and heat pipe evaporator length were varied to achieve the assigned system pressure drop (100, 200, and 

300 kPa) and the 10 W/cm2

 evaporator radial flux limit respectively. 

A sample case is reported in table 5 for 10 cm heat pipe spacing and a 1.25 cm heat pipe inner 

diameter. The resulting facesheet thickness required to achieve the 387 K radiator exit temperature was 

0.25 mm. The heat pipe geometry and facesheet thickness resulted in a fin efficiency of 79 percent. The 

required pump system pressure drop was set at 200 kPa which resulted in a pump power of 478 W for 

each pump. The maximum heat pipe power and radial flux was 456 W and 10 W/cm2

, respectively. The 

total HRS mass was 1511 kg or 7.16 kg/m2

 (based on total surface area), and the radiator panel areal mass 

was 3.27 kg/m2

 (based on total surface area). 
NASA/TM—2006-214121 10

TABLE 5.—SAMPLE RESULTS CASE 

Parameter Value Basis 

Coolant fluid H2O Input 

Coolant inlet temperature (K) 507 Input 

Coolant outlet temperature (K) 387 Input 

Heat pipe spacing (cm) 10 Input 

Heat pipe inner diameter (cm) 1.25 Input 

Facesheet thickness (mm) 0.25 Input 

Pump system pressure drop (kPa) 200 Input 

Fin efficiency (%) 79% Calculated 

Total pump power (W) 478 Calculated 

MAX heat pipe power (W) 456 Calculated 

MAX heat pipe radial flux (W/cm2

) 10.0 Calculated 

Total radiator panel mass (kg) 689.7 Calculated 

Total heat transport mass (with pumps and accumulators) (kg) 821.3 Calculated 

Total HRS mass (kg) 1511.00 Calculated 

Radiator panel areal mass—single sided (kg/m2

) 3.27 Calculated 

Total HRS areal mass—single sided (kg/m2

) 7.16 Calculated 

Based on the results of this paper, there is substantial mass savings for a H2O system versus a NaK-55 

system for the given radiator temperatures. The mass savings is a function of several parameters including 

the system pressure drop and the radiator inlet temperature. The mass savings lessens with increases in 

system pressure drop or radiator inlet temperature. 

Figure 8 shows the variance of the HRS radiator, fluid loop and total mass as a function of heat pipe 

spacing for a heat pipe inner diameter of 1.25 cm and pump system pressure drop of 200 kPa for both 

NaK-55 and H2O coolants. Results show that the minimum mass HRS occurs at a heat pipe spacing of 

about 10 cm for both coolants. Results also show that an HRS with H2O weights about 230 kg less than 

an HRS with NaK-55 at the minimum mass design point for the given parameters. Similar trends were 

predicted for pump system pressure drops of 100 and 300 kPa. 

Figure 9 shows the variation of the total HRS mass, fluid loop mass, and radiator mass as a function 

of pump system pressure drop for a heat pipe spacing of 10 cm and heat pipe inner diameter of 1.25 cm. 

Pressure drop was varied by changing the duct cross-section. Overall duct length and mass flow rate were 

held constant. A smaller duct size increases the pressure drop, but reduces the duct mass, fluid inventory 

and fluid mass. Increasing the system pressure drop from 100 to 300 kPa, results in a weight saving of 

approximately 200 kg for the NaK-55 system and 150 kg for the H2O system. 

A HRS with a H2O fluid loop weighs less than its NaK-55 counterpart, and this weight saving benefit 

is slightly affected by pump system pressure drop. The weight benefit realized through H2O is due 

primarily to differences within the fluid loops and only marginally affected by differences in the radiator 

itself. While a H2O system requires heavier ducts to withstand the higher pressures as compared to NaK55, an overall weight saving is seen due to the smaller duct size and lower fluid inventory. The weight 

benefit of an H2O-based HRS is reduced somewhat as the pump system pressure drop increases. As the 

system pressure drop increases from 100 to 200 to 300 kPa, the mass savings of the H2O system decreases 

from 265 to 227 to 210 kg, respectively. 

The second part of the analysis evaluated performance sensitivities with variable inlet and exit 

temperatures. For each case, the heat pipe spacing and the heat pipe inner diameter were given fixed 

values. Then, the facesheet thickness was varied until the radiator coolant exit temperature reached the 

desired value and the radiator heat load matched the design value of 590 kWt. 
NASA/TM—2006-214121 11

Figure 8.—HRS mass versus heat pipe spacing—200 kPa. 

Figure 9.—HRS mass versus system pressure drop. 
NASA/TM—2006-214121 12

Figure 10 shows the variation of the HRS radiator, fluid loop, and total mass as a function of radiator 

inlet temperature for a heat pipe spacing of 10 cm, heat pipe inner diameter of 1.25 cm, and system 

pressure drop of 200 kPa with both NaK-55 and H2O coolants. There is a significant mass decrease with 

increasing temperature due to the reduction in radiator surface area. The reduced area permitted decreases 

in the duct supply and return lengths, allowing the duct cross-section to be reduced commensurately given 

the assumption of fixed pressure drop. The higher temperatures did cause an increase in duct wall 

thickness as required for the elevated operating pressures. As the radiator inlet temperature increases from 

482 to 507 to 532 K, the mass savings of the H2O system decreases from 388 to 227 to 38 kg, 

respectively. The mass advantage for water is less pronounced at higher temperature since the duct wall 

increases are exacerbated by the much higher operating pressures. 

Figure 11 shows the variance of the HRS radiator area as a function of radiator inlet temperature for 

a heat pipe spacing of 10 cm, heat pipe inner diameter of 1.25 cm and pump system pressure drop of 

200 kPa for both NaK-55 and H2O coolants. Results show that the HRS area (same for both NaK-55 

and H2O coolants) decreases as the radiator inlet temperature increases. An HRS with a radiator inlet 

temperature of 530 K has 35 percent less area than an HRS with a radiator inlet temperature of 480 K. 

Figure 10.—HRS mass versus radiator inlet temperature. 
NASA/TM—2006-214121 13

Figure 11.—HRS area versus radiator inlet temperature. 

Conclusions and Recommendations 

Earlier HRS design trades were conducted addressing heat transport approaches, material and fluid 

options, and deployed radiator geometries. This paper discusses the interplay between radiator coolants 

(NaK-55 and H2O) for various heat pipe spacings and pump system pressure drops for a fixed geometry 

radiator. It also discussed the interplay between radiator coolants (NaK-55 and H2O) for various radiator 

coolant inlet temperatures for a fixed heat pipe spacing and fixed pump pressure drop system. 

Based on the results of this paper, there is substantial mass savings for a H2O system over a NaK-55 

system for the given radiator temperatures. This mass savings is a function of the system pressure drop 

and the radiator inlet temperature. The mass savings for the H2O system decreases as the system pump 

pressure drop increases and as the radiator inlet temperature increases. 

Additional trade studies are needed to further refine the HRS design and make the choice between 

NaK-55 and H2O final. Other considerations must be taken into account in addition to the mass savings. 

These should include, but not be limited to: 

• Health and safety issues (toxic NaK vs. for non-toxic/non-hazardous water). 

• Technology development (NaK fluid loop requires extensive technology development). 

• Compatibility issues (NaK may have long-term) compatibility issues with fluid loop materials. 

• System Packaging (NaK fluid loop requires larger fluid and flex-hose diameters, complicates 

mechanical packaging, water fluid loop requires smaller fluid and flex-hose diameters, simplifies 

mechanical packaging). 

• Structural design and Integrity (low pressure NaK fluid vs. high pressure H2O system). 
NASA/TM—2006-214121 14

References 

Gilmore, D.G., Satellite Thermal Control Handbook, The Aerospace Corporation Press, El Segundo, 

California, 1994, pp. 133–134. 

J.P. Holman, Heat Transfer, 5th Edition, McGraw-Hill, 1981, pp. 210–211. 

Mason, L.S., “A Power Conversion Concept for the Jupiter Icy Moons Orbiter,” AIAA–2003–6007, 

2003. 

Siamidis, J., “Heat Rejection Concepts for Brayton Power Conversion Systems,” AIAA–2004–5654, 

2005. 
NASA/TM—2006-214121 15

Appendix—Nomenclature 

T Temperature (K) 

ΔP Pressure drop (kPa) 

f Friction factor 

L Length (m) 

d Diameter (m) 

r Liquid density (kg/m3

) 

u Velocity (m/s) 
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13. ABSTRACT (Maximum 200 words)

14. SUBJECT TERMS

17. SECURITY CLASSIFICATION

 OF REPORT

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Unclassified Unclassified

Technical Memorandum

Unclassified

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135–3191

1. AGENCY USE ONLY (Leave blank)

10. SPONSORING/MONITORING

 AGENCY REPORT NUMBER

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National Aeronautics and Space Administration

Washington, DC 20546–0001

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

February 2006

NASA TM—2006-214121

E–15456

WBS–22–982–10–30

21

A Comparison of Coolant Options for Brayton Power Conversion Heat

Rejection Systems

John Siamidis and Lee S. Mason

Brayton power conversion system; Brayton heat rejection system; Heat pipe radiator

Unclassified -Unlimited

Subject Category: 20

Prepared for the Space Technology and Applications International Forum (STAIF–2006) sponsored by the University

of New Mexico’s Institute for Space and Nuclear Power Studies (UNM-ISNPS), Albuquerque, New Mexico, February

12–16, 2006. John Siamidis, Analex Corporation, 1100 Apollo Drive, Brook Park, Ohio 44142; and Lee S. Mason,

NASA Glenn Research Center. Responsible person, John Siamidis, organization code RPT, 216–433–3151.

This paper describes potential heat rejection design concepts for Brayton power conversion systems. Brayton conversion systems are

currently under study by NASA for Nuclear Electric Propulsion (NEP) and surface power applications. The Brayton Heat Rejection

Subsystem (HRS) must dissipate waste heat generated by the power conversion system due to inefficiencies in the thermal-to-electric

conversion process. Sodium potassium (NaK) and H2O are two coolant working fluids that have been investigated in the design of a

pumped loop and heat pipe space HRS. In general NaK systems are high temperature (300 to 1000 K) low pressure systems, and H2O

systems are low temperature (300 to 600 K) high pressure systems. NaK is an alkali metal with health and safety hazards that require

special handling procedures. On the other hand, H2O is a common fluid, with no health hazards and no special handling procedures.

This paper compares NaK and H20 for the HRS pumped loop coolant working fluid. A detailed Microsoft® Excel (Microsoft

Corporation, Redmond, WA) analytical model, HRS_Opt, was developed to evaluate the various HRS design parameters. It is capable

of analyzing NaK or H2O coolant, parallel or series flow configurations, and numerous combinations of other key parameters (heat

pipe spacing, diameter and radial flux, radiator facesheet thickness, fluid duct system pressure drop, system rejected power, etc.) of the

HRS. This paper compares NaK against water for the HRS coolant working fluid with respect to the relative mass, performance,

design and implementation issues between the two fluids.