John Siamidis

Analex Corporation, Brook Park, Ohio

Heat Rejection Concepts for Lunar Fission

Surface Power Applications

NASA/CR—2006-214388

November 2006

AIAA–2006–4196
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John Siamidis

Analex Corporation, Brook Park, Ohio

Heat Rejection Concepts for Lunar Fission

Surface Power Applications

NASA/CR—2006-214388

November 2006

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

AIAA–2006–4196

Prepared under Contract NAS3–00145

Prepared for the

Fourth International Energy Conversion Engineering Conference and Exhibit (IECEC)

sponsored by the American Institute of Aeronautics and Astronautics

San Diego, California, June 26–29, 2006
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NASA/CR—2006-214388 1

Heat Rejection Concepts for Lunar Fission 

Surface Power Applications 

John Siamidis

Analex Corporation 

Brook Park, Ohio 44142 

Abstract 

This paper describes potential heat rejection design concepts for lunar surface Brayton power conversion 

systems. Brayton conversion systems are currently under study by NASA for surface power applications. Surface 

reactors may be used for the moon to power human outposts enabling extended stays and closed loop life support. 

The Brayton Heat Rejection System (HRS) must dissipate waste heat generated by the power conversion system due 

to inefficiencies in the thermal-to-electric conversion process. Space Brayton conversion system designs tend to 

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

HRS was developed for a 100 kWe-class Brayton power system that uses a pumped water heat transport loop 

coupled to a water heat pipe radiator. The radiator panels employ a tube and fin construction consisting of regularlyspaced circular heat pipes contained within two composite facesheets. The water heat pipes interface to the coolant 

through curved sections partially contained within the cooling loop. The paper evaluates various design parameters 

including radiator panel orientation, coolant flow path, and facesheet thickness. Parameters were varied to compare 

design options on the basis of H2O pump pressure rise and required power, heat pipe unit power and radial flux, 

radiator area, radiator panel areal mass, and overall HRS mass. 

Nomenclature 

T Temperature (K) 

∆P Pressure Drop (kPa) 

f Friction Factor 

L Length (m) 

d Diameter (m) 

u Velocity (m/s) 

ρ Liquid Density (kg/m3

) 

I. Introduction 

Fission Surface Power Systems (FSPS) are of current interest because surface reactors may be used on the moon, 

(see fig. 1). Potential applications include providing power for human outposts to enable extended stays, as well as 

powering in-situ resource utilization and closed loop life support systems. The reactor power system consists of the 

reactor, power conversion, heat rejection, and power management and distribution subsystems. Closed Brayton 

Cycle (CBC) converters are one of several promising options for power conversion within a reactor power system. 

The Heat Rejection System (HRS) must dissipate waste heat generated by the Power Conversion System (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 K to 600 K. 

Previous design studies examined a possible heat rejection concept for a 100 kWe Brayton PCS for the proposed 

Jupiter Icy Moons Orbiter (JIMO) mission (ref. 1). This paper uses the previous design concept as a starting point 

for the definition of the FSPS Brayton HRS. The HRS consists of a pumped water heat transport loop coupled to a 

water heat pipe radiator. The radiator area is distributed across two planar panels. The radiator panels provide singlesided heat rejection for the horizontal configuration and two-sided heat rejection for the vertical configuration as 

shown in figure 2. 

The PCS includes two Brayton converters to produce the total net output power. Two Braytons could be 

operated at 50 percent or one Brayton operated at 100 percent with the second serving as a redundant spare. The 

Braytons reject heat through a gas cooler to the HRS radiators. The HRS accepts heat from the Brayton power 

converters and rejects it into space through the radiator panels. A water cooling loop connects the Brayton 

converters to the radiator panels. 
NASA/CR—2006-214388 2
NASA/CR—2006-214388 3

II. HRS Design Basis 

Two radiator panel designs were considered in the trade study, as shown in figure 3. In both designs the radiator 

panels use a construction consisting of regularly-spaced circular heat pipes contained within two composite 

facesheets. In the first design, the heat pipes are thermally connected to the facesheets through a square saddle 

extending along the entire axial length of the heat pipe, and a low density filler material is used between the 

facesheets for panel structural integrity. In the second design the heat pipes are thermally connected to the facesheets 

through a cylindrical saddle extending along the entire axial length of the heat pipe and there is no filler material 

between the facesheets. The second design was assumed for this trade study. 

The heat pipes use water as the working fluid in a titanium envelope. 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 radiator has a central manifold that separates the panel into two equal halves, as shown in figure 4. This 

allows the radiator to utilize more conventional and reasonable length heat pipes. For this paper a heat pipe length of 

two meters was assumed, which resulted in a radiator fixed width of just over four meters. The length of the radiator 

was subsequently varied to yield the necessary total area and accommodate the various heat loads. 
NASA/CR—2006-214388 4

Figure 5 shows how the heat pipe radiator is integrated with the pumped water cooling loop. The POCO (POCO 

Graphite, Inc.) foam saddle provides compliance to permit 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 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. 
NASA/CR—2006-214388 5

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

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 provided between the 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 split 

circular as shown in figure 5. The split circular ducts provide a larger contact interface area with the heat pipe 

evaporators as opposed to a single circular duct. Each cooling loop includes a pump and a bellows accumulator. 

III. Analytical Model 

The design of the Brayton HRS depends on many parameters. An Microsoft Excel (Microsoft Corporation, Inc.) 

spreadsheet model, called HRS-Opt, was developed to evaluate the design trade space described in the previous 

design study (ref. 1). For the analysis presented in this paper, the HRS-Opt spreadsheet model was modified to 

accommodate the FSPS HRS design. 

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 was available in the literature (ref. 2) and was adopted in HRSOpt, as discussed in the previous design study. In order to verify the accuracy of this equation and its applicability to 

the panel configuration, a detailed Finite-Difference model of a single heat pipe and half radiator fin was generated 

using SINDA FLUINT. The heat pipe spacing and fin thickness were varied and the results of the finite-difference 

model were compared to HRS-Opt. There was very good agreement between the HRS-Opt based fin efficiency 

calculations and the SINDA FLUINT model results (ref. 1). 

The calculation of fluid loop system pressure drop is based on the simple flow geometry and a closed form fluid 

loop pressure drop calculation is possible. A simple equation, [∆P = f *(L/d) * ρ * (u2/2)] was used to calculate the 

system pressure drop in the coolant duct (ref. 3). 

For the lunar application, the calculation of the radiator equivalent sink temperatures was necessary. A location 

on the equator of the moon was chosen for a few reasons. First, the sun would be parallel to the radiating surfaces 

and secondly, it’s a more desirable location to abort a mission from. It is also the worst case scenario for the sink 

temperatures. 

Two methods of calculating the radiator equivalent sink temperatures were considered. First, an equation for 

equivalent sink temperatures was available in the literature (ref. 4) and was used to predict radiator equivalent sink 

temperatures for Z–93 coating with 0.90 emittance, 0.20 absorptivity, and with 1.365 kW/m2

 solar insolation. 

Figure 6 shows the variation in lunar surface temperature and equivalent sink temperatures for both vertical and 

horizontal radiators based on the derived equation in the literature (ref. 4). The maximum sink temperature is 318 K 

and 270 K respectively for the vertical and horizontal configurations. 
NASA/CR—2006-214388 6

Second, in order to verify the accuracy of the equation, a detailed model of the radiator was generated using 

Thermal Desktop (Cullimore and Ring Technologies, Inc.). The model predicted radiator equivalent sink 

temperatures for the same assumptions and for a variety of radiator heights from the lunar surface. Sink temperature 

may be reduced by placing Multi-Layer Insulation (MLI) directly on the lunar surface beneath the radiator. The 

effect of MLI was analyzed for the vertical configuration. Figure 7 shows the MLI dimensions and location on the 

lunar surface with respect to the radiator. For the horizontal configuration, MLI has no effect on the sink 

temperatures due to the radiator orientation and view to space. 

Figure 8 shows the results of the Thermal Desktop model. The maximum sink temperature for the vertical 

configuration varies as a function of radiator height from lunar surface from 318 K to 221 K for heights of 0.7 to 

6 m respectively. The maximum sink temperature for the horizontal configuration is 270 K and is independent of 

radiator height from the lunar surface due to its orientation and view to space. 

There was very good agreement between the equation-based calculations and the Thermal Desktop calculations. 

Thermal Desktop sink temperature results for the vertical radiator without MLI and for the horizontal radiator were 

within 1 percent of the calculations from the literature equation. 
NASA/CR—2006-214388 7

It is notable, that for the case of vertical radiator orientation with MLI on the lunar surface, the equivalent sink 

temperatures for the upper and lower sections of the radiator are different and vary as a function of radiator height 

from lunar surface. This difference becomes smaller as the radiator is moved further from the lunar surface. For 

vertical radiator orientation where the radiator will be relatively close to the lunar surface, and MLI is used, the 

direction of the coolant flow path within the radiator (upper to lower section, lower to upper section) will effect the 

radiator performance and therefore the HRS mass. 

IV. HRS Design Parameters 

The Brayton HRS described in the previous design study (ref. 1) provided a starting point for the primary 

thermal design requirements and overall configuration. These parameters were updated as needed to accommodate 

the FSPS HRS design. The major design variables were radiator orientation (horizontal, vertical (with and without 

MLI)), coolant flow (series, parallel), radiator area, facesheet thickness, and duct diameter. 

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 (C-C) with a density of 1.92 g/cm3

 and thermal conductivity (normal to the 

direction of the heat pipes) of 239 W/m-K at 500 K and which varies as a function of temperature. The coolant duct 

was titanium with wall-thickness of 0.15 cm. 

Other design variables considered in this study were radiator heat load, and radiator inlet and outlet 

temperatures. For the vertical radiator configuration, MLI was introduced as a variable. In addition, because the 

upper and lower radiator sections have different equivalent sink temperatures for the MLI case, the direction of the 

coolant flow (from upper to lower section and from lower to upper section) was also introduced as a variable for this 

case. 

The design parameters considered and their values or range of values is provided in table 1. For the vertical 

radiator configuration, the three different flow configurations (parallel, series - upper to lower, series—lower to 

upper) are presented graphically in figure 9. The 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 1.—DESIGN PARAMETERS AND VARIABLES USED IN STUDY 

Parameter Value/Range 

Pump efficiency (%) 30 

Fluid duct pressure drop (kPa) 100 

Duct wall-thickness (cm) 0.15 

Heat pipe spacing (cm) 10 

Heat pipe length (cm) 200 

Heat pipe inner diameter (cm) 1.86 

HP tube wall thickness (cm) 0.07 

Heat pipe saddle min. thickness (cm) 0.375 

Evaporator radial flux limit (W/cm2) 10 

Carbon-carbon in-plane thermal conductivity (W/m-K) Function of temperature 

Radiator orientation Variable 

Coolant flow Variable 

Radiator area (m2) Variable (included 10 percent margin) 

Facesheet thickness (cm) Variable 

Duct diameter (cm) Variable 

Radiator heat load (kWt) 81 (25 kWe), 155 (50 kWe), 297 (100 kWe) 

Radiator inlet temperature (K) 507 (25 kWe), 510 (50 kWe), 512 (100 kWe) 

Radiator exit temperature (K) 386 

Radiator orientation Horizontal, Vertical (with and without MLI) 

Coolant flow Series, Parallel 

Coolant flow direction Upper to lower section, Lower to upper section 
NASA/CR—2006-214388 8
NASA/CR—2006-214388 9

V. Analytical Results 

The process used to analyze each configuration was as follows. For a given power level (25, 50, 100 kWe) the 

corresponding radiator heat load (81, 155, 297 kWt) and radiator inlet and outlet temperatures (507/386, 510/386, 

512/386 K) were chosen. For each of these cases, 3 radiator configurations (horizontal, vertical with MLI, vertical 

without MLI) were analyzed. For the horizontal, and the vertical without MLI configurations, 2 coolant flow paths 

(parallel, series) were analyzed. For the vertical with MLI configuration, 3 coolant flow paths (parallel, series upper 

to lower section, series lower to upper section) were analyzed. Seven cases were analyzed for each power level, as 

shown in table 2. 

TABLE 2.—CASES ANALYZED FOR EACH CONVERTER OUTPUT 

Radiator orientation Horizontal Horizontal Vertical Vertical Vertical Vertical Vertical 

MLI on moon surface NO NO NO NO YES YES YES 

Coolant flow path Parallel Series Parallel Series Parallel Series Series 

Coolant flow path for series, Upper to 

Lower (UL) or Lower to Upper (LT) NA NA NA NA NA UL LU 

The radiator area and facesheet thickness were adjusted until the radiator coolant exit temperature reached the 

required value and the radiator heat load matched the required value. At the same time, the duct size and heat pipe 

evaporator length were varied to achieve the assigned system pressure drop (100 kPa) and the 10 W/cm2

 evaporator 

radial flux limit respectively. 

A sample case for a vertical radiator, with series coolant flow path from upper to lower panel, with MLI on lunar 

surface is reported in table 3 for 10 cm heat pipe spacing and a 1.86 cm heat pipe inner diameter. The resulting 

facesheet thickness required to achieve the 386 K radiator exit temperature was 0.21 mm. The heat pipe geometry 

and facesheet thickness resulted in a fin efficiency of 76.2 percent. The required pump system pressure drop was set 

at 100 kPa which resulted in a pump power of 110 W for each pump. The maximum heat pipe power and radial flux 

was 733 W and 10 W/cm2

, respectively. The total HRS mass was 829.1 kg or 5.4 kg/m2 (based on total surface 

area), and the radiator panel areal mass was 3.8 kg/m2

 (based on total surface area). 

TABLE 3.—SAMPLE RESULTS CASE 

Parameter Value Basis 

Electrical power (kWe) 100 Input 

Thermal power (kWt) 297 Input 

Coolant inlet temp. (K) 512 Input 

Coolant outlet temp. (K) 386 Input 

Heat pipe spacing (cm) 10 Input 

Heat pipe inner diameter (cm) 1.86 Input 

Facesheet thickness (mm) 0.21 Input 

Pump system pressure drop (kPa) 100 Input 

Fin efficiency (%) 76.2 percent Calculated 

Total pump power (watts) 110 Calculated 

MAX heat pipe power (watts) 733 Calculated 

MAX heat pipe radial flux (W/cm2) 10 Calculated 

Total radiator panel mass (kg) 575.9 Calculated 

Total heat transport mass (with pumps and accumulators) (kg) 236.9 Calculated 

Total HRS mass (kg) 829.1 Calculated 

Radiator panel areal mass—single sided (kg/m2) 3.8 Calculated 

Total HRS areal mass—single sided (kg/m2) 5.4 Calculated 

Figure 10 shows the variance of the HRS total mass as a function of power level for the different radiator 

configurations with a heat pipe spacing of 10 cm, heat pipe inner diameter of 1.86 cm and pump system pressure 

drop of 100 kPa. Results show that the minimum mass HRS occurs for a vertical radiator with MLI on the lunar 

surface with coolant flow in series from the upper radiator panel to the lower panel. The benefit of upper to lower 

panel coolant flow path is more pronounced at higher power levels. For the vertical configuration, adding MLI on 

the lunar surface reduces the HRS total mass by as much as 33 percent. 
NASA/CR—2006-214388 10

Figure 11 shows the HRS average radiator temperature as a function of power level for two different coolant 

flow paths. This is for a vertical radiator configuration with MLI on the lunar surface with heat pipe spacing of 

10 cm, heat pipe inner diameter of 1.86 cm and pump system pressure drop of 100 kPa. Results show that the 

radiator with coolant flow from the upper to the lower panel is warmer by about 5 K due to its corresponding 

warmer sink temperature as shown earlier (fig. 8). A higher temperature radiator is more efficient, and as shown in 

figure 10, results in 4 to 5 percent mass savings; 14 kg for the 25 kWe power level, 21 kg for the 50 kWe power 

level, and 35 kg mass for the 100 kWe power level, for this configuration and flow path. 

Figures 12 and 13 show the variance of the HRS radiator physical and radiative area respectively, as a function 

of power level for a heat pipe spacing of 10 cm, heat pipe inner diameter of 1.86 cm and pump system pressure drop 

of 100 kPa. The horizontal radiator has a physical area equal to its radiative area, while the vertical radiator has a 

radiative area equal to twice its physical area. This is because the horizontal radiator radiates only from its upward 

facing surface, while the vertical radiator radiates from both sides. Figure 12 shows the variance of the HRS 
NASA/CR—2006-214388 11

physical area as a function of power level, and figure 13 shows the variance of the HRS radiative area as a function 

of power level. Results show that the HRS radiator physical area is greatest for the horizontal orientation and least 

for the vertical with MLI on the lunar surface. Adding MLI on the lunar surface reduces the vertical radiator’s area 

by about 35 percent. When radiative areas are compared, it is the vertical without MLI configuration that requires 

the largest area (in combination with its corresponding sink temperature and view factors) to reject the required 

amount of heat as shown in figure 13. Horizontal and vertical with MLI configurations require about the same 

radiative area. 

Figure 14 shows the variance of the HRS radiator areal mass as a function of power level for a heat pipe spacing 

of 10 cm, heat pipe inner diameter of 1.86 cm and pump system pressure drop of 100 kPa. Results show that the 

horizontal configuration has a much higher areal mass than either one of the vertical configurations. Areal masses 

for the vertical configurations are about the same. 
NASA/CR—2006-214388 12

VI. Conclusions 

The interplay between radiator orientations, coolant flow paths, and the addition of MLI to the lunar surface was 

examined for various power levels of a Fission Surface Power System on the moon. Based on the results of this 

paper, there is substantial area and mass savings for a FSPS HRS in a vertical orientation utilizing MLI on the lunar 

surface. For the vertical orientation, adding MLI on the lunar surface reduces the radiator area by about 35 percent 

and the HRS total mass by about 33 percent as compared to a horizontal radiator configuration. A coolant flow path 

configuration from the upper radiator panel to the lower panel results in an additional 4 to 5 percent mass savings. 

Additional trade studies are needed to further refine the FSPS HRS design. 

References 

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

NASA/TM—2005-2133372005. 

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

1994, pp. 133. 

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

4. Dallas, T., Diaguila, A., Saltman, J., “Design Studies on the Effects of Orientation, Lunation, and Location on 

the Performance of Lunar Radiators,” NASA TM X–1846. 
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

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E–15666

WBS 463169.04.03

NAS3–00145

18

Heat Rejection Concepts for Lunar Fission Surface Power Applications

John Siamidis

Heat rejection; Radiator; Brayton power; Conversion; Heat pipes

Unclassified -Unlimited

Subject Category: 20

Analex Corporation

1100 Apollo Drive

Brook Park, Ohio 44142

Prepared for the Fourth International Energy Conversion Engineering Conference and Exhibit (IECEC) sponsored by the

American Institute of Aeronautics and Astronautics, San Diego, California, June 26–29, 2006. Responsible person,

John Siamidis, organization code RPT, 216–433–3151.

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

conversion systems are currently under study by NASA for surface power applications. Surface reactors may be used for

the moon to power human outposts enabling extended stays and closed loop life support. The Brayton Heat Rejection

System (HRS) must dissipate waste heat generated by the power conversion system due to inefficiencies in the thermal-toelectric conversion process. Space Brayton conversion system designs tend to optimize at efficiencies of about 20 to

25 percent with radiator temperatures in the 400 K to 600 K range. A notional HRS was developed for a 100 kWe-class

Brayton power system that uses a pumped water heat transport loop coupled to a water heat pipe radiator. The radiator

panels employ a tube and fin construction consisting of regularly-spaced circular heat pipes contained within two

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

cooling loop. The paper evaluates various design parameters including radiator panel orientation, coolant flow path, and

facesheet thickness. Parameters were varied to compare design options on the basis of H2

O pump pressure rise and

required power, heat pipe unit power and radial flux, radiator area, radiator panel areal mass, and overall HRS mass.