Paul C. Schmitz

Power Computing Solutions, Inc., Avon, Ohio

L. Barry Penswick

Sest, Inc., Middleburg Heights, Ohio

Richard K. Shaltens

Glenn Research Center, Cleveland, Ohio

A Design of a Modular GPHS-Stirling Power

System for a Lunar Habitation Module

NASA/TM—2005-213991

November 2005

AIAA–2005–5716
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Paul C. Schmitz

Power Computing Solutions, Inc., Avon, Ohio

L. Barry Penswick

Sest, Inc., Middleburg Heights, Ohio

Richard K. Shaltens

Glenn Research Center, Cleveland, Ohio

A Design of a Modular GPHS-Stirling Power

System for a Lunar Habitation Module

NASA/TM—2005-213991

November 2005

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

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AIAA–2005–5716
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Acknowledgments

The work described in this paper was performed for the Exploration Systems Mission Directorate (ESMD) and the

Radioisotope Power System (RPS) Program, which provided funding for these projects.
NASA/TM—2005-213991 1 

A Design of a Modular GPHS-Stirling Power System 

for a Lunar Habitation Module 

Paul C. Schmitz 

Power Computing Solutions, Inc. 

Avon, Ohio 44011 

L. Barry Penswick 

Sest, Inc. 

Middleburg Heights, Ohio 44130 

Richard K. Shaltens 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Lunar habitation modules need electricity and potentially heat to operate. Because of the low amounts of radiation 

emitted by General Purpose Heat Source (GPHS) modules, power plants incorporating these as heat sources could 

be placed in close proximity to habitation modules. A design concept is discussed for a high efficiency power plant 

based on a GPHS assembly integrated with a Stirling convertor. This system could provide both electrical power 

and heat, if required, for a lunar habitation module. The conceptual GPHS/Stirling system is modular in nature and 

made up of a basic 5.5 KWe Stirling convertor/GPHS module assembly, convertor controller/PMAD electronics, 

waste heat radiators, and associated thermal insulation. For the specific Lunar application under investigation eight 

(8) modules are employed to deliver 40 KWe to the habitation module. This design looks at three levels of Stirling 

convertor technology and addresses the issues of integrating the Stirling convertors with the GPHS heat sources 

assembly using proven technology when ever possible. In addition, issues related to the high-temperature heat 

transport system, power management, convertor control, vibration isolation, and potential system packaging 

configurations to ensure safe operation during all phases of deployment will be discussed. 

I. Introduction 

Lunar habitation modules would require significant amounts electricity to operate. While GPHS has been used for 

years on NASA’s deep space missions due to their expense and limited availability of these units power levels of 

greater than 1 KWe have not been available. Advances in Stirling technology however, could allow very efficient 

use of the GPHS generated heat and provide a reasonable path to producing significant amounts of power on the 

lunar surface. NASA’S recent Cassini spacecraft had 3 Radioisotope Thermoelectric Generators (RTG’s) that 

produced 850 watts of electrical power. Each RTG has 18 GPHS modules for a total launched GPHS inventory of 

54 GPHS modules. Converting to a high efficiency Stirling convertor should allow a nearly 5-fold increase in 

electrical output with the same GPHS thermal power input. For this design concept the GPHS modules are linked to 

the Stirling convertor via a radiative coupling while using a sodium (Na) thermal siphon in the lunar gravity 

environment to move the heat. When operating on the lunar surface the GPHS heat source assembly would be on 

the bottom. The GPHS modules would boil the sodium in the pipes, which results in the Na rising and condensing 

on a radiative surface at the top of the assembly. The Stirling convertor is set above the heat source assembly with 

a corresponding lower temperature radiative collector surface integrated into the Stirling heater head. This would 

serve as both the mechanism for transferring heat to the Stirling and to concentrate the heat in the small surface area 

of the Stirling heater head. Heat is rejected at the cold end of the Stirling convertor using a liquid water pumped 

loop. Because of the relatively low amount of radiation emitted by GPHS modules the power plants could be 

placed in close proximity of the habitation modules. This design looks at three levels of Stirling convertor 

technology and integrates these Stirling convertors with the radioisotope (GPHS) heat sources. The GPHS heat 

sources are integrated with the Stirling convertors using heat pipes and the heat is rejected form the convertors using 

a pumped liquid loop. 
NASA/TM—2005-213991 2 

II. Stirling Convertor 

The performance, specific power, and reliability of free-piston Stirling cycle power conversion systems have 

improved dramatically in the past decade due a combination of maturing technology and the realization that the 

development of a convertor must be closely integrated with other elements of the system to provide stable and 

reliable power to the end user. As an example of these improvements in the space power area, hardware 

developments in support of NASA/DOE (ref. 1) has resulted in convertors with overall thermal to electric 

efficiencies approaching 40 percent, specific mass levels <10 Kg/KWe at power levels on the order of 100 We 

(ref. 2), These high convertor efficiencies result in GPHS loading requirements on the order of 1/4 to 1/5 that of 

comparable RTG systems. In addition, efforts are currently under way at NASA/GRC and other groups (ref. 3) to 

further improve on convertor performance parameters without impacting convertor durability and reliability. 

While current Stirling convertor efforts have focused on the use of a small number of GPHS modules (1 to 2) to 

provide the thermal energy, there is no fundamental reason that larger numbers of GPHS modules could not be 

used to provide power levels in the KWe range. This concept has a number of advantages compared to other 

power generation schemes: 1) the GPHS module is fully defined and space launch qualified, 2) emitted radiation 

is very low and would allow easy access and placement of the system close to the end user, 3) heat source 

development costs will be low, and 4) the GPHS heat source can be easily simulated with electrical heaters 

allowing extensive life testing in existing facilities. The negative aspects of GPHS modules are the cost and the 

limited supply. 

Table 1 defines the basic Stirling convertor and/or GPHS related parameters employed in the evaluation process. 

As noted previously these parameters are based on currently demonstrated hardware or technology under active 

development at this time. 

Table 1.—Evaluation Parameters 

Parameter Nominal Value or Range 

Net electrical power to habitat 5.0 KWe 

Controller/PMAD efficiency 0.93 

Stirling convertor electrical output 5.5 KWe 

Effective hot end temperature 925 to 1275 K (650 to 1000 °C) 

Effective cold end temperature >350 K 

GPHS maximum operating temperature 1275 to 1375 K (1000 to 1100 °C) 

 

From the viewpoint of the convertor the parameters having the greatest impact on performance and specific mass, 

are 1) heater head (“hot end”) temperature, 2) rejection (“cold end”) temperature, 3) alternator characteristics, and 

4) interface with the heat source (GPHS modules). Because of radiator and sink temperature constraints on size 

and mass, the effective cold-end temperature of the convertor was constrained to temperatures greater than 350 K. 

The cold-end temperature impacts the convertor in two ways: 1) as the cold-end temperature increases, for a fixed 

hot-end temperature, the effective cycle temperature ratio falls reducing performance and 2) at temperatures 

exceeding approximately 400 K (125 °C) the current high performance NdFeB magnets employed in the linear 

alternator have reached their upper operating temperature limit. The latter will require the change to the SmCo 

magnet material that results in somewhat larger and heavier linear alternators. Since the linear alternator makes up 

approximately 60 to 70 percent of the convertor mass, this increase becomes an important factor in evaluation of 

convertor options. 

Stirling convertor performance is shown in Figure 1 as a function of effective cycle temperature ratio for the 

375 K (100 °C) rejection temperature. Techniques employed to define convertor efficiencies take into account 

thermodynamic cycle losses, alternator losses, along with all thermal parasitic losses and have proven to be quite 

accurate based on a number of hardware development efforts. The vertical lines represent the corresponding 

temperature ratios at the heater head temperatures noted. The lower heater head temperature (650 °C) is 

representative of late 1990’s technology, the mid temperature (850 °C) hardware currently in development and is 
NASA/TM—2005-213991 3 

0.2

0.25

0.3

0.35

0.4

0.45

0.5

2 2.25 2.5 2.75 3 3.25 3.5 3.75 4

Cycle Temperature Ratio (Th/Tc)

Convertor Efficiency (We/Qin)

Ref . Ef f .

Th=650 C

Th=850 C

Th=1000 C

Figure 1.—Convertor performance versus cycle temperature ratio, cold end temperature 375 K. 

Figure 2.—Convertor configurations. 

employed as the reference point for the proposed system, and the highest temperature (1000 °C) representative of 

advanced developments incorporating ceramic materials. In the case of the highest heater head temperature, the 

issue of maximum GPHS operating temperature must be considered. Current estimates place this limitation in the 

1275 to 1375 K (1002 to 1102 °C) range which, when combined with a HTS temperature drop conservatively set 

at 100 °C, yield maximum heater head temperatures in the 1175 to 1275 K (900 to 1000 °C) range. Increasing the 

effective rejection temperature essentially “slides” the heater head temperature lines to the left to lower effective 

temperature ratios. 

The schematic shown in Figure 2 provides a description of the basic configuration and size of the two options for the 

proposed Stirling conversion system. The pool boiler and associated thermal insulation has been removed for clarity. 

The single cylinder configuration is representative of current small space engines (for the SRG two single cylinders 

engines not sharing a common expansion space are placed end-to-end), while the opposed piston configuration 

(common expansion space) has been employed in multi KWe development programs (ref. 4). The opposed piston 

configuration is inherently dynamic balanced eliminating the need for a separate vibration isolation system but 
NASA/TM—2005-213991 4 

represents a more complex interface between the convertor HTS and the heater head assembly. The single cylinder 

configuration, which was selected as the reference configuration for analysis purposes, employs a simple monolithic 

heater head. The latter acts as the condenser surface for the pool boiler employed to transfer the thermal energy from 

the GPHS assembly to the convertor. However, since this type of convertor is not dynamically balanced, either a 

passive or active vibration isolation system would be employed. For simplicity, the isolator would be located within 

the convertor pressure vessel. The use of this type of isolation system does impact the mass of the convertor by 

approximately 10 percent. Convertor specific mass for the single cylinder configuration, including the required 

vibration isolation system, is estimated at 8.25 Kg/KWe yielding a convertor mass of 45 Kg, overall length of 

480 mm and a maximum diameter of 230 mm. The opposed piston configuration is slightly heavier at 51 Kg and 

somewhat larger (745 mm long by 215 mm maximum diameter). It is important to note that the convertor 

performance is identical for both and the impact on system mass of changing between the two configurations is 

minimal. 

To function in a stable and efficient manner the free-piston Stirling convertor must be actively controlled. The 

convertor controller would provide the capability to vary power piston amplitude so as to maintain the design 

operating temperature for the heater under all operating conditions, In addition the controller would adjust the 

convertor electrical power factor without the need for large external capacitors and monitor/control the convertor 

vibration levels. As a final feature, the system would carry out the necessary power conditioning activities so that 

the regulated 160 volt output is available to the end user. 

III. Heat Transport System 

As noted previously, a driving factor in the concept evaluation process was the use of proven or low risk technology 

whenever possible. A number of alternative configurations were considered ranging from a direct coupling of the 

GPHS assembly to the convertor heater head via heat pipes to conductively coupling the convertor to various GHPS 

HTS configurations. The HTS must effectively couple the GPHS assembly with the convertor with minimum 

thermal losses. Since the system would operate in a lunar gravity field it would be possible to employ gravity 

assisted HTS concepts such as a pool boiler or thermosyphon rather than pure heat pipes can be employed. This 

minimizes issues concerning the lifetime reliability of the capillary wick materials required for successful heat pipe 

operation. As shown schematically in Figure 3 the GPHS portion of the HTS is made up of 6 individual boiler 

“pipes” (one shown mounted and another alone for clarity) which employ Na as a working fluid and are attached to 

the carbon composite thermal support structure carrying the GPHS modules. Since the evaporator portion of the 

boiler can be of any configuration it is formed as a curved panel that effectively couples two stacks of GPHS 

modules to a single pipe. The individual modules are “clocked” such that the maximum number of GPHS modules 

are served by an individual boiler pipe. The 6 pipes are attached to the backside of a carbon composite plate that that 

serves as the radiator for the radiative coupling to the convertor. This surface provides the hot surface of the 

radiation-coupling feature of the proposed concept. 

Figure 3.—GPHS HTC configuration. 
NASA/TM—2005-213991 5 

Figure 4.—Radiator size requirements. 

The convertor side HTS would also employs a pool boiler and is based on existing technology. The face of the boiler 

acts as the “cold” receiver, with the boiling of the Na working fluid occurring on the rear side. Vapor from the boiler 

condenses on the convertor heater head with the liquid condensate falling, under the influence of gravity, back to the 

boiler surface. 

To provide maximum utility the thermal energy flow between the GPHS assembly and the convertor is via radiation 

across a relatively narrow gap. Based on thermal power requirements, and the reference convertor heater head 

operating temperature, 1123 K (850 °C), the surface area required for the radiator can be defined as a function of 

GPHS operating temperatures, see Figure 4. Based on existing GPHS module operating temperature constraints a 

radiator surface area of approximately 0.25 m2

 (0.5 m diameter) would be required which also defines the convertor 

pool boiler diameter. It is important to note that under these operating conditions both pool boiler 

condenser/evaporator flux levels are quite low, <5W/cm2, providing significant operating margin based on the Na 

working fluid limits. While not specifically used as a HTS system design criteria, the radiation coupling does allow 

the convertor/GPHS assembly to be decoupled. This would allow for convertor replacement or transport to different 

sites on the lunar surface. 

IV. System Operation 

The use of the combined pool boiler/radiation coupled GPHS heat source/convertor configuration would provide a 

high degree of reliability when operating in the lunar gravity field. However, while in transient neither of the two 

pool boilers would function under “zero g” conditions. Since the GPHS assembly is always generating thermal 

energy a technique must be employed to reject this energy to an alternative sink when the system is not producing 

power on the lunar surface. To remove this energy during times when the system is not producing power the 

insulation package surrounding the GPHS assembly would be “opened” allowing the thermal energy to be radiated 

to space eliminating the need for an independent radiator. A deployable HTS insulation disk would also be utilized 

to “fill” the gap between the GPHS radiator and the convertor absorber in the radiation-coupled portion of the HTS. 

The in transit (and after initial lunar landing) configuration of the system operating in this mode is noted in Figure 5. 

An additional aspect of this capability is that if convertor were to fail while on the lunar surface, the insulation could 

be deployed to reduce the non-functioning convertor hot-end temperature. 
NASA/TM—2005-213991 6 

Figure 5.—In transit configuration—convertor not operating. 

Figure 6.—System configuration during normal power generation—convertor operating. 

Once the convertor heat rejection radiators are deployed and the HTS insulation disk removed, the GPHS insulation 

package is “closed” essentially forcing the thermal energy to flow to the convertor. The configuration of the system 

in this operating condition is noted in Figure 6. 

V. Power Management and Distribution 

The power management and distribution system first converts the low frequency AC power generated from the 

Stirling convertor to DC and then connects the eight (8) Stirling convertors together and moves to the habitat 

module. It was assumed for this study that the load for the habitat modules is 160 volts, the same as for the 

International Space Station. Figure 7 shows the electrical layout and efficiencies of the various components. Each 

Stirling has its own controller and AC to DC rectifier which converts the 100 Hz AC to the 160 volt direct current 

(DC) bus voltage. Each Stirling convertor is attached to common switchgear and remote bus isolator (RBI). The RBI 
NASA/TM—2005-213991 7 

Figure 7.—Electrical layout and efficiencies—PMAD system. 

Figure 8.—DC Transmission line mass versus operating DC voltage. 

switchgear allows the sources to be added or removed as desired without affecting the rest of the bus and the 

provides circuit protection in case of faults (ref. 5). The power is then sent to the habitat modules using aluminum 

conductors. Figure 8 shows the trade of cable mass as a function of voltage for a 1 percent loss. It is assumed that 

the habitat is 20 meters from the habitat module. Both the rectifiers and the cabling, RBI and switchgear are dual 

redundant in their configuration. The switchgear/RBI, cabling and the rectifier are modeled using the Rocketdyne 

model sited in the previous reference. Table 2 shows the mass breakdown for the cabling, remote bus interface and 

the switchgear. 
NASA/TM—2005-213991 8 

Table 2.—Mass breakdown 

Component Total Mass (kg)

DC /RBI Switchgear 62 kgs 

Cabling 61 kgs 

Totals 123 kgs 

VI. Heat Rejection Subsystem—Radiator Cold End Fluids and Materials 

Power system operation on the lunar surface is a challenge because of the lack of atmosphere to reject heat and the 

wide range of heat rejection or sink temperatures, which occur during the lunar diurnal cycle. From both a cost and 

production standpoint the power system designer would want to minimize the amount of isotope required and for a 

thermodynamic cycle this requires low rejection temperatures. The penalty for this low rejection temperature is that 

the area required to reject the waste heat increases with decreasing rejection temperature. This trade between heat 

source mass and size and radiator size leads to mass minimums for the overall system. Studies analyzed the 

advantages and disadvantages of radiator orientation on the lunar surface (refs. 6 and 7). From these studies it was 

found that for vertically oriented radiators the maximum and minimum sink temperatures during lunar day/night 

cycle was about 180 to 314 K and between 180 and 270 K for horizontal radiators which don’t see the lunar surface. 

The trade between the a two sided radiator operating at a higher sink temperature and a single sided radiator 

rejecting heat at a lower temperature may provide different answers depending on how little isotope is available or if 

the system is mass or area constrained. In addition, the large change in sink temperatures will result in changes in 

both the power output of the system and the temperature range requirements of the heat rejection system. 

To reject the waste heat a pumped water loop is flown over the cold end of the Stirling convertor and then sent out to 

a water heat pipe radiator. Stirling convertors operate best when the inlet to exit coolant temperature difference is 

kept to a minimum. In general the temperature rise of a fluid used to remove the waste heat should be about 25 K or 

less to ensure no Stirling cycle performance penalty. The panel design, see Figure 9, selected is similar to that used 

on the space station which consists of heat pipes sandwiched between two outer face sheets. The two outer face 

sheets are used for the vertical radiator panels, for the horizontal radiator panels the bottom sheet is removed. Panel 

mass was approximately 3.5 kg/m2

 for the cases shown for the 2-sided radiator and 2.3 kg/m2 for a horizontal 

radiator. This number does not include the fluid ducts, fluid or pumps, which are accounted separately. The pump 

design selected is also scaled from that used on the space station and is scaled both in efficiency and mass to meet 

the pressure drop and flow rate requirements of the system. At lunar noon the temperature of the radiator without 

any heat should be that of the sink temperature (i.e., 270 K for the horizontal radiator and 314 K for the vertical 

radiator). In order to ensure that a thaw system is not necessary it is assumed that startup occur for the horizontal 

radiator case near lunar noon and for the vertical radiator nearly anytime after dawn. For the horizontal radiator case 

it may be necessary to increase the solar absorptivity of the horizontal radiator surface to raise its temperature, which 

would require a small increase in radiator area. For the vertical radiator surface the temperature is well above the 

melting point of water at 1 Bar. Figure 10 shows the sink temperature for a both a horizontal and vertical radiator on 

the lunar surface. 

Figure 9.—Radiator configuration. 
NASA/TM—2005-213991 9 

Figure 10.—Radiator sink temperature for horizontal and vertical orientations. 

VII. System Trade Studies 

Trade studies for the isotope Stirling power systems were performed using different operating conditions (cold-end 

temperature, convertor design, etc.) and examining how each of these operating conditions affects the overall power 

plant. For each of the subsystems discussed previously, technology assessments were made regarding their 

developmental status and readiness for a conceptual near term lunar mission. Components were then modeled and 

parameters varied to understand how each impacts the system. As an example, a higher temperature ratio across the 

convertor would provide improved efficiency but will also in turn require a larger radiator. Due to high launch costs 

and limited lift capability, launch mass is often a parameter that is minimized. In this study in particular because of 

the high cost of GPHS total landed lunar costs were also considered. Additionally, radiator area, which is limited by 

volumetric constraints of the shroud of the launch vehicle, may prevent the power system from being able to operate 

at its minimum mass point. 

Two technology baselines were considered which were discussed in more detail in the Stirling section of the report. 

Both 375 and 425 K cold end temperatures were considered for this report and reflect the current state-of-the-art. 

Both cases are discussed in this analysis. For both of these systems a reference heater head temperature of 1123 K 

(850 °C) was used. 

VIII. Results 

Figure 11 shows the mass and radiator area as a function of Stirling cold-end temperature for a 5 KWe net output 

Stirling convertor system with both horizontal and vertical radiators. The mass cross over point between the radiator 

orientations occurs at a Stirling cold-end temperature of 430 K. The reason for the crossover is the trade between a 

lower effective sink temperature with the horizontal radiator versus the two-sided radiator using vertical panels but 

with a higher sink temperature. If radiator deployed area is of primary concern this cross over between the two 

radiator orientations occurs at about 400 K. For all cases the lower the rejection temperature the fewer GPHS 

modules required. 
NASA/TM—2005-213991 10 

Figure 11. —Mass and radiator as a function of Stirling cold end temperature. 

Table 3 shows a comparison of the 375 and 425 K cold end along with the minimum mass cases with both 

horizontal and vertical radiators. If the 375 K cold end Stirling is used it is preferable from system mass and radiator 

area to use a horizontal radiator. When using the high-temperature Stirling cold end (i.e., 425 K) the vertical radiator 

orientation has a slight mass (352 vs. 366 kgs) and radiator panel area advantage (15 vs. 11.5 m2

). For the minimum 

mass cases both radiator orientations optimize to 450 K but the vertical radiator orientation has 25 kg mass 

advantage and a 3 m2

 radiator panel advantage. To keep mass relatively close to its minimum, Stirling cold-end 

temperatures should not fall below about 360 K before a rapid increase in system mass occurs due to a increasing 

heat rejection area and mass. Figure 12 shows both system mass and GPHS count as a function of Stirling cold-end 

temperature for a horizontal radiator configured 5 KWe system. GPHS module count would rise linearly with 

increasing temperature but system mass rapidly increases below about 360 K because of the increasing heat 

rejection subsystem. Figure 13 shows a bar chart mass comparison between a 375 and 425 K system with a 

horizontal radiator. Operating at 375 K results in a 16 percent (50 kg from 35 to 410 kgs) mass increase over the 

minimum mass design. The radiator panel area for the minimum mass design is 8 m2

 versus 29 m2 for the 375 K low 

temperature system using a horizontal radiator to minimize the GPHS inventory. For comparison going to the higher 

cold end temperature is an increase in the number of GPHS units required rising from 66 to 73 GPHS modules 

corresponding to 375 and 425 K cold end systems and going to the minimum mass system requires 76 GPHS 

modules. 

Figure 14 shows power variation as a function of the lunar day/night cycle for both beginning of life (BOL) and end 

of life (EOL) operation for a vertically oriented radiator. For this system a life of 10 years is assumed which results 

in a decrease in thermal power from each GPHS from 250 watts to 234 based on its 87-year half-life. Power 

variations for this system are approximately 300 watts. For a horizontal radiator the variation in power is about 

175 watts due to the smaller overall change in sink temperature. 
NASA/TM—2005-213991 11 

Table 3.—5 kWe System Mass Breakdown 

Component 

375 K 

Cold-End 

Horizontal 

Radiator 

425 K 

Cold-End 

Horizontal 

Radiator 

375 K 

Cold-End 

Vertical 

Radiator 

425 K 

Cold–End 

 Vertical 

Radiator 

450 K 

Horizontal 

Radiator 

Minimum 

Mass 

450 K 

Vertical 

 Radiator 

Minimum 

 Mass 

Stirling 

Convertors 

(kgs)

45 49 45 49 51 51 

GPHS Mass 

(kgs/no. modules ) 

96/66 106/73 96/66 106/73 110/76 110/76 

GPHS Assembly 

Structure 

(kgs) 

79 87 79 87 91 91 

GPHS Aluminum 

Cover (kgs)

4 4 4 4 4 4 

Insulation 

(kgs)

17 19 17 19 18 18 

Heat Rejection 

(kgs)

146 78 162 50 63 38 

Power Cond. 

(Control + Conv.) 

(kgs) 

27 27 27 27 27 27 

RadiatorArea (m2) 

 Panel Area (m2

)

29 

29 

15 

15 

87 

43.5 

23 

11.5 

11 

11 

16 

8 

Totals 409 366 427 352 360 335 

Figure 12.—System mass and number GPHS modules as a function of Stirling cold end 

temperature for a horizontal radiator. 
NASA/TM—2005-213991 12 

Figure 13.—Mass breakdown for both 375 and 425 K 5 KWe systems using a horizontal radiator. 

Figure 14.—Beginning of life and end of life power variations as a function of lunar day/night 

cycle, horizontal radiator. 
NASA/TM—2005-213991 13 

IX. Conclusions 

A modular 5 KWe Stirling convertor power system employing GPHS modules as a heat source has been 

investigated and a reference system configuration identified. This system could have a range of applications in the 

further exploration of the lunar surface. The proposed system consciously uses currently existing hardware 

technology or concepts under active investigation at this time. The Stirling convertor itself represents a 

straightforward adaptation of current technology scaled to higher power levels. The convertor configuration options 

include both single cylinders and dual-opposed arrangements incorporating proven vibration isolation schemes. The 

required convertor controller and PMAD systems are an extension of existing technology. A simplified HTS has 

been identified which minimizes the use of potentially life limiting aspects of high temperature heat pipes while 

taking full advantage of the available Lunar gravitational field. Various techniques have been selected to ensure that 

the system would operate reliability under all expected condition. 

This approach has a number of advantages compared to other power generation schemes: 1) the GPHS module is 

fully defined and space launch qualified, 2) emitted radiation is very low and could allowing easy access and 

placement of the system close to the end user, 3) heat source development costs would be low, and 4) the GPHS heat 

source can be easily simulated with electrical heaters allowing extensive life testing in existing facilities. 

References 

1. Schmidt, G.R., “NASA’s Program for Radioisotope Power System Research and Development,” STAIF 2005, 

Albuquerque, NM, 2005. 

2. Chan, J., “Stirling Radioisotope Generator (SRG110) Status,” STAIF 2005, Albuquerque, NM, 2005. 

3. Thieme, L.J. and Schreiber, J.G., “Supporting Development for the Stirling Radioisotope Generator and 

Advanced Stirling Technology Development at NASA GRC,” STAIF 2005, Albuquerque NM, 2005. 

4. Dhar, M., “Stirling Space Engine Program,” NASA/CR—1999-209164, 1999. 

5. Metcalf, K., “Power Management and Distribution (PMAD) Model Development Final Report,” NASA Contract 

NAS3–01140 Rocketdyne, NASA Glenn Research Center, Cleveland, OH, May 2002. 

6. Bien, D.D. and Guentert, D.C., “A Method for Reducing the Equivalent Sink Temperature of a Vertically 

Oriented Radiator on the Lunar Surface,” NASA TM X–1729, NASA Lewis Research Center, Cleveland, OH, 

1969. 

7. Dallas, D., Diaguila, A.J., and Saltsman, J.F., “Design Studies of the Effects of Orientation, Lunation, and 

Location on the Performance of Lunar Radiators,” NASA TM X–1846, NASA Lewis Research Center, 

Cleveland, OH, 1971. 
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19

A Design of a Modular GPHS-Stirling Power System for a Lunar

Habitation Module

Paul C. Schmitz, L. Barry Penswick, and Richard K. Shaltens

Isotope; Stirling; Power system; GPHS; General purpose heat source; Lunar base

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Subject Categories: 12 and 20

Prepared for the Third International Energy Conversion Engineering Conference sponsored by the American Institute

of Aeronautics and Astronautics, San Francisco, California, August 15–18, 2005. Paul C. Schmitz, Power Computing

Solutions, Inc., 4672 Bellerive Way, Avon, Ohio 44011; L. Barry Penswick, Sest, Inc., 18000 Jefferson Park Road, Suite

104, Middleburg Heights, Ohio 44130; and Richard K. Shaltens, NASA Glenn Research Center. Responsible person,

Paul C. Schmitz, organization code RPC, 216–433–6174.

Lunar habitation modules need electricity and potentially heat to operate. Because of the low amounts of radiation

emitted by General Purpose Heat Source (GPHS) modules, power plants incorporating these as heat sources could be

placed in close proximity to habitation modules. A design concept is discussed for a high efficiency power plant based

on a GPHS assembly integrated with a Stirling convertor. This system could provide both electrical power and heat, if

required, for a lunar habitation module. The conceptual GPHS/Stirling system is modular in nature and made up of a

basic 5.5 KWe Stirling convertor/GPHS module assembly, convertor controller/PMAD electronics, waste heat radiators,

and associated thermal insulation. For the specific lunar application under investigation eight modules are employed to

deliver 40 KWe to the habitation module. This design looks at three levels of Stirling convertor technology and addresses

the issues of integrating the Stirling convertors with the GPHS heat sources assembly using proven technology whenever

possible. In addition, issues related to the high-temperature heat transport system, power management, convertor control,

vibration isolation, and potential system packaging configurations to ensure safe operation during all phases of deployment will be discussed.