Paul C. Schmitz

Vantage Partners, LLC, Brook Park, Ohio

Parametric System Model for a

Stirling Radioisotope Generator

NASA/CR—2015-218450

March 2015
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Paul C. Schmitz

Vantage Partners, LLC, Brook Park, Ohio

Parametric System Model for a

Stirling Radioisotope Generator

NASA/CR—2015-218450

March 2015

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NASA/CR—2015-218450 1 

Parametric System Model for a Stirling Radioisotope Generator 

Paul C. Schmitz 

Vantage Partners, LLC 

Brook Park, Ohio 44142 

Abstract 

A Parametric System Model (PSM) was created in order to explore conceptual designs, the impact of 

component changes and power level on the performance of the Stirling Radioisotope Generator (SRG). 

Using the General Purpose Heat Source (GPHS ~ 250 Wth) modules as the thermal building block from 

which a SRG is conceptualized, trade studies are performed to understand the importance of individual 

component scaling on isotope usage. Mathematical relationships based on heat and power throughput, 

temperature, mass, and volume were developed for each of the required subsystems. The PSM uses these 

relationships to perform component- and system-level trades. 

1.0 Introduction 

The Radioisotope Power Systems (RPS) Program Office commissioned a study to help understand 

how Stirling Radioisotope Generators (SRGs) scale in power from the current 140-W Advanced Stirling 

Radioisotope Generator (ASRG) up to 1 kWe. The motivation to consider higher power systems is 

twofold. First in that it may reduce the integration burden placed upon future high-power spacecraft that 

would require many of the lower power ASRGs, and second that higher power levels and optimization of 

a SRG might enable higher specific power (w/kg). A Microsoft Excel-based Parametric System Model 

(PSM) was created to model the individual components associated with a SRG and determine how each 

of these component’s characteristics affects the system. In order to understand these interactions some 

assumptions are needed to relate how the overall system will be arranged. For this study the ASRG (see 

Figure 1) was used as a typical physical arrangement for all of the SRGs considered in this analysis. The 

ASRG layout is a dual-opposed Stirling convertor system with the General Purpose Heat Source (GPHS) 

modules placed near the heater heads and located on the outboard side of each housing end. Surrounding 

the GPHS is thermal insulation that defines the minimum internal dimension of the housing/radiator. For 

all of the cases considered in this study, a cylindrical radiator was assumed, which is different from the 

rectangular block shape housing of the ASRG. The housing/radiator in the ASRG as well as for the SRG 

studied in this paper serves to contain the insulation, provide structural rigidity to the entire assembly, and 

contain an inert cover gas required during launch. 

Figure 2 shows a heat flow diagram representing the SRG’s analyzed. The heat flows and/or 

allowable temperature drops are used to size the components. By making material substitutions, an 

assessment can be made on the impact each substitution would have on overall system performance. Heat 

is generated in the GPHS module(s), which is surrounded by thermal insulation to reduce the heat losses 

to the surroundings. The remaining heat is sent via a conductive interface called the hot-side attachment 

(HSA) to the Stirling convertor. The Stirling convertor produces electrical power and the cycle waste heat 

is rejected to the cold-side adapter (CSA). The waste heat passes through the CSA and is then rejected to 

the environment via the housing/radiator. 

 
NASA/CR—2015-218450 2 

Figure 1.—Advanced Stirling Radioisotope Generator. 

Figure 2.—Heat and power flows in a Stirling Radioisotope Generator (SRG).

GPHS

Hot Side

Adaptor

Heat Loss

Through Insula on

Heat into

S rling

Heat 

Rejected

Electrical

Power

Radiator

Insula on

Cold Side Adaptor 

(CSAF in ASRG)

C

S

A

F

S rling
NASA/CR—2015-218450 3 

2.0 Modeling Overview 

Assuming a Stirling convertor temperature ratio (TR) (Eq. (1)), the Carnot efficiency (Eq. (2)) is used 

to calculate the ideal efficiency of a heat engine. Next, a representation of how close the Stirling engine is 

to ideal cycle (fraction of Carnot efficiency) is used to estimate the overall heat engine efficiency 

(Eq. (3)). The fraction of Carnot efficiency can either be a fixed value or dependent upon heat flow and 

acceptor/rejector temperature. The ASRG operating near a TR of 3.0 that has a fraction of Carnot is about 

63 percent. Next, using an estimate of the alternator efficiency, one can find an overall efficiency of the 

engine/alternator combination (called a convertor) (Eq. (4)). The fraction of Carnot including the 

alternator for ASC is about 56 percent. Equation (5) is then used to calculate the heat into the heat engine. 

GPHS modules are specified as producing ~250 W at the beginning of life (BOL) (an assumed 3 years 

before mission launch). By selecting the electrical power output and the desired time (i.e., isotope decay) 

in the life of the GPHS, one can calculate the minimum number of GPHS modules required. The GPHS 

thermal input subtracted from the Stirling required heat input is the allowable losses through the 

insulation. With the insulation losses now fixed an estimate can be made for the surface area of the 

insulation and in turn the insulation thickness can be calculated. As the insulation thickness increases so 

does the minimum radiator/housing size and the radius of the CSA. All of these components coupled 

together are evaluated over a wide range of TRs within the PSM to achieve the required electrical power 

output. 

C

H

T

T Temperature Ratio (TR)  (1) 

TR

1 Carnot Efficiency  1 (2) 

Heat engineefficiency  Carnot efficiencyFractionof Carnot (3) 

 efficiency Overallefficiency Heat engineefficiency Alternator (4) 

Overall efficiency

Electrical power QIN  (5) 

3.0 Components 

All of the components described below are modeled in the PSM. A brief overview of the 

characteristics of each component and the user-selected options that are available in the PSM are 

discussed. 

3.1 General Purpose Heat Source 

Reference 1 discusses the Department of Energy (DOE) GPHS, which is the heat source building 

block used for this analysis and is used in all current NASA RPS. The GPHS contains Pu-238 in the form 

of PuO2 in four interior iridium capsules. The GPHS Pu-238 fuel has a half-life of approximately 87 years 

that leads to relatively constant heat flux out of the GPHS module during a typical NASA mission. The 

current GPHS in the form of a Step 2 GPHS are used for all space missions. Dimensions of a Step 2 

GPHS module are 5.3 by 9.32 by 9.72 cm. Using the largest face (9.32 by 9.72 cm), the maximum 
NASA/CR—2015-218450 4 

heat flux out of a single GPHS module at BOL with insulation is 2.69 W/cm2. In contrast to the low heat 

flux from the GPHS modules is that required by a Stirling convertor. A typical SRG developed for high 

specific power requires an input heat flux of about 15 W/cm2

. 

The GPHS module temperature limits are set by the iridium cladding around the Pu-238 fuel whose 

temperature must be maintained between 1335 C (1608 K) and 900 C (TE1173 K). The effective 

maximum surface temperature of the GPHS graphite shell is 1100 C (1373 K) in vacuum. This 

temperature is well above the 840 C Stirling convertor upper limit that is allowed by the Stirling heater 

head superalloy (MarM-247). This temperature combined with the temperature drop through the HSA sets 

the temperature of the Stirling convertor heater head. 

Finally, the PSM allows variations in the GPHS initial heat output. The user may select either the 

BOL 250 W nominal value or other values based on lower or higher initial heat loading. Additionally, the 

user may select some future time for the SRG to produce its required power by allowing for the decay of 

the isotope. 

3.2 Heat Source Attachment 

A component called the HSA is used to conductively couple the GPHS module(s) to the Stirling 

convertor. In the ASRG, the HSA is made of nickel and sits over the acceptor dome of the Stirling 

convertor. As electrical power requirements increase and/or efficiency decreases it becomes necessary to 

add additional GPHS. One option that does not require changing the HSA is to stack the GPHS modules 

(Figure 3). This approach can be at best extended to two GPHS modules (see Ref. 2) as analysis has 

shown the temperature drop from most distant GPHS to the Stirling reduces safety margin of the heat 

source. The arrangement used in this study (see Figure 4) is to place the GPHS modules radially outward 

from the Stirling heater head. The advantage is that all of the GPHS modules are operating at the same 

temperature. The disadvantage of this arrangement is that the insulation plus multiple GPHS modules are 

driving the minimum diameter of the SRG. This in turn increases the mass penalty and/or temperature 

drop through the CSA. It is shown in Reference 3 that as power levels increase, GPHS orientation with 

the shortest GPHS dimension (5.32 cm) going radially outward from the centerline axis of the convertor 

is preferred and is used for the remainder of this analysis. 

The mass of the HSA can be found by using the material properties, Stirling and HSA heat flux 

requirements and the distance between the heater head and the GPHS. A comparison between a nickel and 

graphite HSA mass is shown in Figure 5 for various numbers of GPHS modules arranged as shown in 

Figure 4. 

Figure 3.—General Purpose Heat Source (GPHS)/Stirling arrangements. 
NASA/CR—2015-218450 5 

Figure 4.—General Purpose Heat Source (GPHS) arrangements. 

Figure 5.—Hot-side attachment (HSA) adapter as a function of the number of 

General Purpose Heat Source (GPHS) modules and connection material. 

~280 w DC ~420 w DC ~560 w DC ~700 w DC

S rling

Insula on

Heat Source

Connector
NASA/CR—2015-218450 6 

3.3 Insulation 

Multilayer insulation (MLI) is considered as a possible replacement for the MicrothermHT solid 

thermal insulation currently used in the ASRG. High-temperature MLI was the thermal insulation used in 

the GPHS radioisotope thermoelectric generator (RTG), which was used on Cassini, Galileo, and New 

Horizons. The change from a solid insulation to a MLI, which requires a vacuum to be effective, would 

eliminate the SRGs use on planetary bodies with an atmosphere but should provide a higher specific 

power system when used in vacuum. The ASRG is a modified version of the SRG–110 generator 

discussed in Reference 4. The primary difference between the ASRG and the SRG–110 is that the ASRG 

replaced the Infinia Stirling convertors with Sunpower ASCs. Because the housing was a carryover from 

the SRG–110 program, its size and dimensions were set to optimize the SRG–110 that had a Stirling hot 

end temperature of 640 C. During the development of the ASC there was a material change from the 

SRG–110’s Stirling convertor Inconel heater head to the ASRG’s MarM-247. This gave the ASRG the 

potential to have heater head temperatures in excess of 840 C. Unfortunately, since the maximum 

insulation thickness was set by the original SRG–110 beryllium housing dimensions the optimal 

temperature based on balancing heat loss through the insulation and heat into the Stirling convertor 

resulted in a generator peak power temperature of 760 C. It is desirable from a system perspective to 

explore variations in insulation and/or higher performing insulation to decouple the housing/radiator size. 

Figure 6 shows conceptually the trade between coupling the insulation thickness to the radiator diameter 

and by decoupling the insulation size from the insulation. Case A shows the blue radiator/housing 

diameter set at the thickness of insulation. This requires that the radiator must increase the cylindrical 

height in order to increase its surface area. Case B shows a decoupling of the radiator/housing diameter 

from the insulation thickness. This requires a larger CSA but results in a shorter effective fin length (when 

defined from the CSA housing attachment point to the end). Case B also requires additional support 

structure to contain the insulation and this was modeled as a thin-walled aluminum shell. The PSM allows 

the user to select MLI, MicrothermHT in a vacuum, and MicorthermHT in a Mars environment as well as 

consider coupling the housing diameter and the insulation diameter. 

Figure 6.—Insulation sizing and scaling. 

A

B
NASA/CR—2015-218450 7 

3.4 Stirling Convertor Performance 

As was discussed earlier in this paper the performance of a Stirling convertor is related to both the 

Carnot efficiency and the fraction of Carnot efficiency. Stirling development at NASA Glenn Research 

Center and in industry has shown continuous improvements in both efficiency and mass. In 2004 during 

the SRG–110 program, the Stirling Technology Company (STC) created the Technology Demonstration 

Convertor (TDC) and was able to achieve approximately 47 percent of Carnot efficiency at a TR of 2.86. 

With the switch to the Advanced Stirling Convertor (ASC) and the intervening 8 years Sunpower was 

able to demonstrate 56.5 percent of Carnot at a TR of 3.3. Internal differences between the two convertors 

and materials changes resulted in the increased fraction of Carnot performance. It is important to 

understand the impact of higher performance on the overall performance of SRGs in general. The 

Novikov engine represents a semi-ideal heat engine operating at maximum power output in which heat 

transfer is irreversible but other components are ideal. In practice this represents a practical upper bound 

for engine efficiency and is useful to compare with the results currently being achieved in modern Stirling 

convertors. The Chambadal-Novikov efficiency is calculated by 

H

c

T

T Novikovheat engineefficiency  1 (6) 

The PSM allows user inputs of either a fixed fraction of Carnot, the Novikov efficiency along with 

other models of engine efficiency as a function of TR. At a TR of 3.3 the Novikov efficiency prediction is 

56 percent of Carnot (including a 87 percent efficient alternator). Comparing this with the achieved 

56 percent of Carnot for the ASC suggests it is very near the practical limits of what can be achieved with 

a heat engine. 

3.5 Cold End Interface/Housing 

The CSA moves heat from the Stirling to the housing. The CSA is attached to the Stirling convertor 

cold end and extends radially out to the radiator/housing. Figure 7 shows both a conventional solid CSA 

on the left and a heat pipe CSA under development at NASA Glenn on the right. The housing/radiator is a 

cylinder whose inner diameter is set by the housing of the GPHS modules and length set by the required 

area. Temperature drops in the CSA are measured from the outer heat rejection wall of the Stirling 

convertor cold end to the inner surface of the cylindrical radiator. Attachment points to the radiator/ 

housing are assumed to be 1⁄4 of the distance from either end of the final height of the cylinder to make 

each of the 1⁄4 radiator sections reject similar amounts of heat. The interconnect tube, which joins the two 

convertors together is sized based upon Stirling convertor length, radiator length, and the 1⁄4 point 

attachment distance. 

The PSM will either calculate the thickness of the CSA based on allowable temperature drop or the 

thickness can be set with the resulting temperature drop calculated. The greater the temperature drop 

allowed in the flange, the lighter the CSA mass but the lower the overall effective temperature of the 

radiator. This in turn leads to a larger radiator size and higher housing mass. One technology option under 

consideration is the replacement of the solid CSA with a water heat pipe, which would provide both a 

lower mass connection from the Stirling to the housing and also reduce the temperature drop over this 

same distance. Temperature drops into and out of the heat pipe CSA is based upon evaporator and 

condenser heat flux, heat pipe wall thickness, and working fluid selected. The obvious disadvantage in 

using a heat pipe CSA is the potential reliability penalty paid for this new heat transport device. 

Just as in the CSA, the housing can be a solid conductive piece (as the Be housing of the ASRG), or it 

can have heat pipes embedded on the interior surface to augment heat transfer. Trade can be made using 

different materials (i.e., beryllium vs. aluminum) with or without the addition of heat pipes. Fin thickness 

of the housing/radiator can be set by fixing the temperature drop from the outer cold end flange to the 
NASA/CR—2015-218450 8 

Figure 7.—Heat pipe cold-side adapter under test at NASA Glenn 

Research Center.

average radiator surface temperature. Increasing housing thickness or embedding heat pipes in general 

allows the surface area of the housing to be a more effective radiator by decreasing the temperature drop 

from the CSA to the average radiator temperature. The PSM allows the user to select either a solid 

(a variety of material properties are included) or heat pipe CSA. Additionally, the heat pipe material and 

working fluid may be selected. 

4.0 Results 

Table 1 shows a number of trades that were performed over the last year using the PSM. Below are 

some details of a few of these to illustrate the outcome of these studies. 

The PSM was exercised to look at how its projected results compare with the ASRG. Figure 8 shows 

a plot of component and system mass as a function of Stirling cold end temperature. Each cold end 

temperature is associated with a 140-W generator. Each cold end temperature represents different 

convertor efficiency and therefore a different heat input and heat rejection requirement. Several of the 

larger components mass variations are included to help illustrate how the various systems are interacting. 

Notice that as the cold end temperature rises the mass of the radiator drops due to its T4

 dependence on 

heat rejection. As the cold end of the cycle rises the efficiency is dropping. In order to produce 140 W, the 

heat input to the Stirling convertors must increase. This additional heat is obtained by increasing the 

thickness of the insulation surrounding the GPHS modules. This can be clearly seen when looking at the 

insulation mass rapid rise around 360 K. The consequences of this increase in insulation thickness are a 

larger radius and more massive CSA and a larger more massive housing/radiator. It is this interplay that 

results in the step change at around 360 K and forces the system to add an additional GPHS module to 

meet the 140 W requirement while maintaining the 760C/97 C (1033/360 K) TR. Although the 

cylindrical housing assumption will create a somewhat different system optimization than the rectangular 

block housing with fins of the ASRG, the overall results should be similar. Note that the PSM predicted 

maximum power occurs at a Stirling cold end temperature of 320 K while the ASRG maximum power 

point is 311 K. 

In future plots when considering a range of power outputs one could make general conclusions about 

technology insertion trends. These future plots will only relate best specific power points. Each point 

shown in Figure 9 through Figure 12 is obtained by running an optimization similar to that shown in 

Figure 8. 
NASA/CR—2015-218450 9 

TABLE 1.—TRADE STUDY COMPARISON EXAMPLES 

Trade Study Space 

PSM Results vs. ASRG Design 

General Power Scaling Results 

Comparison of Single Convertor with and w/o Balancer 

Hot End Temperature Variations 

Beryllium vs. Aluminum Housing 

Variable Stirling Convertor Performance 

Space Only vs. Multi-Mission Capable SRG 

Single Convertor Systems 

Figure 8.—A 140-W SRG system mass and number of GPHS modules as a function of 

cold end temperature. 

Figure 9.—Five technology insertion options for SRGs.

ASRG

4.0$

4.5$

5.0$

5.5$

6.0$

6.5$

7.0$

7.5$

8.0$

8.5$

0$ 100$ 200$ 300$ 400$ 500$ 600$ 700$ 800$

Specific'Power'(w/kg)'

Power'Output'(wa5 s)'

2 GPHS

“ASRG”

+ MLI

+ HP CSAF

+ HP Rad.

+ Gr Coll.

4 GPHS

6 GPHS 8 GPHS 10 GPHS
NASA/CR—2015-218450 10 

Figure 10.—Best specific power cold end temperature. 

Figure 11.—Multimission capability. 

Hot End and Cold End 

Technologies Added

“ASRG Like”

Case 5

5.0$

5.5$

6.0$

6.5$

7.0$

7.5$

8.0$

8.5$

0$ 100$ 200$ 300$ 400$ 500$ 600$ 700$ 800$

Specific'Power'(w/kg)'

Power'Output'(wa5 s)'

Specific$Power$(w/kg);1033,HT,

4,Cond,Cond$

Specific$Power$(w/kg);1033,MLI,

4,Cond,Cond$

Specific$Power$(w/kg);1033,MLI,

4,HP,Cond$

Specific$Power$(w/kg);1033,MLI,

4,HP,HP$

Specific$Power$(w/kg);1033,MLI,

4,HP,HP,$HS$AF ach$Graphite$

Specific$Power$(w/kg);1033,HT,

4,HP,Cond$

Specific$Power$(w/kg);1033,HT,

4,HP,HP,$HS$AF ach,$Graphite$

Specific$Power$(w/kg);1033,HT,

4,HP,HP,$HS$AF ach,$Ni$

“ASRG”

+ HP CSAF

+ HP Rad.

+ Gr Coll.
NASA/CR—2015-218450 11 

Figure 12.—Heater head temperature. 

4.1 System Trade Space 

In order to quantify each of the material/configuration options available in the PSM impact, the system 

five different combinations were considered as starting points for this analysis and then an additional 

parameter was varied in order to see its impact on specific power. 

The five cases are as follows: 

1. An “ASRG-like” system is used that has similar conductive interfaces with the same materials 

used in the ASRG as well as in solid insulation around the GPHS modules 

2. A space-only system where the solid insulation around the GPHS modules is replaced with 

MLI 

3. A space-only system with the replacement of the solid CSA with a heat pipe CSAF 

4. A space-only system where in addition to the heat pipe CSA heat pipes are added to the 

radiator 

5. A space-only system where a highly conductive graphite heat collector is added to join the 

GPHS modules to the Stirling convertor (4 + graphite) 

These five different cases span a range of possible technology insertions into SRG systems, which should 

have a meaningful impact into how these systems scale. Figure 9 shows the five options inserted in order 

serially from 1 to 5. In general, peak specific power occurs in 120- to 130-W increments that correspond 

to best specific power points associated with each pair of GPHS added to the system. MLI and both heat 

pipe systems add approximately 1 w/kg to specific power over the range of power considered. Above 

400 W specific power decreases unless all technology insertions are implemented. The lumpy nature of 

the specific power plot is a result of the interplay between insulation thickness, CSA thickness, and 

radiator area. Figure 10 shows cold end optimal temperature for both Cases 1 and 5 that further illustrate 

this interplay. Optimal cold end temperature in all but the two GPHS cases occurs between about 400 and 

500 K, which is significantly higher than the ASRG’s 311 K. The reason for this change is because of the 

increasing importance of radiator area, HSA, and CSA to the overall mass of the system. Notice that peak 

0.0#

1.0#

2.0#

3.0#

4.0#

5.0#

6.0#

7.0#

8.0#

9.0#

0# 100# 200# 300# 400# 500# 600# 700# 800#

Specific'Power'(w/kg)'

Power'Output'(wa5 s)'

Specific#Power#(w/kg)<1033,HT,4,Cond,Cond,56%#

Specific#Power#(w/kg)<1033,MLI,4,HP,HP,#56%#

Specific#Power#(w/kg)<1113,MLI,4,HP,HP,#56%#

Specific#Power#(w/kg)<973,MLI,4,HP,HP,#56%#

Specific#Power#(w/kg)<1113,HT,4,Cond,Cond,56%#

Specific#Power#(w/kg)<973,HT,4,Cond,Cond,56%#

700 C

840 C

“ASRG Like”

Case 5
NASA/CR—2015-218450 12 

specific power always occurs about halfway between the maximum and minimum optimized temperature 

range for any given number of GPHS modules. As an example let us look at Case 5 for a SRG producing 

450 W. Optimal specific power of just about 8 w/kg occurs at a cold end temperature of about 450 K. 

This SRG uses eight GPHS modules to generate this power but the actual range of an eight-GPHS SRG is 

from 400 to 550 W. At 400 W the insulation can be very thin and thus the heat losses through the hot end 

are large and still achieve the 400 W required output. Additionally, the Stirling convertor does not have to 

be very efficient and the heat rejection temperature can be high (low Carnot efficiency). As power 

requirement rises above 400 to 550 W, efficiency becomes more important as well as requiring thicker 

insulation and then the optimal cold end temperature drops. 

Multimission capability, which is the ability to operate on the surface of Mars or other planets/moons 

with an atmosphere, is not possible with MLI. Just like in the GPHS RTG, the superior performance of 

MLI allows higher specific powers to be achieved at the expense of Mars surface operation. The vast 

majority of RPS have been used as either orbiters to the outer planets or as vehicles that flyby another 

body. From a SRG design point the superiority of the MLI allows some decoupling of the housing 

diameter from the number of GPHS modules. This decoupling then allows the size (diameter and length) 

of the housing/radiator to be less influenced by the GPHS arrangement and diameter. Figure 11 shows 

comparisons of Cases 1 through 5 and the specific power changes due to the addition of MLI. The solid 

lines are associated with MLI, while the dotted line of the same color use MicrothermHT. In general, 

about 1 w/kg increase in specific power is found when using MLI versus MicrothermHT. The additional 

mass is a direct result of the increased thickness of the insulation and the downstream consequences of a 

larger CSA, higher temperature drops to the radiator, and therefore larger radiator. Case 5 MLI (with the 

other additions) does allow nearly constant specific power with power that is not possible using the 

MicrothermHT solid insulation. The flexibility in decoupling the specific power scaling from the absolute 

power level allows the design of a future SRG to be based on projected spacecraft requirements. 

Just like fraction of Carnot efficiency can decrease isotope consumption, raising the hot end 

temperature can also increase efficiency by raising the Carnot efficiency of the engine. However, the 

higher temperatures lead to reduced material margins and increased heat losses through the insulation 

and/or thicker insulation. Figure 12 shows specific power as a function of power level and heater head 

temperature. For the ASRG-like systems, the range of heater head temperatures explored shows little 

difference between the 700 and 840 C temperatures. This is due to the balance between hot end losses 

and efficiency gains of the system. This is exactly what is seen in the ASRG as it is fully capable of 

higher temperature operation but peak power occurs at 760 C at BOL. With MLI and the other heat 

transfer enhancements for Case 5, one can see some advantages by going to higher heater head 

temperatures with the peak difference being about 0.25 w/kg. 

5.0 Conclusions 

A computer tool was developed to help understand how power output, material substitutions, and 

technology changes impact a Stirling Radioisotope Generator (SRG) mass and volume. The Parametric 

System Model (PSM) has shown that in order to increase specific power at higher power levels than the 

current Advanced Stirling Radioisotope Generator (ASRG), improvements must be made in insulation 

and heat transport systems within the SRG. Additionally, future SRG systems when optimized for highest 

specific power most likely require higher cold end temperatures than that found in the ASRG. Results 

from this analysis show that peak specific power using components similar to those used in the ASRG 

occur at around 6 w/kg and in about 120-W increments. The components, which made the system 

“similar” to the ASRG, were solid insulation and conductive heat input and heat rejection from the 

Stirling convertor. Projecting the insertion of technology advances such as higher performance materials 

and heat pipe heat transport allows the system to have improved specific power with increasing power 

and provide about 8 w/kg above 500 W. 
NASA/CR—2015-218450 13 

References 

1. Pantano, D.R.; and Hill, D.H.: Thermal Analysis of Step 2 GPHS for Next Generation Radioisotope 

Power Source Missions. Space Technology and Applications International Forum, 2005. 

2. Or, C.T.; and Carpenter R.T.: Steady State Temperature Predictions for General Purpose Heat Source 

in Various Environments. Proceedings of the 33rd Intersociety Engineering Conference on Energy 

Conversion, IECEC–98–227, Colorado Springs, CO, 1998. 

3. Schmitz, P.C.; Penswick L.B.; and Shaltens, R.K.: Stirling Isotope Power Systems for Stationary and 

Mobile Lunar Applications. Proceedings of the 4th International Energy Conversion Engineering 

Conference (IECEC–2006), San Diego, CA, 2006. 

4. Chan, J.; Wood, G.; and Schreiber, J.: Development of Advanced Stirling Radioisotope Generator for 

Space Exploration. NASA/TM—2007-214806, 2007.