Gary R. Halford

Glenn Research Center, Cleveland, Ohio

Ashwin Shah

Sest, Inc., Middleburg Heights, Ohio

Vinod K. Arya

University of Akron, Akron, Ohio

David L. Krause and Paul A. Bartolotta

Glenn Research Center, Cleveland, Ohio

Structural Analyses of Stirling Power Convertor

Heater Head for Long-Term Reliability,

Durability, and Performance

NASA/TM—2002-211327

April 2002
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Gary R. Halford

Glenn Research Center, Cleveland, Ohio

Ashwin Shah

Sest, Inc., Middleburg Heights, Ohio

Vinod K. Arya

University of Akron, Akron, Ohio

David L. Krause and Paul A. Bartolotta

Glenn Research Center, Cleveland, Ohio

Structural Analyses of Stirling Power Convertor

Heater Head for Long-Term Reliability,

Durability, and Performance

NASA/TM—2002-211327

April 2002

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NASA/TM—2002-211327 1 

Structural Analyses of Stirling Power Convertor Heater Head for Long-Term 

Reliability, Durability, and Performance

Gary R. Halford 

National Aeronautics and Space Administration 

Glenn Research Center 

 Cleveland, Ohio 44135 

Ashwin Shah 

Sest, Inc. 

 Suite 104, 18000 Jefferson Park 

Middleburg Heights, Ohio 44130 

Vinod K. Arya 

University of Akron 

Department of Theoretical and Applied Mathematics 

Akron, Ohio 44325 

David L. Krause and Paul A. Bartolotta 

National Aeronautics and Space Administration 

Glenn Research Center 

 Cleveland, Ohio 44135 

Summary

 Deep-space missions require onboard electric power systems with reliable design lifetimes of up to 

10 yr and beyond. A high-efficiency Stirling radioisotope power system is a likely candidate for future 

deep-space missions and Mars rover applications. To ensure ample durability, the structurally critical 

heater head of the Stirling power convertor has undergone extensive computational analyses of operating 

temperatures (up to 650 °C), stresses, and creep resistance of the thin-walled Inconel 718 bill of material. 

Durability predictions are presented in terms of the probability of survival. A benchmark structural testing 

program has commenced to support the analyses. This report presents the current status of durability 

assessments. 

Introduction 

 Reliable durability along with ample functional performance must be demonstrated before long-term 

structural systems can be fully accepted for use. Computational analyses and long-term experimental 

input are essential ingredients for this. The approach taken to ensure the structural reliability, durability, 

and performance of a Stirling power convertor heater head is described and discussed in this report. The 

convertor must generate electric power with a high degree of reliability for mission durations up to 10 yr 

and beyond. The structurally critical cylindrical heater head is made of thin-section wrought Inconel 718 

and must operate at temperatures as high as 650 °C. Creep resistance is the dominant durability limitation. 

 Tradeoffs between maximum operating temperature (650 °C), hoop stress, and creep resistance have 

been examined for Inconel 718 in the conditions of proposed use. The internally pressurized cylinder 

(nominal 2.5 MPa) must not be allowed to distort excessively (or rupture) because of creep. The structural 

durability design criterion for the heater head is expressed as a durability surface with coordinates in 
NASA/TM—2002-211327 2 

temperature T, stress σ, and operational lifetime tf. Families of nonintersecting durability surfaces 

reflect various probabilities of successful operation. 

 In support of the durability analysis activities, the NASA Glenn Research Center is generating creep 

and rupture data for the material with specified thickness and specific heat that is to be used in the 

construction of heater heads for flight convertors. A benchmark component testing program is also 

underway. Because of time constraints, creep testing must be restricted to no more than about a 2-yr 

duration, far short of the required 10 yr or more. Fortunately, about three decades ago, an extensive longterm creep and creep-rupture data base had been generated (Brinkman, Booker, and Ding, 1991) on 

Inconel 718 by the Oak Ridge National Laboratories (ORNL) in support of terrestrial electric power 

generation. Certain aspects of their results are being used extensively in the current evaluations, namely, 

long-term results and measured statistical variations. The ORNL creep life data cannot be used directly 

because of quantifiable differences between them and preliminary NASA Glenn data resulting from slight 

variations in alloy chemistry and grain size. Also of importance is the significant difference in thickness 

of the creep test specimens used by the two laboratories. The Glenn creep test specimens were only 

0.5 mm thick (i.e., about four to five times the diameter of a human hair). This thickness was selected to 

be reasonably representative of the thickness of the convertor pressure vessel enclosing the regenerator. 

Preliminary design calculations indicated that the maximum thickness in this area might be as small as 

0.38 mm. Based on information of the current study, however, the hot section thickness has to be 

increased to meet life requirements with high probabilities of success. The new thickness values under 

consideration range from 0.89 to 1.14 mm. By contrast, the ORNL data (Brinkman, Booker, and Ding, 

1991) were generated using test specimens on the order of 20 times thicker (≈10 mm) than those used by 

Glenn. Creep resistance for wrought alloys generally decreases dramatically with decreasing specimen 

thickness (below about 20 grain diam.) and with decreasing grain size. Scaling factors accounting for the 

differences between the ORNL and the Glenn data have been established and will be reexamined as the 

Glenn data base nears completion. 

 The heater head benchmark testing program will assess relatively short-term (1 yr) creep growth 

under realistic operating conditions. The favorable, concave upward shape of the creep curve of Inconel 

718 allows realistic extrapolation to longer times from short time behavior. 

Finite-Element Analysis 

 Elastic finite-element structural and thermal analyses of the cylindrical portion of the heater head 

were performed using the finite-element program MARC (Anon., 1996). Appropriate boundary and 

symmetry conditions were applied at both ends. No credit was taken for any stiffening or strengthening 

imparted by either an internal or external heat-conduction structure that might increase durability. 

 Because of geometric symmetry, only a segment (slice) of the heater head (fig. 1) was modeled and 

utilized in the analysis. Thin-shell elements were employed. The tapered wall thickness of the segment 

was incorporated in the shell elements. 

 A steady mean internal pressure of 2.5 MPa was applied to the heater head. The finite-element 

analysis provided stress values in excellent agreement with those obtained from considerations of the 

basic strength of materials equilibrium, which considers the heater head locally as a thin cylindrical shell. 

Effects on durability resulting from an 82-Hz oscillating internal pressure of ±10 percent of the mean 

have been considered. However, results of detailed calculations have shown these effects to be of secondorder importance and hence are not included in the ensuing analyses. 

 A linear thermal conductivity analysis of the heater head was performed by employing temperature 

boundary conditions of 650 °C at the thicker heater end and 80 °C at the cold, thinner end. The 

temperature distribution over the axial length of the heater head was slightly nonlinear because of the 

wall thickness tapering. The temperature-dependent values of Young’s modulus of elasticity E and the 

coefficient of thermal expansion CTE for Inconel 718 used in the analyses are listed in table I. 
NASA/TM—2002-211327 3 

Figure 1.—Stirling heater head structural analysis segment.

650 °C

80 °C

TABLE I.TEMPERATURE-DEPENDENT PROPERTIES 

OF INCONEL 718 

Temperature, 

°C 

Elastic modulus, 

E, 

MPa 

Thermal expansion, 

CTE, 

10−6

/°C 

21 200 12.2 

204 190 13.3 

427 178 14.0 

538 171 14.4 

650 163 14.9 

 For purposes of parametric studies, three values of the pressure vessel wall thickness at the critical 

hot end of the regenerator were considered: 0.89, 1.02, and 1.14 mm. In all cases, the thickness tapered 

linearly to a mere 0.28 mm at the cold temperature end. These configurations gave rise to a range of 

probabilities of survival for a given mission usage. 

 

Creep Characterization 

 The generation of sufficient long-term (>104

 h), high-temperature creep data for engineering alloys is 

a formidable and time-consuming task. It is expensive because of the duration of testing and the range of 

variables to be covered. Many tests under different temperature and stress conditions generally are 

required for numerous heats of a material to determine general creep and creep-rupture laws describing 

the behavior. Also, large numbers of these tests are required because of the significant scatter typically 

observed in creep and creep-rupture testing. 

 Because of cost and time constraints, large data bases for long-term creep and creep-rupture behavior 

are rarely generated. Current industrial practice is to perform accelerated creep tests and extrapolate the 

results to approximate long-term behavior using what are known as time-temperature parameters, the 

most common being the classical Larson-Miller parameter (1952). As discussed below, we have not had 

to resort to such extrapolation methods because of the existence of a large data base for Inconel 718. 

 In an effort to develop a suitable creep durability model for analyses of the heater head, an 

engineering methodology has been adopted. It utilizes the large creep data base generated by ORNL 

(Brinkman, Booker, and Ding, 1991) for various heats, heat treatments, and product forms of Inconel 718. 

Although there are differences between the material used for the ORNL tests and that to be used in the 
NASA/TM—2002-211327 4 

current heater head, the proposed methodology captures the differences and accounts for their effect on 

creep behavior. The following paragraphs provide details of the analyses of the ORNL data and a 

description of the methodology for the material to be used for the heater head. 

 The ORNL test data encompass 14 heat forms of bar, plate, and forged material over the temperature 

range 538 to 704 °C. The longest reported rupture life is 87 000 h (10 yr). The data cover the pertinent 

temperature and time regimes of the heater head design. The Glenn heater head test material is only about 

5 percent of the thickness of the ORNL test material. As described in the Introduction, the creep behavior 

of this material could be appreciably different from that of the ORNL material. Measures have been taken 

to evaluate correlation constants for the Glenn creep data with respect to those reported by ORNL. The 

methodology focuses on adopting the ORNL master curve model (eq. (1)). This model is thoroughly 

explained by Brinkman, Booker, and Ding (1991): 

e tt * ** = − 

 





 







  exp (1) β

α

1

 

where e

* is the normalized creep strain (with respect to creep strain at the onset of tertiary creep), t* is 

the normalized time (with respect to time at the onset of tertiary strain), and α and β are constants 

quantified from test data. 

 The stress and temperature independence of the general form of equation (1) over the temperature 

range up to 650 °C makes it generic in terms of its application. Constants in the equation had been 

evaluated by ORNL using nonlinear regression analysis. These data were further analyzed to quantify 

their statistical distribution and scatter in the magnitude of the constants. It should be noted that scatter in 

the ORNL results is assessed independently of the product form and other variables. Hence, it is 

reasonable to take advantage of the large data base generated by ORNL to approximate the scatter in the 

relatively small amount of data that will be generated in the limited-time Glenn program. The ORNL 

creep-rupture law (eq. (2)) has been applied directly to the existing Glenn results on the heater head 

material by adjusting only the heat constant Ch, giving rise to rupture lives 

log log log log log (2) (t C C C C CT r h ) =+ + 1 2( ) + ( ) +

2

3

3

σσ σ σ 4

where tr is rupture life in hours, Ch, C1, C2, C3, and C4 are the constants determined from the ORNL 

data (for Inconel 718, Ch = 162.319, C1 = −193.662, C2 = 88.117, C3 = −12.807, and C4 = −0.01052), 

σ is stress in megapascals, and T is temperature in kelvin. 

 These rupture lives are, at worst, one-third of those from ORNL for the same stress and temperature. 

As more Glenn data are generated, this factor of one-third could change. This difference in rupture lives 

reflects the combined effects of specimen thickness, chemistry, and grain size governed by the heat 

treatment. Note that the limited Glenn data are based on creep tests at higher stresses (and shorter times to 

rupture) than anticipated in the actual design. Therefore, the Glenn results must be extrapolated to the 

lower stress, longer life regime. Using the rupture life variation pattern at the higher stresses, the Glenn 

results are extrapolated parallel in life to those from ORNL. Once the rupture life from equation (2) is 

calculated, equation (1) can be used for determining the creep strain behavior. Thus, the rupture life is 

downscaled to give an estimation of the time to the onset of the tertiary creep. Based on the ORNL data, 

the scale factor for the time to reach tertiary creep tter is approximately 70 percent of the rupture life tr. 

The design time to failure is defined as tf, where tf ≡ tter. The selection of the tertiary creep strain as the 

criterion for design lifetime was based on two important considerations: First, it places a deformation 

limit on the cylindrical section of the heater head that prevents the wall from growing excessively, and 

second, this measure of strain was fortunately well documented by ORNL. Equation (1) resulted from the 

recorded data. 
NASA/TM—2002-211327 5 

Design Criteria and Probabilistics 

 Considerable scatter is generally observed in both creep rate and creep-rupture life data. The reliable 

design of high-temperature structural components subject to creep becomes complicated because of these 

uncertainties. Conventional deterministic design approaches use sizeable knockdown safety factors to 

ensure adequate durability. This usually results in overly conservative and heavy designs. Since spacebased power conversion systems generally cannot tolerate excess weight, a rational alternative to 

deterministic design is required. Fortunately, uncertainties in material behavior as well as any design 

variables can be rationally accounted for using a probabilistic approach. 

 This long-range probabilistic approach for the heater head analysis aims at quantifying the 

uncertainties in the constants in equations (1) and (2) and adopting a Monte-Carlo simulation to perform 

probabilistic creep and creep-rupture durability analyses. The methods of most likelihood event (MLE) 

and least squares will be combined to quantify the uncertainties in the constants. Since sufficient data 

for the Glenn heater head material are not available, the uncertainties in the ORNL data have been 

quantified and applied to the Glenn heater head material. ORNL test data covering stress values from 

170 to 1096 MPa and temperatures from 538 to 704 °C were fit to a Weibull distribution using the MLE 

method. Figure 2 shows the cumulative probability distribution of the ORNL test data (in unit normal 

space) and the analytic Weibull distribution of these data. Note that the rupture life has been represented 

as a ratio of observed rupture life to the mean rupture life. The quantified mean value of the ratio is 

1.57, and the scatter is 91 percent. Since the analytically obtained probability distribution curve matches 

fairly well with the test data, it is used to compute the life for a desired probability of survival. 

 Three-dimensional probabilistic durability surfaces (stress, temperature, and design time to failure) 

for ranges of reliability have been generated. An example durability surface for 50-percent probability of 

survival (PoS) for extrapolated creep-rupture life for the ORNL data is shown in figure 3. The durability 

surface is shaded with patterns to show various combinations of temperature and stress that result in the 

range of life represented by a specific shade. Families of nominally parallel surfaces (parallel relative to 

the time axis) would represent various levels of PoS. 

 

Figure 2.—Probability distribution (in unit normal space) of the rupture life,

 ORNL data with stresses of 170 to 1096 MPa and temperatures from 538 

 to 704 °C.

–4.0

–3.2

–2.4

–1.6

–0.8

0.0

0.8

1.6

2.4

3.2

4.0

012345678

Observed rupture life/mean rupture life

Cumulative probability

Analytical

Data

 
NASA/TM—2002-211327 6 

Figure 3.—Failure surface for 50-percent PoS. Extrapolated

 time to rupture, tr.

11

9

7

5

50

70

90

110

500

550

600

650

Temperature,

°C

log tr, h

Stress, MPa

109 to 1011

107 to 109

105 to 107

Durability,

h

TABLE II.FACTORS OF SAFETY ON CALCULATED 

 ALLOWABLE LIFETIME RELATIVE TO 100 000 h 

FOR VARIOUS PROBABILITIES OF SURVIVAL 

Factors of safety 

Maximum thickness, mm 

Probability 

of survival, 

percent 0.89 1.02 1.14 

50 13 52 230 

90 5 21 93 

99 1.2 5 21 

99.9 0.2 1 4 

99.99 0.05 0.2 1 

 The factors of safety on design lifetime are indicated in table II for the three maximum wall thickness 

values (of the pressure vessel enclosing the regenerator) considered. The factor of safety is expressed as 

the ratio of the calculated lifetime to the desired lifetime of 100 000 h for PoS of 50, 90, 99, 99.9, and 

99.99 percent for a constant use temperature of 650 °C and internal pressure of 2.5 MPa. 

 A lifetime ratio >1.0 satisfies the design lifetime requirement. The maximum thickness of 1.14 mm 

is required for the highest PoS shown, whereas a wall thickness of only 0.89 mm would be ample for a 

lower PoS of only 99 percent. The difference in weight between the two configurations is only about 25 g 

and has a negligible impact on the overall weight of the Stirling power convertor system. The greatest 

thickness also has only a small impact on the operating efficiency of the system. 

 To compare the lifetime results of table II to more traditional stress-based evaluations of PoS, it is 

necessary to transform the variables. Graphically, this is straightforward. First, examine the failure 

surface in figure 3 (shown only for 50 percent PoS) and add the other four failure surfaces for the higher 

percentages of PoS. Then take the two-dimensional slice of computed lifetime versus stress at a constant 

temperature of 650 °C. One thus obtains a series of five nonintersecting curves that represent the tradeoff 

between life and stress at 650 °C. For a constant lifetime of, say, 100 000 h, a horizontal line has five 

intersection points with the five PoS curves. These intersection points identify the critical stress values for 
NASA/TM—2002-211327 7 

TABLE III.FACTORS OF SAFETY ON CALCULATED 

CRITICAL STRESS FOR VARIOUS PROBABILITIES 

OF SURVIVAL AT 650 °C 

Factors of safety 

Maximum thickness, mm 

Probability 

of survival, 

percent 0.89 1.02 1.14 

50 1.6 1.8 2.0 

90 1.2 1.4 1.6 

99 1.0 1.2 1.3 

99.9 0.9 1.0 1.1 

99.99 0.8 0.9 1.0 

the five PoS values. If each critical stress is normalized to the stress existing at the thickest section of the 

pressure vessel enclosing the regenerator (at 650 °C), one obtains the safety factor for the stress needed to 

achieve the PoS desired. Results are summarized in table III. Note that for 50 percent PoS, the factor of 

safety for stress ranges from 1.6 to 2.0, depending upon thickness. 

 The proposed durability assessment methodology described above is straightforward and does not 

assume a specific underlying probability distribution. It is devoid of the convergence problems associated 

with highly nonlinear-behaving durability response surfaces and time-dependent reliability algorithms. 

The curves also identify the magnitudes of stress and temperature for a desired life. Thus, it could be used 

for design iterations to optimize and improve a given design. The resultant methodology, although 

currently lacking sufficient intermediate-term creep characteristics of the thin Glenn heater head material, 

promises a workable approach for evaluating design tradeoffs between temperature, stress, and useful 

lifetime with assured reliability. 

 A full probabilistic analysis of the critical factors governing the design life of the heater head is not as 

yet available. In fact, such an analysis could be misleading in the absence of accurate and pertinent 

information. An appropriately thorough assessment must await the more complete evaluation of the creep 

and creep-rupture properties of the specific Glenn material. Another year of coupon and benchmark 

testing is awaiting completion. In addition, procedures and techniques are being evaluated for application 

as further information becomes available. Additional reviews are to be held prior to finalizing important 

details of the heater head design. Clearly, however, relatively small additions of material to increase wall 

thickness in critical locations or even small percentage decreases in operating temperature can have a 

profound influence on increasing the reliability of a given mission lifetime. An easily remembered 

tradeoff is that at a given value of PoS and stress, a 15 °C temperature change causes the creep life to 

change by a factor of 2. Other tradeoffs between life, temperature, stress, and PoS can be assessed from 

the family of computed failure surfaces similar to the single surface shown in figure 3. 

 Armed with the information now available, it is also possible to assess a nonsteady mission loading 

history. For example, one could assess the impact on usable durability of a decreasing temperature of the 

heater head with time reflecting a realistic radioisotope decay process. Preliminary calculations by the 

authors reflect an approximate order-of-magnitude increase in life with an exponential temperature 

decrease of 100 °C over a 10-yr span. 

Benchmark Structural Testing 

 A series of experimental benchmark tests of the heater head are planned to corroborate the structural 

analyses and independently determine creep performance of this component under the design loading 

conditions. This testing is important because predictions of creep strain in general are based on material 

properties determined from uniaxial creep testing. Both the ORNL historical data and the current NASA 

creep data are from uniaxial tests. However, the heater head configuration and pressure loading result in a 

2:1 biaxial stress state, and the effects of this condition will be captured in the benchmark tests. 
NASA/TM—2002-211327 8 

 The benchmark test program includes a first phase of evaluating a prototype heater head (fig. 4) 

at several combinations of temperature and pressure that result in creep strains representing 1, 3, and 

6 months of accelerated creep deformation. These results will be employed in the analyses to more 

accurately predict creep behavior. The second phase of testing applies the design pressure (2.5 MPa) and 

temperature gradient (peaking at 650 °C) to the benchmark heater head for 1 yr or longer. A decision is to 

be made later as to whether a constant temperature or a programmed decreasing temperature history 

profile is used. Creep deformations will be recorded periodically, and an evaluation of experimental and 

analytically predicted values will be performed. This study will then result in the final prediction of the 

creep life under mission conditions for a 10-yr duration. 

 The experimental equipment includes benchtop hardware mounted with temperature-control and gas 

pressurization systems. The prototype test hardware is attached to a water-cooled stainless steel mounting 

flange that has been machined with a metal O-ring groove and threaded holes to match the cold flange. The 

heating system consists of a 4.5-kVA induction heater with water-cooled copper coils designed to provide 

the correct temperature gradient throughout the part. The pressurization system includes a regulated highpressure supply of purified argon gas, a solenoid valve, a direct drive control valve, a proportional pressure 

relief valve, the connecting tubing, and gas passageways machined into the mounting flange. An alumina 

volume fill plug is located inside the test cylinder to aid in control of gas pressurization. 

 A customized digital control system provided by McGaw Technology, Inc., controls pressurization 

and records experimental data. The control system also provides ramping, mean pressure and alternating 

pressure cycles, and synchronized thermal cycles, if needed, for any future test requirement. The 

instrumentation consists of thermocouples to measure metal temperature gradients and monitor argon 

temperature; a pressure transducer to control gas pressure; and strain gages to acquire initial mechanical 

pressure strains at room temperature. Creep deformation at the cylinder-head juncture and at the head 

centerline is continuously monitored with five linear variable differential transformers. 

 At the present time all test hardware is assembled, and nonintegrated initial testing of the induction 

heater and the argon system is complete. A geometrically simplified demonstration specimen has been 

fabricated and is currently being used to verify the performance of the entire test and data acquisition 

system. 

Figure 4.—Benchmark testing hardware.

Mounting 

for gas 

manifold

Induction

heater coils

Heater head

pressure

boundary

Volume

fill plug

Displacement

probes
NASA/TM—2002-211327 9 

Concluding Remarks 

 Preliminary durability analyses emphasizing the probability of survival have been performed to assess 

the potential capabilities of a Stirling power convertor heater head design for long-term space science 

missions (deep-space and Mars rover applications). Mission lifetimes of 100 000 h appear achievable 

with Inconel 718 utilized in thin sections with the maximum thickness of the pressure vessel enclosing the 

regenerator in the range of 0.89 to 1.14 mm at temperatures as high as 650 °C. Confidence in the 

calculated probabilities of survival is currently low, as a very small amount of hardware-specific material 

creep data have been generated in the limited time span available to date. Nevertheless, identified trends 

in the influence of critical factors such as temperature and stress are now usable as valuable input to 

finalizing the optimal heater head design for performance and reliable durability. 

References

1. Bowman, Randy R.: Long-Term Creep Assessment of a Thin-Walled Inconel 718 Stirling Power-Convertor Heater Head. 

Intersociety Energy Conversion Engineering Conference Paper IECEC2001−CT−33, 2001, pp. 435−440. 

2. Brinkman, C.R.; Booker, M.K.; and Ding, J.L.: Creep and Creep-Rupture Behavior of Alloy 718. Proceedings of the 

International Symposium on the Metallurgy and Applications of Superalloys 718, 625, and Various Derivatives, Minerals, 

Metals & Materials Society, Warrendale, PA, 1991. 

3. Larson, F.R.; and Miller, James: A Time-Temperature Relationship for Rupture and Creep Stresses. Trans. ASME, vol. 74, 

pp. 765−775. 
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NASA TM—2002-211327

E–13136

15

Structural Analyses of Stirling Power Convertor Heater Head for Long-Term

Reliability, Durability, and Performance

Gary R. Halford, Ashwin Shah, Vinod K. Arya, David L. Krause,

and Paul A. Bartolotta

Structural analysis; Structural durability; Stirling engine; Creep; Creep-rupture;

Life prediction; Inconel 718

Unclassified -Unlimited

Subject Category: 39 Distribution: Nonstandard

Prepared for the Space Technology and Applications International Forum (STAIF–2002) sponsored by the American

Institute of Aeronautics and Astronautics, Albuquerque, New Mexico, February 3–7, 2002. Gary R. Halford, David L.

Krause, and Paul A. Bartolotta, NASA Glenn Research Center; Ashwin Shah, Sest, Inc., 18000 Jefferson Park, Suite 104,

Middleburg Heights, Ohio 44130; and Vinod K. Arya, University of Akron, Department of Theoretical and Applied

Mathematics, Akron, Ohio 44325. Responsible person, Gary R. Halford, organization code 5000, 216–433–3265.

Deep-space missions require onboard electric power systems with reliable design lifetimes of up to 10 yr and beyond.

A high-efficiency Stirling radioisotope power system is a likely candidate for future deep-space missions and Mars rover

applications. To ensure ample durability, the structurally critical heater head of the Stirling power convertor has undergone extensive computational analyses of operating temperatures (up to 650 °C), stresses, and creep resistance of the

thin-walled Inconel 718 bill of material. Durability predictions are presented in terms of the probability of survival.

A benchmark structural testing program has commenced to support the analyses. This report presents the current status

of durability assessments.

WU–896–50–10–00