Peggy A. Cornell and David L. Krause

Glenn Research Center, Cleveland, Ohio

Glen Davis

Sunpower, Inc., Athens, Ohio

Malcolm G. Robbie and David A. Gubics

QinetiQ North America, Cleveland, Ohio

Lateral Load Testing of the Advanced

Stirling Convertor (ASC–E2) Heater Head

NASA/TM—2011-217103

November 2011

AIAA–2010–6692
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Peggy A. Cornell and David L. Krause

Glenn Research Center, Cleveland, Ohio

Glen Davis

Sunpower, Inc., Athens, Ohio

Malcolm G. Robbie and David A. Gubics

QinetiQ North America, Cleveland, Ohio

Lateral Load Testing of the Advanced

Stirling Convertor (ASC–E2) Heater Head

NASA/TM—2011-217103

November 2011

AIAA–2010–6692

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

8th International Energy Conversion Engineering Conference (IECEC)

sponsored by the American Institute of Aeronautics and Astronautics

Nashville, Tennessee, July 25–28, 2010
Acknowledgments

This work is funded through the NASA Science Mission Directorate. The authors wish to acknowledge the people who made this 

effort possible including Jeff Schreiber, Ralph Pawlik, Wayne Wong, Frank Bremenour, and Sreeramesh Kalluri of NASA Glenn 

Research Center, as well as Debbie Kuhlman and Greg McNelis of Lockheed Martin for their counsel and support throughout 

this test. The authors also thank Ms. Angela Coates of the University of Akron for her real-time evaluation of test article response 

and enthusiastic analysis of the voluminous post-test data.

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identifi cation 

only. Their usage does not constitute an offi cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 
Free-piston Stirling convertors are fundamental to the development of NASA’s next 

generation of radioisotope power system, the Advanced Stirling Radioisotope Generator 

(ASRG). The ASRG will use General Purpose Heat Source (GPHS) modules as the energy 

source and Advanced Stirling Convertors (ASCs) to convert heat into electrical energy, and 

is being developed by Lockheed Martin under contract to the Department of Energy. 

Achieving flight status mandates that the ASCs satisfy design as well as flight requirements 

to ensure reliable operation during launch. To meet these launch requirements, GRC 

performed a series of quasi-static mechanical tests simulating the pressure, thermal, and 

external loading conditions that will be experienced by an ASC–E2 heater head assembly. 

These mechanical tests were collectively referred to as “lateral load tests” since a primary 

external load lateral to the heater head longitudinal axis was applied in combination with the 

other loading conditions. The heater head was subjected to the operational pressure, axial 

mounting force, thermal conditions, and axial and lateral launch vehicle acceleration 

loadings. To permit reliable prediction of the heater head’s structural performance, GRC 

completed Finite Element Analysis (FEA) computer modeling for the stress, strain, and 

deformation that will result during launch. The heater head lateral load test directly 

supported evaluation of the analysis and validation of the design to meet launch 

requirements. This paper provides an overview of each element within the test and presents 

assessment of the modeling as well as experimental results of this task. 

Nomenclature 

AFSPCMAN = Air Force Space Command Manual 

ASC = Advanced Stirling Convertor 

ASRG = Advanced Stirling Radioisotope Generator 

CSAF = Cold Side Adapter Flange 

FEA = Finite Element Analysis 

GPHS = General Purpose Heat Source 

GRC = Glenn Research Center 

LVDT = Linear Variable Differential Transformer 

PIR = Program Information Request/Release

RPS = Radioisotope Power System 

Lateral Load Testing of the Advanced Stirling Convertor

(ASC–E2) Heater Head

Peggy A. Cornell and David L. Krause

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Glen Davis

Sunpower, Inc.

Athens, Ohio 45701

Malcolm G. Robbie and David A. Gubics

QinetiQ North America

Cleveland, Ohio 44135

Abstract

NASA/TM—2011-217103 1
I. Introduction 

ASA’s next generation of Radioisotope Power System (RPS), the Advanced Stirling Radioisotope Generator 

(ASRG), is progressing toward flight qualification. It is intended to provide electrical power for spacecraft and 

planetary probes that cannot rely on solar energy. The ASRG system efficiency of 28 to 32% would reduce the 

amount of radioisotope required for a given power level by a factor of four compared to radioisotope thermoelectric 

generators. Its high specific power enables certain missions and applications.1

A key element in the path to qualification is that the Advanced Stirling Convertors (ASCs) satisfy design as well 

as flight requirements that achieve reliable operation during launch. The ASCs are being developed by Sunpower, 

Inc. under the management and technical assistance of NASA Glenn Research Center (GRC). One of the 

requirements for launch mandated from the Air Force Space Command Manual (AFSPCMAN) 91–710 V3 was 

fulfilled by subjecting an ASC–E2 heater head assembly to the maximum external axial and lateral loads anticipated 

during an ASRG mission and verifying that it successfully withstood the load environment. The load values and 

load application points for this test were documented by Lockheed Martin in the Program Information 

Request/Release (PIR) for derivation of loads to be used for the heater head lateral load test. A secondary objective 

of this test was to qualify the Finite Element Analysis (FEA) used to predict the stress, strain, and deflection within 

the test article. The heater head lateral load test subjected the hardware to thermal, pressure, and external force 

loads, while measuring the stress, strain, and deflection at key locations of interest. 

II. Test Description 

A. Test Article

The test article used to qualify the heater head analysis and to 

provide validated test results to meet launch requirements was an 

ASC–E2 assembly prepared and provided by Sunpower, Inc., of 

Athens, Ohio. The test article is shown in Fig. 1 and consists of the 

following major components: heater head assembly, transition 

assembly, cylinder assembly, pressure vessel, split flange, water 

jacket assembly, load block, and deflection plates. 

The heater head assembly consists of the heater head, heat 

collector body, heater collector plate, and internal acceptor. The 

heater head assembly was fabricated and tested according to 

Sunpower, Inc’s ASC–E2 process document with the exception of 

bolt holes that were placed in the heat collector plate and heat 

collector body. The purpose of the bolt holes was for attachment of

the load block and upper deflection plate unique to this test. The load 

block was fabricated and mounted above the heat collector to permit 

both axial loading through the test article axis and lateral loading 

through a line of action simulating the combined center of gravity of 

components above the cold side adapter flange (CSAF). The upper 

and lower deflection plates were used for mounting the mini-LVDTs 

(Linear Variable Differential Transformers) to measure lateral 

deflection and rotation. 

The transition assembly, which was secured to the heater head 

assembly by laser welding the weld flange on the heater head to the flange on the transition, consists of the 

transition, internal rejector, external rejector, and CSAF. The internal and external rejectors were brazed to the 

transition, while the CSAF was attached to the external rejector by electron beam welding. A water jacket assembly 

was fixed firmly to the top side of the CSAF and was used to control the rejector temperature of the test article. The 

water jacket assembly contains an integral, open channel for heated glycol to be brought into contact with the CSAF. 

The transition assembly as a whole was then joined via a split flange to the pressure vessel that held the lower 

deflection plates. 

The cylinder assembly, which extends into the bore of the heater head, comprised a cylinder, hot cylinder, 

clamping ring, and regenerators. The cylinder was modified from the baseline ASC–E2 design to allow for a 

pressure vessel of shorter length. The pressure vessel was designed specifically to fit over the opening to the cavity 

in the heater head permitting pressurization of the test article. A full-scale pressure vessel was unnecessary given 

N

Figure 1. Heater Head Lateral Load 

Test Article. 3-D model of the primary 

assembly components.

Load Block

Upper 

Deflection

Plate

Lower 

Deflection

Plate

Heater Head

Water

Jacket

Assembly

CSAF

Pressure 

Vessel

NASA/TM—2011-217103 2
that the linear alternator and related components were not required for 

assessment of the structural performance of the heater head. 

B. Test Fixtures and Hardware 

A preexisting test fixture was used to mount the ASC–E2 heater head 

and instrumentation. The rigid fixture provided support and alignment for 

the test article and provided the datum for displacement measurements. 

The fixture was mounted to the test facility bedplate and allowed access to 

the test article for load actuators, LVDTs, strain gages, heating source, and 

pressurization system. The components that made up the fixture include 

mini-LVDT displacement sensors, LVDT mounting plates and brackets, 

load rollers, and a mounting plate and stands. Sunpower, Inc. was 

responsible for designing and fabricating a new mounting plate and stands 

customized for testing the ASC–E2 test article. The remaining preexisting 

components were employed without modification. A view of the test 

article mounted onto the fixture’s mounting plate is shown in Fig. 2. The 

test article CSAF was bolted to the mounting plate. Stiff plates enclosed 

the test article and provided attachment locations for the mini-LVDTs, as 

well as provided a level of protection for test personnel from potential 

burn and burst hazards. 

C. Test Machine and Instrumentation 

As shown in Fig. 3, an Instron in-plane biaxial load frame at the NASA 

GRC Structural Benchmark Test Facility was used to apply external loads 

to the test article using servo-valve-controlled hydraulic actuators. The 

stiffness of the load frame was designed to industry standards for highcycle fatigue loadings up to 200,000 pounds. The four axes’ positions are 

actuated with 110,000-pound-capacity double-acting hydraulic pistons. 

Closed-loop stroke displacement control of the actuators was achieved 

with feedback from LVDTs mounted inside the actuators’ hydraulic 

pistons. Closed-loop load control was achieved with feedback from 2500-

pound-capacity fatigue-rated load cells mounted to each actuator. To 

accommodate the ASC–E2 Heater Head Lateral Load Test, a custom 

bedplate was installed within the load frame, which included a top 5-inchthick T-slotted steel mounting plate rigidly attached vertically and 

horizontally to the load frame. 

Control of the load frame actuators was accomplished with an MTI 

TESTExpress Biaxial Digital Controller, Version 1.0.17 (2009). This 

controller provides programmable proportional-integral-derivative closedloop stroke, load, or strain control of the actuator motion. Actuator control 

can be independent, or linked in any combination including crosscompensated centroidal control. A function generator is also provided to permit preprogramming of loading 

waveforms for each actuator. Furthermore, limit stop and annunciation functionality is provided to safely terminate 

any loading upon reaching a predefined load or displacement limit. The load frame controller, which was powered 

by an uninterruptible conditioning power supply, also includes a basic data acquisition system to record the applied 

loads and displacements from the four actuators, plus eight additional channels for conditioned analog signals such 

as strain gages or LVDTs. 

Bimba Flat-1 Model FS–1252.5 pneumatic cylinders were used on the load frame actuators to provide a 

mechanical overload safety function. They were powered by the laboratory central service air system and controlled 

locally with Marsh Bellofram Type 10EXHR–REG–25–2–120T10 high relief exhausting regulating valves. 

For controlling and maintaining the test article’s temperature, an ethylene glycol solution was pumped to the test 

article water jacket with a Fisher Scientific Isotemp 3016 recirculation water heating-cooling bath. For controlling 

and maintaining the test article’s internal pressure, a K-bottle helium source was manually controlled with a gas 

regulator; the system included a spring-loaded relief valve set to protect the assembly from over-pressurization. 

A National Instruments LabVIEW 5.01 data system was used to collect and save the test data. The collection rate 

is programmable and was synchronized with the load frame controller. The channels that were acquired include four 

Figure 2. Heater Head Lateral Load 

Test Article mounted onto the 

fixture’s rigid mounting plate.

Figure 3. Servo-valve-controlled 

hydraulic actuators in the Instron 

in-plane biaxial load frame. The 

actuators were used to apply external 

loads to the test article.

NASA/TM—2011-217103 3
load and two stroke channels from the load frame controller; 

eight mini-LVDTs; and 18 strain gages. A National Instruments 

SCXI data conditioning system was used to stream the digital 

data, including load, stroke, mini-LVDT, and strain gage 

signals, to the LabVIEW system. Other test data were primarily 

static and recorded by hand. This information included test 

article dimensional data, test article internal pressure, glycol 

bath reservoir, test article rejector temperature, room 

temperature, room relative humidity, and actuator air cylinder 

pressures. 

The axial and lateral loads were controlled and measured 

using two 2,500-pound-capacity Tovey Model FR10–2.5K–

B000 load cells. Loads were also measured redundantly with 

two 12,500-pound-capacity Tovey Model FR20–12.5K–B000 

load cells mounted in series on the actuator. The loads cells 

were conditioned by the load frame controller system that also 

provided analog output signals. The analog signals were 

converted to digital signals by a National Instruments module in 

the test data acquisition system. The axial and lateral load 

locations on the test article are shown in Fig. 4. 

A Noshok Model 615–1000–2–1–2–8 pressure transducer 

and digital readout measured the internal pressure. Two Omega 

Type N thermocouples measured the test article rejector 

temperature through thermocouple access holes with a Fluke 

Model 54II handheld conditioner. The pressure and temperature 

readings were recorded by hand. 

The test article displacements were measured by eight Micro-Epsilon DTA–1G–1.5–SA–F mini-LVDTs. These 

were conditioned with matched Micro-Epsilon in-line conditioners that provided analog output signals, which were 

converted to digital signals by National Instruments modules in the data acquisition system. As shown in Fig. 4, the 

mini-LVDTs were located on the upper and lower deflection plates, denoted by “U” and “L,” and representing “X” 

and “Y” axes, respectively. For the upper deflection plate located on top of the load block, two mini-LVDTs were 

mounted laterally and measured lateral deflection and rotation. The average of the two displacement readings is the 

lateral deflection of the point midway between the measurement points, and their difference divided by their 

separation distance is the torsional rotation of the line connecting their centers. Two mini-LVDTs were mounted 

axially on the upper deflection plate and measured axial deflection and rotation. The average of the two 

displacement readings is the axial deflection of the point midway between the measurement points, and their 

difference divided by their separation distance is the bending rotation of the line connecting their centers. Four miniLVDTs were mounted on the lower deflection plate located beneath the pressure vessel and measured deflections 

and rotations similarly. Comparison of 

deflection and rotation of the upper and 

lower plates provided an evaluation of 

linearity and elasticity of the test article 

and discernment of distortion between the 

heater head and the CSAF. 

Two Vishay Micro-Measurements 

Type EA–06–062AA–120 strain gages, 

one trimmed Type EA–06–031RB–120, 

and five Type EA–06–031RB–120 strain 

gage three-element rectangular rosettes 

permitted measurement of 18 test article 

strains. Four three-element strain gage 

rosettes were located near the predicted 

highest stress area on the MarM-247 heater 

head test article and provided the surface 

two-dimensional state-of-strain through 

standard mechanics of materials strain 

Figure 4. Heater Head Lateral Load Test 

Article load and mini-LVDT locations.

Figure 5. Heater Head Lateral Load Test Article strain gage 

locations. 

2 Single-Element Strain 

Gages (180° apart)

4 Three-Element Strain 

Gage Rosettes (90° apart)

MarM-247 High Stress Area 

for Final Load Case

Strain Gages 

at CSAF

NASA/TM—2011-217103 4
equations. Two single-element strain gages measured principal strains at the predicted high stress area slightly 

removed from geometry-induced stress concentrated areas. Additionally, one rosette and one single-element gage 

measured strains in the CSAF component. National Instruments modules in the test data acquisition system 

conditioned the thermocouple readings. The strain gage locations on the test article are shown in Fig. 5. 

D. Test Methodology

A substitute test article with dimensions and stiffness similar to the heater head was used to determine acceptable 

proportional-integral-derivative settings for the test frame closed-loop controller. This assured that the axial and 

lateral load hydraulic actuators would provide stable, acceptable performance for testing in load control. 

Representative ramped load steps of the test sequence matrix were applied to the substitute test article to assure 

proper operation of controls, instrumentation, and data recording. Finally, the substitute test article was used to 

activate the actuator push-rods to determine correlation factors for the overload safety pneumatic cylinders, which 

relate air pressure to mechanical overload force protection. The substitute test article was removed and the heater 

head test article and all related instrumentation was installed and verified. Alignment of the heater head test article 

within the load frame was determined by observation of strain gage readings upon axial loading to 50 pounds. 

The heater head test article was tested in three combined external load cases. Each case required 100 °C 

temperature at the rejector braze location and the maximum internal operating pressure expected during launch. For 

each load case, the axial or lateral external loads were applied in discontinuous increasing and decreasing ramped 

steps of increasing peak magnitude until the maximum indicated peak load value was reached. This enabled 

structural evaluation of the test article for evidence of material yielding or other failure modes during and after each 

ramp step. The temperature and pressure conditions as well as the following loads were provided by the Lockheed 

Martin PIR. 

1. Load Case 1 – Maximum expected flight axial compressive load 

2. Load Case 2 – Nominal flight axial load plus maximum expected flight lateral load 

3. Load Case 3 – Nominal flight axial load plus failure-inducing lateral load 

For Load Case 1, the peak values of the axial ramped steps were increased incrementally until the indicated peak 

load was reached. For Load Case 2, the peak values of the lateral ramped steps were increased incrementally until 

the indicated peak load was reached. For Load Case 3, the alternating ramped steps of increasing peak magnitude 

concluded with a steady lateral load ramp that was imposed on the test article until a failure-inducing lateral load 

was reached. Failure was defined in the test plan by the first occurrence of significant material yielding, material 

stress rupture, elastic or inelastic buckling, braze failure, fastener fracture, or leakage of the pressurization gas. 

III. Analysis 

Pro/Engineer was utilized with ANSYS simulation software for stress, strain, and deflection predictions of the 

three load cases. The geometry was updated based on measurements obtained from the test article inspection report 

provided by Sunpower, Inc. This allowed for “as-built” configuration geometries of the test article and the test 

fixture to provide the baseline modeling data for this analysis. The parameters for each of the load case simulations 

were provided by Lockheed Martin. Simulations 

were based on the test conditions of maximum 

internal operating pressure, however, the 

thermal conditions were analyzed at room 

temperature rather than operating temperature. 

This thermal state was acceptable for the 

analysis given that the behavior of MarM-247 

material tends to exhibit similar property 

characteristics at temperatures less than 800 °C. 

The boundary conditions for the load cases were 

set by fixing the model on eight mounting 

locations on the CSAF. For Load Case 1, the 

maximum compressive axial force anticipated 

during a mission was applied to the top of the 

load block. For Load Case 2, a compressive axial 

force of reduced intensity as well as the 

maximum anticipated lateral force was applied to 

the load block through a line of action simulating 

Figure 6. Predicted Maximum Stress in Heater Head. The 

red arrow indicates the maximum stress location on the test 

article given an applied lateral load.

NASA/TM—2011-217103 5
the combined center of gravity above the CSAF. Load Case 3 was equivalent to Load Case 2 with the exception of an 

increased lateral load to the point of yield on the heater head. Figure 6 shows the location of predicted maximum stress on 

the heater head for Load Cases 2 and 3, and thus the location of predicted failure in Load Case 3. 

Furthermore, deflection values of the upper and lower deflection plates for each load case in the model were 

determined by ANSYS and used to generate the predicted lateral deflection and rotation of the test assembly. 

This predicted force and deflection data was used during the actual test to corroborate the experimental results. 

IV. Experimental Results 

A. Pre-Test Configuration 

Prior to applying the external loads, the test article was heated to 100 °C at the rejector braze location by 

pumping an ethylene glycol solution of higher temperature to the test article water jacket and maintaining a 

temperature gradient such that the rejector braze location reached the required 100 °C. Upon reaching thermal 

equilibrium, the test article was pressurized internally with helium gas at the specified maximum operating pressure, 

which was maintained by manually regulating a K-bottle helium source. Strain measurements at predicted high 

stress areas indicated that the strain at this state was nominal. 

B. Material Inelasticity Description 

MarM-247 does not have a welldefined yield point like materials such as 

carbon steel. Rather, upon first loading, an 

initially stiff linear stress-strain response 

gradually transforms to lower values until 

the ultimate strength is attained. For such 

metals, the ASTM Standard E6 is often 

used to define the yield strength for the 

material at a particular offset strain value, 

commonly 0.2%. The curve denoted 

“Tensile Test Data” in Fig. 7 illustrates 

the stress-strain behavior, and the dashed 

lines show the determination of the yield 

strength (0.2% offset) from the 

intersection of a line parallel to the initial 

tangent modulus line and offset to +0.2% 

strain. 

When such a material is loaded as was 

performed for the heater head lateral load 

test, the cyclic increasing load steps result 

in the incremental accumulation of 

inelastic strain. At stress levels previously 

not experienced, the nonlinear stressstrain curve is followed; at stress levels at 

or below prior loading history, the deformation is linear and occurs at the initial tangent elastic modulus. Steps of 

unloading follow the elastic modulus, but are offset by the amount of inelastic (plastic) strain. Two increasing load 

steps are shown, for example, in yellow and green in Fig. 7. The designation of the “Yield Point” has no effect on 

this deformation pattern, but rather is chosen to facilitate engineering design of structures using such materials. 

C. Load Case 1 

The first four axial load ramps of Load Case 1 were completed without incident, except that the measured axial 

deflections, although very small in magnitude, greatly exceeded the predicted values. For this reason, it was decided 

to perform the Load Case 2 lateral load ramps before proceeding with the final two axial load ramps of Load Case 1. 

After completing Load Case 2, these final two axial load ramps were completed without incident. The load-time 

history for Load Case 1 is shown in Fig. 8. 

Figure 7. Strain Hardening Illustration. The incremental 

accumulation of inelastic strain during cyclic increasing load steps is 

illustrated on a stress-strain diagram for a generic material with an 

indistinct yield point; linear behavior is manifest for load magnitudes 

up to the prior loading history.

NASA/TM—2011-217103 6
Figure 9 shows the measured axial deflections 

relative to axial load. The average axial deflections 

were measured from the “Y” axis of the upper and 

lower deflection plates (shown in Fig. 4), denoted by 

“UY” and “LY”. The small difference in peak values 

recorded for the upper and lower deflection plates 

indicates that the majority of the deformation 

occurred in the CSAF. Resolution of the discrepancy 

between predicted and measured axial deflections is 

ongoing. With the displacements and longitudinal 

strains (Fig. 10) showing linear behavior and postramp values returning to the initial state, it was 

apparent that yielding did not occur under Load Case 

1. For Fig. 10 and the remaining longitudinal strain 

figures, the lateral load was applied at the 000 

location and the strain measurement locations are 

represented in a counter-clockwise direction from the 

top view. “U” and “L” represent the upper and lower 

strain gage placements (shown in Fig. 5). The test 

article did not exhibit yielding, stress rupture, elastic 

buckling, braze failure, fastener fracture, leakage of 

the internal helium gas, or any other failure mode in Load Case 1. It was determined that the test article passed the 

ASRG maximum expected mission axial load test requirement. 

D. Load Cases 2 and 3a 

After application of the maximum mission axial compressive load, the five lateral load ramps of Load Case 2 

proceeded as planned (Fig. 11). For loads up to the maximum of Load Case 2, the measured test article lateral 

deflections (Fig. 13), lateral rotations (Fig. 14), and longitudinal strains (Fig. 15) closely matched predicted values, 

were linear, and returned to their initial states upon unloading, as indicated in the plots shown by heavy bold lines. 

Similar to Load Case 1, yielding, stress rupture, elastic buckling, braze failure, fastener fracture, leakage of the 

internal helium gas, and other failure modes did not occur in Load Case 2. It was determined that the test article 

passed the ASRG maximum expected mission lateral load test requirement. 

Figure 9. Load Case 1 Measured Axial 

Deflections. Though larger than predicted, the axial 

deflections still create apparently noisy data plots 

due to their small magnitude; inelastic behavior is 

minimal. Angular displacements were imperceptible 

from electronic noise.

Figure 10. Load Case 1 Measured Longitudinal 

Strains. The positive-valued longitudinal strains due 

to internal pressure were decreased during the 

compressive axial loadings of Load Case 1.

Figure 8. Load Case 1 Time History. This plot shows 

application of constant rate axial load ramps with variable 

inspection hold times at loaded and unloaded states for the 

Load Case 1 loading sequence; the final ramp shown is the 

maximum expected mission axial load.

NASA/TM—2011-217103 7
The data plots of Load Case 3a initially replicates Load Case 2, as the increased lateral load ramps of this case 

are an experimental extension of the Load Case 2 ramps. Fig. 12 provides the load-time history for the seven 

increasing peak lateral load ramps of Load Case 3a; the last peak value was more than twice the Load Case 2 lateral 

load value. For the Load Case 3a loadings, the accumulation of inelastic deformation and strain (Figs. 13 to 15) is 

evident, as described in the Material Inelasticity Description (Section B) above. The average lateral deflections 

shown in Fig. 13 were measured from the “X” axis of the upper and lower deflection plates (shown in Fig. 4), 

denoted by “UX” and “LX”. For Fig. 14 and the remaining lateral rotation figures, the total rotation, CSAF rotation 

(ӨROT), and heater head rotation (ӨBEND) are shown. Even at the maximum lateral load of this case, the measured 

residual strain at the high stress area remained well below 0.2%, so it is assured that the defined yield strength (0.2% 

offset) was not reached. In addition, no other failure modes were observed for Load Case 3a. The peak load of Load 

Case 3a was slightly greater than the predicted load to reach the defined yield stress at the most critical location of 

the test article. 

Figure 11. Load Case 2 Time History. This plot 

shows application of constant rate lateral load ramps 

with variable inspection hold times at loaded and 

unloaded states for the Load Case 2 loading sequence; 

the final ramp shown is the maximum expected mission 

lateral load.

Figure 12. Load Case 3a Time History. This plot 

shows application of constant rate lateral load ramps 

with variable inspection hold times at loaded and 

unloaded states for the Load Case 3a loading 

sequence; all ramp peaks are at magnitudes greater 

than the maximum expected mission lateral load.

Figure 13. Load Cases 2 and 3a Measured 

Lateral Deflections. For the prefailure lateral load 

cases, the measured lateral deflections closely 

matched the predicted values, especially after the first 

20 to 30% of the maximum load shown. Very little 

inelastic behavior is noted at and below the maximum 

expected mission lateral load shown in bold.

Figure 14. Load Cases 2 and 3a Measured 

Lateral Rotations. Similar to lateral deflections 

measured for the prefailure lateral load cases, the 

measured lateral rotations matched the predicted 

values. Again, very little inelastic deformation is 

apparent at and below the maximum expected mission 

lateral load shown in bold.

NASA/TM—2011-217103 8
E. Load Case 3b 

To determine the maximum lateral load-carrying 

capacity of the test article and induce gross structural 

deformation or other failure modes, the internal 

pressure and axial compressive loads were maintained 

as in Load Cases 2 and 3a, and a final lateral load 

ramp using the constant rate from previous testing was 

imposed on the test article. The end peak load for this 

ramp was set to a very high value to cause test 

termination by attaining a predefined large lateral 

deflection. The load-time history for Load Case 3b is 

shown in Fig. 16, where a peak load was reached that 

is more than three times greater than the maximum 

expected mission lateral load. Lateral deflections (Fig. 

17), lateral rotations (Fig. 18), and longitudinal strains 

(Fig. 19) were linear up to the magnitude of the peak 

lateral load of Load Case 3a, and then followed an 

elastic-plastic curve that may be typical for the MarM247 heater head material. The average lateral 

deflections shown in Fig. 17 were measured from the 

“X” axis of the upper and lower deflection plates 

(shown in Fig. 4), denoted by “UX” and “LX”. The 

linear response of the lower deflection plate indicated 

that the CSAF remained elastic, while the large 

nonlinear deformation measured at the upper deflection plate, as well as visual inspection of the test article, revealed 

formation of a plastic hinge at the heater head high stress areas. 

The failure mode achieved in Load Case 3b was yielding of the heater head and resultant gross deformation. 

Upon physical inspection after completion of Load Case 3b, no other failure modes were evident. 

Figure 17. Load Case 3b Measured Lateral 

Deflections. The measured lateral deflections for the 

failure load case indicated that the CSAF remained 

linear and elastic, while the heater head yielded 

considerably without losing pressure integrity.

Figure 16. Load Case 3b Time History. This plot 

shows application of a constant rate lateral load ramp 

until failure of the test article is manifest, when the test 

load frame automatically shut down and released the 

load. The peak lateral load attained was more than 

three times the maximum expected mission lateral 

load.

Figure 15. Load Cases 2 and 3a Measured

Longitudinal Strains. Longitudinal strains measured at 

predicted peak stress locations were linear and elastic at 

and below the maximum expected mission lateral load 

shown in bold. At higher lateral loads, inelasticity is 

evident on the bending tensile side (000L) and, to a lesser 

extent, the bending compressive side (180L).

NASA/TM—2011-217103 9
V. Conclusion 

The Heater Head Lateral Load Test was successful in directly supporting qualification of the Advanced Stirling 

Convertor (ASC)–E2 heater head analysis and validation of the design to meet launch requirements. The pressurized 

test article was exposed to anticipated maximum axial compressive and maximum lateral loads during an Advanced 

Stirling Radioisotope Generator (ASRG) launch, and reliably sustained these load conditions. After demonstrating 

that the test article did not fail under flight-like loads, the test continued with increased lateral loading until the 

heater head yielded, sustaining more than three times the maximum expected mission lateral load while maintaining 

pressure integrity. This test result validated the capability of the heater head to meet the launch load requirements 

with sufficient margin. 

References 

1Chan, J., Wood, J.G., and Schreiber, J.G., “Development of Advanced Stirling Radioisotope Generator for Space 

Exploration,” proceedings of Space Technology and Applications International Forum (STAIF 2007), edited by M.S. El-Genk, 

AIP Conference Proceedings 880, pp. 615–623, 2007. 

Figure 18. Load Case 3b Measured Lateral 

Rotations. Similar to the measured lateral deflections, 

the lateral rotation measurements for the failure load 

case indicated that the CSAF remained linear and 

elastic, while the heater head yielded considerably.

Figure 19. Load Case 3b Measured Longitudinal 

Strains. Even at the highest stress area, the 

longitudinal strains under the failure lateral loading 

were linear up to prior history load levels (shown in 

phantom), then followed the shape of a stress-strain 

curve typical for the heater head MarM-247 material.

NASA/TM—2011-217103 10
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Lateral Load Testing of the Advanced Stirling Convertor (ASC-E2) Heater Head 

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Cornell, Peggy, A.; Krause, David, L.; Davis, Glen; Robbie, Malcolm, G.; Gubics, David, A. 

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WBS 138494.05.02.04.01 

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14. ABSTRACT

Free-piston Stirling convertors are fundamental to the development of NASA’s next generation of radioisotope power system, the Advanced 

Stirling Radioisotope Generator (ASRG). The ASRG will use General Purpose Heat Source (GPHS) modules as the energy source and 

Advanced Stirling Convertors (ASCs) to convert heat into electrical energy, and is being developed by Lockheed Martin under contract to 

the Department of Energy. Achieving flight status mandates that the ASCs satisfy design as well as flight requirements to ensure reliable 

operation during launch. To meet these launch requirements, GRC performed a series of quasi-static mechanical tests simulating the 

pressure, thermal, and external loading conditions that will be experienced by an ASC-E2 heater head assembly. These mechanical tests 

were collectively referred to as “lateral load tests” since a primary external load lateral to the heater head longitudinal axis was applied in 

combination with the other loading conditions. The heater head was subjected to the operational pressure, axial mounting force, thermal 

conditions, and axial and lateral launch vehicle acceleration loadings. To permit reliable prediction of the heater head’s structural 

performance, GRC completed Finite Element Analysis (FEA) computer modeling for the stress, strain, and deformation that will result 

during launch. The heater head lateral load test directly supported evaluation of the analysis and validation of the design to meet launch 

requirements. This paper provides an overview of each element within the test and presents assessment of the modeling as well as 

experimental results of this task. 

15. SUBJECT TERMS

Free-piston Stirling convertor; Radioisotope power systems; Advanced Stirling radioisotope generator 

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