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

Cornell, Peggy A.

Davis, Glen

Gubics, David A.

Krause, David L.

Robbie, Malcolm G.

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

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Lateral Load Testing of the Advanced Stirling
Convertor (ASC–E2) Heater Head
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.  
https://ntrs.nasa.gov/search.jsp?R=20110008756 2019-08-30T15:10:33+00:00Z
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Lateral Load Testing of the Advanced Stirling
Convertor (ASC–E2) Heater Head
Peggy A. Cornell, NASA Glenn Research Center
David L. Krause, NASA Glenn Research Center
Glen Davis, Sunpower, Inc.
Malcolm G. Robbie, QinetiQ North America
David A. Gubics, QinetiQ North America
Presented at the 8th International Energy Conversion Engineering Conference 
Nashville, TN
July 25 - 28, 2010 
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Introduction
• Objective: To validate the ASC-E2 heater head design to meet 
launch requirements for the Advanced Stirling Radioisotope 
Generator (ASRG)
– Launch requirements were mandated by AFSPCMAN 91-710 V3 (Range 
Safety User Requirements)
• GRC performed tests subjecting an ASC-E2 heater head assembly 
to the operational pressure, axial mounting force, thermal conditions, 
and axial and lateral launch vehicle acceleration loadings that will be 
experienced during launch
– Stress, strain, and deflection were measured at key locations 
– The load values and load application points 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 
The heater head lateral load test directly supported 
evaluation of the analysis and validation of the design to 
meet launch requirements 
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Test Article Design
The Heater Head Lateral Load 
Test Article includes:
• Heater Head Assembly
• Transition Assembly
• Cylinder Assembly
• Cold Side Adapter Flange 
(CSAF)
• Pressure Vessel
• Water Jacket Assembly
• Load Block
• Deflection Plates
Load Block Upper 
Deflection
Plate
Lower 
Deflection
Plate
Heater Head 
Assembly WaterJacket
Assembly
CSAF
Pressure 
Vessel
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Test Fixture Design
The Heater Head Lateral Load 
Test Fixture includes:
• LVDT Mounting Plates
• LVDT Mounting Plate Brackets
• LVDT (Linear Variable Differential 
Transformer) Displacement 
Sensors
• Mounting Plate 
• Mounting Plate Stands
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Test Machine and Instrumentation
Structural Loads
• Instron hydraulic in-plane biaxial load 
frame 
• Used to apply axial and lateral loads
• Actuators are capable of 100K-pound 
force
• The axial and lateral loads were controlled 
and measured using two 2,500-pound-
capacity load cells for better accuracy
• A dummy test article was used to verify 
the closed-loop PID control system
• 12.5K-pound load cells were used to 
provide back-up data
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Instrumentation – Location of LVDT’s
• Instrumentation recorded test article 
axial & lateral deflections and rotations
– 0.080” high-precision mini-LVDT’s 
for excellent resolution of 
displacements
– Four LVDTs were mounted on the 
upper deflection plate
– Two LVDTs were mounted 
laterally and measured lateral 
deflection and rotation
– Two LVDTs were mounted 
axially and measured axial 
deflection and rotation
– Four LVDTs were mounted on the 
lower deflection plate
– Deflections and rotations were 
measured similarly
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Instrumentation – Location of
Strain Gages
• Instrumentation recorded strains at critical locations to determine yield
– Four three-element strain gage rosettes were located near the predicted 
highest stress area on the MarM-247 heater head
– Two single-element strain gages measured principal strains at the predicted 
high stress area 
– One rosette and one single-element gage measured strains in the CSAF 
component
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
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• Pressure - Maximum internal operating pressure expected during launch
• Internal pressure is provided by local helium K-bottle system
• Temperature - 100 °C at the rejector braze location
• A water jacket assembly was fixed firmly to the top side of the CSAF 
to control the rejector temperature
• Contains an open channel for heated ethylene glycol
Test Methods 
Pressure and Temperature
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• The heater head test article was tested in 
three combined external load cases
• Load Case 1 – Maximum expected 
flight axial compressive load
• Load Case 2 – Nominal flight axial load 
plus maximum expected flight lateral 
load
• Load Case 3 – Nominal flight axial load 
plus failure-inducing lateral load
• For each load case, ramped steps were 
increased incrementally until the indicated 
peak 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
Test Methodology 
Load Cases
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Analysis
• Pro/E was utilized with ANSYS simulation software for stress, strain, and deflection predictions 
of the three load cases
• “As-built” configuration geometries of the test article and the test fixture 
• The boundary conditions for the load cases were set by fixing the model on eight mounting 
locations on the CSAF
• Deflection values of the upper and lower deflection plates were determined 
• Used to predict lateral deflection and rotation of the test assembly
• The predicted force and deflection data was used during the actual test to corroborate the 
experimental results
• Analysis method used to approximate the test results will be revised based on comparison 
with the experimental results
• To reduce risk through greater accuracy in our future structural assessments
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Similar Material Behavior
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Experimental Results – Load Case 1
• Load Case 1 – Maximum expected flight 
axial compressive load
• Majority of deformation (elastic) 
occurred in CSAF 
• Measured axial deflections, although very 
small in magnitude, exceeded the 
predicted values
• Still investigating discrepancy 
between predicted and measured 
axial deflections
• The displacements and longitudinal 
strains showing linear behavior and post-
ramp values returning to the initial state 
indicated no yield
The test article did not fail under the flight-like 
axial load conditions
Rigid Body Rotation
+ Heater Head 
Deflection
Rigid Body Rotation 
at CSAF
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Experimental Results – Load Case 2 & 3a
• Load Case 2 – Nominal flight axial load 
plus maximum expected flight lateral load
• Measured test article lateral deflections, 
lateral rotations, and longitudinal strains 
closely matched predicted values, were 
linear, and returned to their initial states 
upon unloading
• Load Case 3a – Initially replicated ramped 
steps of Load Case 2 while increasing to 
failure-inducing lateral load
• 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
The test article did not fail under the flight-like 
lateral load conditions
Rigid Body 
Rotation at
CSAF
Rigid Body Rotation
+ Heater Head Deflection
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Experimental Results – Load Case 3b
• Load Case 3b – Nominal flight axial load 
plus failure-inducing lateral load
• To induce failure, a final continuous 
lateral load ramp was imposed on the 
test article
• Predicted lateral force was within 25% 
of measured value
• Deflections and strains were linear through 
Load Case 3a, and then followed an 
elastic-plastic curve that may be typical for 
the MarM-247 heater head material 
• CSAF remained elastic
• Still investigating whether initial failure was 
on the tensile or compressive side
• Failure mode was yielding of the heater 
head and resultant gross deformation
The test article sustained more than three 
times the maximum expected mission lateral 
load while maintaining pressure integrity
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Conclusion
• The Heater Head Lateral Load Test was successful in directly 
supporting qualification of the ASC–E2 heater head. 
• The pressurized test article was exposed to maximum axial 
compressive and maximum lateral loads anticipated during an 
ASRG launch, and 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 failed, 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
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Acknowledgment
This work is funded through the NASA Science Mission
Directorate. Any opinions, findings, and conclusions or
recommendations expressed in this article are those of
the authors and do not necessarily reflect the views of the
National Aeronautics and Space Administration. 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.


Lateral Load Testing of the Advanced Stirling Convertor (ASC-E2) Heater Head

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