CP'A+/\N I L\ ~ 

200\ZL.-0q b6 

NASA/TM-2001-211315 

\ 

Stirling Technology Development 

atNASAGRC 

Larmy G. Thieme, Jeffrey G. Schreiber, and Lee S. Mason 

Glerm Research Center, Cleveland, Ohio 

December 2001 

https://ntrs.nasa.gov/search.jsp?R=20020014360 2018-05-15T14:21:16+00:00Z
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NASA/TM-2001-211315 

Stirling Technology Development 

atNASAGRC 

Lanny G. Thieme, Jeffrey G. Schreiber, and Lee S. Mason 

Glenn Research Center, Cleveland, Ohio 

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 

National Aeronautics and 

Space Administration 

Glenn Research Center 

December 2001 
Acknowledgments 

The authors would like to acknowledge the Office of Space Science (Code S) and the Office of Aerospace 

Technology (Code R) at NASA Headquarters for providing funding for this project. 

Trade names or manufacturers' names are used in this report for 

identification only. This usage does not constitute an official 

endorsement, either expressed or implied, by the National 

Aeronautics and Space Administration. 

Available from 

NASA Center for Aerospace Information 

7121 Standard Drive 

National Technical Information Service 

5285 Port Royal Road 

Hanover, MD 21076 Springfield, VA 22100 

Available electronically at http://gltrs.grc.nasa.gov / GLTRS 
Stirling Technology Development at NASA Glenn 

Lanny G. Thieme,1 Jeffrey G. Schreiber,2 and Lee S. Mason3 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

1216-433-6119, Lanny.G.Thieme@grc.nasa.gov 

22 16-433-6144, Jeffrey.G.Schreiber@grc. nasa.gov 

32 16-977-7106, Lee.S.Ma on@grc.nasa.gov 

Abstract. The Department of Energy. Stirling Technology Company (STC), and ASA Glenn Re earch Center 

( ASA Glenn) are developing a free-piston Stirling convertor for a high-efficiency Stirling Radioisotope Generator 

(SRG) for NASA Space Science missions. The SRG is being developed for multimission use, including providing 

electric power for unmanned Mars rovers and deep space missions. NASA Glenn is conducting an in-house technology 

project to assi t in developing the convertor for space qualification and mission implementation. Recent testing of 

55-We Technology Demon ll'ation Convertors (TDC's) built by STC include mapping of a second pair of IDC's. 

single TDC testing, and TDC electromagnetic intelference and electromagnetic compatibility characterization on a nonmagnetic test tand. Launch environment te t of a ingle TDC without its pressure ve sel to better understand the 

convertor intemal structural dynamic and of dual-opposed TDC' with everal engineering mounting tructure with 

different natural freq uencies have recently been completed. A preliminary life assessment has been completed for the 

TDC heater head, and creep testing of the IN71 8 material to be used for the flight convel10rs is underway. Long-term 

magnet aging tests are continuing to characterize an y potential aging in the strength or demagnetization resistance of 

the magnets u ed in the linear altemator (LA). Evaluations are now beginning on key organic materials u ed in the LA 

and piston/rod urface coating. NASA Glenn is also conducting fll1i te element analyse for the LA, in part to look at 

the demagnetization margin on the pelmanent magnets. The world's fU'st known integrated test of a dynamic power 

system with electric propulsion was achieved at NASA Glenn when a Hall-effect thru ter was successfully operated 

with a free-piston Stirling power source. Cleveland State University is developing a multidimensional Stirling 

computational fluid dynamics code to Significantly improve Stirling loss predictions and a ist in identifying convertor 

area for further improvement . This paper will update the status and re ults for these efforts. 

INTRODUCTION 

The Department of Energy (DOE), Stirling Technology Company (STC), and NASA Glenn Research Center 

( ASA Glenn) are developing a free-piston Stirling convertor for a high-efficiency Stirling Radioisotope Generator 

(SRG) for NASA Space Science missions. The SRG is being developed for multimission use, including providing 

electric power for unmanned Mars rovers and deep space missions. The SRG is being evaluated as a high-efficiency 

power source alternative to replace Radioisotope Thermoelectric Generators (RTG's). The SRG system efficiency 

of 20 to 25 percent will reduce the required amount of radioisotope by a factor of 3 or more compared to RTG's. 

This significantly reduces radioisotope cost, radiological inventory, and system cost and provides efficient use of the 

scarce domestic supply of radioisotope resources. 

STC is developing the Stirling convertor under contract to DOE (White, 2001). Technology Demonstration 

Convertors (TDC's), operating as single units and in dynamically-balanced opposed pairs, are now being tested by 

STC and ASA Glenn. Power outputs of 55 to 65 We and efficiencies of 25 to 28 percent have been demonstrated 

for TDC operating conditions of 650 °C hot-end temperature and cold-end temperatures varying from 80 to 120 0c. 

Long life has also been demonstrated on a similar STC 10-We radioisotope terrestrial convertor, RG-IO, that has 

been on life test at STC for over 65 ,000 hours (7.4 years) with no convertor maintenance and no degradation in 

performance. An assessment of the free-piston Stirling convertor's capability for long life is given by Schreiber 

(2000). NASA Glerm is providing technical consulting for the TDC development effort under an Interagency 

Agreement with DOE. 

A procurement for a SRG System Integration Contractor is now being conducted by DOE. Three contractors have 

developed SRG conceptual designs. First SRG's are expected to have a specific power exceeding 4 We/kg. Growth 

options have been identified to significantly improve the specific power to 6 to 8 We/kg and then to 8 to 10 We/kg. 

NASAffM-2001-211315 
NASA Glenn is conducting an in-house technology project to assist in developing the convertor for space 

qualification and mission implementation. This include independent verification testing of the TDC's, heater head 

life assessment, dFeB magnet aging characterization, electromagnetic and thermal analyses of the linear alternator, 

launch environment characterization testing, electromagnetic inteIierence ancl electromagnetic compatibility 

(EMI/EMC) characterization and reduction, and organic materials evaluation. NASA Glenn also supports reli ability 

studies being led by Westinghouse for DOE. NASA Glenn has also initiated effoltS to develop advanced Stirling 

technologies and to evaluate Stirling convertors for further applications. As part of these efforts, a multidimensional Stirling computational fluid dynamics (CFD) code is being developed, and a demonstration of an 

integrated dynamic power system with electric propulsion was accomplished when a Hall-effect thruster was 

successfully operated with a free-piston Stirling power source. This paper will discuss the status and recent results 

for this work. 

HEA TER HEAD LIFE ASSESSMENT 

Heater head life is a critical element for achieving the 100,000+ hour life of the convertor. The heater head is a thinwalled pressure vessel fabricated from Inconel 7 18 (IN718). ASA Glenn materials and structures personnel have 

developed an approach to characterize the long-term durability of the heater head using relatively short-term 

extrapolation methods. This approach involves both deterministic and probabili tic methods and uses I 71 8 

material testing, IN718 data from the literature, and heater head structural benchmark tests to calibrate and validate 

the models. 

An initial assessment of the TDC heater head life has been completed (Halford, 2002). These initial analytic studies 

have indicated that the pressure vessel wall thickness at the hot end of the regenerator will need to be increased over 

(I) 

-I'tI 

CJ) 

-

10.7 ...-----------""""---, 

10.

11 

a relatively short span to achieve the required lifetime. A 

constant maximum hot-end temperature of 650 °C for very long 

lifetimes was originally thought to be achievable with the current 

TDC design, using extrapolated creep-rupture curves for IN718 

obtained from handbook information. However, the handbook 

curves are generally based on data of less than 10,000 hours. 

Based on an extensive long-term creep and creep-rupture 

database (up to 87,000 hours) taken on IN718 by the Oak Ridge 

>-

ational Laboratories (ORNL) , the ruptw'e life vs. stress 

~ 10·12-'-_ __ ~ _____ __ relationship for IN718 becomes nonlinear after about 10,000 ..........J 

$ 100 200 300 400 600 1000 hours at 650 °C, making extrapolations using short-term data non- CJ) Stress (MPa) conservative. The ORNL data is being used by ASA Glenn to 

factor in long-term results and also measured statisti cal variations. 

In addition, ASA Glenn is now conducting creep testing of the 

IN718 material that will be used to build the flight convertors. 

This material differs somewhat from that tested by ORNL in heat 

treats, grain sizes, slight variations in alloy chemistry, and 

FIGURE L Steady-State Creep Rates for 

Thin (-0.05 cm) IN7 18 Samples Compared 

to Typical Values Reported in the Literarure 

for Thick Specimens. 

FIGURE 2. IN718 Creep Testing at 

NASA Glenn. 

especially in the thickness of the test specimens. Scaling factors 

accounting fo r the differences between the ORNL data and the NASA 

Glenn data will be established as the NASA Glenn database nears 

completion. Trade-off assessments are continuing that will determine a 

recommended pressure vessel wall thickness at the hot end of the 

regenerator that will provide adequate life (100,000+ hours) with 

margin based upon probability of survival. 

Most published creep data on IN71 8 has been taken on thick samples; 

however, thin samples that are relevant to the TDC heater head wall 

thickness tend to have less creep resistance as shown in figure I 

(Bowman, 2001). The data shown for thin samples include test data 

taken at NASA Glenn on several heats of material. In thin samples, the 

number of grains across the thickness is an important parameter. NASA 

Glenn has completed creep tests to study this effect of wall thickness to 

grain size ratio and used this data to determine the optimum grain size 

NASArrM- 2001-211315 2 
for the IN71 8 material that will be used for the flight conveJtors. 

ASA Glenn has received this material, and samples are now being 

used for creep testing at various stress levels. A disc ussion of these 

various aspects of assessing IN71 8 for long-term creep resistance is 

given by Bowman (2001). Figure 2 shows some of the uni axial creep 

frames that are being used for this testing at ASA Glenn. 62,639 total 

hours of creep testing have been accumulated on 19 samples as of 

August 31, 2001. 

Material creep propeJties are generally determined from uniaxial creep 

tests, including both the long-term ORNL data and the data now being 

FIGURE 3. Heater Head Structural taken at NASA Gleml. However, the heater head configuration and 

Benchmark Te t Rig. pressure loading re ults in a biaxial stress state. To factor this effect 

into the life assessment, a structural benchmark test rig, shown in 

figure 3 and described by Halford et al. (2002), has been a sembled to test prototypical heater head in accelerated 

life tests. Elevated pressure and/or temperature will be used to accelerate the testing. Several tests will be run to 

first assist in the calibration of the life assessment methods and then a longer test run fo r validation purposes. The 

test rig has recently completed checkout testing, and heater head testing will be initiated once the heater head design 

is finalized and sample heater heads have been obtained. 

TDC PERFORMANCE AND EMI TESTING 

TOC's SIN 5 and 6 were delivered to NASA Glenn in August of 2000. They were tested at full piston stroke and 

also over a range of reduced strokes while in the dual-opposed configuration (Schreiber, 2002). This configuration 

leads to dynamically balanced operation that results in very low levels of vibration. An additional set of convertors, 

known as TDC's SIN 7 and 8, has been delivered to NASA Glenn and has completed acceptance tests. TDC's SIN 7 

and 8 are nominally identical to TDC's SIN 5 and 6 and are being tested in the same dual-opposed configuration. 

During the acceptance test, TDC's SIN 7 and 8 generated 110.1 watts AC of power at the design condition of 6S0 °C 

hot-end temperature and 120 °C cold-end temperature. These convertors are presently undergoing variable-stroke 

performance mapping and the results are generall y the same as those for TOC's SIN Sand 6. The performance map 

was not completed in time to include in this report. Current plans caU for TDC's SIN 7 and 8 to be tested for 

approximately 1,000 hours, followed by operation during a vibration test at ASA Glenn to simulate launch. The 

convertors will then be put on extended test to accumulate another 2,000 hours of operation. This sequence will 

approximate operation on earth prior to launch, the launch environment, and steady state operation fo llowing a 

launch. The conveltors will then be disassembled and carefully inspected to check for any evidence of wear or 

possible degradation. 

Variable-stroke performance mapping of TDC's SIN S and 6 has been completed, and they are now used to support 

a variety of other tests. One facet of this testing is to operate each TOC as a single unit to characterize performance. 

The fundamental difference in operation as a single convertor stems from the motion of the convertor housing. In 

the dual-opposed mode of operation, there would be no casing motion if the convertors were identical dynamically 

FIGURE 4. TDC' SIN 7 and 8 in 

Foreground (Dual-Oppo ed) and SIN 

5 and 6 in Distance (Single Units). 

ASA/TM-2001-21131S 

and if the connecting structure was perfectly rigid. In this case, the 

dynamics of the piston and displacer in each convertor would be that of a 

nonlinear two-body dynamic system. In reality, any connecting structure 

will have some compliance, and there will be some finite motion of each 

housing. The single convertor test stand has been designed to approximate 

the dynamics that may exist in a space power generator. Figure 4 shows 

TDC's SIN 5 and 6 on the single convertor test stand. Performance data for 

this configuration was not available for this report. 

Performance mapping of the TDC's made use of a laboratory controller that 

uses control logic with a zener diode to prevent overstroke of the piston. 

However, it imposes a sharp load twice per cycle and thus introduces a 

significant content of higher harmonics to the output current and voltage 

waveforms. To obtain the EMl/EMC levels necessary for most space 

science missions, it may be preferred that the controller produce little 

3 
harmonic content. A controller has been developed at ASA Glenn that uses digital logic to regulate a resistive 

load o n the AC o utput of the linear alternator. Initial tests with the NASA Glenn contro ller indicated that the 

harmonic content was less than 5 percent. TDC's SIN 5 and 6 have been operated with this controller, but full 

performance mapping has not yet been done. Tests have been pelformed to characterize the AC magnetic emissions 

with both controllers. A total of 64 measurements were made over a two-day test. The data is currently being 

reduced and readied for interpretation. These initial tests will help develop strategies to manage the EM!. 

LAUNCH ENVIRONMENT TESTS 

Testing and characterization of the TDC's in the ASA Glenn Structural Dynamics Laboratory (SDL) has become a 

key part of the technology project. The SDL has two capabilities that are utilized. One is a facility known as the 

Microgravity Emissions Laboratory (MEL), which is used to measure the vibratory emissions that emanate from an 

operating device. The SDL also has shake tables that are used to pelform more typical vibration tests. During 2001 , 

TDC's SIN 5 and 6 have been used in one MEL test and two vibration tests. More details on these tests are given by 

Schreiber (2001) and Suarez (2002). 

The MEL facility is shown in figure 5. The MEL tests were performed on TDC's SIN 5 and 6 operating in the dualopposed configuration. The purpose of the test was to determine the effect of alignment tolerances on disturbances 

generated during operation. The MEL fixture was designed to allow the alignment of the convertors to be adjusted 

by lateral offsets and angular misalignments. Also, tests were performed to compare rigidly mounting the 

convertors to using a mounting structure with a resonant frequency either 40 percent below or 40 percent above the 

TDC operating freq uency. The convertors were operated at the full piston stroke during all MEL tests and thus were 

generating their maximum vibratory signature. The results of the test showed the emissions to be distinctly tonal 

and generally benign. 

FIGURE 5. The MEL Facility at NASA 

Glenn. 

FIGURE 6. TDC's SIN 7 and 8 

Mounted in the Engineering Mounting 

Structure Test Stand. 

The TDC was shown to be able to operate at full power and suffer no 

damage when subjected to high levels of random vibration in previous 

vibration tests (Goodnight, 2000). The response of the internal 

components and the remaining margin were not known. To better 

understand the internal structural dynamics, a test was devised that used a 

single convertor mounted on the shake table with the pressure vessel 

removed. A suite of accelerometers, optical probes, and force sensing 

bolts was developed to measure the response of the key components, 

primarily the piston rod and linear alternator. The convertor, TDC SIN 6, 

was motored at the resonant frequency of 73 Hz, which is a lower 

frequency than for typical operation as a power convertor since it was now 

being operated at a mean pressure of one atmosphere. Random vibrations 

were imposed in the axial direction and then in the lateral direction, Witll 

the TDC operating at full stroke and half stroke. The level of random 

vibration used in these tests was 2.17 Grms. This is 25 percent of the 

flight acceptance level and should allow for sufficiently detailed data that 

a structural dynamicist could develop an understanding of the structural 

response. A series of sine sweeps were also performed from 5 Hz to 2000 

Hz at levels up to 0.5 G. The tests have been completed, and the data are 

being analyzed (Suarez, 2002). 

Random vibration tests were also conducted with dual-opposed TDC's 

SIN Sand 6 mounted in an Engineering Mounting Structure. The purpose 

of these tests was to stud y the interaction of dual-opposed Stirling 

convertors when subject to random vi bration through a coupling that may 

be representative of the structure of the final SRG. Mounting structures 

were devised with resonant freq uencies of 11 Hz, 36 Hz, and 104 Hz. 

These included structures with resonant frequencies lower and higher than 

the operating freq uency of the convertor. Again, random vibration was 

kept to 2S percent of the flight acceptance level. The test fixt ure with the ii-Hz mounting structure is shown in 

figure 6. The tests have been completed, but the data have not yet been processed. The TDC's operated in a well 

NASAfTM-2001-21i315 4 
_.

behaved manner, even during sine sweeps when the mounting tructure allowed a high degree of motion as the te t 

passed through resonance. 

PERMANENT MAGNET AGING CHARACTERIZATION 

NdFeB pelmanent magnets are used in the TDC linear alternator. Long-term magnet aging tests with margin on the 

magnet operating temperature and in a demagnetizing fie ld are being run on NdFeB magnet samples to quantify any 

potential magnet degradation with time and temperature. Such degradation, if any, could affect both the remanent 

magnetization and the demagnetization resistance. The aging te ts will be run for up to 12.000 hours at 120 °C and 

with a DC demagnetizing field of 6 kOe. For a 120 °C convertor cold-end temperature, the magnets are expected to 

operate at temperatures around 75 °C. The magnet aging test fixture can hold up to 10 magnet samples. Over 

4300 hours of testing have been completed as of August 3 1, 2001. Magnet samples are removed periodically, 

characterized, and compared to their baseline characterizations completed before the start of testing to determine the 

rate of aging. 

Short-term aging tests were first run on various NdFeB magnet types to assist in selecting the magnets for the longterm tests. Tests were completed for 100 hours on th.ree magnet types at 120 °C and in a 6-kOe demagnetizing fi eld; 

no measurable changes were found after these tests. 200-hour tests were then run on six magnet types at 150 °C and 

in a 5-kOe demagnetizing fie ld to see if this higher temperature would produce any measurable aging. These tests 

showed that, depending on magnet type, remanence losses ranged from 0 to 7 percent and intrinsic coercivity losses 

varied from about I to 4 percent. In general, measurements were re olvable to about I percent. In some cases, the 

coercivity losses were not recoverable by recharging the magnet sample, indicating a structural change of the 

material. These results are di cussed by iedra (2001). 

PERMANENT MAGNET DEMAGNETIZATION MARGIN 

NASA Glenn ha s developed a three-dimensional finite element method (FEM) approach for evaluating linear 

alternators using Ansoft's Maxwell 3D finite element software. The analyses of the TDC linear alternator was 

recently extended to include the effects of the load current. The demagnetization margin for the NdFeB permanent 

magnets was evaluated over the operating cycle and al so for several magnet temperatures. The ASA Glenn 

analysis tools and the e results are discussed by Geng (2001). Figure 7 shows the magnetic field strength (H) vs. 

distance along a pair of magnets attached to one stator leg for the mover positioned at the end of stroke. The model 

predicts that this is the most severe magnet loading over the entire stroke and that the maximum localized 

demagnetization field occurs on the inside surface (mover side) of the uncovered magnet. 

800 

700 Inside Surface 

Center 

Outside Surface 

600 

~ 500 E 

-... 

12 400 \ 

i 

1 '-' 

::r: 300 

200 Center 

Outside SUliace 

100 

0 --~--- Magnet--:-------

0 5 10 15 20 25 30 

Distance Along Magnets (mm) 

FIGURE 7. Magnetic Field Strength (H) vs. Distance Along 

Magnets at 75 °C: End of Stroke. 

ASAffM-2001-211 315 5 

Magnet temperatures were calculated for 

several convertor cold-end temperatures using 

a thermal model developed with the 

ANSYSTM finite element software. For each 

condition, the maximum localized 

demagnetization field and the 

demagnetization field averaged over the 

magnet volume were determined. These were 

then compared to the knee of the normal 

induction curve for the TDC magnets 

(demagnetization field limit), based on the 

magnet manufacturer's data, to determine the 

margin to demagnetization. The results are 

given in Table 1. 

Design margins increase with decreasing 

convertor cold-end temperature. The final 

design value of convertor cold-end 

temperature will be based on system analysis 

and has not yet been selected. A value for 

acceptable design margin to demagnetization 

also remains to be determined. Variation in 
individual magnet demagnetization resistance and aging effects, if any, on this demagnetization resistance should be 

taken into account. 

TABLE 1. Magnet Demagnetization Margin for the TDC dFeB Pennanent Magnets. 

Convertor Ave. Magnet Max. Localized Demag. Field Demag. Margin Based Margin Based 

Cold-End Temp. (0C) Demag. Field Ave. Over Field On Max. On Ave. 

Temp. eC) (kA/m) Magnet Vol. Limit Loc. Demag. Demag. 

(kA/m) (kAlm) Field (%) Field (%) 

80 53 760 550 1108 31 50 

100 62 745 539 1018 27 47 

120 75 725 527 884 18 40 

EV ALUA TION OF CONVERTOR ORGANICS 

SRG multimission use could include a mission to the environment of Jupiter, such as to the moon Europa. The 

ionizing radiation around Jupiter is much more severe than that produced by the GPHS radioisotope heat source. 

The organic materials in the TDC have been replaced as necessary, with radiation-hard organic materials to meet 

this need (Golliher, 2001). These organics are used in the piston, displacer, and rod surface coatings; piston 

bumpers; and alternator adhesives and electrical insulations. These radiation-hard organics were first used on the 

TDC's that are now on test at NASA Glenn, and these convertors are serving as a test bed for these materials. 

In addition, the ASA Glenn Polymers Branch and Tribology and Surface Science Branch are further evaluating 

two of the key organic materials. Thin-film samples of the Xylan material used in the piston bearing coatings are 

being subjected to testing in a radiation exposure facility, using X-rays as the radiation source. Both the original 

material, which was Teflon-based, and the one now being used for radiation hardness, which is based on 

molybdenum disulphide, MoS2, are being tested. A one-hour exposure provides a dose of 4 Mrad to the interior of 

the .OOS-cm thick film. These thin films will then be "scratch-tested" to evaluate changes in their mechanical 

properties. 

3MlM Scotch-WeldlM 2216 B/A Gray epoxy adhesive is used to bond the NdFeB permanent magnets to the stator 

laminations in the linear alternator. A series of tests will first be run on test coupons to experimentally quantify this 

bond strength under various loading modes and as a function of temperature. Pure tension, pure compression, 

sandwich shear, and single cantilever loads will be included. Fatigue curves will also be generated for the adhesive 

and interface. Once the general characteristics of the bond have been established component-scale tests will be 

conducted to evaluate the performance and lifetime of the bond in the in-service operating envi.ronment and under 

the combi.ned stress states that the bond sees in actual operation. The loading conditions will be based on results of 

FEM modeling of the linear alternator. 

MULTI-DIMENSIONAL CFD PERFORMANCE CODE 

A grant has been awarded to Cleveland State University (CSU) to develop a multi-dimensional Stirling 

computational fluid dynamics (CFD) code to significantly improve Stirling loss predictions and assist in identifying 

convertor areas for further improvements. The University of Minnesota (UMN) and Gedeon Associates are teamed 

with CSU for this effort. Key existing one-dimensional Stirling performance and design codes agree reasonably 

well with each other in terms of overall performance but can differ significantly in the magnitude of specific internal 

losses. Also, one-dimensional codes do not rigorously model manifolds between heat exchangers and 

expansion/compression spaces in the convertor. In certain cases, hardware experiences have shown that large 

performance gains can be made by varying manifolds and heat exchanger designs to improve flow distributions in 

the heat exchangers. It is expected that this CFD code should give a significant improvement in the understanding of 

these losses and convertor design areas and, thus, help lead to further performance improvements. 

NASA/TM- 2001 -211315 6 
Two CFD codes have been evaluated. The first i a two-dimen ional incompressible flow code, CAST, Computer 

Aided Simulation of Turbulent Flows. CAST was used for initial two-dimensional modeling of "StirUng machinelike" components during the SP-IOO program, aimed at eventual development of a Stirling CFD code. CAST was 

then modified to model compressible, non-acoustic flow (Tew. 2001). The other code under consideration is CFDACE+, a commercial CFD code developed by the CFO Research Corporation. CFO-ACE+ can model two- or 

three-dimensional geometries and includes acoustics. Results from the two codes were compared with test data 

taken from 1) a gas spring and 2) a single piston/cylinder and aJmular heat exchanger (Ibrahim, 2001 and Tew, 

2001). CFO-ACE+ has now been chosen for proceeding on the development of the Stirling CFD code. Thi choice 

was based primaJ'ily on CFD-ACE+'s higher state of development, including its ability to model three-dimensional 

geometries and it sophisticated pre-and post proces ors. The two codes were similar in their execution time. 

FIGURE 8. Cleveland State 

University Oscillating Flow Test Rig 

(phOto counesy of CSU). 

FIGURE 9. University of 

Minnesota 0 cillating Flow Test Rig 

(photo courtesy of UMN). 

It is important to validate the Stirling 

CFD code as its development 

continues. The CAST and CFD-ACE+ 

codes agreed rea onably we ll with the 

gas spring test data, but there were 

some significant differences between 

the two code and with the test data in 

predicting temperatmes and heat 

transfer for the piston cylinder and heat 

exchanger. EffOlts are now underway 

to obtain further validation test data. 

Existing test rigs at both CSU and 

UMN are being modified and tests 

sections are being designed and 

fabricated to look at key areas relative to the multi-dimensional modeling. These include flow area changes such as 

occur in the manifolding between heat exchangers and the cylinder. Figures 8 and 9 show the CSU and UMN test 

rigs. The UMN test rig is a large-scale rig that runs at low frequencies, allowing detailed measmements in the flow 

streams; it relies on achieving dynamic similarity of the key flow parameters. The CSU rig operates at frequencies 

similar to actual Stirling convertors but is also large enough to allow access for detailed measurements. 

INTEGRATED HALL THRUSTER TEST 

An important aspect of implementing SRG 's on future missions is the integration of the generator and controller 

with potential spacecraft loads. Some recent studies have indicated that the combination of SRG 's and electric 

propulsion devices offer ignificant trip time and payload fraction benefits for deep space missions (Oleson, 2001 ). 

To begin to understand the interactions between Stirling generators and electric thrusters, an electrically-heated STC 

RG-350 (350-watt output) Stirling convertor was coupled to a 300-watt SPT -50 Hall-effect thruster built for NASA 

by the Moscow Aviation Institute (RIAME). The RG-350 and the SPT -50, shown in fi gures 10 and 11 , were 

installed in adjacent vacuum chamber ports at NASA GleIm's Electric Propulsion Laboratory, Vacuum Facility #8. 

The Stirling controller interfaced directly with the Hall thruster power processing unit (PPU), both of which were 

located outside of the vacuum chamber. The PPU accepted the 48-VDC output from the Stirling controller and 

distributed the power to all the loads of the SPT -50 including the magnets, keeper, heater, and discharge. 

FIGURE 10. RG-350 

Stirling Convenor. 

NASA/TM-2001-2113 15 

FIGURE 11. SPT-50 HallEffect Thruster. 

On February 28 , 2001 , the ASA Glenn test team 

successfully operated the Hall-effect thruster with the 

Stirling convertor. This is the world's first known test of 

a dynamic power source with electric propulsion. The 

RG-350 successfully managed the transition from the 

purely resistive load bank within the Stirling controller to 

the highly capacitive PPU load. The Stirling convertor 

was operating at a hot-end temperature of 530 °C and a 

cold-end temperature of -6 °C. The linear alternator was 

producing approximately 250 watts at 109 V AC while the 

PPU was drawing 175 watts at 48 VDC. The majority of 

power was delivered to the discharge circuit operating at 

7 
115 VDC and 0.9 amps. Future testing will examine the possibility of directly driving the Hall thruster discharge 

circuit using rectified and filtered output from the Stirling alternator. 

CONCLUDING REMARKS 

The Stirling technology development project at ASA Glenn has produced substantial test results over the last year 

in the areas of peIiormance validation, IN718 creep testing, NdFeB magnet aging characterization, launch 

environment testing, and EMI/EMC characterization. The e efforts continue a do a heater head life assessment, 

FEM analysis of the linear alternator, an eva luation of key organic materials, support for DOE reliability 

assessments, and development of a Stirling multi-dimensional CFD code. This project will contillue to provide 

critical support to the overall DOE efforts to develop the SRG as work moves into the next stage after election of a 

Stirling System Integration Contractor. The integrated test of a Stirling convertor driving a Hall thruster indicates a 

further promi ing application for Stirling technology. 

REFERENCES 

Bowman, R.R. , "Long-Term Creep Assessment of a Thin-Walled Inconel 71 8 Stirling Power-Convel10r Heater Head." 

Proceedings of rhe 36r/7 /nrersociery Energy Con version Engineering Conference, Savannah, GA, 2001 

(IECEC2001-CT-33). 

Geng, S.M., Schwarze, G.E., Niedra, J.M., and Regan, T.F., "A 3-D Magnetic Analysis of a Stirling Convenor Linear Alternator 

Under Load," Proceedings of rh e 36r/i /nrersociery Energy COI1l'ersion Engineering Confe rence, Savannah, GA, 2001 

CIECEC2001-CT-34), NASAfTM- 2oo1-21 1084, July, 2001. 

Golliher, E.L. and Pepper, S.V., "Organic Materials Ionizing Radiation Susceptibility for the Outer Planets/Solar Probe 

Radioisotope Power Source," Proceedings of rhe 35rh /nlersociety Energy Conversion Engineering Conference, Las Vegas, 

NY, 2000 (AlAA- 2000--3077), NASA(TM-2001-210245. January, 2001. 

Goodnight, T.W., Hughes, W.O., and McNelis, M.E., "Dynamic Capability Of An Operating Stirling Convenor," Proceedings of 

the 35tl1 lntersociery Energy Conversion Engin eering Conference, Las Vegas, NY, 2000 (AlAA- 2000-2839), 

NASAfTM-2000-210527, November, 2000. 

Halford, G.R., Arya, V.K. , Krause, D.L., Shah, A., and Bartolotta, P.A. , "Su1Jcturai Analyses of Stirling Power Convenor Heater 

Head for Long-Term Reliability, Durability, and Performance," in these proceedings, 2002. 

Ibrahim, M.B., Tew, R.C. Jr., Zhang, Z., Gedeon, D., and Simon, T.W., "CFD Modeling of Free-Piston Stirling Engines," 

Presented at the 36th Intersociety Energy Conversion Engineering Conference, Savannah, GA, 2001 (IECEC2001-CT-38). 

Niedra, I.M .. "ShoI1-Tenn Aging of NdFeB Magnets for Stirling Linear Alternator Applications," NASNCR-200l-210952, 

July, 2001. 

Oleson, S., Gefen, L., Patterson, M., SchJeiber, J., McAdams, J., and Ostdiek, P., "Outer Planetary Exploration with Advanced 

Radioisotope Elecu'ic Propulsion," to be presented at the 27th International Electric Propulsion Conference, Pasadena, CA, 

2001. 

Schreiber, J.G., "Assessment of the Free-Piston Stirling Convenor as a Long Life Power Convenor for Space," Proceedings of 

rhe 35rh inrersociery Energy Conl'ersion Engineering Conference, Las Vegas, NY, 2000 (AlAA- 2000--3021). 

Schreiber, J.G. and Thieme. L.G., "Update on the Stirling Convenor Te ting and Technology Development at NASA GRC," 

Proceedings of the 36th /nrersociery Energy COil version Engineering Conference, Savannah, GA, 2001 

(IECEC200 1-CT -26). 

Schreiber, J.G. and Skupinski, R.C., "Accomplishments in Free-Piston Stirling Tests at NASA GRC," in these proceedings, 2002. 

Suarez, V.1., Goodnight, T.W., Hughes, W.O., and Samorezov, S., "Impact Testing of a Stirling Convel10r Linear Alternator," in 

the e proceedings, 2002. 

Tew, R.C. Jf. and Ibrahim, M.B., "Study of Two-Dimensional on-Acoustic Modeling of Stirling Machine Type Components," 

Proceedings of the 36th /nTersociery Energy Con version Engineering Conference, Savannah, GA, 2001 

(IECEC2oo1-CT-27), NASAfTM-2oo l-211066, July, 2001. 

White, M., Qiu, S. , Augenblick, J., Peter on, A., and Faultersack, F., "Status Update of a Free-Piston Stirling Convertor for 

Radioisotope Space Power Systems," in proceedings of Space Technology and ApplicaTions /nrernarional Forum (STAlF2001 ) , edited by M. EI-Genk, AlP Conference Proceeding 552, New York, 2001 , pp. 998-1004. 

NASAfTM-2001-21 13 15 8 

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Decembe r 2001 Technical M emorandum 

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Stirling Technology Development at NASA GRC 

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and 755-A4-14-00 

Lanny G. Thieme, Jeffrey G. Schreiber, and Lee S. Mason 

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National Aeronautics and Space Administration 

John H. Glenn Research Center at Lewis Field E-13119 

C leveland, Ohio 44135-3191 

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AGENCY REPORT NUM BER 

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Washington, DC 20546-0001 NASA TM-2001-2 11315 

11. SUPPLEMENTARY NOTES 

Prepared for the Space Technology and Appljcatio ns International Forum (STAIF-2002) spon sored by the American 

Institute of Aeronautics and Astronautics, Albuq uerque, New Mex ico, February 3- 7, 2002. Responsible person, 

Lanny G. Thieme, organization code 5490, 216-433-6119. 

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Unclassified - Unlimited 

Subject Categories: 20 and 44 D istribution : Nonstandard 

Available electronically at http:U!;ltrs.grc .lIasa.gov/GLTRS 

This publication is available from the NASA Center for AeroSpace Information, 301-62 1-0390. 

13. ABST RACT (Max imum 200 word s) 

The Department of Energy, Stirling Technology Company (STC), and NASA Glenn Research Center (NASA Glenn) are developing a free-piston 

Stirling convertor for a high-efficiency Stirling Radioisotope Generator (SRG) for NASA Space Science missions. The SRG is being developed for 

multimission use, including providing electric power for unnlanned Mars rovers and deep space missions. NASA Glenn is conducting an in-house 

technology project to assist in developing the convertor for space qualification and mission implementation. Recent testing of 55-We Technology 

Demonstration Convertors (TDC's) built by STC includes mapping of a econd pair of TDC's, single TDC testing, and TDC electromagnetic interference and electromagnetic compatibi.lity characterization on a nonmagnetic test stand. Launch environnlent tests of a single TDC without its pressure 

vessel to better understand the convertor internal structural dynanlics and of dual-opposed TDC's with several engineering mounting structures with 

different natural frequencies have recently been completed. A preliminary life assessment has been completed for the TDC heater head, and creep testing 

of the IN71 8 material to be used for the flight convertors is underway. Long-tenn magnet aging test are continuing to characteri ze any potential aging in 

the strength or demagnetization resistance of the magnet u ed in the linear alternator (LA). Evaluations are now beginning on key organic materials 

used in the LA and piston/rod surface coating. NASA Glenn is also conducting finite element analyses for the LA, in part to look at the demagnetization 

margin on the permanent magnets. The world 's first known integrated test of a dynamic power system with electric propulsion was achieved at NASA 

Glenn when a Hall-effect thruster was successfully operated with a free-piston Stirling power ource. Cleveland State University is developing a 

multidimensional Stirling computational fluid dynamics code to significantl y improve Stirling loss predictions and assist in identifying convertor areas 

for further improvements. This paper will update the status and results for these effort . 

14. SUBJECT TERMS 15. NUMBER OF PAGES 

Stirling engines; Nuclear electric power generation; Deep space; Mars: Roving vehicles; Random 14 

vibration ; Vibration tests; Electromagnetic interferen ce: Life (durability); Linear a1temator; 16. PRICE CODE 

Penn anent magnets; Ionizing radiation 

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