NASA's dynamic power convertor development in support of high-efficiency RPS is progressing as planned, and shows promise. Ongoing research utilizing existing hardware supports viability of dynamic power conversion for Radioisotope Power Systems

Oriti, Salvatore

NASA Technical Reports Server

Dynamic Radioisotope Power Systems
Salvatore Oriti
NASA Glenn Research Center
Thermal Energy Conversion Branch
National Aeronautics and Space Administration
www.nasa.gov 
Conference on Advanced Power Systems for Deep Space 
Exploration
October 24, 2018
https://ntrs.nasa.gov/search.jsp?R=20190001011 2019-08-30T10:32:47+00:00Z
National Aeronautics and Space Administration 2
Energy Conversion Options for 
Radioisotope Power Systems 
Static
• Thermoelectrics (Seebeck effect, heat direct to emf)
• Solid-state, no moving parts
• Demonstrated reliability and long life (Voyager, 
Cassini, New Horizons, Mars Curiosity)
• Other options : Thermo-photovoltaic, thermionic
Dynamic
• Heat engines (Stirling, Brayton, Rankine)
• Moving parts (Pistons, turbines, alternators)
• Heat to mechanical, to electrical energy
• Can be designed to eliminate wear mechanisms, and 
have infinite fatigue life
• Never flown in space as power convertor
• Multiple free-piston Stirling cryocoolers have operated 
long-term in space, up to 20 years
National Aeronautics and Space Administration 3
Dynamic-Conversion Power System Background
SRG110, using flexure-bearing Stirling convertor 
(image credit : Lockheed Martin)
SRG-110
• ~114 We output
• Infinia’s Technology Demonstration Convertor (TDC)
• 2 Pu-238 GPHS modules
• Overall efficiency = 23%
• 4.2 We/kg (before engineering unit build)
• Developed during 2001 to 2006 timeframe
ASRG, using gas-bearing Stirling convertor 
(image credits : Sunpower, Lockheed Martin)
HEAT SOURCE (2)
STIRLING 
CONVERTER
ASSEMBLY (2)
CONTROLLER
S/V Interface (4)
GAS MANAGEMENT
VALVE
PRESSURE RELIEF
DEVICE
INSULATION
SYSTEM (2)
FIN
SV Mounting 
Interface (4)
Piston Position 
Telemetry
Shunt
Pressure Relief 
Device (PRD)
Gas Management 
Valve (GMV)
GPHS (2)
Thermal 
Insulation
Assembly (2)
Advanced Stirling 
Convertor (ASC) (2)
ASC 
Controller 
Unit (ACU)
Cold-Side 
Adapter Flange 
(CSAF) (2)
Heat Source 
Support (2)
Shunt 
Dissipator 
Unit (SDU)
AC Power
Analog 
Telemetry 
Connector
DC Power 
Connector 
to SV
TLM/ 
Command to 
SV
ACU 
Mounting 
Interface (6)
ASC 
Interconnect
Tube
Ground 
Test
HEPA 
Filter
ASRG
• ~140 We output
• Sunpower’s Advanced Stirling Convertor (ASC)
• 2 GPHS modules
• Overall efficiency = 28%
• 4.4 We/kg 
• Developed during 2006 to 2013 timeframe
Convertor Generator
National Aeronautics and Space Administration 4
Current Dynamic Convertor Development
Goals make convertor designs 
applicable to a wide range of missions
Item Performance Goal
Life 20 years
Efficiency ≥ 24% at Tcold > 100 °C
Specific Power ≥ 20 We/kg (convertor only)
Partial power Can be throttled down to 50%
Degradation < 0.5% / year
Hot-End Temp < 1000 °C
Cold-End Temp 20 to 175 °C
Random Vibe Launch qual
Static Accel 20g for 1 minute, 5g for 5 days
Radiation 300 krad
Size Enables generator that can fit in DOE shipping container
Robustness goals also defined:
• Design has margin to tolerate events outside expected environments
• Fewer single-point-failures is more robust
• Number of fasteners minimized
• Tolerant of loss of electrical load
• Tolerant of operational error
• Manufacturability not dependent on specialized workmanship
Lunar
(Far side & South Aitken Basin) Europa
Titan
National Aeronautics and Space Administration 5
Convertor Development Timeline
Status Date Description
 2016-Aug RFP Release
 2016-Nov Proposal review
 2017-Jul Contract awards (4)
 2017-2018 Phase 1 - Design
 2018-Apr Decision Gate 1
Ongoing 2018-2020 Phase 2 – Fab & Test
Future 2020-2021 Phase 3 – IV&V
Future 2021 Tentative Decision Gate 2
Future 2021 Goal : Begin DOE flightgenerator development
Item
Flexure Isotope
Stirling Convertor
(FISC)
American 
Superconductor, Inc.
Turbo-Brayton
Convertor
(TBC)
Creare, LLC
Thermo-Acoustic
Power Convertor
(TAPC)
Northrop Grumman
Sunpower Robust
Stirling Convertor
(SRSC)
Sunpower, Inc.
Power (We) 70 355 110 65
Efficiency (%) 31 26 26 29
Hot-end Temp (°C) 650 730 700 720
Mass (kg) 3.3 15.5 6.4 2.0
Specific Power (We/kg) 21 22 17 33
Phase 2 awarded Yes Yes No Yes
Convertor development contracts awarded in 2017:
National Aeronautics and Space Administration
Flexure Isotope Stirling Convertor (FISC)
American SuperConductor (AMSC), formerly Infinia Tech Corp.
6
• Flexure-bearings, beta arrangement free-piston Stirling convertor
• Derivative of Technology Demonstration Convertor (TDC) from SRG-110 project
• TDCs have established long operational life via convertor testing at GRC
• Design deltas relative to TDC to improve the following:
1. Higher radial stiffness flexures, overstroke tolerance, hot-end temperature margin
2. Independently verifiable subassemblies
3. Higher efficiency alternator, higher cold-end temp capability
4. System integration : Tailored interfaces
Power Output 70 We
Efficiency 31% @ TCOLD=100°C
Fraction of Carnot 0.52
Hot-end Temp 650 °C
Mass 3.3 kg (~21We/kg)
FISC Characteristics
70 W Flexure Isotope Stirling Convertor (FISC) Notional 420 We generator concept with 100% convertor redundancy
Alternator
Heat source
6 GPHS modules
Phase 2 awarded : 2018-April
Component fabrication has begun
Prototype delivery : 2019-Oct
National Aeronautics and Space Administration
Turbo-Brayton Convertor (TBC)
Creare, LLC
7
• Closed Brayton continuous flow cycle with recuperation
• Scaled-down from previous designs
• Life-limiting engineering : Hot-end material creep from centrifugal stress
• Recuperator is large portion of convertor mass
• Two counter-rotating units permits redundancy, and nullifies angular momentum
• Flexible component placement on spacecraft
355 We Turbo-Brayton Convertor (TBC)
Notional 355 We generator concept with 100% convertor redundancy
Power Output 355 We
Efficiency 26% @ TCOLD=100°C
Fraction of Carnot 0.41
Turbine Inlet Temp 730 °C
Mass 15.5 kg (22 We/kg)
TBC Characteristics
Radiator
Heat source
6 GPHS modules
2x convertors
Phase 2 awarded : 2018-April
Component fabrication has begun
Prototype delivery : 2019-Oct
National Aeronautics and Space Administration
Sunpower Robust Stirling Convertor (SRSC)
Sunpower, Inc.
8
• Gas-bearing based, beta arrangement free-piston Stirling convertor
• Derivative of Advanced Stirling Convertor (ASC) from ASRG Project
• Enables wide generator design space
• Design deltas relative to ASC to improve the following:
1. Higher radial gas bearing stiffness, overstroke tolerance
2. Regenerator robustness, debris tolerance
3. Higher cold-end temp and static acceleration capability
65 We Sunpower Robust Stirling Convertor (SRSC) Notional 500 We generator concept with 25% convertor redundancy
SRSC Characteristics
Alternator
Cold
End
Hot
End
Phase 1 : Complete
Phase 2 awarded in process
Prototype delivery : 2019-Jan
Power Output 65We
Efficiency 29% @ TCOLD=100°C
Fraction of Carnot 0.46
Hot-End Temp 720 °C
Mass 2.0 kg (33 We/kg)
National Aeronautics and Space Administration
Path to Flight
9
Surrogate Mission Team (SMT), chartered by RPS Program
• NASA, DOE, JPL, APL, GSFC
• Formulated requirements to provide mission pull
• Integrated with convertor contract progress monitoring
• Formulated a TRL evaluation method
• Providing failure mode and probability of success analysis
• Work phases and deliverables tied to TRL advancement
Goal:
Achieve convertor TRL 6, then initiate generator flight development
NASA definition of TRL 6: “System/subsystem model or prototype demonstration in a relevant environment (ground or space)”
Relevant environments can be simulated 
Phase 1 : Design
Preliminary design
Power, thermal, structural 
analyses
FMEA
Req. compliance matrix
Design review
TRL 4
Phase 2 : Fab and Test
Demonstrate steady-state perf.
Performance mapping
Initial extended operation
Validate physics-based models
TRL 4,6 Phase 3 : IV&V
Independent modeling and analysis
Component accelerated tests
Launch vibration exposure
Static acceleration exposure
Performance mapping
Half-power operation
Durability/robustness tests
Extended operation
TRL 6
Gate 1
Flight 
Generator
Development
National Aeronautics and Space Administration
First Mission Potential
10
20 year life requirement is atypical
• Demonstrating 2x life via experiment is not realistic
• Statistical reliability analysis will have small number of hardware data points
• Fabrication of tens of hardware data points not possible on current timeline
• Convertor-level accelerated testing not possible
• Convertor component accelerated testing is possible
• Convertor redundancy has significant effect on generator reliability
First flight-mission use of any new conversion technology must accept some risk
0.5
0.6
0.7
0.8
0.9
1
1 2 3
Ge
ne
ra
to
r R
el
ia
bi
lit
y
Convertor Redundancy
R=0.5
R=0.8
R=0.9 96%
Lunar mission is an attractive first use
• Short cruise time (days, not years)
• Short mission duration (2 years instead of 20)
• Significant science return
• Many candidate missions enabled or enhanced by 
nuclear power:
o 330-hr darkness
o Permanently-shadowed craters
National Aeronautics and Space Administration
Generator and Convertor Risk Mitigation
11
Is dynamic conversion worth the risk?
What can be done to encourage adoption?
Convertor risk mitigation:
• Long-term material property data (metals and organics)
• Radiation endurance
• Component accelerated tests
• Robustness demonstrations (perhaps test to destruction)
• Develop enhancing products (e.g. debris-free regenerator)
Ideal spacecraft power source (target these traits):
• Reliable, always producing power
• Consistent output through every mission environment 
(behaves as a constant-voltage power source)
• High power density and specific power
• No disturbance to spacecraft (EMI, vibration, thermal)
• Simple con-ops (for fueling, launch, EDL, cruise)
• No human-in-the-loop needed at any mission stage
• No ground-command intervention needed
• Robust (capable of unexpected situations outside norm, 
capable of internal component failure)
Generator risk mitigation:
• Demonstrate concept with convertor redundancy
• Demonstrate radiant heat source coupling to convertor
• Simple controller development, with fault tolerance
• Test multiple generators on spacecraft electrical bus
Dynamic Conversion Advantages:
• Higher thermal-to-electric efficiency (up to 40%)
• Lower waste heat to output power ratio
• No degradation
• Low generator power decline (fuel decay only)
• Large multi-mission generator design space
• Extensible to high power levels
National Aeronautics and Space Administration
Stirling Convertor Reliability Demonstrations
12
NASA GRC has demonstrated zero-degradation long-term 
operation of several flight-relevant convertors
Dynamic RPS
Cumulative Per-Convertor Runtime as of September 2018
*Current record-holder for maintenance-free heat engine runtime
**Have undergone launch-vibe portion of life certification
TDC #13
extended operation setup
ASC-E3 pair
extended operation setup
TDC #14 disassembled and inspected after 12 years of operation:
• No evidence of degradation
• Robustness demonstrated
• Tolerated debris, oxygen ingress, and overstroke
• Further disassembly commencing
• Will enable inspection of flexure bearings
ASC-E3 #3 will also be inspected (after 2.5 yrs of operation)
• Suspect an assembly error causing fluctuations in performance
Project &
Provider Test Article
Bearing
Technology
Years of
Operation
(Cumulative)
Status
SRG-110
Infinia, Corp.
TDC #13*
Flexure
12.8 On-going
TDC #14 12.1 Shutdown for disassembly and inspection
TDC #15 11.9 On-going
TDC #16 11.9 On-going
SES #2** 0.5 On-going
ASRG
Sunpower, Inc.
ASC-0 #3**
Gas
8.6 On-going
ASC-E3 #3 2.5 Shutdown for disassembly and inspection
ASC-E3 #4** 3.5 On-going
ASC-E3 #6** 2.8 On-going
ASC-E3 #8 2.3 On-going
ASC-E3 #9 1.9 On-going
ASC-L** 4.4 On-going
National Aeronautics and Space Administration
Conclusions and Next Steps
13
NASA’s dynamic power convertor development for high-efficiency 
RPS is progressing as planned, and will result in advancements in 
power options for Exploration and Science missions
• 3 DPC contracts have passed Decision Gate 1, and have been awarded Phase 2 (convertor 
prototype fabrication and test)
• NASA GRC is preparing for DPC prototype IV&V, ~2020
• Ongoing research utilizing existing hardware supports viability of dynamic power 
conversion for RPS
• Next steps:
1. Finalize IV&V and risk mitigation plans
2. Execute these plans
3. Finalize generator flight development path


Dynamic Radioisotope Power Systems

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server