Rodger W. Dyson and Geoffrey A. Bruder

Glenn Research Center, Cleveland, Ohio

Progress Towards the Development of a

Long-Lived Venus Lander Duplex System

NASA/TM—2011-217018

May 2011

AIAA–2010–6917
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Rodger W. Dyson and Geoffrey A. Bruder

Glenn Research Center, Cleveland, Ohio

Progress Towards the Development of a

Long-Lived Venus Lander Duplex System

NASA/TM—2011-217018

May 2011

AIAA–2010–6917

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 was supported by the NASA Science Mission Directorate, Radioisotope Power System Project Office Technology 

Development Project. The Advanced Stirling Duplex Team is fortunately comprised of the leading experts in their specific 

discipline areas. The authors would like to humbly thank the team for their energy and creativity.

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ideas from a technical peer group.
NASA/TM—2011-217018 1

Progress Towards the Development of a Long-Lived

Venus Lander Duplex System

Rodger W. Dyson and Geoffrey A. Bruder

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Abstract

NASA has begun the development of a combined Stirling cycle power and cooling system (duplex) to 

enable the long-lived surface exploration of Venus and other harsh environments in the solar system. The 

duplex system will operate from the heat provided by decaying radioisotope plutonium-238 or its 

substitute. Since the surface of Venus has a thick, hot, and corrosive atmosphere, it is a challenging 

proposition to maintain sensitive lander electronics under survivable conditions. This development effort 

requires the integration of: a radioisotope or fission heat source; heat pipes; high-temperature, corrosionresistant material; multistage cooling; a novel free-displacer Stirling convertor for the lander; and a 

minimal vibration thermoacoustic Stirling convertor for the seismometer. The first year effort includes 

conceptual system design and control studies, materials development, and prototype hardware testing. A 

summary of these findings and test results is presented in this report.

Nomenclature

ASD Advanced Stirling Duplex

ASRG Advanced Stirling Radioisotope Generator

FSP Fission Surface Power

REP Radioisotope Electric Propulsion

RPS Radioisotope Power System

TRL Technology Readiness Level

DAQ Data Acquisition

PLC Programmable Logic Controller

VEXAG Venus Exploration Analysis Group

TASHE Thermoacoustic Stirling Heat Engine

Introduction

Radioisotope power systems (RPSs) have been used for decades in space to enable scientific missions 

to the farthest reaches of our solar system except in the most extreme high-temperature environments 

such as occur on the surface of Venus, Jupiter, Mercury, Io, or near the Sun. Of those extreme 

environments, Venus is in many ways the most challenging to operate within due to the high-temperature, 

high-pressure, and corrosive atmosphere. All of the currently proposed Venus missions have a very 

limited technology base to enable even short duration (5 to 10 hr) surface exploration the power and 

cooling requirements are particularly challenging. Fortunately, Glenn’s Advanced Stirling Duplex, which 

is being supported by the RPS Program Office’s Technology Development Project, can provide both the 

electrical power and cooling protection required for long-term exploration (>1 year), while reducing the 

overall development costs of the spacecraft (Refs. 9, 10, 11, 12, 13, and 14).
NASA/TM—2011-217018 2

Stirling Triad of Enabled Missions Overview

Stirling systems have been selected as the power conversion technology for the Advanced Stirling

Radioisotope Generator (ASRG), Fission Surface Power (FSP), and the Advanced Stirling Duplex (ASD)

project because of their high efficiency. These three technology development efforts will enable longduration missions to any location in the solar system. Often non-nuclear power technologies such as solar 

and chemical are preferred whenever solar radiation is available or the mission is of short duration. But in 

many cases, space exploration requires radioisotope- or fission-based power systems.

ASRG

ASRG and its lower and higher power cousins provide modest power levels (100’s W to about 1 kW)

in difficult environments such as the outer planets and Moon. For example, ASRG enables outer planet 

exploration and it also enables a long-lived international lunar network. A higher power convertor based 

on the same basic technology is also being considered. The ASRG is designed to operate for 17 years to 

support outer planet science missions.

FSP

FSP provides high power levels (1 to 100’s kW) in difficult environments such as the Moon, Mars, 

and outer planets for human outposts or large flagship missions. It utilizes the heat from a fission reactor 

to provide very high power levels. It is intended to operate for at least 8 to 10 years and is the only viable

power option for high power levels (Refs. 8 and 9).

ASD

ASD (Figs. 1 and 2) enables exploration in areas no other power technology can provide due to 

excessively high temperatures. ASRG and FSP systems cannot operate in hot environments (>300 °C) 

since they do not provide cooling and their organic and magnetic components would fail. Nearly all other 

power technologies cannot survive either in the extreme environments that exist on Venus, Mercury, 

Jupiter, and Io. One exception is high-temperature batteries, which can operate at high temperatures for a 

limited time. However, any long-lived mission in an extreme environment would benefit from ASD 

technology to provide not only a long-lived power supply, but to refrigerate other sensitive components 

such as electronics, sensors, and motors.

Figure 1.—Duplex block diagram. Figure 2.—ASD.

Expansion Space

Compression Space 

Displacer

Power Piston

Displacer

Compression Space 

Expansion Space

HXREGEN CHX

Heater

Regenerator

Cooler

HX

Warm Heat 

Exchanger

Regenerator

Chiller

Stirling Heat Engine

Stirling Heat Pump

Stirling

Duplex

Radiator

Heat Pipe

Heat Source

Stirling Convertor

Two-Stage Cooler

Electronics 

Coldbay

Pressure 

Vessel
NASA/TM—2011-217018 3

Figure 3.—RPS technology roadmap.

RPS Technology Roadmap

As shown in Figure 3, the development path is defined by the technology readiness level (TRL). The 

current plan envisions the use of ASRG for outer planet missions over the next decade. FSP is anticipated 

for mission use starting in the next decade. Notice ASD is intended to be demonstrated in the 2016 

timeframe to TRL 6 and would be flight qualified by 2020.

Advanced Stirling Duplex Team

The ASD Team comprises NASA, industry, and academia. The stakeholders include the mission 

planning organizations, project office, technology development organizations, and research management. 

The team makeup is designed to quickly develop and test a working duplex prototype. Multiple parallel 

efforts are underway to develop both power and cooling technologies that can operate at high temperature 

on the surface of Venus. The supporting technologies being developed by this team include a freedisplacer Stirling power convertor, a thermoacoustic power convertor, a multistage Stirling cooler, control 

system, thermophotovoltaic power convertor, variable conductance heat pipe, extreme high-temperature

materials, full-scale extreme environment test chamber, and spacecraft-cooling-related integration 

technologies such as insulation, radiator, and heat collection systems. The mission planning centers, 

which include Jet Propulsion Laboratory, Goddard, and Applied Physics Laboratory have provided 

challenging requirements that must be met for this technology to be useful under a variety of Venus 

mission scenarios. Several space grant consortium projects from Arizona, North Carolina, and Maryland 

have also contributed to the lander design. The National Research Council’s planetary science 

subcommittee and decadal survey have also highly ranked missions to Venus. The Venus Exploration 

Advisory Group has proposed specific missions requiring the ASD.

Clearly, many organizations have a stake in the successful development of this duplex system, and a 

team consisting of discpline leaders have been formed to expeditiously demonstrate this new enabling 

technology.

Future Technologies
NASA/TM—2011-217018 4

Top Level System Requirements

Duplex mass Heat engine 

efficiency

Heat pump 

efficiency

Self-induced 

vibration

Lifetime Heat lifted

<50 kg >50% of carnot >25% of carnot <2.3e-3 N 3 years 400 W internal 

1100 W parasitic

Figure 4.—Schedule, requirements, and milestones.

Development Schedule, Requirements, and Milestones

As shown in Figure 4, the technology development project is expected to have a 5-year development 

period. The test chamber and power convertors are being developed initially, followed by coolers that are 

then integrated into a duplex and eventually integrated into a mock lander and aeroshell for a complete 

system demonstration.

Venus Exploration Approach

While Venus has been explored in the past, the ability for extensive scientific return has had to wait 

until new technologies such as the Advanced Stirling Duplex could be developed to answer some key 

questions including the following: “Was there ever an ocean on Venus?”; “Why does Venus not have a 

magnetic field?”; and “Did Venus ever have plate tectonics?”. Perhaps more significant is the runaway 

greenhouse gas effect causing the planet to overheat. Some of the lessons learned about Venus may apply 

to Earth as well. Venus is not considered to be habitable due to the high-pressure, high-temperature, and 

corrosive atmosphere, however, as Landis (Ref. 15) has suggested, floating cities in the mild climate 

above the sulfuric acid clouds are a possibility.

The approach for returning to Venus has been the development of multiple mission scenarios by 

independent teams in which new technology is assumed to be available. A Venus mission was already 

recommended in the last decadal survey and more recently, a Discovery mission announcement 

opportunity has Venus ranked in the top three. Some technology development has begun for these 

proposed missions including the ASD, high-temperature electronics, and balloon bellows for Venus 

Mobile Explorer. These technologies are required to provide power, cooling, communications, and 

mobility over extended durations.

Since current technology only permits very short-lived missions, it is critical that ASD be 

demonstrated soon so these missions can be properly baselined to include long-lived in-situ surface 

elements.

Past and Proposed Missions

Numerous missions to the Venus surface have been attempted. The longest surviving was the Soviet 

Venera 13. It lasted for 127 min and was cooled with a phase change material and utilized batteries for 

power.

Project Began 

Dec. 3

Controller

Free 

Displacer

High Power 

TASHE

Thermophoto

-voltaic Study

Chamber

Design

Spacecraft 

Study

Cooler 

Concept

Heat pipe & 

Radiator

Materials

Power 

Prototype

Cooler 

Prototype

Chamber 

Phase I

Duplex 

Prototype

Chamber 

Phase II

Lander 

Integration

System 

Test

TRL 6 

Lander 

Demo

Deliver 

Duplex 

TRL 6

Duplex

Pulse-tube 

Cooler

Stirling 

Cooler

Chamber 

Fabrication

2009 2010 2011 2012 2013 2014 2015
NASA/TM—2011-217018 5

Several mission scenarios have been proposed for exploring the surface of Venus including the use of 

rovers, landers, and buoyant vehicles. In addition, long-lived weather stations and seismometers on the 

surface have been proposed. One of the most significant studies was recently completed by the Venus 

Science and Technology Definition Team in which a flagship mission was proposed for launch in the 

2025 timeframe. In that mission, two landers, two balloons, and an orbiter were proposed. Since a Venus 

capable Stirling duplex has not been demonstrated, the baseline mission assumed the landers would only 

survive for 5 hr. In a separate mission study (Ref. 16), the Venus Mobile Explorer is proposed in a 

decadal white paper in which a bellows system inflates and deflates to enable hopping across the planet. 

In this system, the ability to actively cool the lander enables extensive time on the surface.

In all of those studies in which only passive cooling is used, the near surface vehicle must exercise all 

of its instruments in a concerted dash to complete its science mission. This requires a much higher peak 

power load than if only some of the instruments are operating at a time. Since the Stirling duplex will 

enable at least a year’s worth of operations, the electrical power requirement is likely to be considerably 

less than the baselined missions currently require (Ref. 15).

Venus Environment

Venus is perhaps the most challenging exploration target in the solar system. Some of the 

environmental parameters are shown in Figure 5. In addition to the hot surface and high pressure, the 

environment contains potentially very corrosive and reactive species. On the surface the atmosphere is 

mostly supercritical carbon dioxide and nitrogen, but it also contains trace amounts of sulfur dioxide, 

argon, water, carbon monoxide, helium, neon, hydrogen chloride, and hydrogen fluoride. Hydrogen 

sulfide and sulfuric acid are found above the surface. The dense atmosphere also limits the planetary entry 

angle permitted and limits the solar radiation reaching the surface. The surface of the planet is basalt and 

rocky. It is thought the thick atmosphere is due to extreme volcanic activity. The lack of old craters on the 

surface suggests a sublimation occurs every million years or so.

The challenging Venus environment requires new technology development beyond ASD to fully 

explore the surface including high temperature electronics, sensors, motors, and materials. These new 

technologies must be demonstrated in a simulated Venus environment for flight qualification. Since 

Venus is in many ways the harshest environment in the solar system, any technology developed to survive 

there can likely be used in other challenging locations in which high temperature limits mission life such 

as Mercury, Jupiter, Io, and near the Sun. Fortunately, radiation exposure is minimized on Venus due to 

the thick atmosphere.

Figure 5.—Venus environment.

H = 0 km; 

T = 462 C; 

p = 92 bar;

v(mean) = 0.6 m/s

H = 15 km; 

T = 348 C; 

p = 33 bar;

v(mean) = 16 m/s

H = 52 km; 

T = 150 C; 

p = 0.8 bar;

v(mean) = 61 m/s

H = 70 km; 

T = 40 C; 

p = 3.7E-2 bar;

v(mean) = 92 ms

Mid level 

Landers

High level 

Low level 

Near Surface 
NASA/TM—2011-217018 6

Extreme Environment Test Chamber

All eight of the currently proposed missions to Venus would benefit significantly from active cooling. 

But key to demonstrating component and system performance is testing in a suitable Venus environment. 

While a few organizations have very small testing chambers suitable for low TRL test tube and coupon 

testing, the international community is in need of a larger facility.

The capability for testing and validation of technology systems in extreme environments such as on 

the surface of Venus, Jupiter, Mercury, and on Jupiter’s moon, Io, does not currently exist. Several 

mission studies have concluded the need for developing surface exploration technologies such as Stirling 

duplex for power and cooling, high temperature sensors, electronics, motors, telecom, and sample 

acquisition systems for operating in those extreme environments. This section provides a brief description 

of a proposed test chamber that would enable the development and flight qualification of these critical 

technologies.

Background

Due to the challenging Venus environment, mission planners have a very limited technology base 

from which to choose to achieve near planetary surface scientific goals. An ASD has been identified as a 

key technology development priority since it will provide both the electrical power and cooling necessary 

to enable long-lived surface missions. But key to demonstrating component and system performance is 

testing in a suitable Venus environment.

Proposed Facility

Since the initial need is for materials characterization and power system performance testing in a 

Venus environment, a small 3- by 5-ft chamber is being fabricated such as shown in Figure 6. A larger 

chamber would subsequently be added to this smaller chamber to allow complete system testing. The 

modular nature of this design also allows for simultaneous tests from several organizations and the 

addition of much larger chambers if required by the Agency.

Figure 6.—Extreme environment chamber.
NASA/TM—2011-217018 7

Features

As shown in Figure 6, the test chamber facility will include test article controls, DAQ, and PLC racks 

for test chamber control, optical port holes, removable liner, rolling door access, and overhead crane

access. Notice the chamber consists of several sections to allow for modular construction that is consistent 

with budget and testing requirements.

Mission Phases

The test chamber will be capable of supporting both a vacuum and high pressure to simulate the entire 

mission sequence including launch, cruise, entry, descent, and surface operations. The chamber will

support both on surface testing and higher atmospheric operations. The pressure, temperature, wind 

speed, and atmospheric composition can be varied within the chamber. The wind speed is below 1 m/s at 

the surface, but significantly impacts radiator sizing, seismometer, weather, and lander stability 

considerations.

Lander Concepts

Shown in Figure 7 are four proposed lander concepts (Refs. 2, 3, 10, and 11) that could be tested in 

the extreme environment chamber. Originally, the VISM Team proposed a lander in which the 

seismometer was attached to an actively cooled pressure vessel in 1993. Next, the Arizona Space Grant 

Consortium recommended a hybrid dual chamber approach in which only some of the instruments were 

cooled. The NASA Science Technology Definition Team develop a passively cooled lander concept in 

2009. Most recently, the ASD Team developed an actively cooled lander concept similar to the original 

VISM concept, but without a seismometer attached. The pressure vessel size is approximately 3 ft in 

diameter and the landers are typically less than 500 lb. The test chamber is intended to initially test only 

the power, cooling, and materials components in the 3- by 5-ft chamber. Note that it is necessary to 

include the entire lander in the Stirling duplex test because the exact parasitic heat load into the vessel is 

not known at this time. Moreover, the multistage cooling requires separately cooled and insulated 

compartments to be successfully tested (Ref. 16).

Also, since Venus has approximately the same gravity as Earth, the test chamber can very realistically 

simulate all known physics occurring on the Venus surface.

Figure 7.—Lander concepts.
NASA/TM—2011-217018 8

TABLE 1.—POWER OPTIONS TABLE 2.—COOLING OPTIONS

 

Advanced Stirling Duplex

The high temperatures on Venus require a high-efficiency power and cooling system because the high 

heat rejection temperatures create a small temperature ratio for power conversion. Since the refrigerated 

temperatures are approximately 30 °C, and the environment is approximately 460 °C, most power 

technologies cannot operate efficiently enough to be useful.

Power and Cooling Trade-Space

Shown in Tables 1 and 2 are a comparison of power conversion options and cooler options. Notice all 

the Stirling cycle power and cooling systems have a higher overall efficiency. One of the properties of

Stirling convertors is their ability to maintain high efficiencies even when the temperature ratio gets 

small. Another important consideration is in how to integrate the power and cooling systems into a single 

duplex. For example, a Stirling cycle has an oscillating working fluid whereas a Brayton cycle has 

rotating turbomachinery. Attempting to join an oscillation with a rotation can limit the overall system 

efficiency because pneumatic connections between the two systems are not possible. Instead, either an 

electrical or mechanical connection must be utilized that each create additional losses, respectively.

Effect of Hot-End Temperature

Notice in the figure on the right a comparison is made of three power convertors based on the Stirling

cycle: free-piston, free-displacer, and thermoacoustic. It is currently believed that free-piston is the most 

efficient power conversion option available, followed by free-displacer, and then by thermoacoustic. 

However, as the dashed line shows, if the free-piston convertor cannot be operated with a temperature 

hotter than 950 °C due to having moving parts in the hot-end (Ref. 17) and if both the free-displacer and 

thermoacoustic designs can be operated at much higher temperatures since they have no moving parts at 

the hot-end, then the dashed line in Figure 8 shows the free-displacer will eventually out-perform the freepiston at 1025 °C and the thermoacoustic outperforms the free-piston at 1130 °C. For these reasons, these 

alternative technologies are being developed under this project with a goal of 1200 °C at the hot-end.

It should also be noted that thermophotovoltaic technology is not shown as a power system since it 

cannot operate at Venus temperatures unless refrigerated. It is potentially possible to combine a thermoacoustic duplex with a thermophotovoltaic system to provide cooling and power with no moving parts.

Life-Limiting Mechanisms

The Stirling cycle devices have been improved over the past decades to the point they are extremely 

efficient and reliable. Some potential life-limiting mechanisms might include wear, fatigue, creep, 

permeation, and radiation. Fortunately, as shown in Figure 9, some of these mechanisms have been 

mitigated since noncontacting operation is now possible using either flexures or gas bearings.
NASA/TM—2011-217018 9

Figure 8.—Efficiency comparison.

Figure 9.—Life limiting mechanisms.

Notice that the free-piston configuration does not have a mechanical connection between the displacer 

and the piston. The relative motion of these elements is determined by physical resonance conditions and 

the non-contact operation is achieved with gas bearings. This configuration is currently the world’s most 

efficient heat engine achieving 55 percent of Carnot efficiency. 

In the last example shown in the figure, a free-displacer convertor is depicted in which the displacer is 

shortened and power is taken off of the displacer. In that case, the efficiency of the engine decreases due 

to the increased dead volume. A related technology is the thermoacoustic convertor in which the displacer 

is entirely removed and replaced by an acoustical circuit.

One of the design issues is the working fluid. Often helium is used, but in the case of the Stirling 

duplex, there appear to be advantages to utilizing alternative working fluids such as hydrogen or carbon 

dioxide. One of the challenges of using alternative fluids is they tend to permeate through the walls or can 

damage magnets. If a suitable material is identified for fluid containment then modest improvement in 

system efficiencies (~1 percentage point) (Ref. 18) are possible for the heat engine and cooler.

Another design issue is the thickness of the heater head. Ideally, the walls would be thin to reduce 

thermal losses from the hot-end to the cold-end. Thin walls can lead to excessive material creep, 

especially at the high temperatures. This creep can cause the oscillating flow to bypass the regenerator.
NASA/TM—2011-217018 10

One possible approach to minimizing this effect is to use the Venus atmosphere as the working fluid

(Ref. 18). In this way, the inside of the duplex would always be directly connected to its environment. 

Hence, the pressure forces would always be in static equilibrium and creep effects are minimized.

Fatigue can occur on the moving parts, especially those on the hot-end of the convertor due to the 

repeated oscillating pressure forces acting on them. While more research is required to quantify the exact 

limitations, this project is investigating every option. Also, parts such as springs or flexures have to be 

carefully designed for millions of oscillations over the entire operating life. Finally, radiation is 

potentially a concern, particularly for magnets and organics. Preliminary tests indicated minimal impact at 

expected exposure limitations. Also, one of the benefits of the thick Venus atmosphere is minimal 

radiation actually reaches the surface (Ref. 6).

Principle of Operation

The Stirling duplex can be designed with a variety of power and cooling configurations (Ref. 1). Each 

of those power and cooling technologies can be joined into a duplex using either a pneumatic, electrical, 

or mechanical connection. The least desirable is the mechanical mode due to the need for gear lubrication 

and the associated frictional losses. The electrical mode requires converting the mechanical motion into 

electrical and then back into mechanical. Each of those conversion steps results in overall energy losses 

due to the imperfect efficiency of linear alternators and compressors. The most desirable is the pneumatic 

mode because the acoustical energy is directly used for cooling without the gear friction or alternator 

conversion losses.

Cooling System Staging

One of the keys for developing a successful Venus lander cooling system is to take advantage of the 

thermodynamic efficiency of a multistage cooler. 

Single-Stage Cooler

In a single-stage cooler, the heat is removed from the payload bay in a single step. Most previous 

Venus lander studies assumed a single-stage cooler would be utilized and this resulted in very inefficient 

designs. As shown in Figure 10, all the electronics are contained within a single pressure vessel. This is 

the same basic pressure vessel structure used for all previous Venus landers (Refs. 3, 5, and 7).

Notice that the cooler has to reject the heat produced by the electronics in the payload and the heat 

that enters from outside the pressure vessel due to the hot Venus environment. Shown in this figure is the 

original design from 1993 in which a double acting Stirling engine with 8 GPHS was assumed. 

Figure 10.—Single-stage cooling system.

All previous studies assumed single-stage cooling. 

Most detailed study was Venus Interior Surface Mission, 1993 

Double-ended

Stirling Engine

Pneumatic 

Coupling

Heat Rejection 

Fins

Vacuum and 

Helium Duct

Heat Absorption 

Fins

Stirling Cooler

Engine Heat 

Source

Pilot Chute

Stem 

Deployer

Actuator

Seismometer 

and 

Windshield

Refrigerated 

Payload Bay

Pressure Shell Vacuum Shell

Protected 

Payload
NASA/TM—2011-217018 11

Figure 11.—Two-stage cooler integration.

The cooler has a piston that was activated from the helium pressure wave and the piston then acted 

through a gear system to power the cooler. The heat absorption fins were used to collect the heat so the 

cooler could pump it out. The heat rejection fins would reject the cold-end waste heat from the Stirling 

convertor. Notice this duplex had a pneumatic coupling but connected to a kinematic cooler. This original 

design was overly optimistic in the heat engine and cooler efficiencies assumed, the electrical payload 

requirements were understated, and uneven cooling in the payload was never addressed.

Two-Stage Cooler

The two-stage cooler configuration shown in Figure 11 is a much more recently designed 

configuration (Ref. 2). Notice the number of GPHS modules ranges from 8 up to 40 depending on the 

power and cooling required. Now the overall efficiency of the duplex is higher because of the significant 

thermodynamic advantage that occurs by intercepting the heat coming from the Venus environment at a 

higher temperature. The heat collection and rejection is also enhanced via modern variable conductance 

heat pipes. This allows the payload bay to be evenly cooled (Ref. 4).

Prototype Convertors

A number of prototype heat engine convertors have been developed in the past under different 

projects that demonstrate high hot-end operating temperatures are currently possible.

High-Temperature Stirling

A Component Test Power Convertor (CTPC) was developed under the SP-100 program to operate at 

777 °C while producing 12 kWe and operating with a mean pressure of 15 MPa. Since the Venus 

environment is approximately 460 °C, it is possible to use this engine on Venus if the cold-end were 

protected with refrigeration as in a duplex configuration. Moreover, the high operating pressure actually 

exceeds the Venus atmospheric pressure. This was demonstrated nearly 20 years ago.

More recently, the Advanced Stirling Convertor was demonstrated to operate at 850 °C while 

achieving 55 percent of Carnot efficiency with over 2000 hr of continuous operation. It is anticipated that 

this convertor would operate with 17 percent efficiency on the surface of Venus if the cold-end were 

protected in a duplex configuration. While not demonstrated yet, it is thought by leading designers the 

highest temperature possible with ASC technology is 950 °C due to cyclic fatigue on the moving hot-end 

parts (Ref. 17).

Outer Pressure Vessel

First Stage Cooler

• Lifts 400W

• Rejects 1358W

Second Stage Cooler

• Lifts 2050W

• Rejects 5363W

GPHS modules

15449Wth

1200o C

Stirling Duplex

(Pneumatic) Stirling Duplex

(Pneumatic)

Insulated Inner Vessel

Protected Instruments

Heat leak-in from

Venus surface

700W at 500o C
NASA/TM—2011-217018 12

Most recently, several attempts at eliminating moving parts in the hot-end were successful by 

developing small power convertors utilizing thermoacoustic technology. While they operated at only 

around 600 °C on the hot-end, they demonstrated the efficiency that is possible with this approach. In 

general, they are about 80 percent as efficient as the free-piston-based Stirling cycle commonly used 

when operated at the same temperatures. However, as mentioned previously, if their hot-end temperature 

can approach 1200 °C, their overall efficiency may surpass the ASC as shown in Figure 8 if the ASC is 

limited to 950 °C. Higher power thermoacoustic engines have also been demonstrated as discussed in the 

next section (Ref. 17).

Clearly, high-temperature and high-pressure Stirling systems have already been demonstrated, and an 

approach for raising the temperature further has been demonstrated. The cold-end temperatures are less of 

a concern because they can be cooled in a duplex configuration.

High-Power Thermoacoustic

A thermoacoustic engine/driver that was initially developed under an Air Force Small Business 

Innovation Research (SBIR) phase II effort, was successfully tested as shown in Figure 12. This proof-ofconcept prototype operates at the high power levels required for a Venus mission. It is unique in that it 

was designed to operate as a duplex for the ARES upper stage rocket’s cryogenic propellant densification 

system. It is the second largest thermo-acoustic engine ever constructed and it is designed to be mated 

with a two-stage cryocooler. Due to funding discontinuation, the engine and cryocooler were never 

operated and have sat in mothball for over 2 years. The Advanced Stirling Duplex project was able to 

leverage this previous work to quickly assemble and test this hardware. Since the engine is the first at the 

higher power levels required for Venus, it is an ideal risk reduction tool to confirm our analytical models 

so we can confidently design for the Venus environment.

Shown in Figure 12 is the thermoacoustic engine with one of the consultants on the project, Dr. Greg 

Swift. The engine operates at 30 Hz with 500 psi mean helium pressure. It is designed to accept 12000 W 

of thermal energy and convert that to 4000 W of acoustical energy. This engine/driver can be connected 

to linear alternators and pulse tube coolers to provide electrical power and refrigeration. 

While this engine is not designed to operate at Venus temperatures, it provides an important proof-ofconcept for understanding the physics of these large engines. Moreover, the already fabricated cryocooler 

can be quickly added to this engine to form a prototype duplex early in our project to help guide future 

development efforts.

Figure 12.—High-power thermoacoustic milestone completed.

Heat Exchanger Hot-end Power Convertor
NASA/TM—2011-217018 13

Also shown in Figure 12 are a large resonator and dummy masses to contain the inertial forces. Note 

that since this engine was originally designed to provide cryogenic cooling, its operating frequency is

30 Hz. This low frequency induces vibration and requires a large resonator. However, for our Venus 

application of this technology, the cooler can operate at much higher frequencies since the refrigerated 

temperatures are much higher. This significantly reduces vibrations and reduces the size of the resonator 

required. The closeup of the engine shown in Figure 12 shows the electrical heating leads and liquid 

cooling lines.

One of the critical components of any thermoacoustic engine is the jet pump (Ref. 18). This 

component prevents Gedeon streaming from occurring within the engine and it is very difficult to 

analytically predict its behavior. The jet pump on this engine will be resized several times to optimize the 

performance of the engine. After that, the next step for this early prototype demonstration effort is the 

addition of a cryocooler to confirm the duplex operation matches analytical predictions. Once all physical 

behavior can be explained and our analytical predictions are validated, then this duplex can be cost 

effectively and confidently modified to operate under Venus conditions.

Technical Challenges

Many technical challenges as shown in Figure 13 remain to be addressed under this current effort. 

First, a Stirling cycle heat engine must be developed that can operate at a high hot-end temperature with 

high efficiency, and be compatible with the Venus environmental conditions. Second, a high-efficiency 

Stirling cycle multistage cooler must be developed that can fit within the lander’s pressure vessel and 

operate at very high efficiency with minimal vibration. Third, the heat engine and multistage cooler must 

be compatible with each other, share the same working fluid, and maintain an overall high system 

efficiency. Fourth, the variable conductance heat pipe technology must be developed to efficiently 

transport heat energy to the heat engine and cold fingers while rejecting heat to radiators that are exposed 

to the Venus environment. Fifth, the entire system must operate within the pressure vessel of the lander 

while being mass efficient. Finally, the Stirling duplex, heat pipes, radiator, and insulated pressure vessel, 

as shown in Figure 2 must be demonstrated to operate within the Venus environment for up to 1 year.

Figure 13.—Strategic technologies and commercialization.
NASA/TM—2011-217018 14

Conclusions

Clearly the development of a Stirling duplex is possible and desirable for a host of reasons including:

solar system exploration in harsh environments such as Venus and Mercury; commercial opportunities

such as efficient home power and cooling; and environmental benefits such as climate change model 

verification. This technology development project is anticipated to continue for the next several years 

with a demonstration unit scheduled for completion by 2015. Success of this effort will be when the 

Stirling duplex can operate at TRL 6 in a Venus-like environmental test chamber for 1 year while 

maintaining the payload bay of a mock Venus lander within acceptable refrigerated limits.

References

1. Kadambi, J.R., Sastry, S. “A Thermoacoustic Engine Refrigerator System for Space,” Ph.D. 

Dissertation, Mechanical Engineering Dept., Case Western Reserve Univ., Cleveland, OH, 2010.

2. Dyson, R.W., Schmitz, P.G., Penswick, L.B., Bruder, G.A., “Long-Lived Venus Lander Conceptual

Design: How to Keep it Cool,” AIAA International Energy Conversion Engineering Conference, 

AIAA–2009–4631, Denver, CO, 2009, Aug. 2–5.

3. Stofan, E., Nock, K., Dudenhoefer, J., Vitali, N., Schock, A., Neuman, J., “Venus Interior Structure 

Mission (VISM),” Advanced Stirling Duplex [online database], URL:

http://www.grc.nasa.gov/WWW/ASDT/documents/VISM%20Documents.pdf

4. Hall, J.L., Bullock, M., Senske, D.A., Cutts, J.A., Grammier, R., “Venus Flagship Mission Study, 

Final Report of the Venus Science and Technology Definition Team,” Jet Propulsion Lab [online 

database], URL:

[cited 15 May 1993].

http://vfm.jpl.nasa.gov/files/Venus+Flagship+Mission+Study+090501-

compressed.pdf [cited 17 April 2009].

5. Mellott, K.D., “Electronics and Sensor Cooling With a Stirling Cycle for Venus Surface Mission,” 

AIAA–2004–5610, 2004.

6. Bugby, D., Seghi, S., Kroliczek, E., Pauken, M., “Novel Architecture for a Long-Life, Lightweight 

Venus Lander,” AIP Space, Propulsion & Energy Science International Forum, CP1103, SPESIF2009, Washington, DC, 2009, pp. 39–50.

7. Wilson, K.B., “The Use of Sage Simulation Software in the Design and Testing of Sunpower’s Pulsetube Cryocooler,” M.S. Thesis, Mechanical Engineering Dept., Ohio University., Athens, OH, 2005.

8. Arnold, E.D., “Handbook of Shielding Requirements and Radiation Characteristics of Isotopic Power 

Sources for Terrestrial, Marine, and Space Applications,” Oak Ridge National Lab., Rept. ORNL3576, Oak Ridge, TN, Apr. 1964.

9. Palac, D.T., Mason, L.S., Harlow, S., “Fission Surface Power Technology Development Status,” 

AIAA–2008–7812, 2008.

10. Coburn, A., Daniel, E., Frazier, M., Grimsley, E., “Venus Lander Concept of Design Report,” EmbryRiddle Aeronautical University, December 2008.

11. Balint, T., Cutts, J., Bullock M., Garvin, J., Gorevan S., Hall, J., et al., “Technologies for Future 

Venus Exploration,” Venus Exploration Analysis Group (VEXAG) [online database], URL:

http://www.lpi.usra.edu/decadal/vexag/techWhitePaper.pdf [cited 9 September 2009].

12. Radioisotope Power Systems Committee, NRC, Radioisotope Power Systems: An Imperative for 

Maintaining U.S. Leadership in Space Exploration, National Research Council, ISBN-13: 978-0-309-

13857-4, 2009, pp. 68.

13. Rodgers, D., Gilmore, M., Sweetser, T., “Venus Sample Return: A Hot Topic...,” IEEE 0-7803-5846-

5/00, 2000.

14. O’Brien, R.C., Ambrosi, R.M., Bannister, N.P., Howe, S.D., Atkinson, H.V., “Safe radioisotope 

thermoelectric generators and heat sources for space applications,” J. of Nuclear Materials, Vol. 377, 

2008, pp. 506–521.
NASA/TM—2011-217018 15

15. Landis G.A., “Human Telerobotic Exploration of Venus: A Flexible Path Design Study,” Space 

Technology and Applications Int. Forum-STAIF 2003, AIP Conference Proc., Jan. 28, 2003-Volume 

654, pp. 1193–1198.

16. Glaze, L. et al., “Venus Mobile Explorer (VME): A Mission Concept Study for the National Research 

Council Planetary Decadal Survey,” 7th International Planetary Probe Workshop, 2010.

17. Wood, G., Personal Communication, Sunpower, Inc., 2010.

18. Swift, G., Haberbusch, M., “Thermoacoustic Duplex Technology for Cooling and Powering a Venus 

Lander,” SBIR Phase III, Sierra-Lobo, Inc., 2010.
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NASA has begun the development of a combined Stirling cycle power and cooling system (duplex) to enable the long-lived surface 

exploration of Venus and other harsh environments in the solar system. The duplex system will operate from the heat provided by decaying 

radioisotope plutonium-238 or its substitute. Since the surface of Venus has a thick, hot, and corrosive atmosphere, it is a challenging 

proposition to maintain sensitive lander electronics under survivable conditions. This development effort requires the integration of: a 

radioisotope or fission heat source; heat pipes; high-temperature, corrosion-resistant material; multistage cooling; a novel free-displacer 

Stirling convertor for the lander; and a minimal vibration thermoacoustic Stirling convertor for the seismometer. The first year effort 

includes conceptual system design and control studies, materials development, and prototype hardware testing. A summary of these findings 

and test results is presented in this report. 

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Stirling duplex spacecraft power cooling 

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