**9. Dynamic Isotope Power System (DIPS)**

The Dynamic Isotope Power System (DIPS) program was initiated in 1975 to provide increased power from radioisotope heat sources by using more efficient dynamic conversion systems (Brayton and Rankine). The precedent had been established in the 1950s with the SNAP-1 program (see Section 2) with its mercury Rankine conversion system and the SNAP-2 (3 kWe) and SNAP-8 (30 to 60 kWe) mercury Rankine space reactor programs. In terms of mass and specific power DIPS fills the gap between RTGs and nuclear reactors; in short, it could be the next logical step for increased RPS power after RTGs. Figure 14 illustrates the basic features of a representative DIPS (either Brayton or Rankine) (Bennett and Lombardo, 1989).

Fig. 14. Functional diagram of a generic Dynamic Isotope Power System (DIPS) (Bennett and Lombardo, 1989).

The original DIPS program was focused on producing a 1.3 kWe radioisotope power source with a mass of ≤204 kg using either a Brayton conversion system or an organic Rankine conversion system. The Brayton conversion system built upon the experience of NASA and its contractors (e.g., Garrett Corporation) dating from 1965 in developing a 2 to 10 kWe closed Brayton cycle (CBC) power system. In parallel, work on ground-based Rankine cycle systems led what was then Sundstrand Corporation to propose using Dowtherm A or toluene as a working fluid in order to avoid the corrosion issues with liquid-metal Rankine systems (Bennett and Lombardo, 1989).

Based on actual hardware tests, both the CBC and the organic Rankine cycle (ORC) were shown to be capable of meeting the DIPS goals. The organic Rankine cycle was chosen for further testing. The lack of a mission led to the termination of the program in 1980 but in 1986 the U.S. Air Force expressed interest in having a DIPS for its Boost Surveillance and Tracking System (Bennett & Lombardo, 1989). The DIPS program was restarted with Rocketdyne as the systems contractor and Sundstrand and Garrett as subcontractors.

Modeling of the MMRTG performance indicates that the MMRTG will be able to provide

The Dynamic Isotope Power System (DIPS) program was initiated in 1975 to provide increased power from radioisotope heat sources by using more efficient dynamic conversion systems (Brayton and Rankine). The precedent had been established in the 1950s with the SNAP-1 program (see Section 2) with its mercury Rankine conversion system and the SNAP-2 (3 kWe) and SNAP-8 (30 to 60 kWe) mercury Rankine space reactor programs. In terms of mass and specific power DIPS fills the gap between RTGs and nuclear reactors; in short, it could be the next logical step for increased RPS power after RTGs. Figure 14 illustrates the basic features of

Fig. 14. Functional diagram of a generic Dynamic Isotope Power System (DIPS) (Bennett and

The original DIPS program was focused on producing a 1.3 kWe radioisotope power source with a mass of ≤204 kg using either a Brayton conversion system or an organic Rankine conversion system. The Brayton conversion system built upon the experience of NASA and its contractors (e.g., Garrett Corporation) dating from 1965 in developing a 2 to 10 kWe closed Brayton cycle (CBC) power system. In parallel, work on ground-based Rankine cycle systems led what was then Sundstrand Corporation to propose using Dowtherm A or toluene as a working fluid in order to avoid the corrosion issues with liquid-metal Rankine

Based on actual hardware tests, both the CBC and the organic Rankine cycle (ORC) were shown to be capable of meeting the DIPS goals. The organic Rankine cycle was chosen for further testing. The lack of a mission led to the termination of the program in 1980 but in 1986 the U.S. Air Force expressed interest in having a DIPS for its Boost Surveillance and Tracking System (Bennett & Lombardo, 1989). The DIPS program was restarted with

Rocketdyne as the systems contractor and Sundstrand and Garrett as subcontractors.

the necessary power to enable MSL to achieve its objectives (Hammel, et al., 2009).

a representative DIPS (either Brayton or Rankine) (Bennett and Lombardo, 1989).

**9. Dynamic Isotope Power System (DIPS)** 

Lombardo, 1989).

systems (Bennett and Lombardo, 1989).

This time the CBC was chosen and Rocketdyne developed a basic 2.5-kWe DIPS module that could be used in space or on planetary surfaces. Figure 15 illustrates the basic components of a 2.5-kWe modular DIPS power conversion unit (PCU). While again changing national priorities did not allow DIPS to be developed into a flight system, the basic technology exists to provide an RPS with powers spanning the range from 2 to ≥10 kWe (Rockwell, 1992). Section 10 describes a lower power successor to DIPS, the Advanced Stirling Radioisotope Generator (ASRG) that will provide increased efficiency and use less Pu-238 fuel than existing RTGs

Fig. 15. Components of the proposed 2.5-kWe modular DIPS Power Conversion Unit (PCU).

Overall dimensions for the 2.5-kWe module are 2.44 m x 3.55 m x 0.5 m. (Image credit: Rocketdyne).

## **10. Advanced Stirling Radioisotope Generator (ASRG)**

The ASRG employs an advanced, high efficiency, dynamic Stirling engine for heat-toelectric power conversion. This process is roughly four times more efficient than presently utilized thermoelectric devices. As a result, the ASRG produces comparable power to the MMRTG with only one quarter of the Pu-238, extending the supply of radioisotope fuel available for future space science missions. The higher efficiency provided by dynamic systems, such as the ASRG, could become an enabling power system option for higher power kilowatt class power systems envisioned for flagship class science spacecraft, large planetary rovers, and systems in support of human exploration activities.

The ASRG utilizes an advanced Stirling free-piston heat engine consisting of two major assemblies, the displacer and piston, that reciprocate to convert heat to electrical power as shown in Figure 16. Heat from the GPHS module is conductively coupled to the heater head (not shown). Helium is used as the working fluid and is hermetically contained within the convertor enclosure. The displacer shuttles helium between the expansion

U.S. Space Radioisotope Power Systems and Applications: Past, Present and Future 491

Figure 17 shows one of the two GPHS heat sources, one for each advanced Stirling convertor (ASC). Each GPHS module is surrounded by insulation to minimize heat leakage thus maximizing heat input to the convertors. The displacer side of the ASC is toward the GPHS module and cold-side is attached to the housing via the cold-side adapter flange (CSAF). The housing and attached fins provide a view to the environment to maintain sufficient heat rejection. During ground storage and launch pad operations a slightly positive pressure of inert gas is maintained via the Gas Management Valve. The gas within the housing helps dissipate heat not rejected by the ASC via the CSAF. This gas is permanently vented to vacuum by the Pressure Relief Device (PRD) to achieve full operating power in space

Operating frequency of the Stirling convertors is 102.2 Hz AC. The controller converts AC current to DC current for a typical 28-34 V spacecraft electrical bus. The shunt maintains a required load on the ASRG when it is not connected to a spacecraft, such as during storage or spacecraft integration. The controller also maintains synchronized displacer/piston movement of the two directionally opposed Stirling convertors to minimize induced disturbance to the spacecraft and its precision instrumentation. The ASRG is capable of producing 45% of total power should one ASC fail to operate. Health monitoring of the ASRG is provided by telemetry signals to the spacecraft and then transmitted back to Earth. The ASRG has an autonomous control system since space distances do not allow for direct operator control. The controller has electronics for each ASC plus a third redundant circuit to replace a failed card thus increasing overall system

The ASRG is being developed under joint sponsorship by NASA and DOE for potential flight on a future NASA mission opportunity. Projected mass of the flight unit is 32 kg or less. The flight units are anticipated to produce over 130 We in the vacuum of space and at an effective sink temperature of 4 K (deep space). Other applications on planetary bodies would either increase or decrease the power output depending on the temperature and

ASRG efforts leading up to its flight readiness began in 2000. An engineering unit (EU) was built in 2008 by Lockheed Martin Space Systems incorporating the advanced Stirling

Fig. 17. Cutaway view of the ASRG (Image credit: Lockheed Martin).

vacuum.

reliability.

atmosphere of the environment.

space where heat is received and compression space, where waste heat is removed. The changes in pressures and volumes of the convertor working spaces drive the power piston that reciprocates to produce AC electrical power via a permanent magnet linear alternator (Hoye, et al. 2011).

Fig. 16. Advanced Stirling Convertor (Image credit: Kristin Jansen, NASA Glenn).

space where heat is received and compression space, where waste heat is removed. The changes in pressures and volumes of the convertor working spaces drive the power piston that reciprocates to produce AC electrical power via a permanent magnet linear alternator

Fig. 16. Advanced Stirling Convertor (Image credit: Kristin Jansen, NASA Glenn).

(Hoye, et al. 2011).

Fig. 17. Cutaway view of the ASRG (Image credit: Lockheed Martin).

Figure 17 shows one of the two GPHS heat sources, one for each advanced Stirling convertor (ASC). Each GPHS module is surrounded by insulation to minimize heat leakage thus maximizing heat input to the convertors. The displacer side of the ASC is toward the GPHS module and cold-side is attached to the housing via the cold-side adapter flange (CSAF). The housing and attached fins provide a view to the environment to maintain sufficient heat rejection. During ground storage and launch pad operations a slightly positive pressure of inert gas is maintained via the Gas Management Valve. The gas within the housing helps dissipate heat not rejected by the ASC via the CSAF. This gas is permanently vented to vacuum by the Pressure Relief Device (PRD) to achieve full operating power in space vacuum.

Operating frequency of the Stirling convertors is 102.2 Hz AC. The controller converts AC current to DC current for a typical 28-34 V spacecraft electrical bus. The shunt maintains a required load on the ASRG when it is not connected to a spacecraft, such as during storage or spacecraft integration. The controller also maintains synchronized displacer/piston movement of the two directionally opposed Stirling convertors to minimize induced disturbance to the spacecraft and its precision instrumentation. The ASRG is capable of producing 45% of total power should one ASC fail to operate. Health monitoring of the ASRG is provided by telemetry signals to the spacecraft and then transmitted back to Earth. The ASRG has an autonomous control system since space distances do not allow for direct operator control. The controller has electronics for each ASC plus a third redundant circuit to replace a failed card thus increasing overall system reliability.

The ASRG is being developed under joint sponsorship by NASA and DOE for potential flight on a future NASA mission opportunity. Projected mass of the flight unit is 32 kg or less. The flight units are anticipated to produce over 130 We in the vacuum of space and at an effective sink temperature of 4 K (deep space). Other applications on planetary bodies would either increase or decrease the power output depending on the temperature and atmosphere of the environment.

ASRG efforts leading up to its flight readiness began in 2000. An engineering unit (EU) was built in 2008 by Lockheed Martin Space Systems incorporating the advanced Stirling

U.S. Space Radioisotope Power Systems and Applications: Past, Present and Future 493

The Stirling duplex concept combines a power convertor as in the ASRG with a thermodynamic Stirling cycle cooler. This concept provides both power in the range of 100- 300 We and 1100 Wth cooling, allowing its potential use for missions in harsh high-

To this point, discussions have focused on lower-power science mission objectives. However, the concept of high efficiency energy conversion could find application in human exploration, particularly relating to the Moon and Mars. Multi-kilowatt power level radioisotope systems have application for the Moon due to its long 356-hour night period and on Mars due to its distance from the Sun, atmospheric dust attenuation and short

Fig. 19. A concept for a multi-Kilowatt radioisotope power system deploying a fission reactor on the surface of Mars, (Artist concept credit: Bob Souls, John Frassanito and

Radioisotope power systems provide continuous power avoiding necessity of solar arrays and battery or fuel cell storage (for night time energy) and the wait to recharge them. Relative mission risk could also be reduced since solar array size for many of these

temperature environments.

**12. Human exploration missions** 

winter day periods at higher latitudes.

Associates, Courtesy of NASA).

convertor manufactured by Sunpower. Characterization testing was performed for typical launch and space environments.

Fig. 18. ASRG engineering unit readied for extended testing at NASA Glenn Research Center (Photo Credit, NASA Glenn).

After successful characterization tests the EU was put on extended operation with electrically heated GPHS simulators as shown in figure 18. A cold gas is passed over the EU to maintain proper thermal operating conditions. The EU is planned to demonstrate 14,000 hours of operation in validating the design's viability for flight on a NASA science mission (Lewandowski and Schreiber, 2010). The EU has achieved over 11,000 hours (April, 2011) with a prototypical flight controller.
