**2. Early SNAP program**

474 Radioisotopes – Applications in Physical Sciences

1947, Gendler and Kock, 1949). Radioisotopes were also considered in early studies of

Fig. 1. Cutaway view of a radioisotope power source (RPS) (Image credit: DOE).

1954).

In 1951, the U.S. Atomic Energy Commission (AEC) signed several contracts to study a 1 kWe space power plant using reactors or radioisotopes. Several of these studies, which were completed in 1952, recommended the use of RPS (Corliss and Harvey, 1964). In 1954, the RAND Corporation issued the summary report of the Project Feedback military satellite study in which radioisotope power was considered (Lipp and Salter, 1954). Paralleling these studies, in 1954, K. C. Jordan and J. H. Birden of the AEC's Mound Laboratory conceived and built the first RTG using chromel-constantan thermocouples and a polonium-210 (210Po or Po-210) radioisotope heat source (see Figure 2). While the power produced (1.8 mWe) was low by today's standards, this first RTG showed the feasibility of RPS. A second "thermal battery" was built with more Po-210, producing 9.4 mWe. Jordan and Birden concluded that the Po-210 "thermal battery" would have about ten times the energy of ordinary dry cells of the same mass (Jordan and Birden,

nuclear-powered aircraft (Corliss and Harvey, 1964).

The AEC began the Systems for Nuclear Auxiliary Power (SNAP) program in 1955 with contracts let to the Martin Company (now Teledyne) to design SNAP-1 and to the Atomics International Division of North American Aviation, Inc. to design SNAP-2. (Under the AEC nomenclature system, the odd-numbered SNAPs had radioisotope heat sources and the even-numbered SNAPs had nuclear fission reactor heat sources.) SNAP-1 was to provide 500 We using the then readily available fission product radioisotope cerium-144 (144Ce) (Corliss and Harvey, 1964).

The Martin Company began with a 133-We RPS design using 144Ce as the radioisotope fuel and a Rankine thermal-to-electric conversion system. From this came the 500-We SNAP-1 RPS design based on 144Ce fuel and a Rankine conversion system (see Figure 3) (Corliss and Harvey, 1964). The use of a dynamic conversion system in the first RPS is a key historical fact in understanding the current focus on developing an Advanced Stirling Radioisotope

U.S. Space Radioisotope Power Systems and Applications: Past, Present and Future 477

As of 2010, as shown in Table 1, the U.S. has launched 45 RTGs, hundreds of RHUs and one space nuclear fission reactor. Of the RTGs flown, two different types of thermoelectric materials have been employed: telluride-alloy based or silicon-germanium-alloy based. The following sections will discuss these RTGS to be followed by sections discussing current

The initial and current thermoelectric material of choice is based on telluride technology alloyed with lead (Pb-Te) that, to a first approximation, can be used from room temperature to about 900 K before materials properties become an issue. Above 900 K, the U.S. has had great success with a silicon-germanium alloy (Si-Ge) that has operated exceedingly well at

For the upcoming Mars Science Laboratory (MSL) mission, the U.S. will use a telluridebased thermoelectric material because it meets the requirements of being able to operate both in space on the way to Mars and on the surface of Mars with its dusty, cold, carbon dioxide atmosphere (see Section 8). The successes of the earlier (1976 era) Viking Mars

The SNAP-3B RTG evolved out of the overall SNAP-3 program with the goal of providing 2.7 We to the U.S. Navy's Transit 4A and Transit 4 B navigational satellites. In particular, the SNAP-3B RTGs were to provide power to the crystal oscillator that was the heart of the electronic system used for Doppler-shift tracking, a precursor of today's global positioning system (Dick and Davis, 1962, JHU/APL, 1980). Both RTGs provided power to their respective spacecraft for over 10 years (Bennett, et al., 1983). Figure 5 shows models of the

Landers 1 and 2 using SNAP-19 telluride-based technology support this decision.

Fig. 4. Light-Weight Radioisotope Heater Unit (LWRU) (DOE)

efforts in radioisotope power sources.

**3. The early telluride-based RTGs** 

SNAP-3B RTG and the successor SNAP-9A RTG.

temperatures of about 1300 K.

**3.1 SNAP-3B RTGs** 

Generator (ASRG) (see Section 10). Depending on the design, dynamic conversion systems can provide double, triple and even quadruple the efficiency of state-of-practice thermoelectric conversion systems which means much less radioisotope fuel would be used to achieve the same electrical power (or, conversely, much more electrical power can be produced for the same quantity of radioisotope fuel used in an RTG).

Fig. 3. SNAP-1 turbomachinery package with the shaft assembly shown separately, ruler dimensions are in inches (TRW via Corliss and Harvey, 1964).

In parallel with the SNAP-1 program a series of radioisotope power sources were studied under the umbrella of the SNAP-3 program that was based largely on using thermoelectric elements in the converter. The early SNAP-3 generators were to use 210Po as the fuel but by the late 1950s it was clear that sufficient quantities of 238Pu would be available to provide the fuel for small RTGs. Plutonium-238 provided a number of features that made it more attractive than 144Ce or 210Po, including a longer half-life (87.7 years) and a more benign radiation emission (alpha particles, which can be stopped by material as thin as a sheet of paper) (Corliss and Harvey, 1964).

Safety is the principal design requirement in the use of RPS, so the heat source is designed to contain or immobilize the fuel throughout a range of postulated accidents such as explosions and atmospheric reentries. Over the years this safety design work has led to the development of the general-purpose heat source (GPHS) module, which is the basic building block of U.S. RPS (Bennett, 1995).

All of the U.S. RPS that have flown have been either RTGs or RHUs, (see Fig. 4).

Fig. 4. Light-Weight Radioisotope Heater Unit (LWRU) (DOE)

As of 2010, as shown in Table 1, the U.S. has launched 45 RTGs, hundreds of RHUs and one space nuclear fission reactor. Of the RTGs flown, two different types of thermoelectric materials have been employed: telluride-alloy based or silicon-germanium-alloy based. The following sections will discuss these RTGS to be followed by sections discussing current efforts in radioisotope power sources.
