**4. Flight systems**

#### **4.1 SNAP-3B**

432 Radioisotopes – Applications in Physical Sciences

one heat exchanger, expanded through the turbine, passed through a gas cooler and pressurized by the compressor thus completing the cycle. A rotary alternator attached to

The history of RPS began in the early years of the Cold War, when surveillance satellites were a major impetus for the early space race. The Manhattan project and the years leading up to it had yielded a wealth of knowledge on nuclear physics, particularly the radio-decay properties of actinides and other alpha particle-producing materials. The energy released from the radioactive decay of different elements had become well characterized, and it was recognized early on that radioisotopes could provide power for military satellites and other remote applications. An early study by the North American Aviation Corporation had considered radioisotopes for space power. Then a RAND Corporation report in 1949 evaluated options for space power, and concluded that a radioactive cell-mercury vapor system could feasibly supply 500 We (watts-electric) for up to one year. In 1952, RAND issued a report with an extensive discussion on radioisotope power for space applications, which spurred interest in applying the

Recognizing the viability of nuclear power for reconnaissance satellites, the Department of Defense (DOD) requested in August 1955 that the Atomic Energy Commission (AEC) perform studies and limited experimental work toward developing a nuclear reactor auxiliary power unit for an Air Force satellite system concept. AEC agreed, but wanted to broaden its examination to both radioisotope and reactor heat sources. This marked the beginning of the SNAP program, which was structured into parallel power plant efforts with two corporations. Odd-numbered SNAP projects focused on RPS and were spearheaded by the Martin Company, while even-numbered SNAP projects using reactors were performed by the Atomics International Division of North American

In these early days, efforts focused on dynamic energy conversion. The work of the Martin Company progressed through an early SNAP-1 effort that used the decay heat of Cerium-144 to boil Mercury and drive a small turbine in a Rankine cycle. In early 1954, a new simpler static energy conversion method was conceived by Kenneth Jordan and John Birden of the AEC's Mound Laboratory in Miamisburg, Ohio. Having been frustrated in their efforts to use radioisotope heat sources to generate electricity via steam turbines, these two researchers considered using two metals with markedly different electrical conductivities to generate electricity directly from an applied heat load. This thermoelectric method was patented by Jordan and Birden, and has remained the basis for all RTGs to the present day. In 1958, work began on two thermoelectric demonstration devices at Westinghouse Electric and 3M, while AEC contracts with other companies explored the development of

The project to develop a generator based on thermoelectric energy conversion was given the designation, SNAP-3. The 3M Company delivered a workable converter to the Martin Company in December 1958. Shortly thereafter, a complete radioisotosope-powered generator was delivered to the AEC as a proof-of-principle device, producing 2.5 We with a

the turbine shaft produces alternating current (AC) electrical power.

**3. The early years** 

technology on satellites.

Aviation, Inc.

demonstration thermionic units.

half charge of Polonium-210 (Po-210) fuel.

The first successful use of RTGs in space took place with the U.S. Navy's Transit satellite program. Also known as the NNS (Navy Navigation Satellite), the Transit system was used by the Navy to provide accurate location information to its ships. It was also used for general navigation by the Navy, as well as hydrographic and geodetic surveying, and was the first such system to be used operationally. The Johns Hopkins Applied Physics Laboratory (APL) developed the system, starting in 1957. Many of the technologies developed under the Transit program are now in use on the Global Positioning System (GPS).

Several of the Transit developers had been considering the use of RPS since the beginning of the program. Although solar cells and batteries had powered the first six Transit satellites, there was concern that the battery hermetic seals would not meet the five-year mission requirement. Thus, APL accepted an offer from the AEC to include an auxiliary nuclear power source on the satellite. At that time, however, the radioisotope fuel of choice, Plutonium-238 (Pu-238), was unavailable due to AEC restrictions, and APL refused to use beta-decaying Strontium-90 because of the excessive weight associated with its necessary shielding. The AEC eventually acquiesced and agreed to provide the Pu-238 fuel. The SNAP-3 was converted from use of Po-210 to Pu-238, and acquired the new designation, SNAP-3B. The SNAP-3B RTGs on board these spacecraft supplemented solar cell arrays and demonstrated operation of nuclear systems for space power applications.

Radioisotope Power: A Key Technology for Deep Space Exploration 435

Fig. 7. Integrated Transit payload. Transit satellite is positioned at bottom of stack.

Guard, and Weather Bureau for navigation lights and weather stations on Earth.

(BOM) was nearly an order of magnitude greater than the SNAP-3B.

known as the INSRP (Interagency Nuclear Safety Review Panel).

After the success of SNAP-3B, the team consisting of the AEC, Martin, 3M, Mound Laboratory and APL proceeded to develop the SNAP-9A for the next series of Transit satellites. There was also a growing demand for isotope power for terrestrial applications. For instance, the SNAP-7 series of devices was under development for the Navy, Coast

DOD decided to continue using RTGs for its navigational satellites because of their resistance to radiation. A high-altitude nuclear explosive test in 1962 had adversely impacted the solar cells of earlier Transit satellites, and DOD was concerned with their susceptibility to radiation and other space effects in the future. The SNAP-9A was essentially an expanded version of the SNAP-3B, and was the first RTG employed as the primary spacecraft power source. Its power capability of 26.8 We at beginning of mission

Each 12.3-kg SNAP-9A was designed to provide continuous power for five years in space after one year of storage on Earth. The thermal inventory of 525 Wt (watts-thermal) was supplied by Pu-238 metal encapsulated in a heat source of six fuel capsules maintained in a segmented graphite heat-accumulator block. As shown in Fig. 8, the main body was a sealed cylindrical magnesium-thorium shell containing six heat-dissipating magnesium fins. The unit was 26.7 cm tall and had a fin-to-fin diameter (fin span) of 50.8 cm. The 70 pairs of series-connected Pb-Te thermoelectric couples were assembled in 35 modules of two couples each. Hot junction temperature was calculated at about 790 K at beginning of life. Some waste heat from the RTG was used to maintain electronic instruments in the satellite at a

The SNAP-9A missions in 1963 also marked the beginning of a formal launch safety review process. Although the launches were for DOD systems, NASA was invited to participate in the reviews, which were made a responsibility of the joint AEC/NASA Space Nuclear Power Office. It was during these early launches that efficient and comprehensive review and approval procedures were developed. As early as January 1963, a model charter had been developed for an ad-hoc interagency review committee. Eventually this became

**4.2 SNAP-9A** 

temperature near 293 K.

A schematic of the SNAP-3B generator is shown in Fig. 6. Each unit had a mass of 2.1 kg and an initial power output of 2.7 We, and was designed to last five years. Although this power level was quite low, the RTG performed the critical function of powering the crystal oscillator that was the heart of the electronic system used for Doppler-shift tracking. It also powered the buffer-divider-multiplier, phase modulators and power amplifiers. The heat source produced approximately 52.5 Wt from 92.7 grams of encapsulated plutonium metal, which had an isotopic mass composition of 80% Pu-238, 16% Pu-239, 3% Pu-240, and 1% Pu-241. The power conversion assembly consisted of 27 spring-loaded, series-connected pairs of Lead-Telluride (Pb-Te) thermoelectric elements operating at a hot-juncture temperature of about 783 K and a cold-juncture temperature of about 366 K. The power system had a power-conversion efficiency of 5 to 6 percent and a specific power of 1.3 We/kg.

Fig. 6. SNAP-3B Schematic.

Transit 4A was launched, along with two other satellites (Fig. 7), on June 29, 1961 aboard a Thor-Able rocket. Transit 4B was launched soon afterward on November 15, 1961. Even for this first use of nuclear power in space, there was controversy stemming from concerns over launch safety. The State Department, in particular, expressed concern with its trajectory over Cuba and South America. As part of the aerospace nuclear safety philosophy at that time, the generators were designed for burnup and high altitude fuel dispersal to concentrations below the background radiation attributed to atmospheric nuclear weapons testing. In addition, the spacecraft were placed into 1,100-km orbits, which provided orbital lifetimes (>1,000 years) sufficient for the fuel to decay to these background levels. The Transit 4A generator operated for 15 years, and was shutdown in 1976. The last reported signal from Transit 4B was in April 1971.

Fig. 7. Integrated Transit payload. Transit satellite is positioned at bottom of stack.

#### **4.2 SNAP-9A**

434 Radioisotopes – Applications in Physical Sciences

A schematic of the SNAP-3B generator is shown in Fig. 6. Each unit had a mass of 2.1 kg and an initial power output of 2.7 We, and was designed to last five years. Although this power level was quite low, the RTG performed the critical function of powering the crystal oscillator that was the heart of the electronic system used for Doppler-shift tracking. It also powered the buffer-divider-multiplier, phase modulators and power amplifiers. The heat source produced approximately 52.5 Wt from 92.7 grams of encapsulated plutonium metal, which had an isotopic mass composition of 80% Pu-238, 16% Pu-239, 3% Pu-240, and 1% Pu-241. The power conversion assembly consisted of 27 spring-loaded, series-connected pairs of Lead-Telluride (Pb-Te) thermoelectric elements operating at a hot-juncture temperature of about 783 K and a cold-juncture temperature of about 366 K. The power system had a

Transit 4A was launched, along with two other satellites (Fig. 7), on June 29, 1961 aboard a Thor-Able rocket. Transit 4B was launched soon afterward on November 15, 1961. Even for this first use of nuclear power in space, there was controversy stemming from concerns over launch safety. The State Department, in particular, expressed concern with its trajectory over Cuba and South America. As part of the aerospace nuclear safety philosophy at that time, the generators were designed for burnup and high altitude fuel dispersal to concentrations below the background radiation attributed to atmospheric nuclear weapons testing. In addition, the spacecraft were placed into 1,100-km orbits, which provided orbital lifetimes (>1,000 years) sufficient for the fuel to decay to these background levels. The Transit 4A generator operated for 15 years, and was shutdown in 1976. The last reported

power-conversion efficiency of 5 to 6 percent and a specific power of 1.3 We/kg.

Fig. 6. SNAP-3B Schematic.

signal from Transit 4B was in April 1971.

After the success of SNAP-3B, the team consisting of the AEC, Martin, 3M, Mound Laboratory and APL proceeded to develop the SNAP-9A for the next series of Transit satellites. There was also a growing demand for isotope power for terrestrial applications. For instance, the SNAP-7 series of devices was under development for the Navy, Coast Guard, and Weather Bureau for navigation lights and weather stations on Earth.

DOD decided to continue using RTGs for its navigational satellites because of their resistance to radiation. A high-altitude nuclear explosive test in 1962 had adversely impacted the solar cells of earlier Transit satellites, and DOD was concerned with their susceptibility to radiation and other space effects in the future. The SNAP-9A was essentially an expanded version of the SNAP-3B, and was the first RTG employed as the primary spacecraft power source. Its power capability of 26.8 We at beginning of mission (BOM) was nearly an order of magnitude greater than the SNAP-3B.

Each 12.3-kg SNAP-9A was designed to provide continuous power for five years in space after one year of storage on Earth. The thermal inventory of 525 Wt (watts-thermal) was supplied by Pu-238 metal encapsulated in a heat source of six fuel capsules maintained in a segmented graphite heat-accumulator block. As shown in Fig. 8, the main body was a sealed cylindrical magnesium-thorium shell containing six heat-dissipating magnesium fins. The unit was 26.7 cm tall and had a fin-to-fin diameter (fin span) of 50.8 cm. The 70 pairs of series-connected Pb-Te thermoelectric couples were assembled in 35 modules of two couples each. Hot junction temperature was calculated at about 790 K at beginning of life. Some waste heat from the RTG was used to maintain electronic instruments in the satellite at a temperature near 293 K.

The SNAP-9A missions in 1963 also marked the beginning of a formal launch safety review process. Although the launches were for DOD systems, NASA was invited to participate in the reviews, which were made a responsibility of the joint AEC/NASA Space Nuclear Power Office. It was during these early launches that efficient and comprehensive review and approval procedures were developed. As early as January 1963, a model charter had been developed for an ad-hoc interagency review committee. Eventually this became known as the INSRP (Interagency Nuclear Safety Review Panel).

Radioisotope Power: A Key Technology for Deep Space Exploration 437

decentralization of technical and administrative support so as to directly involve more of the

Noting the success of the SNAP-3A, NASA requested the AEC to evaluate the feasibility of a 50-We RTG for an upcoming Nimbus weather satellite. Nimbus was the first U.S. weather satellite system to make day and night global temperature measurements at varying levels in the atmosphere, and all earlier satellites had been powered exclusively by solar cells. The request led to design and integration studies by the AEC and establishment of the SNAP-19 technology improvement program. With Nimbus, the SNAP program received its first

The unit that eventually flew on Nimbus, SNAP-19B, was used as an auxiliary system. As shown in Fig. 10, each Nimbus satellite carried two SNAP-19B RTGs, which provided about 20% of the total power delivered to the spacecraft bus. This extra continuous power enabled full-time operation of a number of extremely important atmospheric-sounder experiments. Without the RTGs, the total delivered power would have fallen below the load line about

laboratories and facilities of both AEC and NASA.

opportunity to test and demonstrate an RTG on a NASA spacecraft.

Fig. 10. Nimbus III. First NASA application of Radioisotope power.

of 23.5 We yielded a specific power of 2.1 We/kg, the same as SNAP-9A.

SNAP-19B was very similar to the SNAP-9A in terms of configuration and performance. It had a height of 26.7 cm and a fin span of 53.8 cm. It's mass of 13.4-kg and BOM power level

The SNAP-19B was unique in its use of a new 645 Wt heat source, called the Intact Impact Heat Source (IIHS), in conjunction with an array of 90 Pb-Te thermocouples. The IIHS was designed to contain the fuel under normal operating conditions and to limit probability of contaminating the environment in the event of a launch abort or accident. In contrast to the SNAP-9A fuel design, the fuel form for SNAP-19B was changed from Plutonium metal to small Plutonium oxide (PuO2) microspheres carried in capsules. Even in a worst-case

**4.3 SNAP-19 – Nimbus** 

two weeks into the mission.

Fig. 8. SNAP-9A RTG.

After a period of program delays, Transit 5BN-1 (Fig. 9) was launched successfully on September 28, 1963, followed by Transit 5BN-2 on December 5, 1963. The third and last launch of the Transit 5BN-3 on April 21, 1964 was not as successful. A mission abort occurred after the payload had reached an altitude of 1,000 miles over the South Pole. Preliminary data indicated that the payload reentered the atmosphere over the Mozambique Channel at a steep angle. The Pu-238 fuel was designed to burn up into particles of about one millionth of an inch in diameter and disperse widely so as not to constitute a health hazard. Balloon samples taken over the next few years confirmed that the generator's fuel had indeed burned up as expected after the spacecraft failed to achieve orbit.

Fig. 9. Transit 5BN-1.

Although there was a commitment to fly higher power NASA missions, the loss of Transit 5BN-3 led to concerns that the dispersion approach would be unsafe with larger inventories of fuel. Thus, the basic safety concept changed from designing for burn-up and dispersion to designing for intact reentry. By the time that new approach was integrated into an RTGpowered space mission, however, the mechanisms for interagency review and meticulous safety analysis were well established. Another change was the mobilization and decentralization of technical and administrative support so as to directly involve more of the laboratories and facilities of both AEC and NASA.

#### **4.3 SNAP-19 – Nimbus**

436 Radioisotopes – Applications in Physical Sciences

After a period of program delays, Transit 5BN-1 (Fig. 9) was launched successfully on September 28, 1963, followed by Transit 5BN-2 on December 5, 1963. The third and last launch of the Transit 5BN-3 on April 21, 1964 was not as successful. A mission abort occurred after the payload had reached an altitude of 1,000 miles over the South Pole. Preliminary data indicated that the payload reentered the atmosphere over the Mozambique Channel at a steep angle. The Pu-238 fuel was designed to burn up into particles of about one millionth of an inch in diameter and disperse widely so as not to constitute a health hazard. Balloon samples taken over the next few years confirmed that the generator's fuel

Although there was a commitment to fly higher power NASA missions, the loss of Transit 5BN-3 led to concerns that the dispersion approach would be unsafe with larger inventories of fuel. Thus, the basic safety concept changed from designing for burn-up and dispersion to designing for intact reentry. By the time that new approach was integrated into an RTGpowered space mission, however, the mechanisms for interagency review and meticulous safety analysis were well established. Another change was the mobilization and

had indeed burned up as expected after the spacecraft failed to achieve orbit.

Fig. 8. SNAP-9A RTG.

Fig. 9. Transit 5BN-1.

Noting the success of the SNAP-3A, NASA requested the AEC to evaluate the feasibility of a 50-We RTG for an upcoming Nimbus weather satellite. Nimbus was the first U.S. weather satellite system to make day and night global temperature measurements at varying levels in the atmosphere, and all earlier satellites had been powered exclusively by solar cells. The request led to design and integration studies by the AEC and establishment of the SNAP-19 technology improvement program. With Nimbus, the SNAP program received its first opportunity to test and demonstrate an RTG on a NASA spacecraft.

The unit that eventually flew on Nimbus, SNAP-19B, was used as an auxiliary system. As shown in Fig. 10, each Nimbus satellite carried two SNAP-19B RTGs, which provided about 20% of the total power delivered to the spacecraft bus. This extra continuous power enabled full-time operation of a number of extremely important atmospheric-sounder experiments. Without the RTGs, the total delivered power would have fallen below the load line about two weeks into the mission.

Fig. 10. Nimbus III. First NASA application of Radioisotope power.

SNAP-19B was very similar to the SNAP-9A in terms of configuration and performance. It had a height of 26.7 cm and a fin span of 53.8 cm. It's mass of 13.4-kg and BOM power level of 23.5 We yielded a specific power of 2.1 We/kg, the same as SNAP-9A.

The SNAP-19B was unique in its use of a new 645 Wt heat source, called the Intact Impact Heat Source (IIHS), in conjunction with an array of 90 Pb-Te thermocouples. The IIHS was designed to contain the fuel under normal operating conditions and to limit probability of contaminating the environment in the event of a launch abort or accident. In contrast to the SNAP-9A fuel design, the fuel form for SNAP-19B was changed from Plutonium metal to small Plutonium oxide (PuO2) microspheres carried in capsules. Even in a worst-case

Radioisotope Power: A Key Technology for Deep Space Exploration 439

Fig. 11. Pioneer SNAP-19.

Fig. 12. SNAP-19s installed on Pioneer.

scenario involving release and dispersal of the microspheres, the particles would be too big for inhalation. Additional safety design requirements included survival upon reentry and containment/immobilization of the fuel upon impact.

Launch of the Nimbus-B-1 took place on May 18, 1968. Unfortunately an error in setting a guidance gyro caused Nimbus-B-1 to veer off course. The Range Safety Officer sent the destruct signal 120 seconds into flight, thus blowing up the Agena stage at an altitude of 100,000 feet. The upper portion of the stage, including the satellite, fell into water depths of 300 to 600 feet about two to four miles to the north of San Miguel Island in the Santa Barbara Channel. The unit was found in September 1968, and was sent back to the Mound Laboratory for reuse. A second Nimbus satellite (Nimbus III or Nimbus-B-2) was launched and successfully placed into orbit on April 14, 1969. The SNAP-19B RTGs used here had slightly more fuel than their predecessors due to the use of less efficient but more stable thermoelectrics. The units operated fine for approximately 20,000 hours (2.5 years) until they experienced a sharp degradation in performance. This decline was attributed to the sublimation of thermoelectric material and loss of the hot junction bond due to internal cover gas depletion.

Nimbus was the first and last time RTGs were used in Earth orbit by NASA. At that time, solar photovoltaics were still relatively new. With advancement in this area, NASA did not feel that RTGs were warranted for applications where solar cells could work. In addition with the more structured launch safety review process, it was much more cost effective to use solar cells whenever possible.

#### **4.4 SNAP-19 – Pioneer and Viking**

The successful demonstration of Nimbus III encouraged NASA to commit to use of SNAP-19 on the Pioneer and Viking missions, arguably NASA's most exciting science missions of the 1970's. The SNAP-19 design for these applications (Fig. 11), however, had to be modified. For Pioneer, this was driven by the need for a mission life of up to six years. Other modifications were required to deliver a higher power, and to withstand the unique environments of Mars and deep space. For Pioneer, the most significant modification was incorporation of TAGS/Sn-Te thermoelectric elements (thermocouple legs consisting of Tellerium, Antimony, Germanium, Silver and Tin), which increased efficiency, lifetime and power performance. The generator height was also increased to 28.2 cm, and the fin span was reduced to 50.8 cm. This yielded a power output of 40.3 We. The resultant specific power of 3.0 We/kg was nearly 50% higher than the Nimbus design.

Pioneers 10 and 11 were launched on 2 March 1972 and 6 April 1973, respectively. Pioneer 10 was the first spacecraft to travel through the asteroid belt and to make direct observations of Jupiter, which it encountered on 3 December 1973. According to some definitions, Pioneer 10 became the first artificial object to leave the solar system, on 13 June 1983. Pioneer 11 also encountered Jupiter, and in addition to conducting measurements, the spacecraft used a Jupiter gravity assist maneuver to alter its trajectory toward Saturn. After nearly five years, Pioneer 11 encountered Saturn in September 1979, and provided the first local measurements of this planet and its rings before it followed an escape trajectory out of the solar system.

The most noteworthy aspect of the SNAP-19s used for these missions (Fig. 12) was the extremely long time the units continued to operate past their primary tasks and baseline mission lifetimes. Both of these spacecraft continued to transmit data far beyond the orbit of Pluto, and more than fulfilled the original expectations for their operation.

Fig. 11. Pioneer SNAP-19.

scenario involving release and dispersal of the microspheres, the particles would be too big for inhalation. Additional safety design requirements included survival upon reentry and

Launch of the Nimbus-B-1 took place on May 18, 1968. Unfortunately an error in setting a guidance gyro caused Nimbus-B-1 to veer off course. The Range Safety Officer sent the destruct signal 120 seconds into flight, thus blowing up the Agena stage at an altitude of 100,000 feet. The upper portion of the stage, including the satellite, fell into water depths of 300 to 600 feet about two to four miles to the north of San Miguel Island in the Santa Barbara Channel. The unit was found in September 1968, and was sent back to the Mound Laboratory for reuse. A second Nimbus satellite (Nimbus III or Nimbus-B-2) was launched and successfully placed into orbit on April 14, 1969. The SNAP-19B RTGs used here had slightly more fuel than their predecessors due to the use of less efficient but more stable thermoelectrics. The units operated fine for approximately 20,000 hours (2.5 years) until they experienced a sharp degradation in performance. This decline was attributed to the sublimation of thermoelectric material and loss of the hot junction bond due to internal

Nimbus was the first and last time RTGs were used in Earth orbit by NASA. At that time, solar photovoltaics were still relatively new. With advancement in this area, NASA did not feel that RTGs were warranted for applications where solar cells could work. In addition with the more structured launch safety review process, it was much more cost effective to

The successful demonstration of Nimbus III encouraged NASA to commit to use of SNAP-19 on the Pioneer and Viking missions, arguably NASA's most exciting science missions of the 1970's. The SNAP-19 design for these applications (Fig. 11), however, had to be modified. For Pioneer, this was driven by the need for a mission life of up to six years. Other modifications were required to deliver a higher power, and to withstand the unique environments of Mars and deep space. For Pioneer, the most significant modification was incorporation of TAGS/Sn-Te thermoelectric elements (thermocouple legs consisting of Tellerium, Antimony, Germanium, Silver and Tin), which increased efficiency, lifetime and power performance. The generator height was also increased to 28.2 cm, and the fin span was reduced to 50.8 cm. This yielded a power output of 40.3 We. The resultant specific

Pioneers 10 and 11 were launched on 2 March 1972 and 6 April 1973, respectively. Pioneer 10 was the first spacecraft to travel through the asteroid belt and to make direct observations of Jupiter, which it encountered on 3 December 1973. According to some definitions, Pioneer 10 became the first artificial object to leave the solar system, on 13 June 1983. Pioneer 11 also encountered Jupiter, and in addition to conducting measurements, the spacecraft used a Jupiter gravity assist maneuver to alter its trajectory toward Saturn. After nearly five years, Pioneer 11 encountered Saturn in September 1979, and provided the first local measurements of this planet and its rings before it followed an escape trajectory out of the solar system. The most noteworthy aspect of the SNAP-19s used for these missions (Fig. 12) was the extremely long time the units continued to operate past their primary tasks and baseline mission lifetimes. Both of these spacecraft continued to transmit data far beyond the orbit of

power of 3.0 We/kg was nearly 50% higher than the Nimbus design.

Pluto, and more than fulfilled the original expectations for their operation.

containment/immobilization of the fuel upon impact.

cover gas depletion.

use solar cells whenever possible.

**4.4 SNAP-19 – Pioneer and Viking** 

Fig. 12. SNAP-19s installed on Pioneer.

Radioisotope Power: A Key Technology for Deep Space Exploration 441

The Viking missions were a complete success. In addition to characterization of the Mars environment, the Landers provided over 4,500 high quality images of the Martian landscape. All four SNAP-19 RTGs easily met their original 90-day requirement, thus allowing the Viking Landers to operate for years until other system failures led to a loss of data. When the last data were received from Viking 1 in November 1982, it had been estimated that the RTGs were capable of providing sufficient power for operation until 1994,

Interest in RTGs for Navy navigation satellites continued after the earlier Transit missions. The next DOD application of RTGs took place with TRIAD, the first in a series of three experimental spacecraft designed to test and demonstrate improvements to the NNS. These were all developed under the Transit Improvement Program (TIP), which was established in 1969 to provide a radiation-hardened satellite that could maintain its correct position for

The Transit-RTG was designed to serve as the primary power source for the satellite, with auxilliary power provided by four solar-cell panels and a 6 Amp-hr Nickel Cadmium battery. The 13.6-kg Transit RTG was modular in design, and was 36.3 cm tall and approximately 61 cm across its lower attachment (Fig. 15). The RTG delivered 35.6 We at BOM, and used a SNAP-19 heat source. The Transit RTG was the first to employ radiative heat coupling between its heat

source and thermocouples, although this was accomplished at some loss in efficiency.

Fig. 14. Viking Lander.

**4.5 Transit-RTG (TRIAD)** 

18 years beyond the original mission requirement.

over five days without an update from the ground.

Fig. 15. Cutaway of TRANSIT RTG.

The modifications for Viking went further to ensure the RTG, which is shown in Fig. 13, could withstand high temperature sterilization procedures in support of the planetary quarantine protocol, storage during the flight to Mars, and the severe temperature extremes of the Martian surface.

Fig. 13. Viking SNAP-19.

The landers were sterilized before launch to prevent contamination of Mars by terrestrial microorganisms. Among the modifications to the Pioneer SNAP-19 design was the addition of a dome reservoir to allow a controlled interchange of gases. This minimized heat source operating temperatures prior to launch, while maximizing electrical power output at the end of mission. This resulted in the Viking SNAP-19 being slightly larger and more massive than the version used on Pioneer (40.4 cm tall, 58.7 cm fin span, 15.2 kg mass, and 2.8 We/kg specific power).

Vikings 1 and 2 were identical spacecraft (Fig. 14), each of which consisted of a Lander, with a robot laboratory to study the nature of the surface, and an Orbiter, designed to serve as a communications relay to Earth. Each Lander carried two SNAP-19s. Viking 1 was launched on 20 August 1975 from Cape Canaveral. It reached Mars orbit on 19 June 1976, and reached the surface on 20 July 1976 on the western slope of Chryse Planitia. Viking 2 was launched on 9 September 1975, and it touched down on the surface on 3 September 1976 at Utopia Planitia.

Fig. 14. Viking Lander.

440 Radioisotopes – Applications in Physical Sciences

The modifications for Viking went further to ensure the RTG, which is shown in Fig. 13, could withstand high temperature sterilization procedures in support of the planetary quarantine protocol, storage during the flight to Mars, and the severe temperature extremes

The landers were sterilized before launch to prevent contamination of Mars by terrestrial microorganisms. Among the modifications to the Pioneer SNAP-19 design was the addition of a dome reservoir to allow a controlled interchange of gases. This minimized heat source operating temperatures prior to launch, while maximizing electrical power output at the end of mission. This resulted in the Viking SNAP-19 being slightly larger and more massive than the version used on Pioneer (40.4 cm tall, 58.7 cm fin span, 15.2 kg mass, and 2.8

Vikings 1 and 2 were identical spacecraft (Fig. 14), each of which consisted of a Lander, with a robot laboratory to study the nature of the surface, and an Orbiter, designed to serve as a communications relay to Earth. Each Lander carried two SNAP-19s. Viking 1 was launched on 20 August 1975 from Cape Canaveral. It reached Mars orbit on 19 June 1976, and reached the surface on 20 July 1976 on the western slope of Chryse Planitia. Viking 2 was launched on 9 September 1975, and it touched down on the surface on 3 September 1976 at

of the Martian surface.

Fig. 13. Viking SNAP-19.

We/kg specific power).

Utopia Planitia.

The Viking missions were a complete success. In addition to characterization of the Mars environment, the Landers provided over 4,500 high quality images of the Martian landscape. All four SNAP-19 RTGs easily met their original 90-day requirement, thus allowing the Viking Landers to operate for years until other system failures led to a loss of data. When the last data were received from Viking 1 in November 1982, it had been estimated that the RTGs were capable of providing sufficient power for operation until 1994, 18 years beyond the original mission requirement.

#### **4.5 Transit-RTG (TRIAD)**

Interest in RTGs for Navy navigation satellites continued after the earlier Transit missions. The next DOD application of RTGs took place with TRIAD, the first in a series of three experimental spacecraft designed to test and demonstrate improvements to the NNS. These were all developed under the Transit Improvement Program (TIP), which was established in 1969 to provide a radiation-hardened satellite that could maintain its correct position for over five days without an update from the ground.

The Transit-RTG was designed to serve as the primary power source for the satellite, with auxilliary power provided by four solar-cell panels and a 6 Amp-hr Nickel Cadmium battery. The 13.6-kg Transit RTG was modular in design, and was 36.3 cm tall and approximately 61 cm across its lower attachment (Fig. 15). The RTG delivered 35.6 We at BOM, and used a SNAP-19 heat source. The Transit RTG was the first to employ radiative heat coupling between its heat source and thermocouples, although this was accomplished at some loss in efficiency.

Fig. 15. Cutaway of TRANSIT RTG.

Radioisotope Power: A Key Technology for Deep Space Exploration 443

Fig. 18. Use of SNAP-27 on the Moon. Alan Bean deploying SNAP-27 on Apollo 12.

represented a 10% increase over the Pioneer SNAP-19.

Thermal energy from the fuel capsule was transferred to the generator hot frame by radiative coupling. When deployed on the lunar surface, the fuel capsule operated at 1005 K, while the Inconel 102 alloy hot frame was 880 K. The hot junction temperature ranged between 855 K and 865 K, reflecting an overall temperature drop of 15 to 25 K. On the Moon's surface, where temperatures can vary from 350 K during the lunar day to a frigid 100 K during the lunar night, the generator's cold side temperature operated at 545 K. Pb-Te served as the TE material and the couples were assembled in a series-parallel electrical arrangement to prevent string loss. The power capability for the 19.6 kg RTG was at least 63.5 We at 16 Vdc for one year after lunar emplacement. The converter was 46 cm tall and 40 cm wide across the fins. The specific power was greater than 3.2 We/kg, which

The five units deployed on the lunar surface from 1969 to 1972 operated flawlessly. Telemetry data from their operation stopped in 1977 when the ALSEPs were intentionally

The only potential problem with SNAP-27 occurred with the Apollo-13 mission, when there was concern over the SNAP-27 onboard the LEM reentering the Earth's atmosphere. Normal reentry trajectory and velocity were achieved as had been assumed in the prelaunch review accounting for this type of event. The detached LEM broke up on reentry, as

shutdown. Until then, their degradation in performance matched all predictions.

Fig. 17. SNAP-27.

The 12-sided converter used Pb-Te thermoelectric "Isotec" panels operated at a low hot-side temperature of 673 K in a vacuum, thus eliminating the need for hermetic sealing and a cover gas to inhibit thermoelectric material sublimation. Each of the 12 Isotec panels contained 36 Pb-Te thermocouples arranged in a series-parallel matrix with four couples in a row in webbed, magnesium-thorium corner posts with Teflon insulators.

The TRIAD satellite (Fig. 16) was launched on September 2, 1972 from Vandenburg Air Force Base into a 700 to 800 km orbit. The short-term objectives of the TRIAD satellite were successfully demonstrated, including a checkout of RTG performance. However, a telemetry-converter failure onboard the spacecraft caused a loss of telemetry data about a month into the mission. This, in turn, precluded measuring the Transit-RTG power level versus time. However, the TRIAD satellite continued to operate normally for some time and provided magnetometer data using power from the RTG.

Fig. 16. Transit TRIAD Satellite.
