“ There are only three possible sources of which can be used to generate in space – chemical, nuclear, or . Each energy source, . . . has its own intrinsic characteristics, and their differences determine which source is uniquely the best for a specific mission.” Systems for Nuclear Auxiliary …A Report by the Commission – 1964

viii The Early Years Space 1 Systems Take Flight

nly a few years separate the operation of mankind’s first at the University of Chicago in 19421 and the first U.S. research on the use of . Shortly after the Oend of World War II, control of was transferred from military to civilian hands when Congress enacted the Atomic Energy Act of 1946.2 e Act created the Atomic Energy Commission (AEC), which began operation on January 1, 1947. Although responsibility for atomic was now under the new civilian agency, its development continued to remain tied to military purposes.

By the late 1940s and early 1950s, studies by the AEC and the Department of Defense (DoD) began to show that the energy generated from the decay of radioisotopes and the process of nuclear fission held much promise for uses other than atomic . Performed against the backdrop of the early days of the between the and the , in which each country sought military and technological prowess over the other, those studies envisioned radioisotope and reactor power for military reconnaissance and a nuclear reactor propulsion system for intercontinental ballistic missiles.

Around the same time, the United States began efforts to expand the peacetime development of atomic energy. In his for Peace speech before the General Assembly in on December 8, 1953, President Dwight D. Eisenhower presented a vision for the international management of atomic energy as well as its development and use for peaceful purposes. e following year, in 1954, Congress passed a new Atomic Energy Act that opened the door for the development of nuclear power by private industry and improved exchange of nuclear with other nations.3

On August 1, 1946, President Harry S. Truman signed the bill creating the U.S. Atomic Energy Commission. (Photo: DOE Flickr)

Photo Credit 1 The Early Years Space Nuclear Power Systems Take Flight

e studies of atomic energy to electricity. Using the principle Power (SNAP), which focused feasibility for power were of thermoelectricity, otherwise on the use of nuclear reactors soon bolstered by a demonstration known as the Seebeck effect, and radioisotopes for satellite- of that feasibility. In early 1954, the researchers used the based electrical power generation, two Monsanto scientists at the from the to and 2) Rover, which focused on AEC Mound Laboratory in Ohio induce a differential development of a nuclear . demonstrated a device that was across a . e Shortly after these early research to convert the heat from the natural result was the generation of 1.8 and development efforts began to decay of the radioisotope milliwatts of electricity (mWe) bear fruit, the launch of the first polonium-210 and demonstration of the world’s man-made satellite placed in space first radioisotope thermoelectric (Sputnik I), by the Soviet Union in generator (RTG).4, 5 October 1957, gave added impetus for the development of these new ese early efforts gave rise to power systems. Soon an entire two major AEC programs in 1955: industry emerged as scientists and 1) Systems for Nuclear Auxiliary engineers learned to harness and

Dr. Ken Jordan and Dr. John Birden with rst RTG. (Photo: Mound Museum Cover of Atomic Power In Space, A Association) History, published by DOE in 1987.

2 Atomic Power in Space II Chapter 1

use the energy of the in ever Seebeck Effect: Producing Electricity from Heat newer and more creative ways, In 1821, German scientist Thomas Johann Seebeck observed that an electric eventually leading to the far is produced when two dierent conductive materials are joined in a reaches of space. closed circuit and the two junctions are kept at dierent . The pairs of junction are called a thermoelectric couple, or thermocouple. e space nuclear power system and programs undertaken in the United States Basic operation since the mid-1950s include radioisotope power systems (RPSs) (both static and dynamic) as well 1 Nuclear (e.g., -238) decays spontaneously producing heat as reactor systems developed for space power and propulsion. An 2 convert heat directly into electricity earlier history (pre-1987) of space nuclear power development and 3 Electricity is tapped from terminals connected to thermocouples use, which focused largely on RTG 4 Power output technology, is presented in Atomic Power in Space.3 e history and technologies captured in that and Hot junction other works describe the pioneering DC-to-DC efforts that solved many of the converter 2 technical problems of using nuclear power in space and demonstrated P (+) 1 Heat 3 4 that nuclear power is well-suited N (-) rejected for certain types of space missions. ese early efforts are briefly reviewed in this chapter because Electric current they set the stage for the programs Cold junction and missions discussed in the remainder of this book. ey also provide a basis for understanding the evolution of space nuclear power system technologies and some of the political and social environments that influenced their development.

3 The Early Years Space Nuclear Power Systems Take Flight

SNAP Takes Hold designate RPSs while even numbers thermodynamic cycle, the unit was (i.e., SNAP-2) were assigned to never fully developed for space use e SNAP program consisted of reactor systems. Letters of the due to factors that included the two separate but parallel efforts. alphabet were used to indicate need for a system with a longer Development of systems for the design differences among the same operating life and the advent of conversion of heat generated from system. Although systems were with the natural decay of radioisotopes developed for space and terrestrial high efficiency.3 was awarded to the Martin Nuclear uses, this book focuses only on Division of the Martin Company space nuclear power systems. As development of SNAP-1 out of Baltimore, Maryland. progressed, Martin subcontracted Development of systems to employ e first RPS developed by the with Westinghouse Electric the heat generated from the fission Martin Company, SNAP-1, used the and the Minnesota Mining and process within nuclear reactors was heat from the decay of -144 Company (3M) awarded to the to boil to drive to develop an RTG, a type of Division of North American a small , with the goal of power system with no moving Aviation, Inc. To differentiate the generating 500 of electric parts similar to the concept radioisotope systems from the power (We) for a 60-day lifetime. demonstrated by Dr. Ken Jordan reactor systems, AEC devised a Although testing demonstrated and Dr. John Birden. Such systems simple numbering scheme—odd the feasibility of the first dynamic are referred to as static RPSs. In numbers (i.e., SNAP-1) were used to RPS that used a Rankine December 1958, 3M delivered an RTG that used a pellet of polonium-210 encapsulated by the AEC Mound Laboratory to produce 2.5 We. e small , dubbed SNAP-3, was displayed by President Eisenhower in the Oval Office on January 16, 1959.3

Public debut of the SNAP-3 RTG technology demonstration device displayed on President Eisenhower’s desk, January 16, 1959. Pictured left to right: President Eisenhower, Major General Donald Keirn, AEC Chairman John McCone, Colonel Jack Armstrong, and Lt. Colonel Guveren Anderson. (Photo: DOE Flickr)

4 Atomic Power in Space II Chapter 1

With development of RTG technology rapidly progressing, it wasn’t long before the first RTG found its way to space. e for that arose from the desire for a navigational satellite for use by naval ships and planes—a precursor to today’s global positioning system (GPS). e Navy Transit program desired a that would enable a satellite to operate for five years. Unsure that a standard chemical battery would last that long, John Dassoulas of the Johns Hopkins University Applied Laboratory learned about the SNAP program during a chance conversation with G. M. Anderson of AEC while on a return flight from a conference. Following a visit to the Martin facility in Baltimore, Dassoulas received Installation of the SNAP-3B device to the Navy’s navigational satellite at Cape permission from AEC to use an Canaveral by technicians of the Johns Hopkins Laboratory. RTG with the Transit satellite. e This is the rst atomic power supply used in space. (Photo: DOE Flickr) 3M SNAP-3 RTG was modified to use plutonium-238 rather than polonium-210, thereby taking 1976, well beyond its intended While the SNAP-3B unit provided advantage of the much longer half- lifetime. Another SNAP-3B RTG supplementary power for the life of the plutonium isotope (88 was launched aboard the Transit- Transit-4 satellites, the goal years versus less than five months 4B satellite in November 1961 was to demonstrate the feasibility for polonium-210).3 and operated until 1971. e of using an RTG as the sole source successful use of the SNAP-3B of power for a Navy satellite. e modified RTG, named RTGs, which supplemented the Driven by the desire for a power SNAP-3B, produced 2.7 We and systems aboard the source with improved survivability, was launched aboard a Transit- satellites, clearly demonstrated the the SNAP-9A RTG was created, 4A satellite in June 1961, thereby feasibility of the RTG for use as a which was used on the Navy marking the world’s first use space nuclear power system. e navigational satellites Transit- of nuclear power in space. e satellites and their RTGs remain in 5BN-1, 5BN-2, and 5BN-3. Like Transit-4A satellite operated until orbit above .3, 6 SNAP-3B, the SNAP-9A unit was

5 The Early Years Space Nuclear Power Systems Take Flight

fueled with plutonium-238 but re-entered the atmosphere. space nuclear power systems to one was designed to generate Consistent with the - of intact re-entry.3 25 We, almost 10 times the power dispersion safety design philosophy of the SNAP-3B unit. Transit in use at the time, the SNAP-9A Following the 1964 Transit-5BN-3 satellite 5BN-1 was successfully unit and its plutonium fuel accident, four years passed before launched on September 28, 1963, burned up, resulting in its another RTG was launched into and Transit-5BN-2 was successfully dispersion into the atmosphere. space. During those years, AEC launched on December 5, 1963. Although there were no and its contractors continued e third satellite, Transit-5BN-3, unacceptable health risks, with RTG development, including was launched on April 21, 1964. larger quantities of plutonium fuel incorporation of the intact re-entry However, when the satellite failed planned for future RTGs, AEC safety philosophy into new RTG to achieve orbit, the SNAP-9A RTG changed its safety philosophy for designs and development of new plutonium fuel forms to replace the metal fuel used in the SNAP-3A and SNAP-9A RTGs.

SNAP Reactors Heat Up

As success mounted with the early RTG efforts, development of space nuclear reactor concepts under the SNAP program also began to bear fruit. Under contract to AEC, Atomics International began development of a compact - reactor for use as a heat source in space nuclear power systems in the mid-1950s. By 1959, AEC and the Air had initiated a joint program, dubbed SNAP-2, to develop a power system that utilized a reactor coupled with a liquid-metal (mercury) Rankine power conversion unit (the AEC Chairman Glenn T. Seaborg, left, compares a SNAP-9A “atomic battery” (bottom center) with a full-scale model of a SNAP-3B atomic battery held by Major Robert T. Carpenter, AEC-SNAP project engineer. (Photo: 434-N-AEC-63-7042. General Records of the Department of Energy, RG 434, National Archives Still Picture Branch, College Park, Maryland)

6 Atomic Power in Space II Chapter 1

conversion technology used in the e SNAP-10A reactor power orbit, reactor startup was initiated by SNAP-1 program). Development system was launched into space from remote . For 43 days the system of the mercury Vandenberg Air Force Base, located operated as designed and produced conversion system was led by the in southwestern , on 500,000 -hours of electricity. ompson-Ramo-Wooldridge April 3, 1965, in a flight test named However, a voltage regulator on the Company. However, due to the lack SNAPSHOT. Once in its 700-mile failed, causing the reactor of a mission, the SNAP-2 program was redirected into a broader space nuclear power program in 1963.7

Under the SNAP- 10 program, a 300-We reactor power system was designed that utilized a -germanium thermoelectric generator developed by the Radio Corporation of America (RCA). Completed in 1959, the program was subsequently redirected in 1960 to develop a higher-power reactor system that incorporated the SNAP-2 reactor. at redirection resulted in the SNAP-10A program, under which the SNAP-2 reactor developed by Atomics International was coupled with the RCA thermoelectric generator to produce a 950-pound (430-kilogram), 500-We space reactor power system for use in a proof of principle flight.7

SNAP-10A space nuclear power unit. The reactor is located at the top end of the cone (radiator), and shown in inset, top left. (Photo: DOE Flickr)

7 The Early Years Space Nuclear Power Systems Take Flight

With a power output of SNAP-10A remains a significant milestone for the approximately 23.5 We, the SNAP- 19B RTG used a new heat source U.S. space program. that reflected the intact re-entry safety design philosophy adopted by AEC following the Transit- to shut down, putting the satellite Further research into reactor-based 5BN-3 accident in 1964. As such, out of commission. SNAP-10A space power systems continued the SNAP-19B heat source was remains a significant milestone for through the 1960s, but was largely designed to contain its plutonium the U.S. space program. Not only discontinued in 1973 due to fuel (microspheres) under normal did it demonstrate the feasibility changing national priorities. As operating conditions and during of operating a liquid-metal-cooled discussed in later chapters, interest abnormal conditions such as a reactor power system safely and in space nuclear reactor systems launch abort or re-entry. e first reliably in space, with remote startup would be renewed under programs use of the new RTG occurred and control, it was also the world’s such as the Space Power Advanced in 1968 when NASA launched first reactor power system to be Reactor (SPAR) project and the a 8 successfully launched and operated SP-100 program. On the RTG carrying two SNAP-19B RTGs from 7 in space. front, however, no such slow down Vandenberg Air Force Base. occurred, as the young RTG Although SNAP-10A remains technology was soon put to use by Approximately two minutes after the most notable achievement in an even younger national space lift-off, the rocket, carrying the the SNAP reactor program, other agency. satellite and its SNAP-19B RTGs, notable SNAP reactor programs veered off course, prompting a include SNAP-8 and SNAP-50. e RTGs Bring Power to mission-abort command. e abort- SNAP-8 program was initiated in NASA Missions induced destroyed the 1959, at the request of the National launch , after which the two Aeronautics and Space Agency NASA’s interest in space nuclear RTGs fell into the Santa Barbara (NASA), to develop a reactor for power systems grew against the Channel just north of San Miguel use in a nuclear electric propulsion backdrop of the Navy Transit Island off the coast of California. system. e resultant reactor program and the ongoing space Five months later, the SNAP-19B power system employed a mercury power system development efforts units were recovered – intact – Rankine cycle to generate 30 to 60 of the SNAP program. NASA’s from the ocean floor at a depth of kilowatts of (kWe). first use of the new space nuclear approximately 300 feet (90 meters). Under the SNAP-50 program, technology was in Earth orbit e SNAP-19B heat sources had development efforts focused on aboard a Nimbus weather satellite. performed as designed and were demonstration of a -cooled A desire to supplement a returned to Mound Laboratory, reactor coupled with a potassium 200-watt solar power system with where the fuel was recovered and Rankine cycle capable of generating approximately 50 We of RTG reused in a later flight. 300 to 1,000 kWe for use in electric power led to development of the propulsion, as well as power SNAP-19B RTG. supplies for space and large manned satellites.7

8 Atomic Power in Space II Chapter 1

In April 1969, NASA successfully landing, a new RTG (SNAP-27) eventually used in that capacity. All launched a Nimbus-3 weather was used to power the experiments of the systems worked exceedingly satellite, again carrying two SNAP- packages. Built by General Electric well and provided power to more 19B RTGs. e Nimbus-3 weather (GE) under an AEC contract, than 50 scientific experiments, satellite marked the first successful the SNAP-27 design employed as well as to the use of an RTG by NASA, thus the -telluride thermoelectric equipment that relayed data back beginning a partnership with conversion technology used by to Earth, until the Lunar that soon found the Martin Company in previous Surface Experiments Packages were itself on the .9 SNAP RTGs but in a system that shutdown in 1977.4 was designed to have the heat As part of the Apollo scientific source inserted on the moon. Although never deployed on the missions in the 1970s, several Similar to other Martin RTGs, the moon, another SNAP-27 RTG science stations (i.e., Apollo Lunar thermoelectrics were produced was launched aboard the ill- Surface Experiments Packages) by 3M. Designed for a power fated mission in 1970. were placed on the moon. output of approximately 63.5 We, Following an explosion on the Beginning with the second lunar a total of five SNAP-27 RTGs were main craft, the lunar module (with the SNAP-27 RTG onboard) was jettisoned from the command module upon return to Earth. During re-entry, the lunar module disintegrated and the RTG fell into the Pacific Ocean in the vicinity of the Tonga Trench. Subsequent monitoring found no detectable radioactivity, indicating that the RTG had survived re-entry intact.

Although used during the Apollo missions, the SNAP-27 RTGs were not the first use of nuclear energy on the moon. On the first Apollo mission, the Early Apollo Scientific Experiment Package deployed by Neil Armstrong and Edwin “Buzz” Aldrin included two radioisotope heater units (RHUs), each providing 15 watts of thermal power to keep the experiment warm during the long (14 Earth-days) and cold lunar SNAP-19B intact re-entry heat sources fabricated at Mound Laboratory to power the nuclear generators for the Nimbus-B advanced weather satellite. (Photo: DOE Flickr)

9 The Early Years Space Nuclear Power Systems Take Flight

nights. e experiment package designed for a heat output of one from was finally lost in used solar cells for electrical power, watt, to protect instruments and January 2003, over 30 years after which meant that the experiments from low temperatures.10 it was launched. e space probes, shut down during the lunar night.10 Solar panels were unworkable for powered solely by the RTGs, these missions because the energy provided invaluable information During this same period, NASA available from the at such a about , , and the also found nuclear power sources great distance was insufficient to outer . As one NASA to be useful in power the spacecraft’s systems and historian wrote, “e program, beyond the moon. Pioneer 10 experiments. e RTGs performed perhaps this is an understatement, was launched in March 1972, and perfectly, supplying power long after was a huge success. Such success in April 1973, to travel the original 30-month missions would not have resulted without beyond the asteroid belt, fly past had been achieved.3 Radio contact the four RTGs on each spacecraft Jupiter, and leave the solar system. with Pioneer 11, which passed by providing power.”4 Each spacecraft carried four 40-We Saturn as well as Jupiter, stopped in SNAP-19 RTGs and 12 RHUs, each November 1995. e radio signal After the Pioneer missions, NASA continued its use of RTGs when it sent two landers to in 1975. 1 and , each powered by two SNAP-19 RTGs, were launched in August and September 1975, respectively. Originally designed to operate in the of space, the SNAP- 19 RTGs had to be modified to operate in the atmosphere of Mars. NASA chose RTGs over solar panels because of the threat of dust collecting on the panels, reducing power generation. e RTGs enabled the characterization of the Martian environment, the transmission of thousands of pictures from the Martian surface, and the first testing of the Martian surface, and they operated for years beyond their original 90-day requirement.3 with the Viking 2 Lander was lost in

Apollo 12 mission with Alan Bean removing the SNAP-27 heat source from its carrying cask to insert it into the RTG housing. (Photo: NASA.gov)

10 Atomic Power in Space II Chapter 1

April 1980, and the Lander in November 1982.11

As RTGs provided power for NASA missions throughout the 1970s, the technology again found application aboard a Navy navigation satellite in 1972. In the first of a series of three experiments designed Johns Hopkins to test a -hardened University-Applied satellite and demonstrate other Physics Laboratory improvements, the Transit-RTG personnel install the was used aboard the TRIAD nuclear heat source experimental satellite launched into the Transit RTG. on September 2, 1972, from (Photo: JHU-APL) Vandenberg Air Force Base. e Transit RTG used a SNAP-19 heat source to provide approximately 37 We as the primary power source for the satellite. e system operated as designed for about one month, at which time a telemetry- converter failure precluded further monitoring of the RTG power level. Continued operation of the satellite for several years thereafter, however, indicated the RTG continued to provide power.10

A model of the AEC-developed fuel capsule for each of the four SNAP-19 nuclear generators to power the NASA Pioneer spacecraft to Jupiter in early 1972 is displayed by Bernard J. . (Photo: DOE Flickr)

11 The Early Years Space Nuclear Power Systems Take Flight

A Changing Landscape

While the Apollo, Pioneer, and Viking missions of the 1970s were hugely successful, the decade brought a major change to U.S. nuclear research. As a result of the Energy Reorganization Act of 1974,12 the AEC was abolished and its functions split between a new Energy Research and Development Administration (ERDA) and the Nuclear Regulatory Commission. Two years later, ERDA, along with the Federal Energy Administration, the Federal Power Commission, and other agencies, were integrated into a new Department of Energy (DOE) upon enactment of the DOE

Cutaway of the multi-hundred watt RTG. (Image: INL RPS Program)

Power Systems for Space 105

Qualitative regimes (power vs. 104 duration) where dierent space power Nuclear Reactors systems are generally applicable. 103 Nuclear power sources are particularly applicable to long-duration and, 102 especially in the case of nuclear Nuclear Reactors reactors, high-powered missions. Solar+Solar Dynamic 101 Chemical Nuclear power systems also have the Dynamic Isotope bene t of lower vulnerability to harsh Power Systems, Solar Electric power level (kWe) level Electric power 100 environments (e.g., natural radiation Solar Radioisotope Thermoelectric Generators, Solar around Jupiter, meteoroids, or Mars 10-1 dust storms). 1 hour 1 day 1 month 1 year 10 years Duration of use

12 Atomic Power in Space II Chapter 1

Organization Act On March 14, 1976, the Air Force e grand finale of RTG flights of 1977. e new successfully launched Lincoln in the 1970s included one of the department Experimental Satellites 8 and 9, most daring space missions ever began each of which was powered by two undertaken — a pair of spacecraft operations on multi-hundred watt (MHW) RTGs. known as and . October 1, 1977. Designed by GE, the MHW-RTG Launched in August and September of 1977, respectively, the Voyager 1 and Voyager 2 spacecraft were each The grand finale of RTG flights in the 1970s powered by three MHW-RTGs that provided a total of 475 We. was one of the most daring space missions ever Each spacecraft also carried nine undertaken — a pair of spacecraft known as 1-Wt heater units similar to those used on Pioneer. e MHW-RTGs Voyager 1 and Voyager 2. enabled the Voyager spacecraft to operate for extended mission lifetimes, making it possible to Space nuclear power applications provided 157 We at the beginning explore Jupiter, Saturn, Uranus, were not ignored when these of the mission. e use of silicon- and Neptune. It was the longest changes were taking place in germanium thermocouples, mission ever planned at that time. the Executive branch. Efforts rather than the lead-telluride e dramatic photographs sent were spent developing advanced thermoelectrics used in the back from Voyager impressed the thermoelectric materials SNAP-19 RTGs, allowed operation public, and the new data from such as selenides, with the at a higher temperature, which the missions changed scientists’ hope of increasing the overall improved overall efficiency. With understanding of the solar system.3 power conversion efficiency the higher temperature, the As of 2014, the Voyager spacecraft of thermoelectric generators. MHW-RTG provided a significant were still working after more than Building on that began in improvement in specific power 34 years of service, continuing to the early 1960s, NASA and DOE (4.2 We/kilogram) over the SNAP-19 send scientific data back to Earth began new development efforts (3 We/kilogram). e satellites were —Voyager 1 from interstellar space associated with technologies that launched aboard the same launch and Voyager 2 from the edge of our utilized Brayton and Rankine vehicle but were subsequently solar system. thermodynamic cycles, as interest moved into separate orbits. Once in in the higher efficiency of dynamic orbit, the satellites linked with each As the 1970s drew to a close, the power conversion was renewed. In other and with surface terminals to NASA space program maintained addition to ongoing research and provide communication across a high profile in spite of the development, DoD turned once more than three quarters of the termination of the Apollo lunar again to RTG technology.a surface of Earth.3 missions. e Viking missions to

a. Efforts undertaken by DOE (and its predecessors) and NASA to develop advanced thermoelectric materials and dynamic isotope power systems are described in Chapter 3, Advanced Isotope Power Systems.

13 The Early Years Space Nuclear Power Systems Take Flight

b Mars and the Voyager missions A Rocket Named Rover Lawrence Radiation Laboratory to the outer planets kept space (LRL), now known as Lawrence exploration in the public eye Like its nuclear power counterpart, Livermore National Laboratory, to and helped maintain research development of a nuclear thermal develop a nuclear and development funding. e propulsion system had its origins in for use in a propulsion system for advancement of RTG and heat 1955, when the possibility of using ballistic missiles.13 source technology led to safer and a nuclear reactor as a propulsion more powerful RTGs. However, system for ballistic missiles found Although the AEC laboratories space radioisotope and reactor renewed interest within the Air initially worked in parallel, funding power systems were not the only Force and AEC. at interest had limitations led to the consolidation focus for DOE and its predecessors been spurred by a 1953 paper of all reactor work to LASL in in those early years. In an effort by R. W. Bussard in which the early 1957. Ironically, the new largely parallel to the SNAP case was made for reactor-based LASL effort was named Rover, the program, a considerable effort was being superior to chemical name by which the former LRL also devoted to the development of propulsion for heavy payloads. nuclear rocket division had called technology for space-based nuclear With hopes for such a system, itself (LRL was subsequently given thermal propulsion as well as research programs were initiated at responsibility for work on a nuclear nuclear electric propulsion. Los Alamos Scientific Laboratory ramjet program called Pluto14). (LASL), later renamed Los Alamos National Laboratory (LANL), and As the new Rover program got underway, the Nuclear Rocket Development Station was developed to support testing of nuclear rockets. e station was located in the southwest corner of the Test Site in an area known as Jackass Flats. It served as the center for U.S. nuclear rocket propulsion testing from 1958 to 1973. It became home to an that eventually included three reactor test cells/ stands; two maintenance, assembly, and disassembly buildings15 (one for reactors and the other for nuclear

Rover rocket concept. (Image: grin.hq..gov) b. For a detailed account of the Rover and NERVA programs, see “To the End of the Solar System – e Story of the Nuclear Rocket” by James A. Dewar.

14 Atomic Power in Space II Chapter 1

); a technical operations Over time, the Rover program grew Ground Testing the complex; and various support to include five major elements: 1) a Kiwi Reactors buildings. Reactors were moved reactor and fuel-development effort, between the assembly/disassembly called Kiwi, conducted by LASL e first step toward a nuclear and test facilities via a railroad, and Rocketdyne; 2) a nuclear thermal propulsion system involved humorously referred to as the development project supported by a series of reactor tests called “Jackass and Western Railroad.” Aerojet and Westinghouse; Kiwi, named after the flightless Years later, while speaking of his 3) development of advanced reactor because they were only days managing the Rover program designs, called Phoebus and Pewee, intended for ground testing and for AEC and NASA, by LASL; 4) a reactor-in-flight test not for flight. e Kiwi tests were was quick to note that “Jackass project run by Lockheed; and 5) a designed to demonstrate concept Flats is where we did our reactor nuclear fuel-testing project feasibility and basic nuclear rocket and engine testing, but it is not operated by LASL. Although reactor technology such as high- descriptive of nor named for the each subproject had different temperature and long-life fuel people working on the program objectives, they were all designed elements. e Kiwi reactors were here.”16 to lead to a high-powered nuclear designed to demonstrate reactor thermal propulsion system with and fuel feasibility first at a power While the reactor research work demonstrated reliability. Such a task level of 100 megawatts of thermal was initially performed under the would prove to be easier said than power (MWt) (Kiwi-A reactor auspices of AEC and the Air Force, done, however, when considered in series) and then at a power level of defense missions soon gave way to the of the description offered 1,000 MWt (Kiwi-B reactor series). a civilian space focus. By the late years later by Glenn T. Seaborg, To that end, the Kiwi test series 1950s, DoD had dropped further then Chairman of AEC: sought to establish basic testing work on nuclear-powered ballistic procedures, demonstrate that a missiles. Responsibility for nuclear “…What we must do is build a high-power- reactor could rocket work was subsequently flyable reactor, little larger than heat a quickly and to transferred to the newly formed an office desk, that will produce high temperatures, and determine NASA in 1958. While the new the 1,500 megawatt power level of material interactions at the high space agency assumed overall Hoover Dam and achieve this power operating temperatures. responsibility for the Rover in a matter of minutes from a cold program, the responsibility for start. During every minute of its From July 1959 to October 1960, its nuclear aspects remained with operation, high-speed pumps must three Kiwi-A test reactors (Kiwi-A, AEC. Responsibilities between force nearly three tons of , Kiwi-A’ [A-prime], and Kiwi-A3) the two agencies were formally which has been stored in liquid were tested. Although testing established, as was a joint Space form at 420°F below zero, past the revealed problems in reactor and Office (SNPO), reactor’s white-hot fuel elements, fuel design, the problems were upon the signing of a Memorandum which reach a temperature of addressed and the feasibility of a of Understanding in August 1960.16 4,000°F. And this entire system 100-MWt reactor design had been must be capable of operating for demonstrated. By the end of 1960, hours and of being turned off and LASL had also completed the first restarted with great reliability.”17 Kiwi-B reactor design using much

15 The Early Years Space Nuclear Power Systems Take Flight

nuclear rocket. is gives promise of some day providing a means of even more exciting and ambitious exploration of space, perhaps beyond the moon, perhaps to the very end of the solar system itself.”18

Kennedy’s commitment soon turned to action. In June 1961, NASA and AEC awarded a contract to Westinghouse and Aerojet for the Nuclear Engine for Rocket Vehicle Application (NERVA) program. e goal of NERVA was to demonstrate a nuclear-powered rocket for flight testing based on the Kiwi-B reactor design. While NERVA referred to the entire , including the reactor and the various propulsion components, the overall Kiwi-A reactor on a transfer cart. Note the Kiwi bird depicted on the side. development program continued to (Photo: LANL Flickr) be referred to as Rover.

With the NERVA program up and of the technology foundation ongoing expansion of the Soviet running, attention soon returned to established during the Kiwi-A tests. space program were not taken the Kiwi reactor testing. Between In addition, NASA had initiated lightly, and an American response December 1961 and September the reactor-in-flight test program, quickly followed. On May 25, 1964, five Kiwi-B reactor tests looking forward to the day when 1961, newly elected President (Kiwi-B1A, Kiwi-B1B, Kiwi-B4A, the first nuclear rocket might be John F. Kennedy spoke before a Kiwi-B4D, and Kiwi-B4E) were launched. joint session of Congress. e conducted in the ongoing effort speech, which famously included to demonstrate the feasibility Amidst this progress, an event a commitment to go to the moon of a nuclear rocket. As with the in Earth’s orbit soon brought within the decade, also included a Kiwi-A series, testing revealed additional impetus to the young commitment to the Rover program: problems with reactor and fuel reactor development program. On design. One notable example was April 12, 1961, the Soviet Union “…Secondly, an additional 23 million the Kiwi-B4A test conducted sent Yuri Gagarin into orbit above dollars, together with 7 million in November 1962. Although the Earth, the first man ever to dollars already available, will the startup was do so. e early successes and accelerate development of the Rover successful, “…paralleling the rapid

16 Atomic Power in Space II Chapter 1

resumed. He was adamant that the With the NERVA program up and running, problem be thoroughly understood and corrected before hot testing attention soon returned to the Kiwi reactor testing. resumed.

Subsequent cold testing showed increase in power was a rapid Unfortunately, the test had another that the extremely high flow rate increase in the frequency of flashes unintended consequence. After of hydrogen propellant through of light from the nozzle; on reaching visiting the test site, President the reactor had caused severe 500 megawatts, the flashes were so Kennedy decided to slow down vibration within the core, which spectacular and so frequent that flight testing activities until the in turn caused cracking of the the test was terminated and shut cause of the reactor failure was fuel elements. After appropriate down procedures began. Initial addressed, and a follow-on test changes to the core design were disassembly confirmed that the successfully completed. e completed, SNPO authorized flashes of light were reactor parts program was subsequently put resumption of hot testing, being ejected from the nozzle; on hold by SNPO in January which resumed with Kiwi-B4D. further disassembly and analysis 1963 when Harold Finger, then e final Kiwi test, Kiwi-B4E, revealed that over 90 percent of manager of SNPO, insisted that was successfully performed in the reactor parts had been broken, cold (non-nuclear) flow tests be September 1964. Having operated 17 mostly at the core’s hot end.” completed before nuclear testing the rocket engine at nearly full power for 2.5 minutes, comparable to the performance of a chemical rocket, AEC and its contractor team had demonstrated the feasibility of the 1,000-MWt nuclear thermal reactor. Materials and operational issues were no longer an issue, and the final Kiwi-B design provided a baseline that was used in subsequent efforts to develop an integrated NERVA nuclear rocket.17

President John F. Kennedy speaking before Congress, May 25, 1961. (Photo: NASA.gov)

17 The Early Years Space Nuclear Power Systems Take Flight

More importantly, cost estimates for the nuclear rocket flight program continued to escalate. As a result, the planned flight demonstration was canceled in late 1963.

e final Kiwi experiment, Kiwi- (70 kilograms), was located 750 feet TNT, was a deliberate destructive (229 meters) from the reactor; test of a Kiwi reactor that had been smaller pieces of the core were modified to allow a rapid, large, found at even further distances.17 positive reactivity – a sudden burst of power that exceeded the reactor Developing a NERVA design limits. Conducted in 1965, Flight Engine the test was designed to learn what would happen under an extreme Regardless of the success of the Kiwi reactor event and provide tests and tentative plans for use information on the energy produced of a nuclear rocket by NASA, the in the reactor core and the energy Rover program began to fall victim released during the subsequent to declining budgets and changing excursion, including the dispersion priorities in the early 1960s. More of fission products. Although such importantly, cost estimates for testing would be extremely unlikely the nuclear rocket flight program under modern regulatory and safety continued to escalate. As a result, environments, the test provided real the planned flight demonstration data to support safety and accident was canceled in late 1963, which analyses of interest to the Rover led to a redirection of NERVA flight program. For example, the away from the qualification of a reactor core reached a temperature specific engine system and toward of approximately 2,160 and a program of general nuclear rocket generated a total number of fissions 20 technology improvement. en that approached 3.1×10 . Only began a series of full-power reactor about 50 percent of the core tests in early 1964 that continued material could be accounted for to move the nuclear rocket concept within a 25,000-foot (7,600-meter) down a path of development. radius; the remainder was presumably burned in the air or e shift in NERVA to a technology NERVA engine mockup. The spheres dispersed downwind. e heaviest improvement program in 1964 at the top contained hydrogen which, piece of debris, a section of resulted in the redefining of its after passing through the reactor vessel weighing program goals, including operating (center), was discharged out the nozzle approximately 150 pounds (bottom of picture). (Photo: LANL Flickr)

18 Atomic Power in Space II Chapter 1

for 60 minutes at full power; Major contractors involved in the was responsible for producing restarting at any point in the NERVA program included the instrumentation. Construction reactor’s life cycle; demonstrating Rocketdyne Division of North of the nuclear reactor was the rapid temperature increases and American Aviation (later part of responsibility of the Westinghouse decreases; cooling down using Boeing), which was responsible Electric Corporation Astronuclear only liquid hydrogen; startup of for building a liquid hydrogen Laboratory.17 the engine without an external and nozzle; Aerojet, power source; and determination of responsible for a flow control With program goals and the project system operational margins, limits, system; ACF-Erco, responsible team established, nuclear rocket and reliability.17 for manufacturing the pressure engine testing began in September shell; and EG&G, Inc., which 1964 under the name Nuclear

The rst ground experimental nuclear rocket engine (XE) assembly (left) is shown here in cold ow con guration, as it makes a late evening arrival at Engine Test Stand No. 1 at the Nuclear Rocket Development Station in Jackass Flats, Nevada. (Photo: grin.hq.nasa.gov)

19 The Early Years Space Nuclear Power Systems Take Flight

Rocket Experimental (NERVA From December 1968 through Phoebus, Pewee, and the NRX). Over a period of three years, September 1969, XE-Prime was Nuclear Furnace four tests were conducted (NRX-A2, successfully operated over a period NRX-A3, NRX-A5, and NRX-A6). of 115 minutes with 28 separate As visions of space travel turned e NRX test series led to testing restarts, thereby demonstrating again to a manned trip to Mars of the first down-firing prototype the feasibility of an integrated (long a goal within NASA), a nuclear rocket engine, XE-Prime. 1,000-MWt nuclear rocket engine. program was initiated to develop advanced reactors capable of power levels on the order of 5,000 MWt. Conducted in parallel with the NERVA program, the Phoebus and Pewee reactors were designed and tested by LASL for such operation. Phoebus, a 5,000-MWt prototype reactor, was first tested in 1965. By 1968, Phoebus’ final version (the most powerful nuclear rocket ever built) ran at over 4,000 MWt for more than 12 minutes.17 e Pewee reactor was a small test bed used to test full-size Phoebus and NRX fuel elements and other components, allowing components to be developed in parallel to reduce lead times and costs.

As restrictions on radioactive emissions began to tighten, the Nuclear furnace was built to allow testing without releasing radioactivity into the atmosphere. e furnace was a modular 44-megawatt reactor, where the core portion could be switched out for separate experiments.17 e reactor effluent filters produced a hydrogen jet without detectable fission products.19

Phoebus 1A reactor at LANL, 1965. (Photo: LANL Flickr)

20 Atomic Power in Space II Chapter 1

Accomplishments and the incremental performance, reliability 100-MWt Kiwi-A reactors to the Face of Changing National and lifetime improvements.” In 4,000-MWt NERVA XE reactor Priorities addition, the Rover program through 20 different reactor tests. demonstrated a model by which As the Rover program turned two government agencies could Against the backdrop of such the corner on a new decade, it effectively manage a major technical accomplishments, soon found itself facing a very technology development program.21 lessons had been learned that different future. After the urgency of reaching the moon passed and the Apollo missions ended Against the backdrop of such technical in 1972, it became clear that a manned Mars mission wouldn’t accomplishments, lessons had been learned and be the next step. Due to changing priorities on national budgets, provided a foundation for future development. further development of the NERVA nuclear rocket was terminated in the fiscal year 1972 budget. Looking Forward provided a foundation for future development. In addition, an From 1955 to 1971, the United As the first 30 years of space entire infrastructure had been States spent approximately nuclear power drew to a close built across the DOE complex to $3.5 billion (in 1960 dollars) on in the mid-1980s, DOE, NASA, support ongoing development, 19 the Rover and NERVA programs and others had many reasons testing, and use of space nuclear (compared with $19.4 billion spent to celebrate. irty-five RTGs, systems. Most importantly, a new on the from ranging in power from the small technical discipline had been 20 1960 through 1973 ). During that 2.7-We SNAP-3B unit to the defined, an industry established, period, 17 reactors, one nuclear 157-We MHW-RTG, had been and a foundation laid that safety reactor, and two ground successfully launched into space would allow a new generation of experimental engines were to power lunar experiments and space nuclear technologists to developed and tested. e planetary orbiters, while other carry the of space nuclear feasibility of a spacecraft were on their way to the power into another 30 years reactor/nuclear rocket engine had end of the solar system. Multiple of testing, development, and been clearly established “...at space RPS concepts had been accomplishments. temperature, pressure, power levels, developed and tested under the and durations commensurate with SNAP program, and one space today’s [1991] propulsion system nuclear reactor power system requirements...” e technology had had been successfully placed also been demonstrated to an into Earth’s orbit. On the nuclear extent such that “…future nuclear rocket front, the feasibility of propulsion development associated nuclear thermal propulsion had with new space exploration been successfully demonstrated, initiatives can be directed to evolving from initial testing of the

21 The evolution of RTG technology through the 1970s well represented the idea of taking something good and making it better. Looking back, however, we see that the best was yet to come.

22 and The General Purpose 2 Heat Source RTG

ollowing their conception in 1954, RTGs evolved from the small, 2.7-We SNAP-3B system in 1961 to the MHW-RTG unit that generated a nominal 157 We. Improvements were made in the fuel Fform, thermoelectrics, and safety aspects of these static power systems. e evolution of the “quiet technology” of the RTG through the 1970s well represented the idea of taking something good and making it better. Looking back, however, we see that the best was yet to come.1

At the end of a 12-year period during which DOE and NASA prepared for the parallel missions of Galileo and Ulysses, the pull of and the push to look at the old ways of doing things in new, creative ways led to the development of the most powerful RTG ever to be used in space applications. e general-purpose heat source (GPHS)-RTG, as it came to be called, would prove to be the most efficient RTG ever built (as well as the unit with the highest specific power) and would power NASA missions for decades to come.

Powering New Missions

While the MHW-RTGs were performing as planned on their Earth- satellite and outer solar-system missions, DOE continued its efforts to advance RTG technology throughout the 1970s. ose efforts were centered largely on the development of advanced thermoelectric materials called selenides, which were to replace the silicon-germanium materials used in the MHW-RTG; a new modular heat source, the GPHS; and an improved MHW heat source that featured improved -alloy fuel cladding and advanced graphitic materials for the aeroshell.2, 3, 4

e new GPHS was being developed at LANL for use in a wide range of power conversion systems, power levels, and space missions. With a focus that included improved safety, development plans for the new heat source included an extensive safety testing and qualification program.5, 6

While advances in heat source technology sought to improve safety, advances in thermoelectric materials sought to improve the overall efficiency by which thermal power was converted to useable electricity. To that end, Teledyne Energy Systems (TES) was working on advanced

The Galileo and Ulysses missions marked the beginning of new assembly and testing operations at Mound, circa 1985. (Photo: Mound Museum Association)

Photo Credit 23 Galileo and Ulysses The General Purpose Heat Source RTG

power conversion technology using e next phase in SIG development its options against mission needs selenide-based thermoelectric would combine the selenide and schedules, DOE decided to materials under development at the thermoelectrics with the new abandon further development of 3M Corporation. Early laboratory- modular GPHS for use on a multi- the selenide materials and return scale testing of the selenide national mission originally called to the proven silicon-germanium materials indicated potential the International Solar Polar unicouples for the needed conversion efficiencies of at least Mission (ISPM). As conceived, the RTGs. For the Galileo mission, 10 percent, which represented a hopeful improvement of approximately 50 percent above With the broader set of mission uses, the that of the silicon-germanium materials used in the MHW- ISPM-RTG was soon rebranded the GPHS-RTG, RTG. Amidst such favorable a name that would find its place in the annals of expectations, DOE planned to develop a new RTG (called the RTG history. selenide isotope generator [SIG]) that would eventually utilize the new selenide thermoelectric materials and the new GPHS.7 mission was a joint effort between development of the MHW-RTGs NASA and the European Space continued but with the proven e first phase in developing the Agency (ESA) to provide the first silicon-germanium thermoelectrics. SIG would combine the selenide polar-orbital survey of the sun. For the ISPM mission, thermoelectrics with a MHW heat Each agency would supply one responsibilities for development source for use on a mission that spacecraft, both powered by RTGs of the RTG were subsequently came to be called Galileo (initially supplied by DOE, to make scientific contracted to GE and the new RTG called Jupiter Orbiter Probe), measurements above and below the was named (at least briefly) the named after the 17th-century ecliptic —the plane of Earth’s orbit ISPM, or Solar-Polar, RTG.6 Italian astronomer who discovered about the sun. Domestic budget four of Jupiter’s . Long-term in the early 1980s resulted As the developmental success of the surveys of Jupiter would be made in cancellation of the NASA ISPM-RTG progressed, changing using an RTG-powered orbiter, spacecraft; ESA continued the mission plans for Galileo, which while a smaller probe would collect mission, and it was subsequently included a desire for more power, atmospheric and other information renamed Ulysses.9 eventually gave way to a decision about the giant during a one- to use the ISPM-RTG in lieu of the time pass through its atmosphere.8 Despite high hopes for the new improved MHW-RTG. With the selenide thermoelectric materials, broader set of mission uses, the by 1979, testing of prototypic ISPM-RTG was soon rebranded selenide thermocouples had the GPHS-RTG, a name that would uncovered significant material- find its place in the annals of instability and conversion- RTG history.7 efficiency issues. After considering

24 Atomic Power in Space II Chapter 2

As the dust of program and The General Purpose GPHS-RTG Development - mission planning settled, DOE Heat Source Key Contractors was responsible for developing The systems contractor for the the new GPHS-RTG for use on Initial design and development of GPHS-RTG development eort the two missions. Ultimately, the new GPHS resided with LANL was the Astrospace Division of GE three flight-qualified GPHS-RTGs but was later enhanced by GE and (later incorporated into Lockheed- and one spare unit were needed. Fairchild. Design of the new heat Martin). Program execution was e new GPHS, around which source included several goals, one widely distributed and involved the RTG was to be built, was still of which was the idea of modularity, numerous contractors and national under development; safety testing in which individual modules could laboratories: and a final design, necessary to be combined to provide the amount demonstrate acceptability for use in of power required by a specific System Contractors space, remained to be completed. mission. Another goal was to • General Electric In addition, the launch vehicles develop a heat source that would be • Teledyne Energy Systems planned for launching the Galileo compatible with multiple static and Technical Support and Ulysses spacecraft into space dynamic power conversion systems. • Fairchild Space Company were also still under development. A high of at least • Battelle Columbus Laboratories On top of that, the production 75 watts (thermal) per pound was capability for the silicon germanium also desired. In keeping with the Technology unicouples had to be re-established. intact re-entry safety philosophy • Years later, the effort to develop the adopted in the 1960s, the primary • General Electric GPHS-RTG would aptly be referred safety objective was to keep the to as a “mission of daring” by many fuel contained or immobilized to Safety Organizations of the individuals directly involved.10 prevent inhalation or ingestion • Applied Physics Laboratory by people. Ultimately, mission • Los Alamos National Laboratory As had been performed previously, planners had to be confident that • Naval Ocean Systems Center DOE completed this mission the public would be protected in • NUS Corporation using a small but diverse group case of any foreseeable accident.5,12 Heat Source Production and RTG of companies and national Assembly/Testing laboratories, a unique set of e final GPHS design consisted of • Monsanto Research Corporation technical capabilities and assembly a rectangular-shaped -carbon • Oak Ridge National Laboratory and test facilities, and a group module into which the encapsulated • Savannah River Laboratory of highly dedicated individuals fuel would be placed. e design • Savannah River committed to program success. An included several protective features interagency agreement signed by to guard against , fires, Reliability and Quality Assurance DOE and NASA defined the overall impacts, projectiles, and the heat • Sandia National Laboratories roles and responsibilities of the of Earth re-entry. ese features respective agencies. Responsibility included the fuel, iridium-alloy metal for development of the GPHS-RTG cladding, fine-weaved pierced fabric resided with the DOE Office of (FWPF), and a carbon-bonded, Special Applications.11 carbon-fiber sleeve component.

25 Galileo and Ulysses The General Purpose Heat Source RTG

oxidation also provides for good post-impact protection. A small frit vent, welded into one end of the iridium cladding, allows gas produced from the of the plutonium fuel to escape while retaining the plutonium, thereby precluding the buildup of pressure that could crack or breach the cladding. e fuel clad set, commonly referred to as a fueled clad, has the shape of a cylinder with rounded edges and is approximately one inch long and one inch in diameter.

A third protective feature of the new heat source was the use of FWPF in several components, including impact shells and the Cutaway of the GPHS (Step-0). (Image: SNL) GPHS module. e impact shells provide protection from projectiles and debris, and impacts on the e first protective feature was the and tends to fracture into pieces ground. A final protective feature fuel itself. e GPHS fuel pellet generally too big to inhale. was the use of a carbon-bonded consisted of approximately 150 carbon-fiber sleeve component, grams of plutonium oxide that e fuel pellet was encapsulated which serves to protect the fueled produced roughly 62.5 watts of in a strong iridium alloy metal clads against high temperatures thermal power. As with previous cladding, another protective associated with launch vehicle RPS fuel, plutonium emits alpha feature. Developed at the DOE Y-12 fuel fires and the heat that results radiation, which is easily shielded Plant, and referred to as DOP-26, during atmospheric re-entry, such and can only become a health the cladding provided high material as might occur following a failure concern if it enters the body, such strength while retaining good in space. Fully assembled, a GPHS as through ingestion or inhalation. ductility, a physical property that module containing four fueled clads To minimize the risk, the allows a metal to bend and stretch, weighed approximately 3.3 pounds plutonium oxide was produced in a without breaking under conditions (1.5 kilograms) and generated form similar to the MHW of high strain. e iridium alloy is approximately 250 watts of ceramic fuel spheres. e ceramic also chemically compatible with the thermal power. fuel pellet is chemically stable, has plutonium oxide fuel and graphite a high melting and vaporization components and has a high melting temperature, is highly insoluble, temperature. Its resistance to

26 Atomic Power in Space II Chapter 2

The GPHS-RTG As design and development of the new heat source As design and development of the new heat source progressed, so did progressed, so did the new power conversion unit. the new power conversion unit. Developed by the GE Astrospace Division, the GPHS-RTG consisted been shut down following these Replication of the RCA process of a structural housing inside of missions. us, GE had to re- was not without its challenges. which 572 interconnected silicon- establish a new production capability For example, it was discovered germanium thermoelectrics, called that duplicated the RCA process. that the production practices unicouples, converted heat into employed by RCA personnel had useable electricity. e housing e process by which the small embellished upon documented in which the thermocouples silicon-germanium thermoelectric specifications and processes; as were located was constructed of elements had been made by RCA a result, the additional practices aluminum and designed to support consisted of several steps, each of and other changes required a stack of 18 GPHS modules. e which had to be re-established. incorporation into production selection of aluminum for the housing was part of the overall GPHS-RTG Specifications safety design of the system, ensuring that it would readily melt The GPHS-RTG generated approximately 300 We from the nominal 4,400 watts if exposed to the high temperatures of thermal heat generated by its 18 GPHS modules. Fully assembled, the of re-entry, and that the GPHS GPHS-RTG was approximately 16 inches (0.4 meters) in diameter and 43 inches modules would be released as (1.1 meters) long, and weighed approximately 123 pounds (56 kilograms), individual units. Other design resulting in a speci c power of approximately 5.4 We per kilogram when rst features included the use of eight assembled. (Image: SNL) heat rejection fins, electrical power connectors, and externally mounted cooling loops that could provide cooling for the RTG when located inside a confined area such as the bay.

As GE transitioned into production of the new power conversion system, it needed a production capability for silicon-germanium thermocouples. e production capability, originally established by Radio Corporation of America (RCA) in support of the Voyager and Lincoln Experimental Satellites-8 and -9 missions, had

27 Galileo and Ulysses The General Purpose Heat Source RTG

procedures to improve process Guided by Safety new space transportation system rigor and control.13 For the four (STS), which included the space GPHS-RTGs planned for the Integral to the design and shuttle. Once the shuttle reached Galileo and Ulysses missions, over development of the GPHS-RTG was its parking orbit, the spacecraft 2,000 individual unicouples were a rigorous safety testing program, would be released from its cargo eventually fabricated for use in the conducted to determine how bay, and an upper-stage propulsion units. Additional unicouples were the heat source would respond system would be used to propel the also produced for the engineering to various postulated accident spacecraft towards its destination. and qualification units used to conditions that might occur demonstrate readiness of the RTG during launch or once in orbit. For In the evolution of planning for flight use, as well as for an example, a launch vehicle explosion associated with the use of the unfueled spare converter. on the launch pad or during ascent new STS, which was then under might subject the RTG and its heat development, a decision regarding Along with re-establishing the sources to high-pressure shock the specific upper-stage launch production process for the waves from liquid and solid fires, vehicle to be used for Galileo unicouples, GE conducted a launch vehicle and spacecraft and Ulysses changed frequently, rigorous testing program to address fragment impacts, and ground alternating between a solid-fuel all facets of the power conversion impacts onto , concrete, or inertial upper stage and a system and the fully assembled sand. A late launch accident, such as rocket that was fueled with liquid RTG. Testing was performed in a one involving orbital or suborbital hydrogen/liquid . Each progressive manner, beginning with re-entry, might also subject the upper-stage vehicle had unique individual unicouples followed by RTG and its heat sources to high hazards and characteristics that 18-couple modules. e unicouple temperatures and subsequent influenced development of the testing was followed by tests of ground impacts. e modular requisite safety analysis. As a result full-scale component engineering design of the GPHS modules also of ongoing changes regarding test units for structural, thermal, ensured that the chances of impacts which upper-stage vehicle to use, and material properties, which from spacecraft debris would a preliminary safety analysis was demonstrated the design of an be very low, if not impossible. based on use of an inertial upper electrically heated engineering Subsequent safety testing at stage but was later revised to reflect unit. Non-nuclear testing of LANL and SNL demonstrated the a Centaur system. As discussed the engineering unit included protective safety features of the later in the chapter, this would not thermal and vibration tests to GPHS module under a variety of be the last change in the launch ensure the thermoelectrics and postulated accident scenarios.5, 15 system to be used.15 other components of the power conversion system would properly e testing was planned and While upper-stage vehicle planning operate. Testing culminated with conducted against a backdrop perturbations were frustrating, a nuclear-heated qualification of evolving mission plans that tests were nonetheless devised to unit, which served to verify that centered on a new NASA launch simulate a broad range of possible the GPHS-RTG would operate vehicle. For the Galileo and Ulysses pressures, temperatures, and other as designed and meet all mission missions, NASA planned to environmental conditions. In a requirements.14 launch the spacecraft aboard its series of safety verification tests

28 Atomic Power in Space II Chapter 2

designed and carried out in the during an accident. Other tests A Formal and Exhaustive early 1980s, data and information studied the response of the GPHS Safety Review Process were gathered for use in the safety module to a solid-rocket-propellant Federal agencies collaborate to and environmental analyses needed fire, as well as long-term exposure complete two separate safety to support a launch decision. of heat source plutonium oxide processes before the United States fuel to aquatic and terrestrial launches a spacecraft carrying e tests were planned and environments.15 . The rst is the conducted using facilities and production of an environmental equipment at DOE sites (i.e., LANL is testing demonstrated the impact statement (EIS) by the agency and SNL). For example, launch robustness of the new heat source launching the spacecraft (with vehicle explosions and the resulting and provided information to DOE support) incorporating public overpressures were simulated support the safety analyses and review. DOE produces a nuclear risk using a shock tube at SNL. Impact review processes needed for launch assessment that is used as input to tests were simulated using a rocket approval. For example, fueled the EIS. The second process is an sled at SNL and the Isotope Fuels GPHS modules impacted against independent safety evaluation by an Impact Test Facility at LANL, concrete at approximately 116 miles ad-hoc Interagency Nuclear Safety which could accelerate heat source per hour (52 meters per second), its Review Panel (INSRP) formed for the components to high velocities terminal velocity, which resulted in speci c mission in question, which for impact against different no fuel release. Impacts of fueled ends in an Executive Branch review materials, such as concrete and clads without the protection of the that requires nal launch approval steel. Similarly, the SNL rocket sled GPHS aeroshell against sand at by either the President directly or used aluminum plates to simulate velocities up to and including 560 by the Director of the White House the impact of fragments or other miles per hour (250 meters per Oce of Science and Technology debris that might be generated second) and against concrete up to Policy. To support this process, DOE prepares a detailed Safety Analysis Report that describes the potential launch accidents and the nuclear Mission and system response to these accidents DOE launch vehicle and their associated probabilities. It data (NASA) also describes the mission, spacecraft, Safety analysis DOD Office of the report (DOE) power system, and testing that has President been completed to evaluate the risk of Accident descriptions EPA Office of the release of nuclear material under (NASA) Science and various accident scenarios. The INSRP Safety evaluation Technology produces a Safety Evaluation Report NASA Policy report (INSRP)* that evaluates the DOE Safety Analysis System design Report. The two review processes are *Interagency Nuclear Other and test data agencies both comprehensive and detailed. The Safety Review Panel (DOE) full process for RTG launch approval generally takes four to eight years.

29 Galileo and Ulysses The General Purpose Heat Source RTG

134 miles per hour (60 meters per second) yielded similar results. The Galileo and Ulysses missions brought new No releases occurred as a result of explosion over pressure opportunities for the space nuclear power system when tested, as did testing at over workforce at Mound. pressures up to 2,200 pounds per square inch (psi). At the conclusion of the safety testing, design, and development efforts, the new GPHS system workforce at Mound. Mound recalled years later, “We was finally ready for production In the late 1970s, DOE decided had technical problems which led and use.15 to transfer RTG assembly and to schedule issues and working 24 testing operations from its system hours a day, seven days a week. contractors, GE and TES, to But it was a good group and we all Production Takes 17 Center Stage Mound. Although radioisotope worked well together...” heat source assembly and other related operations had been After months of preparation, e effort to produce the GPHS- conducted at Mound for over training, and reviews, Mound RTGs and their heat sources was 30 years, the new RTG assembly finally put its new RTG assembly widely distributed and involved and testing work required that a and testing capability to work. numerous DOE contractors and facility, equipment, operations, and In April 1983, Mound received national laboratories. For example, personnel be in place and ready to a GPHS-RTG qualification unit, the iridium-alloy raw materials support the planned launch dates. designated Q-1, for fueling used to fabricate the cladding and e new operation was located in and testing. Assembly of the frit vents were produced at ORNL. Building 50, the RTG Assembly qualification unit was performed Fabrication of the iridium cladding and Testing Facility, which had in the new Inert Atmosphere and frit vents was performed been used for other activities since Assembly Chamber, where at the Mound Laboratory. the early 1970s. operators assembled the stack Fabrication and encapsulation of of 18 fueled GPHS modules the plutonium oxide fuel pellets Modifications to accommodate the and inserted the stack into the was performed in the Plutonium new RTG operations began in the generator. e RTG was then Fuel Fabrication Facility at early 1980s and were completed in sealed and backfilled with an inert Savannah River Site (SRS). e 1983. Equipment was received and gas, which served to protect the encapsulated fuel pellets were then set up, procedures were developed, thermoelectrics from deleterious transferred to Mound, where they and workers were trained to effects of atmospheric oxygen were encapsulated in the graphitic accommodate the new assembly during storage and testing components during GPHS module and testing operations.16 It was a operations. assembly operations. hectic time, and the pressure to have RTGs ready for the aggressive e Galileo and Ulysses missions launch schedule meant long brought new opportunities hours and work weeks for those for the space nuclear power involved. As Wayne Amos of

30 Atomic Power in Space II Chapter 2

GPHS-RTG Identification

GPHS-RTGs are identi ed following a convention based on their status as either a quali cation unit or a ight unit that has been certi ed by DOE as being ready for mission use. A quali cation unit receives a Q designation, followed by a sequential number representing its assembly order. For example, a Q-1 designation means the unit is the rst quali cation unit for the GPHS-RTG design. Similarly, an F-1 designation means the unit is the rst ight-certi ed version of the GPHS-RTG design.

A technician works on the assembly of a nuclear generator. (Photo: DOE Flickr)

Following assembly, the nuclear- testing of the qualification unit from Mound to the Kennedy fueled qualification unit was put provided assurance that the Space Center (KSC) in preparation through an extensive series of new GPHS-RTG would meet for the Galileo and Ulysses tests that checked the system for mission needs and verified the missions, which were scheduled resistance to leaks, - and analytical models used to predict to launch later that year. A fifth gamma-radiation emission rates, performance.16 unit, F-2, had been prepared but and pressure decay. Vibration inadvertent exposure to air during testing was performed using a Following successful completion of operational processing resulted shaker table, which simulated the RTG qualification and safety- in slight power degradation, and launch conditions. Long-term testing programs, DOE and NASA use of the unit was deferred to a power testing was performed in a were confident that the GPHS- later mission. DOE provided the large vacuum chamber to simulate RTG was ready for flight. In 1985, GPHS-RTG units in time for the RTG performance in space. Mound completed the assembly scheduled flights, and they were and acceptance testing of four ready to power the next step in Upon completion of the testing, a GPHS-RTGs – three for use in the space exploration. long-term life test was initiated to Galileo and Ulysses missions and demonstrate the longevity of the one spare unit. In January 1986, power conversion system over a the four RTGs (designated F-1, period of several years. Successful F-3, F-4, and F-5) were transferred

31 Galileo and Ulysses The General Purpose Heat Source RTG

Launch Plans Put On Hold systems. After the integration work -assist maneuver resulted in was completed, the RTGs were the need for additional re-entry Only days after the GPHS-RTGs returned to Mound in 1986 for safety tests and analyses.9, 19 arrived at KSC, the Space Shuttle servicing, monitoring, and long- e results prompted iterative Challenger exploded shortly term safekeeping pending their final changes to the original Earth fly-by after takeoff on January 28, 1986. transfer to KSC for launch. maneuvers and resulted in a slightly Following the accident, all parts of delayed arrival at Jupiter. the U.S. civil space program that depended on the space shuttle were put on hold, and investigations into Only days after the GPHS-RTGs arrived at KSC, the the cause of the accident quickly exploded shortly after ensued. e accident had major ramifications for the Galileo and takeoff on January 28, 1986. Ulysses missions. First, the original 1986 launch dates for the missions were eventually moved to 1989 for Against the backdrop of the In addition to the need for the Galileo and 1990 for Ulysses. e Challenger accident and the risks gravity-assist maneuver, the accident also prompted questions associated with use of the Centaur decision to use a solid-fuel upper regarding the risks of launching rocket with its liquid-oxygen/ stage resulted in the need for new nuclear payloads aboard the liquid-hydrogen propellant, NASA tests and analyses to ensure the shuttle.18 In addition, questions Administrator James Fletcher GPHS could contain its fuel under arose regarding the risks of the announced, in June 1986, a decision postulated accident conditions Centaur upper-stage launch vehicle, to cancel the Centaur program. e involving large pieces of the shuttle with its liquid hydrogen propellant, decision ended the plans to use the solid-rocket-booster. In a series of aboard the shuttle. Finally, the Centaur rocket with the Galileo tests conducted at SNL, scientists Challenger accident, coupled with spacecraft. As a consequence, NASA simulated the interaction of large the catastrophic failure of a - subsequently decided to use a less solid rocket booster fragments with 34D rocket during launch in April energetic solid-fuel inertial upper a GPHS-RTG. Components used 1986, led NASA to completely stage (IUS) rocket for the Galileo in the testing included the GPHS- re-evaluate its launch vehicle failure and Ulysses missions. Because the RTG engineering unit that was modes and associated accident IUS booster didn’t have the power to being tested at Mound and GPHS environments and probabilities.19 send the spacecraft on a direct path modules containing fueled clads to Jupiter, Galileo mission planners of uranium oxide used to simulate As investigations into the accidents devised a new flight plan that the plutonium oxide. In a series of progressed and plans developed included a -Earth-Earth three separate tests, solid-rocket- to address the causes, the GPHS- (VEEGA) to set the booster fragments were shot into RTGs, still at KSC, were connected spacecraft on its path to Jupiter. In sections of the GPHS-RTG at to the Galileo and Ulysses the VEEGA maneuver, the gravity of varying velocities and orientations spacecraft in a series of hot tests the two planets would be used to to determine impact effects on to ensure proper integration and increase spacecraft velocity and the fueled clads.19 e test results operation with the spacecraft reduce travel time to Jupiter. e provided information for use in

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updated safety and environmental Gravitational Assists analyses. Jim Turi, retired from A gravitational assist is an essential tool for some interplanetary missions. It DOE, noted one special feature of allows a spacecraft to change direction or speed by taking advantage of the the testing: “What is little known gravitational pull of planets and moons. A gravitational assist is typically used is that in testing the RTGs, we when a spacecraft requires a large change in velocity, such as when traveling actually used some leftover pieces to the outer planets or even Mercury, as the technique enables the spacecraft from the Challenger accident after to achieve velocities that are beyond the capability of its rockets. This enables the investigation was finished. We missions that would not otherwise be possible, and also reduces the amount of put pieces of the space shuttle on fuel or time required for a trip. Use of a gravitational assist requires a high level of rocket sleds at Sandia and ran them planning and navigational accuracy. With the planets, including Earth, in constant into the RTGs…”.20 motion, a particular gravitational-assist maneuver may only be available within a short time period. The time period when a spacecraft can be launched that will allow it to successfully complete its mission is known as a launch window.

When a spacecraft approaches the orbiting body that will provide the assist, the gravity of that body pulls the spacecraft towards it. Objects in orbit are not stationary, but travel at great velocity as they orbit around the sun. If the spacecraft approaches the moving Earth Flyby (1) December 8, 1990 Earth Flyby (2) December 8, 1992 planet from behind, it inherits some Launch October 18, 1989 of the planet’s velocity as it is pulled Earth forward by gravity. Similarly, if the Venus Sun spacecraft approaches an oncoming Venus Flyby February 10, 1990 Asteroid planet from the front, it will be slowed. belt With careful planning and execution, the spacecraft will exit the gravitational inuence of the planet in the proper IDA (asteroid) Gaspra (asteroid) direction and with the desired speed.

Mariner 10, launched in 1973, was the rst spacecraft to use a gravitational assist when it traveled to Mercury. Pioneer II used a gravitation assist at

Probe release - July 13, 1995 Jupiter to reach Saturn, and Voyagers 1 and 2 used gravitational assists to Jupiter - Arrival December 7, 1995 explore the outer planets.

Galileo Venus-Earth-Earth gravitational assists. (Image adapted from NASA/JPL illustration)

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By 1989, DOE and NASA had e concerns culminated in protests the adrenaline, the happiness. It updated the safety and environmental at KSC and the filing of a first- was just amazing. It was just totally analyses to support the planned ever lawsuit against NASA, which unexpected when it occurred.”20 Galileo launches. e updated threatened to halt the Galileo launch. Galileo was successfully deployed analyses reflected the use of a After hearing arguments from both from the shuttle, once it reached solid-fuel upper stage, the VEEGA, sides of the case, the presiding orbit, and sent safely on its way to and the results from the large judge ruled in favor of NASA. e Jupiter via the gravity assists. fragment tests, and concluded Galileo mission was finally a go. that the risks of launch with the After the launch of Galileo, the During the flyby of Earth in GPHS-RTG onboard the shuttle Ulysses mission would see similar December 1990, Galileo provided were acceptably low. Reflecting concerns and protests. Although the first photographs taken by an upon the Challenger accident and the protests and lawsuit didn’t stop unmanned spacecraft of the Earth its impact on the space nuclear the launch, they did contribute to and moon together and confirmed program, Roy Zocher, one of the the establishment of pre-launch the existence of a huge ancient heat source designers from LANL, contingency planning. Such planning impact basin in the southern part noted, “[T]hat accident…brought provides for response to radiological of the far side of the moon that us to explore other possibilities in incidents in the unlikely event of a had never been mapped. e first terms of accident environment, and launch accident.1, 9, 18 multispectral study of the moon so a good deal of the extensive safety showed evidence of more extensive testing originated as a result of the The Launch and lunar volcanism than previously Challenger accident...”.21 With the new Discoveries of Galileo thought.22 On its six-year outbound safety testing and review processes journey to Jupiter, the spacecraft completed, Galileo and Ulysses were After more than 10 years of provided the first close-up once again ready to fly, at least as far development, testing, and observations of asteroids (Gaspra as DOE and NASA were concerned. preparations that spanned three and Ida) and also recorded the Some within the public had a presidential administrations, the collision of the Shoemaker–Levy different opinion, however, and soon Galileo spacecraft and its two with Jupiter. took the opportunity to make their GPHS-RTGs finally made their voices heard. way to space aboard the Space A probe was released from the orbiter Shuttle Atlantis (STS 34) when it on July 13, 1995. Once released, it In the months leading up to the was launched on October 18, 1989. could no longer be commanded and Galileo launch, anti-nuclear groups For all involved, such launches followed its predetermined path for like the Florida Coalition for Peace produce an almost indescribable 147 days before entering the Jovian and Justice, the Foundation on feeling of pride and sense of atmosphere. After the probe was Economic Trends, and the Christic accomplishment, as described by released, the orbiter trajectory was Institute (a public interest law firm) Jim Turi: “…I can remember the altered to fly over the site where the began to express their opposition. first launch I was down there for, probe encountered the planet so Concerns were voiced over the Galileo. I’m not a very emotional it could data from the probe possibility of accidents that could person but I actually had tears in back to Earth. e orbiter began result in plutonium contamination my eyes when it got off the ground. circling Jupiter on December 7, 1995, of the space coast and other areas. It was just the excitement, the thrill, becoming the first spacecraft to orbit

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an outer planet. On the same day, the probe, running on its life-limited 580-watt-hour batteries, entered the Jovian atmosphere to take direct measurements for the first time. For the next 22 months, the orbiter traveled between several moons, including those discovered by Galileo Galilei (, , , and ), using their gravity to set its course and complete its primary mission. Although it was the fifth mission to observe Jupiter, the Galileo spacecraft was the first to enter orbit around the planet, and it changed mankind’s understanding of Jupiter, its moons, and the outer solar system.

At the end of its planned eight-year (71,000-hour) mission in December 1997, the two GPHS-RTGs were still providing 482 We, well beyond the mission required 470 We. Even after the launch delay and the extended VEEGA route to Jupiter, the GPHS- RTGs produced sufficient power to Artist’s conception of the Galileo spacecraft. The GPHS-RTGs are located at the end enable NASA to extend the mission of the booms on the back of the craft. (Photo: NASA.gov) by several years, enabling the craft to ultimately complete 35 orbits of Jupiter. In February 2003, with the Although it was the fifth mission to observe spacecraft running out of propellant, Jupiter, the Galileo spacecraft was the first to the flight team extracted the last scientific data from Galileo and sent enter orbit around the planet, and it changed the spacecraft into a collision with Jupiter on September 21, 2003. e mankind’s understanding of Jupiter, its moons, planned collision eliminated the and the outer solar system. possibility of a crash on Europa that could have left contamination on the moon, which was understood to host an ocean important to the study of .1, 10

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Ulysses’ Solar Odyssey

e launch of Ulysses went as smoothly as that of Galileo. e spacecraft, along with its GPHS-RTG, was launched on October 6, 1990, aboard the (STS 41). A two-stage inertial upper stage and a booster sent the spacecraft out of Earth’s orbit on a direct 16-month trip to Jupiter, where the gravity of Jupiter was then used to shift the spacecraft trajectory out of the ecliptic plane of the planets and over the poles of the sun.10

Ulysses studied the solar , a steady stream of particles ejected from the sun that produces a in interstellar space called the . Ulysses provided the first map of the heliosphere from the solar equator to a maximum latitudinal inclination of approximately 80 degrees (relative to the North and South Poles) during 1994 and 1995. e mission also improved the understanding of sunspot behavior, solar flares, solar x-rays, solar radio noise and plasma waves, the sun’s magnetic field, cosmic rays, and both interstellar and interplanetary gas and dust. Having successfully accomplished its primary mission, Ulysses continued to operate on its second orbit of the sun to study the solar The —carrying the Galileo orbiter and atmospheric probe— lifts o from KSC. (Photo: NASA.gov)

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wind during the sun’s maximum at the Jet Propulsion Laboratory your RTG is going to spend 100 of solar activity cycle. (JPL) during preparations for the its 300 watts… just to supply those Galileo and Ulysses missions, heaters. So if you put the RHU At the end of its planned five-year points out the benefit: “… [B]y there, that solves all the wiring (42,000-hour) mission in August using these RHUs just at certain difficulties. It… gives you an extra 1995, the GPHS-RTG was still places where they are needed… 100 watts saving on the RTG power providing 248 We, slightly above it completely saves having to run that doesn’t have to be used… for the mission required 245 We. With electric cables out, separate electric those little electrical heaters to heat the ongoing power provided by cables out to each one of these up those units.”23 the RTG, the Ulysses mission was things, just to have an electric able to be extended several times. heater there. And then you are First used in the mission Ulysses started its third south using the power supplied by the in a configuration that produced 15 polar pass in November 2006, and RTG to run all the way out there, watts of thermal power, a smaller completed its third north polar to supply these instruments with one-watt version was developed for pass in March 2009. On June 30, heater power. So it’s more draining use in the of the 2009, after 18 years of successful on how much electrical power 1970s. e one-watt Pioneer RHU operation, electronic equipment you’re going to be able to use for was subsequently modified for use problems and negligible thruster the rest of the spacecraft. It’s about on the Voyager 1 and Voyager 2 fuel precluded further operation 100 RHUs. at’s a demand of 100 spacecraft launched in 1977. Fueled of the spacecraft, resulting in watts total. But now all of a sudden and encapsulated at Mound, the termination of the Ulysses mission.

Radioisotope Heater Units

As the Galileo and Ulysses missions came to an end, this chapter would not be complete without mentioning one of the simplest but most unsung heroes of space nuclear technology: RHUs. RHUs are used to keep sensitive instruments at desired operating temperature without using electrical power and without producing the electromagnetic interference produced by electrical heaters. Bob Campbell, who worked

Lightweight RHU components. (Photo: ORNL)

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RHU produced its one watt of lower RHU for use on the inches [32 millimeters versus thermal power from approximately Galileo orbiter and probe. e new 47 millimeters]) but somewhat 0.01 pounds (2.7 grams) of heater unit, called a light-weight larger in diameter (1 inch versus plutonium oxide fuel. Similar to (LWRHU), 0.9 inches [26 millimeters versus RTGs, the RHU was designed with produced one watt of thermal 22 millimeters]) than the RHU. several protective layers and other power from approximately 0.01 e new LWRHU was designed safety features to contain and/or pounds (2.7 grams) of plutonium with several safety-related features, immobilize the plutonium oxide oxide fuel similar to the GPHS- including a ceramic plutonium fuel during accidents.24 RTG. e LWRHU, however, was oxide fuel pellet; - (0.08 pounds versus 0.1 alloy cladding with a frit vent in In 1978, LANL began development pounds [40 grams versus 57 grams]) one end to allow the release of of an improved version of the and shorter (1.3 inches vesus 1.8 helium from the natural decay of

An artist’s impression of the Ulysses spacecraft at Jupiter. (Image: NASA/European Space Agency)

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the plutonium (whereas the RHU similar in size to a modern-day capability of the new lightweight was designed to contain helium); a C-cell battery.25 heater unit was evaluated in pyrolytic graphite thermal safety tests similar to those for the to keep the clad from melting e LWRHU development effort GPHS-RTG. e tests included in case of re-entry; and a FWPF originally planned for design, testing, overpressures, impact by aluminum aeroshell for re-entry protection production, and delivery to be alloy bullets, extended exposure (an improved material relative to completed in 14 months; however, to burning solid-fuel propellant, that used in the RHU). Completely the postponement of the Galileo impact on a hard surface in various assembled, the LWRHU was a launch enabled development of orientations at velocities greater than cylinder approximately 1.25 inches a design with an increased safety terminal, and immersion for nearly (3 centimeters) long and one inch margin and allowed for additional two years at a pressure equivalent to (2.5 centimeters) in diameter, testing. e fuel 6,000 meters of seawater.25 Following the development and testing effort, assembly of 134 LWRHUs for the “ When you look at the photos from the Galileo and Galileo mission was completed at LANL in 1985; however, only Ulysses missions, people marvel at the pictures 120 of the units were used on the spacecraft, with the remainder being and the science… And you realize, you know, that set aside as spares. couldn’t have happened if it wasn’t for these RTGs, Looking Back and usually the RTGs aren’t even mentioned…. e success of the GPHS-RTG But those who are in the community, we know, and LWRHU on the Galileo and Ulysses spacecraft closed another and… there’s just an awful lot of pride… when you chapter in a long history of the think about how you played a small part in these quiet technology. e GPHS-RTGs extended mission life and provided missions’ successes and advancing science...that the power necessary to transmit you helped make these missions successful, and countless pictures, data, and other information that enhanced the I think that’s why I and others just have a warm understanding of our solar system. e LWRHUs provided heat that place in our heart for these programs. Let’s face kept scientific instruments and other it, space exploration is still sexy and it will be for a electronics warm (and functioning) in the coldness of deep space. And in long time, I think.” both cases, with the GPHS-RTG and LWRHU, DOE had developed units –Jim Turi, DOE that would power and heat NASA missions for decades to come.

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