The objective of this article is to describe the design concepts of and the programs to develop our nuclear rocket propulsion capability for spaceflight and the nuclear ramjet systems intended for missile delivery and to describe the mission capabilities they could provide. In both systems, the nuclear reactor replaces the combustion chamber as the source of high-temperature energy used to provide the thrust in place of their chemical alternatives. The nuclear rocket was, and still is, considered for deep space missions, including missions, such as human flight to Mars, whereas the stimulus for the nuclear ramjet was its potential performance as a high-speed, almost limitless range, low-altitude and, therefore, almost undetectable missile delivery system. In the early phases of research on the nuclear rocket, it too had been considered as a possible missile system.
A drawing of the nuclear rocket propulsion system is shown in Fig. 1, including identification of the major components of the system. In this system, low molecular weight hydrogen is heated to high temperatures in the nuclear fission reactor to produce a specific impulse that is about twice (900 seconds compared to the 452 seconds of the hydrogen-fueled combustion Space Shuttle Main Engine) and could eventually be even much greater than the level of the best chemical rocket systems. This system also required the advanced development of large capacity hydrogen turbopump systems and also hydrogen-cooled jet nozzles that operate at the high temperatures of the jet exhaust hydrogen gas; this technology was not yet available in the early phases of the program. In the nuclear ramjet, which is shown with its major components in Fig. 2, the air flowing through the system, as a result of velocity imparted to the vehicle by a separate launch system, is also heated to high temperature in the nuclear reactor. No propellant fuel must be supplied and carried, so it could have a very long range. However, the inlet system must be designed to reduce pressure losses efficiently through the inlet shock waves and to reduce the air velocity through its inlet diffuser from its supersonic entry to subsonic levels so that the air can be heated in the reactor core and then accelerated efficiently in the jet nozzle. Those systems and, of course, high-temperature reactors were not yet proven when the program started. However, analysis and technology development was underway in various laboratories on the reactors, pumps, diffusers, and nozzles for rocket and ramjet systems.
Nuclear rocket engine.
Figure 1. Nuclear rocket engine.
Nuclear ramjet propulsion system.
Figure 2. Nuclear ramjet propulsion system.
Of course, in both systems, the hardware weight of the entire system must be limited so that the various mission benefits resulting from the nuclear energy source are not counterbalanced by that total weight. Major design and development requirements include selecting materials that have high-temperature strength and are compatible with hydrogen propellant in the rocket and air in the ramjet, appropriate neutronic characteristics, controlled and stable start-up and operation, component design and reliability, and are safe. All of these matters must be and were comprehensively researched and developed to provide systems that could achieve the desired performance. These were all high priority tasks in both the nuclear rocket and nuclear ramjet programs, and they are discussed in this section, as are the important management organizations and technical capabilities established to carry them out.
Though significant progress was made in these programs, neither the nuclear rocket nor the nuclear ramjet program was carried to actual application in its proposed and anticipated missions. In the nuclear rocket case, the space program has not yet committed to mission objectives that require the high payload and deep space capability of that system. However, nuclear rocket propulsion continues to be recognized as necessary for large, deep space missions. In the nuclear ramjet case, existing chemical rocket systems provide all of the military missile requirements that are foreseen. Similar to the nuclear ramjet development, the extensive work that had been done on nuclear-powered turbojet engines, that is mentioned in this section, was also discontinued because chemically fueled turbojets provide all of the capability that was required.

Origins of The Nuclear Rocket and Ramjet Programs

The twentieth century produced an outstanding array of scientific and technological developments that have had major effects on our planet, on the economies of the various nations, on national security, and on individual lives around the world. Several of them combined to provide the basis for establishing the theoretical and experimental background and the broad capabilities required to achieve the developments and missions discussed throughout this topic and, very specifically, the flight propulsion developments in this section. Among these scientific and technological innovations that were the originating stimulus and foundation for the concepts of nuclear rocket and ramjet propulsion were Orville and Wilber Wright’s first powered aircraft flights on 17 December 1903 at Kitty Hawk, North Carolina (1); Robert Goddard’s concepts and development of his first rocket flight on 16 March 1926 and his continuing developments and flights beyond that (2), and then, of course, the discovery of the nuclear fission process in the late 1930s and the demonstration of the first controlled nuclear chain reaction on 2 December 1944 under the grandstands of Stagg Field Stadium at the University of Chicago that ”marked the birth of the nuclear age” (3,4). Certainly, these achievements were the stimulus for applying nuclear energy to flight systems.
But it must also be recognized that many of the basic concepts that led to these developments were originally theorized and suggested and, in some cases, even purely imagined by diverse individuals and organizations throughout the United States and in many other nations throughout the world. The continuing research and development of these and many other individuals and organizations in and from various nations generally led to achieve the spaceflight capabilities described here. For example, as P.E. Cleator (5) points out in his 1936 topic entitled Rockets Through Space – The Dawn of Interplanetary Travel, some were even imaginatively fictional space propulsion conceptions that encouraged suggestions of advanced concepts to overcome Earth’s gravity—such as Jules Verne’s shot into space by a huge cannon and H.G. Wells’ ”conveniently discovered” substance he called Cavorite ”to which the earth was definitely repellant.” Both of these citations also indicate the long fascination with travel in space. P.E. Cleator suggested some of these very advanced and imagined space propulsion concepts, including improved fuels and the use of solar energy and, even before Stagg Field’s chain reaction, ”atomic energy” recognizing ”that matter contains tremendous stores of potential energy, if only we knew how to release and use it.” He used the 10-fold increase in energy released by the disintegration of radium per gram compared to that of coal as the basis for his ”fanciful and remote— possibility of utilizing atomic energy.” In 1943, Robert H. Goddard speculated on the use of ”inter-atomic energy” so a ”large body could be sent from the solar system—after the problem of atomic disintegration has been solved” (6). Many others among the early scientists and those who were involved in early nuclear physics research perceived realistic possibilities of atomic energy.
The fundamental discoveries and technological achievements within the first half of the twentieth century stimulated advancements that led to the development of turbojet aircraft engines, to extensive work on nuclear aircraft propulsion, to chemical rocket and air breathing missile development, and to various nuclear-powered propulsion concepts, including the nuclear rocket and ramjet systems discussed here. And, of course, from the objective of achieving deep spaceflight, a host of other innovative propulsion energy concepts were also identified, including ion propulsion, plasma propulsion, gas core and particle bed nuclear rocket reactors, nuclear bomb propulsion (the Orion Project), solar sails, and antimatter propulsion; some of them are still being explored today.
It must also be recognized that in the 1940s and 1950s, we were at war. In its aftermath, various international antagonisms, crises, and strong competition emphasized and stimulated the need to demonstrate visibly our international technological leadership to ensure national security. Certainly, the Soviet Union’s launch of its Sputnik I in October, 1957 contributed greatly to this need. President Eisenhower referred to that event when he said, ”whatever our hopes may be for the future—for reducing this threat or living with it—there is no escaping either the gravity or the totality of its challenge to our security and survival—a challenge that confronts us in unaccustomed ways in every sphere of human activity.” Then on 25 May 1961, only a month after Yuri Gagarin’s first manned orbital flight, President John F. Kennedy delivered his famous ”Special Message—on Urgent National Needs” (7) to a joint session of the Congress. In that message, he proposed ”that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the moon and returning him safely to the earth.” His second space proposal was to ”accelerate development of the Rover nuclear rocket” which ”gives promise of some day providing a means for even more exciting and ambitious exploration of space, perhaps beyond the moon, perhaps to the very end of the solar system itself.”

Existing Capabilities in the 1950s

Achieving and ensuring such internationally visible technological strength and preeminence required the active involvement of many organizations that could provide sound mission analysis and research and technology development in areas of basic technical and scientific knowledge, such as those mentioned before, to achieve the planned goals. Fortunately, many of these capabilities already existed in the 1950s. They included leadership, support, and technical and management capabilities of the various military organizations, including the U.S. Army, the Navy, and the Air Force (USAF) that depended on aircraft systems and missiles to fulfill its national security responsibilities; the National Advisory Committee for Aeronautics (NACA) which the 1915 Naval Appropriations Bill had created to conduct research on aircraft systems for ”the scientific study of the problems of flight, with a view to their practical solution” (8); NACA became the foundation of the National Aeronautics and Space Administration (NASA) when that organization was established on 1 October 1958 in the U.S. response to the Soviet Union’s demonstration of its space capability, and the U.S. Atomic Energy Commission (AEC) which was created under the Atomic Energy Act of 1946 (9) to provide civilian control over the development of nuclear energy, including military as well as peaceful civilian applications. President Truman’s Executive Order transferred control of the nuclear weapons development program from the Army to the AEC on 1 January 1947. The AEC later became part of the Energy Research and Development Administration and then the Department of Energy.
Progress in developing nuclear rocket and ramjet propulsion was the result of research, design analysis, and technology development led and carried out by technical organizations within these various government agencies, generally working collaboratively among themselves but also working very closely with private companies and with universities. The government laboratories included some of those that had previously been established to develop our first atomic bombs, such as the AEC’s Los Alamos Scientific Laboratory, the Lawrence Liv-ermore Radiation Laboratory, the Oak Ridge National Laboratory, and the
Argonne National Laboratory. In addition, the NACA/NASA laboratories, such as the Langley and Ames Research Centers, had been heavily involved in high-speed aircraft research; the Dryden Flight Research Center had been doing high-speed aircraft flight testing; and the Lewis Research Center (recently renamed the John H. Glenn Research Center) had been working on propulsion systems, including turbojet, rocket, and ramjet engine research and the hydrogen components that would go with advanced rocket engines, well before the space program was formally initiated (10). That Laboratory had originally been named the NACA Aircraft Engine Research Laboratory when George W. Lewis broke ground for it on 23 January 1941 (8). The Wright-Patterson Air Force Base was involved early in military aircraft development. The work of these various research and development organizations was generally conducted in close collaboration with the development and engineering capabilities of many companies throughout the nation. Among these were the Aerojet General Corporation; Westinghouse Electric Corporation; Rocketdyne Division and Atomics International of North American-Rockwell Corporation; General Electric Company; Glenn L. Martin Company which later became part of Lockheed Martin; Pratt & Whitney Aircraft which later became part of the United Technology Corporation; Thiokol; the DuPont Company; Edgerton, Germeshausen, and Greer; General Atomics; Mar-quardt; Chance-Vought and many other companies and government and university laboratories. The collaborative efforts and capabilities of these various organizations—government, industry, and universities—have made advanced deep space missions realistically achievable when decisions are made to commit to them. Several of those companies also continued to play a major role in developing and providing our U.S. aircraft and missile system capability.

Management Arrangements for Nuclear Flight System Programs

As a result of the interests of the USAF and its discussions with Dr. Vannevar Bush of the Office of Scientific Research and Development, as well as with Major General Leslie Groves, who headed the Manhattan Project’s atomic bomb development, studies of the feasibility of nuclear-powered propulsion for aircraft were undertaken (11). These led the Pentagon to establish, in 1950, an Aircraft Nuclear Propulsion Program (ANP), which was conducted as a joint effort of the AEC and the Air Force. About a year later, an Aircraft Nuclear Propulsion Office (ANPO) was organized within the AEC staffed jointly by the AEC and the Army Air Corps with Major General Donald J. Keirn as its Director. Brigadier General Irving L. Branch succeeded General Keirn in 1959. Consistent with the Atomic Energy Act provisions and intent, the AEC provided the nuclear phases of the work and the USAF was responsible for the nonnuclear and aircraft aspects, but all were managed by this new, single, joint organization. At that time, the objective of the program was raised from evaluating the feasibility of nuclear-powered aircraft flight to include actual demonstration of nuclear-powered flight. General Electric and Pratt & Whitney (which was working with the AEC’s Oak Ridge National Laboratory) were under contract to conduct research and development on direct- and on indirect-cycle concepts of nuclear aircraft propulsion, respectively, for the ANPO.
During that period, in the first half of the 1950s, Los Alamos and Livermore were also involved in conducting work on nuclear rocket propulsion using non-project research funds available to them (12, pp. 3-6). In response to an inquiry from the AEC in September 1955, the Department of Defense (DOD) ”indicated an intense interest in this program” but on the basis that it not interfere with the Los Alamos and Livermore nuclear weapons programs. In essence, that was interpreted by the ANPO to mean proceeding with a ground test and development program ”to determine the feasibility by ground operation of a nuclear rocket engine by about 1959, looking forward to a possible flight in (deleted) if this appeared desirable.” A formally designated nuclear rocket propulsion program was then established under the management of the ANPO, and budget funding was requested. However, in 1956, Secretary Wilson of the DOD sent a letter to Chairman Strauss of the AEC modifying that requirement and removing the sense of urgency that had been previously indicated by asking the AEC to ”continue on a moderate scale to develop a reactor suitable for nuclear propulsion of missiles, satellites, and the like” (12).
As a result, ”funds were reduced and a decision was made to go to a single laboratory approach” for the nuclear rocket propulsion program. Los Alamos continued its work on the nuclear rocket program, whereas Livermore dropped its nuclear rocket work and, in January 1957 (12), was assigned work on the nuclear ramjet program called Project Pluto. Both of these programs are the major subjects of this article. The Rover name that Livermore had used for its nuclear rocket work replaced the Los Alamos Condor designation for the nuclear rocket program. The ANPO provided the overall management of both the nuclear rocket and the ramjet projects and, also, had responsibility for developing radioisotope and reactor space power systems under its Missile Projects Branch.
While these actions were being taken by the AEC, the Navy, Army, and Air Corps were conducting major activities to develop advanced missiles and research on using advanced fuels (13). At the same time, during the 1940s and 1950s, the NACA laboratories were continuing their research, mentioned before, to achieve high-speed aircraft flight, including vehicle aerodynamic advancement and advanced propulsion systems work that involved work on turbojet, rocket, and ramjet systems at the Lewis Research Laboratory. That work was regularly reported and presented to broad government and industry audiences (10) in classified (since then declassified) conferences during the 1940s and 1950s, well before NASAwas established, and conferences continued after NASAwas formed.
When NASA was formally activated on 1 October 1958, responsibility for the nuclear rocket’s nonnuclear components, subsystems, and integrating them with the AEC nuclear reactor into the engine system and in flight vehicle development was transferred by Executive Order from the USAF to NASA (11). NASA staff was assigned to work with the ANPO, Los Alamos, and other participants in the program to fulfill NASA’s responsibility. Recognizing the AEC’s legislatively assigned responsibility for nuclear reactor systems research and development, it was proposed soon after NASA was established, that a joint office of the AEC and NASA should be formed that would carry out the Rover Program’s reactor and nonnuclear component development and their application to engine systems. However, as is indicated in his diary (14) ”The Birth of NASA,” it took Dr. T. Keith Glennan’s—NASA’s first Administrator—persistent effort and great patience to establish the joint office. In all such AEC organization as well as program and budget arrangements, the Joint Committee on Atomic Energy (JCAE), which was established by the Congress in 1947 to provide congressional oversight of all AEC activities, was very actively involved and played powerful and influential roles. Dr. Glennan had experience with the AEC and JCAE because he had served as one of the five Commissioners from 1950 to 1952 (15), and in 1955, he began serving on a committee under the JCAE direction (14).
The major obstacle in establishing the joint NASA and AEC office related to selecting the person to head that new joint office that would replace the ANPO as manager of the nuclear rocket program. Members of the JCAE including the powerful Senator Clinton P. Anderson (14) and some others (16) favored the appointment of Air Force Colonel Jack L. Armstrong. He was then the Deputy Chief of the ANPO and was well known to the JCAE. NASA favored Harold B.
Finger, the author of this paper, who had been a research engineer at the NACA Lewis Research Center starting in May 1944, and was working in the NASA Headquarters on nuclear systems; he was the key person who had been assigned as the link with the AEC-Air Force ANPO on the nuclear rocket program when NASA was established.
Finally, Dr. Glennan and the AEC Chairman John A. McCone signed a space nuclear rocket Memorandum of Understanding on 26 August 1960 and 29 August, respectively (17), and on 31 August 1960, the joint office was publicly announced by the two agencies. The author was named Manager of the AEC-NASA Nuclear Propulsion Office, and Mr. Milton Klein, who had been Assistant Manager for Technical Operations of the AEC’s Chicago Operations Office, was designated as his Deputy. In that Memorandum of Understanding and in the public announcement, the responsibilities of the two agencies were clearly defined. The AEC had primary responsibility ”for conducting research and development on all types of nuclear reactors and reactor components, including those required for aeronautical and space missions specified by NASA,” and NASA had ”primary responsibility for research and development on nonnuclear components and integration of the nuclear components in engines and vehicles of rocket systems.” At that point, the nuclear rocket responsibility was transferred from the ANPO to the new AEC-NASA joint office, and key people, including two USAF officers, were transferred to the new office. Therefore, experienced people from the Air Force, the AEC, and NASA staffed the office.
That joint office was very soon more descriptively renamed the AEC-NASA Space Nuclear Propulsion Office (SNPO). That clearly emphasized the objective of its work and distinguished it from the original consideration of potential ICBM applications that had stimulated the Air Force’s original interest in nuclear rocket propulsion but was canceled by the Defense Department in 1956. The resulting AEC and NASA organizational structure is shown in Fig. 3. The Nevada, Albuquerque, and Cleveland Extensions shown on the chart were established by the SNPO to provide its management support for on-site activities and, in Cleveland, for managing its engine contract work. The Cleveland Extension drew on the technical support of the Lewis Lab because of its experience and continuing work on hydrogen pumps, nozzles, and other components needed for the nuclear rocket engine as well as reactor support. In 1965, the author was also named the AEC’s Director, Space Nuclear Systems Division to head the AEC’s space nuclear power systems development.
Organization—nuclear activities.
Figure 3. Organization—nuclear activities.

Mission Performance Benefits of Nuclear Rockets

Although the nuclear rocket program was started by the Air Force as a possible approach to strengthening its potential missile capabilities, it was also considered very early by those identified before as a possible space launch system. Important mission benefits were and still are anticipated from nuclear rocket propulsion systems compared to chemical rockets, if nuclear systems could be satisfactorily developed. For example, the statement of Dr. Hugh L. Dryden, Deputy Administrator of NASA (18), on Project Rover at the 27 February 1961 Hearings of the House of Representatives Committee on Science and Astronautics emphasized this point. “We in the NASA are particularly aware of the large advantages that nuclear energy offers in our space program.—Our evaluations have made it clear that the space program will require the application of nuclear energy sources in order to provide the large amounts of energy that are required to move about freely in space.—We are confident that—probably the first means will be chemical, but when you get to transporting large amounts of material, nuclear energy is essential—manned exploration of the moon and the planets rests on the mission capabilities afforded by nuclear propulsion systems. For this reason, we consider the development of nuclear propulsion systems as our major advanced propulsion development program.” And, on 28 August 1961, shortly after he was appointed NASA’s second Administrator, Mr. James E. Webb stated in testimony before the Joint Committee on Atomic Energy (19), “We look to the nuclear rocket primarily for application to missions beyond the first manned lunar expeditions; for providing the heavy payloads that may one day be required to support lunar bases and for manned exploration of the planets. Nuclear energy is essential for such missions.” That is still the case.
Even before Wernher von Braun and his rocket team in the Army Ballistic Missile Agency were being phased into NASA, starting in November 1959 and culminating in March 1960 when they became the NASA Marshall Space Flight Center, they had already been deeply involved in developing the large Saturn launch vehicle which various NASA analyses (20) had indicated was necessary for its eventual space missions, whereas the Department of Defense Advanced Research Projects Agency (ARPA) could not anticipate missions that would require such a large launch vehicle. During that time, the Manager of the SNPO emphasized to members of the von Braun team and in meetings, including the Administrator, Deputy Administrator, and others in NASA Headquarters, that the Saturn vehicle should be made as large in diameter as possible in order to ensure that it could accommodate nuclear-powered upper stages containing the large volume hydrogen tanks that would be required. As a result, the diameter was set as large as was permitted by the hook height in the vehicle assembly building at the Marshall Center. NASA emphasized (19) that its chemical rockets were being designed so that nuclear upper stages could be applied to them to increase ”payloads for various missions including lunar missions.”
Figure 4 (18) presents the increase in escape payload for one of the early Saturn vehicle concepts by using a nuclear third stage on the first two stages of the Saturn C-2 configuration, compared to a three- and a four-stage chemical Saturn C-2. (The Saturn C-2 was one of several configurations being examined at the time this figure was originally presented.) The upper level of the shaded Saturn line is for the four-stage chemical vehicle, and the lower level is for the three-stage vehicle. Even at a reactor thermal power of 1000 megawatts, which would provide about 50,000 pounds of thrust, the nuclear system would deliver an escape payload of twice the all-chemical system. As the reactor power and resulting thrust increase, that increase in payload grows significantly so that at 4000 Mw, the escape payload of the nuclear system is three times that of the all-chemical Saturn.
Nuclear stage increases Saturn escape payload.
Figure 4. Nuclear stage increases Saturn escape payload.
Figure 5 compares the payload that could be delivered in various unmanned solar system missions with a chemical or a nuclear third stage on the fist two stages of the large Saturn V vehicle which served as the first stages of the launch system for the Apollo mission. The nuclear third stage vehicle provides at least twice the all-chemical system payload. The benefits of nuclear propulsion are far greater for human missions. To accomplish a human round trip mission to Mars, the total weight required in Earth orbit with a nuclear powered vehicle is less than half that of the best chemical rocket in the most favorable Mars-Earth planetary alignment with a significant reduction in trip time. That advantage of nuclear propulsion increases substantially at other less opportune times since the Earth orbital weight required for the chemically propelled vehicle’s Mars mission accomplishment increases significantly and rapidly compared to the nuclear propelled mission.
Unmanned solar system missions—application of nuclear third stage to Saturn V.
Figure 5. Unmanned solar system missions—application of nuclear third stage to Saturn V.
These performance benefits can be summed up as they were reemphasized in the 1991 report (21) of a Synthesis Group on America’s Space Exploration Initiative chaired by Thomas P. Stafford, which pointed out the important role of nuclear thermal rocket propulsion in space exploration and singled out Mars exploration:
Nuclear thermal propulsion has approximately twice the performance of chemical rockets, with reduced propellant requirements. This leads to reduced mass in low Earth orbit, faster trip times (increasing crew safety) and increased launch windows.

The Nuclear Rocket Rover Program

Formally designated on 30 August 1961, the SNPO initiated major program actions, and the Los Alamos Scientific Laboratory (LASL) continued its responsibility and effort for the nuclear rocket reactor program aimed at developing and testing its nuclear reactors in the facilities that had been built under Los Alamos direction at our Nuclear Rocket Development Station (NRDS) in the Jackass Flats area of the AEC’s Nuclear Test Station in Nevada. Those early reactors were named after the flightless KIWI bird. When the SNPO was established, Los Alamos had already tested two reactors, the KIWI-A on 1 July 1959 and the KIWI-A’ on 8 July 1960 (both discussed later).
The early SNPO and NASA program actions are listed in Fig. 6. They indicate the broad scope of the program and the goals of full nuclear rocket engine and flight vehicle development aimed at flight testing. RIFT was the Reactor In-Flight Test rocket stage; NERVA was the Nuclear Engine for Rocket Vehicle Application; the MAD Building was the engine Maintenance and
1. 31 August 1960: AEC-NASA Space Nuclear Propulsion Office established.
2. September 1960: Contracts with Convair, Douglas, Lockheed,
Martin for flight testing nuclear rockets (RIFT).
3. December 1960: Contract with Parsons team on master plan for required nuclear rocket engine development facilities.
4. February 1961: Issued RFP for NERVA contractor.
Proposals due April 3.
5. 7 June 1961: Aerojet General-Westinghouse Team selected for
NERVA contract. 10 July 1961: NERVA contract signed.
6. July 1961: RIFT studies extended.
7. August 1961: Contract with Vitro to design Engine MAD Building.
Construction started in 1962.
8. 11 July 1962: RIFT development contract awarded to Lockheed.
Figure 6. Early Space Nuclear Propulsion Office Program actions.
Disassembly Building that was required, so that the system disassembly and any modifications after ground testing could be done using shielded remote manipulator operation. The AEC and Los Alamos had already constructed a reactor MAD building, and it had already been used to disassemble the KIWI-A and A’ reactors after Los Alamos had tested them.
The Reactor and Engine Development Test Program. The sequence of the resulting reactor and engine development testing activities is shown in Fig. 7, and the operating time and power level are given in Fig. 8. All of these tests were conducted at the NRDS in Jackass Flats which included two reactor test facilities, an engine test stand equipped to permit test operation under space vacuum conditions, and a reactor and an engine maintenance and disassembly building.
As pointed out in the Introduction of this section of the topic, a major issue involved in designing the nuclear rocket reactor was and is the choice of materials to withstand the high temperatures required and to ensure compatibility with the hydrogen propellant and among the various other materials in the system. In addition, the neutron capture cross-section of the materials becomes a significant property in determining the type of reactor and fissionable fuel loading required. Numerous analyses and tests have been conducted on various potentially suitable materials for rocket reactor design by various participants involved in research and analysis of nuclear rocket propulsion. Among them are the work of R.W. Bussard (22) of LASL, Frank E. Rom (23) of the Lewis Research Center, and Donald P. MacMillan (24) of LASL. Among the materials considered have been graphite as both the fissioning fuel element and neutron-moderating material, tungsten, molybdenum and others as well as various carbides for the fuel material with beryllium or beryllium oxide for moderator and reflector materials. The Argonne National Laboratory became involved in research on tungsten-fueled fast neutron reactor concepts, and the Lewis Research Center worked with reactor systems using tungsten fuel elements and water moderation systems that required the enriched tungsten-184 isotope to reduce the high neutron capture cross-section of natural tungsten (23).
NERVA/Rover reactor system test sequence.
Figure 7. NERVA/Rover reactor system test sequence.

Date Test Article NRDS
Maximum Time at Maximum Power
1 July 1959 8 July 1960 10 October 1960 7 December 1961
I September 1962 30 November 1962 13 May 1964
28 July 1964
10 September 1964
24 September 1964 15 October 1964
23 April 1965
20 May 1965
28 May 1965
25 June 1965
3, 16, 23 March 1966 23 June 1966 23 February 1967 13 December 1967
26 June 1968
3 – 4 December 1968
II June 1969
29 June – 27 July 1972
Chronology of Rover and NERVA reactor/engine tests. A A A A A A
Chronology of Rover and NERVA reactor/engine tests. Chronology of Rover and NERVA reactor/engine tests.

Figure 8. Chronology of Rover and NERVA reactor/engine tests.
After considering the various high-temperature materials alternatives in its extensive early research and development work that started in 1955, LASL selected graphite in spite of the fact that, unless effective protective coatings were found, the hydrogen reactor coolant and rocket propellant would severely corrode the graphite and form methane or acetylene at the high operating temperatures required for successful nuclear rocket operation. Materials research including such coatings and protective concepts were an important part of the KIWI and follow-on Rover research and development program. The graphite reactor systems were the principal reactor development objective throughout the Rover program, including both the Los Alamos KIWI reactors and the Westing-house reactors for the NERVA rocket engine, which was based heavily on the Los Alamos KIWI work, and for the later, higher power LASL Phoebus reactor designs. Although work continued on other reactor concepts, including the tungsten fuel element systems mentioned before and also gas core reactor work, the major emphasis through the entire program was on the graphite reactor development effort conducted by Los Alamos and then the graphite reactor-engine system work by Aerojet-General and Westinghouse. The ultimate outcome of that work was successful achievement of all of the objectives that had been set. That put this country in a solid position to undertake flight system development and mission accomplishment with a high degree of confidence when required missions are defined. However, such missions have not yet been defined. The KIWI-A Reactors. All of the KIWI-A series reactor tests (A, A0, and A3) listed in Figs. 7 and 8 were aimed at about 100 MW thermal power levels. Work directed toward those test reactors had started in the mid-1950s. All of those tests were run with gaseous hydrogen stored in high-pressure tanks as the reactor coolant and used water-cooled Rocketdyne nozzles. Their purpose was primarily to check out basic elements of the reactor operations and design and to serve as facility checks. Though problems were encountered, including some that were anticipated, the Kiwi-A series demonstrated that high hydrogen temperatures could be produced in these heat transfer reactors with controlled start-up, stable operations, and shutdown.
Dr. Raemer E. Schreiber, Chief of N-Division in Los Alamos and responsible for the Rover Program, described the first test of the KIWI-A (12), as ”a test device for our own education in order to get us the first information on an integral system which has some of the characteristics which we are looking for in actual propulsion engines.—The relationship between this and a flyable device is pretty tenuous.” That KIWI-A reactor test system is shown in Fig. 9 on the railroad car moving it to Test Cell A from the reactor assembly building. All ofthe reactor tests were fired upward. The KIWI-A reactor system (25) consisted of uranium-235 (UO2)-loaded graphite fuel plates in an annular section around a central section containing heavy water (D2O) and the rods that controlled the fission process, including those that could scram the reactor in an emergency. The fuel element plates were 1/4 inch thick and were spaced 0.050 inches apart to provide for the hydrogen flow. This first test had no protective coating on the graphite fuel. Although part of the center island blew out at start-up, the test was run for about 5 minutes, limited by pressurized hydrogen capacity, up to a power level of 70 MW. Because no corrosion protection was used, it was not surprising that heavy corrosion occurred, but those results ”showed no disagreement with laboratory results” (25). However, in addition, most elements were cracked and part of the central island blew out through the nozzle during start-up.
The configurations of the second and third test reactor, the KIWI-A’ and the KIWI-A3, were very similar to each other but very different from KIWI-A. The
KIWI-A Reactor in transit to Test Cell A.
Figure 9. KIWI-A Reactor in transit to Test Cell A.
KIWI-A’ test was run on 8 July 1960 using a cylindrical fuel element configuration that began to approach the hexagonal fuel elements planned for future reactors. However, the KIWI-A’ fuel elements had four flow passages through each fuel element rather than the 19-hole hexagonal fuel elements in the later KIWI-B liquid hydrogen reactor test systems. The KIWI-A3, which was run on 19 October 1960, had seven flow passages (26) in each of its long cylindrical fuel elements. Both of these reactors had hydrogen corrosion protection applied through chemical vapor deposition of niobium carbide (NbC) on the fuel element flow passages. The KIWI-A’ test ran for about 7 minutes up to a power level of 85 MW. Seven of its fuel elements broke, and all others showed heavy cracking. The protective coating looked satisfactory except for some blistering. The KIWI-A3 test ran for about 5 minutes up to 100 MW. Remote disassembly and examination after the test indicated less corrosion than KIWI-A’: one element broken and significant fuel cracking. Though these various KIWI reactor tests indicated significant inadequacies, they also provided design, materials, structural, control, and operating data that were useful in the continuing development of the higher power reactors that were to follow.
The KIWI-B Reactors. Although the KIWI-A series was aimed at about 100 MW thermal power, the KIWI-B series (there were several designs being examined at that time) was designed for 1000 MW (27). The goal was achieving as high an operating temperature as possible through the development program and a high power density. The KIWI-B reactors were to serve as the basis for the NERVA engine reactors (25) to provide an engine thrust of50,000 pounds and the high specific impulse expected of nuclear rocket propulsion. Although much was learned from the KIWI-A series, the KIWI-B reactors were significantly different. The KIWI-B’ used a beryllium reflector, in which the control rods were located, around the periphery of the reactor core. After detailed examination of the different core designs, including the structural results of the KIWI-A test series and considering facility availability, it was decided first to test the KIWI-B1 with gaseous hydrogen at reduced power of 300 MW. The fuel elements in that reactor were similar to the seven flow-hole cylinders in KIWI-A3 and were located in a graphite matrix (26). The operating time and power of that test (Fig. 8) were limited by an emergency reactor scram and a fire caused by a leak in the seal between the nozzle and pressure vessel. In spite of serious concerns about the structural integrity of that design, another KIWI-B1 was run on 1 September 1962 using liquid hydrogen, primarily to obtain data on potential two-phase flow problems. In that test, the liquid-hydrogen-cooled nozzles and hydrogen pump developed by Rocketdyne were used. The core failed and started ejecting fuel out of the nozzle as it increased in power level to about 900 MW. It was clearly not a suitable design. However, the test did provide significant information. It laid to rest the various apprehensions about of two-phase hydrogen flow (27), the turbopump and nozzle in this first liquid hydrogen test operated well, and the control drums in the peripheral reflector effectively started up and controlled the reactor.
The next reactor to be tested was the KIWI-B4A design. This reactor was clearly the preferred design; it was expected to serve as the basis for the NERVA engine reactor design (28). It had the 19-hole hexagonal fuel element assembly configuration. Six of the uranium-fueled graphite fuel elements were placed around the unfueled graphite center support element, and the entire module was supported by a hot end graphite cluster support block and a steel rod running through that central support system that was tied to a cold inlet support plate (29). The coolant passages in the fuel elements and the exterior surfaces at the hot end were coated with niobium carbide (NbC) to protect the fuel elements from corrosion. However, the high expectations for that test were quickly dimmed when the test started on 30 November 1962 and flashes of light in the nozzle exhaust indicated reactor core damage as the power was increased over about 250 MW. After disassembly, it was found that almost all of the fuel elements had broken (27) as a result of severe vibrations in the entire core. Recovery from the KIWI-B4A Failure. In a meeting in the SNPO offices on 3 and 4 January 1963 with Dr. Norris Bradbury, the Director of Los Alamos; his deputy Dr. Raemer Schreiber, and some of their key people; the Deputy Manager and others from the SNPO; and representatives from NASA’s Langley and Lewis Research Centers and the Marshall Space Flight Center, the author, Manager of SNPO, expressed his decision that there would be no further hot testing of a full reactor until thorough work was done to identify the causes of the problems that had occurred and to develop well-defined solutions to those problems. The Los Alamos Director objected saying that the Manager would kill the program if there were not continued reactor testing. The author responded that, on the contrary, the program would be killed if reactors continued to have major test failures. Dr. Bradbury then said that Los Alamos could make a ”quick fix,” but he did not define any specific fix in that meeting, nor subsequent to it. Any undefined quick fix was rejected.
The decision prevailed and a comprehensive collaborative program to understand and solve the problems was developed among the parties involved. It included reactor design analysis and extensive testing by Los Alamos and West-inghouse (30), including component and subsystem and vibrational tests. The Manager also decided that a cold flow, nonfueled and, therefore, nonfissioning test of KIWI-B4A should be conducted with sufficient instrumentation to confirm vibrations as the cause of the failures and to identify their sources. That test, run on 15 May 1963, confirmed a faulty design feature that resulted in interstitial flows between fuel elements and induced vibration and failure. Los Alamos had placed a peripheral seal at the inlet of the core so that the low exit pressure in the periphery permitted the interstitial or interfuel element flow to expand the core outward and induce element vibration. Westinghouse had actually been concerned about the need to bundle the core to limit the interstitial flow corrosion effects, so they had set the seal in its NERVA reactors at the core exit and were working on increased lateral support. Using the exit seal, the peripheral pressure was inward and increased the core bundling. (30, p. 398). The cold flow test clearly proved that the vibrations in KIWI-B4A were flow induced (28).
Based on those results and extensive analysis and component and section or ”pie” testing at both Los Alamos and Westinghouse, changes in the peripheral seal and lateral support designs, similar to those that were being made in the KIWI and NERVA designs, were incorporated into a second cold flow test (KIWI-B4B-CF) in August 1963. No vibrations occurred in that cold flow reactor redesign. In October, based on those results, further supporting tests, and a comprehensive review conducted at Los Alamos, approval was given to go forward with the redesign and with building and hot testing the KIWI-B4D reactor.
During the second half of 1963, NASA and the AEC were busy preparing budget requests for FY 1965. The Commission strongly favored requesting funds for flight testing the nuclear rocket RIFT system, even though full resolution of the KIWI-B4A problems had not yet been conclusively demonstrated. In a meeting with the Commission, the author proposed instead a comprehensive ground-based development program that would establish a sound technical basis for eventual commitment to flight systems and space missions. Although the Commission Chairman, Dr. Glenn T. Seaborg, and the NASA Administrator, James E. Webb, proposed the flight-test program, the President rejected it in their budget discussions. A revised budget request submitted by the Administrator and the Chairman to the President to cover the ground-based program was approved as the basis for the FY 1965 nuclear rocket budget of both agencies. As a result, the RIFT stage development was cancelled, but the NERVA development and further advanced work by Los Alamos on the higher power Phoebus reactor system was continued following the completion of the KIWI reactor tests in 1964.
The KIWI-B4D reactor was operated first as a cold flow unit in February 1964 without a problem. In May, the KIWI-B4D hot test was run. However, the test was cut short as a result of a hydrogen leak and resulting fire at the nozzle at the system’s design power of 1000 MW. After disassembly, the reactor was in excellent shape and had no broken fuel elements. That test generated great euphoria throughout the program. That was followed by the KIWI-B4E tests in September, including the short restart (Fig. 10). Although some corrosion was apparent, the core held together, but it did have some broken fuel elements. NERVA and Phoebus Testing. That KIWI-B4E test completed the Los Alamos KIWI reactor work, which Westinghouse and Aerojet General then extended to the NERVA reactor (NRX), engine breadboard (EST), and the full engine system (XE) tests and development. Los Alamos moved on to the Phoebus reactor work aimed at achieving higher power and temperatures (Fig. 8). Those Aerojet-Westinghouse NERVA tests continued to pave the way for an operational nuclear rocket engine, and the Los Alamos Phoebus work (Fig. 11) provided the information for larger engines. On 13 December 1967, the NRX-A6 achieved the program operating target of 60 minutes at 1100 MW (Fig. 8). On 11 June 1969, the full XE engine, shown in Fig. 12 in the downfiring Engine Test Stand with the full turbopump, nozzle, and propellant tank, ran successfully at 1100 MW and went through 28 starts from December 1968 to August 1969. The Phoebus reactor tests achieved more than 4000 MW, equivalent to more than 200,000 pounds of thrust.
It is important to recognize that, in addition to overcoming the early structural problems discussed before, the tests conducted from 1964 through 1969 applied continued improvements in fuel material and protection from hydrogen corrosion (31). They all had fuel elements containing small pyrocarbon-coated uranium carbide spheres in the graphite matrix and used niobium or zirconium carbide to protect the carbon in the matrix from hydrogen corrosion. Work to evaluate further fuel element improvements was conducted by Los Alamos in the Nuclear Furnace fuel element tests, which ran for 109 minutes at high power densities and temperatures in a radiative environment. Those tests also included a scrubber to remove fission products from the exhaust. Conclusion of Nuclear Rocket Rover Program. In spite of the view in NASA that nuclear propulsion using the NERVA engine would be needed to resume lunar exploration in the 1980s and other advanced missions, its proposed FY 1972 budget continued the ”engine development at a minimum rate—due to fiscal constraints” (32). However, in the face of FY 1972 space program budget ceilings set by the Office of Management and Budget (OMB), NASA offered termination of the NERVA engine development program although it was ”the only large scale advanced propulsion system underway.” The program was killed. As pointed out in Reference 33, ”This decision ended a longstanding NASA policy of developing advanced engines well before there was need for them.”
KIWI-B4E at the test cell.
Figure 10. KIWI-B4E at the test cell.
Based on the progress that had been made in advancing U.S. nuclear rocket technology in the program, there is no question that these results indicate that technology is available that is suitable for application in high-payload, deep space missions when such missions are established as space objectives. However, such missions have not yet been defined in spite of continued interest over decades in ultimate human missions to Mars. The headline in the Wall Street Journal on 11 June 1964 after the successful KIWI-B4D test recognized that ”Nuclear Rocket’s Technical Problems Seem Solved, but Space Mission for it is Lacking.” T.A. Heppenheimer makes the same point in his recent topic ”The Space Shuttle Decision” (33) when, referring to the progress made after the KIWI-B4A reactor failure, he points out that ”The rapid pace of advances in Nevada contrasted painfully with the lack of plans in Washington. With NASA having no approved post-Apollo future, it was quite possible to anticipate a time when Aerojet might build a well-tested NERVA, ready for flight, only to find that NASA had no reason to use it.”
Phoebus 1B being moved to the test cell.
Figure 11. Phoebus 1B being moved to the test cell.
The XE Engine in the engine test stand for full engine test.
Figure 12. The XE Engine in the engine test stand for full engine test.
However, some analysis, testing, and reporting on the capabilities of nuclear rocket propulsion based on the advances provided by the NERVA and Phoebus work and other nuclear rocket concepts for deep space missions has continued. Especially as NASA continues to examine the possibility of Mars missions extending to human exploration, that past work continues to be currently relevant. Reference 34 is an excellent example of ongoing nuclear rocket activities and examination as are continued conferences conducted in Russia, discussed later. The so-called Tom Stafford report (21) further emphasizes this point indicating, ”Advanced nuclear propulsion techniques can shorten the transit time, provide flexible surface stay time, significantly reduce the propellant mass to low Earth orbit and increase the available launch opportunity.” The interest in Mars exploration is great, the ultimate need for nuclear propulsion for such human exploration is apparent, the technology is available, but the commitment to such missions is still remote today. A serious question is how the knowledge base established by the nuclear rocket program can be retained until the time when such a mission commitment is made.
Russian Nuclear Rocket Development Activities. As indicated before, Russia continues to hold conferences devoted to space nuclear systems. The first of these open conferences was held in 1990 in Obninsk. The tenth conference was held in 2000, again, in Obninsk. Those conferences have included participants from the major organizations and activities involved in space nuclear system development in the Commonwealth of Independent States (CIS) and the United States. As a follow-up to attendance at the Third Specialist Conference on ”Nuclear Power Engineering in Space: Nuclear Rocket Engines,” held in September 1992 in Semipalatinsk-21 in the Republic of Kazakhstan, the U.S. Department of Energy organized a team to visit the many Institutes in the CIS that have been involved with space nuclear power and propulsion research and development in addition to the Russian Ministry of Atomic Energy and the then relatively new Space Agency. That team was led by the Director of the Office of Space and Defense Power Systems in DOE and included representatives from the DOE and its laboratories, from the DOD including its Phillips Laboratory and its Strategic Defense Initiative Organization, and consultants that have experience in the areas being examined. The author was part of that team. A principal purpose of these visits was to familiarize the DOE and DOD with Russian capabilities that might suggest cooperative activities in areas considered important to the United States. Such a cooperative space effort had been suggested by the Minister of Atomic Energy. Much of the information concerning the Russian nuclear rocket programs presented here is drawn from the discussions of that team at the conference and in visits with the Institutes, which were fully reported in Reference 35. That report concludes that there are many areas where cooperative efforts could be beneficial; some joint work has been conducted.
The history of the Soviet work in nuclear energy was summarized at the start of the Conference by its Chairman, Dr. N.N. Ponomarev-Stepnoi of the Kurchatov Institute in Moscow. He pointed out that 4 years after controlled fission was achieved in Chicago, the Kurchatov Institute achieved its first fission in a still operating nuclear pile. Their first work was aimed at research and plutonium production, but later they went on to power reactors and then ship and aircraft propulsion. No nuclear aircraft were ever flown. In his comments on nuclear rocket propulsion safety, the Chairman emphasized his view that sub-criticality must be ensured before the rocket stage achieves its high operational orbit. This was similar to the view NASA expressed from the start of its role in Rover and was specified for the NERVA design.
Turning to development of nuclear rocket reactors, he and the following speaker from the Institute of Scientific Industrial Association (SIA) “Luch” described the nearby reactor test facilities that were later visited by the DOE Team. The Impulse Graphite Reactor (IGR) (similar to the U.S. TREAT – Transient Reactor Test facility) went critical in June 1960, and the first nuclear thermal propulsion fuel tests were started there in 1964 when gaseous hydrogen was made available for tests. Another reactor built at the Baikal site (64 km away) was the IVG-1, which could test up to seven clusters of fuel assemblies in the central test section using gaseous hydrogen. The first tests in that reactor were started in 1975, and 30 hot firings were conducted up to 1985. A second reactor, the IRGIT, was built at that site to test more typical rocket reactor cores or fuel clusters. It became operational in 1978 but after only four test series were run, its pressure vessel failed due to hydrogen embrittlement. All of this indicates that the former Soviet Union program on nuclear rocket propulsion started well after the U.S. program. It is also apparent that the available reactor test facilities have no liquid hydrogen capability, no reactor testing was done there using liquid hydrogen, and no full reactor or engine tests were ever conducted on nuclear rocket propulsion. However, substantial effort was devoted to and progress was made on materials development suitable for fuel elements and structures of nuclear rocket reactors. Some work continues in those areas.
In the conference and in its meetings with eight Institutes, it became apparent to the Team that the former Soviet Union had established a large, competitive, and overlapping infrastructure for work on space nuclear systems. It was also apparent that some of this capability had been established for work in developing nuclear weapons. Though the start of the Soviet work followed U.S. activity, much of it continued well after the U.S. nuclear rocket program ended in 1973. The indication that the Soviet program knew ofthe U.S. activities was apparent in much of the work that was presented during the various visits, but it was also emphasized when the Director of the Research and Development Institute of Power Engineering (RDIPE) said that they knew of the U.S. work even though no one had given them the SNPO reports. That Director of RDIPE later became the Minister of Minatom, the Ministry of Atomic Energy. There is no question that Russia has had, and continues to have, interest in nuclear rocket propulsion for use when potential space missions require it.

The Nuclear Ramjet Pluto Program

As pointed out before, the Lawrence Livermore Radiation Laboratory (LRL) was assigned to work on the nuclear ramjet program in 1957. The Marquardt and later, the Chance-Vought companies were involved in the program with LRL under Atomic Energy Commission and Air Force contracts; Atomics International provided some fuel development support. Several different names were used for the program. The development work at Livermore was referred to as the Pluto project, and their ramjet reactors to be tested were named Tory reactors. The overall nuclear ramjet missile system was the Supersonic Low Altitude

Missile (SLAM).

A schematic drawing of the nuclear ramjet is shown in Fig. 2, which identifies its principal components. As in the nuclear rocket program, major emphasis was required and placed on selecting and developing high-temperature materials suitable for the reactor system. However, in the ramjet, they would have to be compatible with the oxidizing atmosphere of the air working fluid. In addition, keeping the reactor small and light was also required to ensure that the ramjet missile delivery system could carry a significant missile payload. The missile speed was expected to be at least twice the speed of sound as it flew its low-altitude, low-detection trajectory. In fact, NACA research had indicated that wingless conical or circular ramjet vehicles could achieve reasonable lift/drag ratios at Mach numbers between 2 and 4; above Mach 5, they are equal to the best winged vehicles (36). However, as also discussed in the Introduction to this article, such speeds introduce significant design requirements and development work on the inlet system, and they lead to high pressure drops and stresses in the reactor, in addition to those from temperature variations. The nuclear ramjet design aimed at speeds of three times the speed of sound (19).
Dr. Theodore Merkle, the Director of the Pluto program and Associate Director at Livermore, described the system and discussed the very important reactor material considerations and their selection in the hearings of Reference 12 and in Reference 11. The strength of materials at high temperature and resistance to oxygen were the key issues in material selection for the nuclear ramjet. For example, some of the high-temperature materials such as graphite and tungsten and carbides such as zirconium and tantalum carbide, which were considered and some applied in the rocket program, could not be used in the ramjet because they burn up in oxygen. The conclusion was that certain ceramics would best meet the reactor requirements; this led to a mixture ofuranium oxide (UO2) in a beryllium oxide moderator as the preferred fuel element material (38).
Preparing for tests after designation as the Pluto development organization in 1957 involved major facility construction at the Nevada Test Site (NTS) in an area next to the Nuclear Rocket Development Station (NRDS) at Jackass Flats (36). For example, in addition to the test stand and the disassembly building, a hot nuclear critical assembly building was built and a very high capacity source of high pressure air—a million pounds—was required to simulate the ramjet flight conditions and to serve as the reactor coolant and thrust fluid of the system (37). That pressurized air system required 25 miles of oil-well casing and used compressors from the Navy’s Groton submarine base to supply the high-pressure air that also was heated to simulate the system’s supersonic flight. In addition to the extensive facility work, LRL was heavily involved in its design, criticality testing, and fuel materials development, to arrive at its test reactor designs.
The first test reactor, the Tory II-A1, was a small, low-power test reactor. The main purpose of testing it was to verify the predicted integrity of the reactor core materials and to study the aerothermodynamic behavior of the core-air heat exchange system under conditions that simulated low-altitude supersonic flight (19). General Electric’s Aircraft Nuclear Propulsion Department fabricated the fuel elements for Tory II-A1. The graphite peripheral reflector was water cooled, and control rods were located in the reflector. Sections at the inlet and outlet ends of the reactor made of unfueled beryllium oxide also acted as neutron reflectors (39). The reactor was assembled at LRL and went critical there on 7 October 1960. It was then moved to the NTS where it was initially tested at 40 MW, about a third of its design power, on 14 May 1961 for about 60 seconds at an inlet air temperature of 400°F and a maximum fuel element temperature of 2250°F (19). The results exceeded expectations. Further test runs were successfully made at full power in September and October 1961 (40). The delay in full power testing resulted from the need to replace the air ducting in the test bunker with high-temperature duct alloy. Although the planned reactor test schedule (21) called for further tests of a second backup Tory II-A reactor, the results of the Tory II-A1 were sufficient to eliminate the need for that second, small, low-power test system.
The program was then directed toward the development and testing of the Tory II-C flight type reactor system. Its configuration differed significantly from the Tory II-A. Its control system was contained within the core; it had a thin air-cooled reflector and improved fuel elements. Its fuel was made by Coors Porcelain Co. (38). That Tory II-C reactor was assembled and went critical at Livermore in July 1963; criticality testing continued through September. After shipment to the test site (NTS), facility welding deficiencies were found that delayed testing of the reactor till 20 May 1964 (40).
Conclusion of the Nuclear Ramjet Pluto Program. Though the Tory II-C test in May was fully successful, the Department of Defense Director of Defense Research and Engineering (DDR&E), Dr. Harold Brown (a former Director of the Livermore Laboratory), notified the Air Force on 6 July 1964 that the Low-Altitude Supersonic Vehicle (LASV) program would be cancelled by DOD, and he notified the AEC that the DOD would not support a flight test of the nuclear ramjet. For several years, in spite of Air Force and Navy encouragement of nuclear ramjet development, the program had been going through major questioning in congressional hearings and in the DOD concerning its anticipated application to a military mission. In fact, the House Appropriations Committee had almost eliminated the AEC FY 1965 funds for the program and threatened to cut the DOD funds if the DOD did not plan to move forward to develop the vehicle system required to flight test the reactor that had been successfully tested. Based on the success of the Tory II-C reactor testing, some argued that the next logical step was to prepare for a flight test of the system to prove its readiness for mission application. However, many considered the cost of such a flight program excessive in the face of no clear need or application for the system. The DDR&E took that position, as did Secretary of Defense McNamara, who indicated that the chances of deploying the system were slight (41). As a result, the program was cancelled in spite of the sound technical progress that demonstrated the engine feasibility of the system.
The front page headline of the Smithsonian’s April/May 1990 issue of Air & Space referring to Gregg Harken’s article (37), which in some places expressed concerns that were considered by some to be exaggerated and even erroneous, summed up the conclusions of many: ”Project Pluto: How America almost built a nightmare missile.” The article was entitled ”The Flying Crowbar” which was the description of the nuclear ramjet system used by Dr. Merkle in his testimony in Reference 12 to indicate its structural solidity. Fundamentally, the existing reactor technology could be further advanced, but in view of the already existing and adequate chemical rocket ballistic missile capability, the lack of any mission requirement eliminated that need. There are no missions foreseen for the nuclear ramjet.

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