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Kiwi
Kiwi A Prime on test stand
Country of originUnited States
DesignerLos Alamos Scientific Laboratory
ManufacturerLos Alamos Scientific Laboratory
ApplicationResearch and development
SuccessorNERVA
StatusRetired
Liquid-fuel engine
PropellantLiquid hydrogen
Performance
Thrust, vacuum245,000 N (55,000 lbf)
Chamber pressure3,427 kilopascals (497.0 psi)
Specific impulse, vacuum834 seconds (8.18 km/s)
Burn time480 seconds
Restarts1
Dimensions
Length137.2 centimetres (54 in) (core)
Diameter81.3 centimetres (32 in) (core)
References
References[1]
NotesData is for Kiwi B4E version.

Project Rover was a US project to develop a nuclear thermal rocket that ran from 1955 to 1973 at the Los Alamos Scientific Laboratory (LASL). It began as a United States Air Force project to develop an upper stage for or an intercontinental ballistic missile (ICBM). It was transferred to NASA in 1958 after the Sputnik crisis triggered the Space Race. It was managed by the Space Nuclear Propulsion Office (SNPO), a joint agency of the Atomic Energy Commission, and NASA. Project Rover became part of NASA's Nuclear Engine for Rocket Vehicle Application (NERVA) project. Project Rover dealt with the research into nuclear rocket reactor design, while NERVA involved the development and deployment of nuclear rocket engines, and the planning of space missions.

Nuclear reactors for Project Rover were built at LASL Technical Area 18 (TA-18), also known as the Pajarito Canyon Site. They were tested there at very low power and then shipped to Area 25 (known as Jackass Flats) at the AEC's Nevada Test Site. Testing of fuel elements and other materials science was done by the LASL N-Division at TA-46 using various ovens and later the Nuclear Furnace. Project Rover resulted in the development of three reactor types: Kiwi (1955 to 1964), Phoebus (1964 to 1969), and Pewee (1969 to 1972). Kiwi and Phoebus were large reactors, while Pewee was much smaller, conforming to the smaller budget available after 1968.

The reactors were fueled with uranium-235, with liquid hydrogen used as both a rocket propellant and reactor coolant. Nuclear graphite and beryllium were used as neutron moderators and neutron reflectors. The engines were controlled by drums with graphite or beryllium on one side and boron (a nuclear poison) on the other, and the energy level adjusted by rotating the drums. Because hydrogen also acts as a moderator, increasing the flow of propellant also increased reactor power without the need to adjust the drums. Project Rover tests demonstrated that nuclear rocket engines could be shut down and restarted many times without difficulty, and could be clustered if more thrust was desired. Their specific impulse (efficiency) was roughly double that of chemical rockets.

The nuclear rocket enjoyed strong political support from the influential chairman of the United States Congress Joint Committee on Atomic Energy, Senator Clinton P. Anderson from New Mexico (where LASL was located), Senator Howard Cannon from Nevada, and Senator Margaret Chase Smith from Maine. This enabled it to survive multiple cancellation attempts that became more serious in the cost-cutting that prevailed as the Vietnam War escalated and after the space race ended with the Apollo 11 Moon landing. Projects Rover and NERVA were cancelled over their objection in January 1973, and none of the reactors developed ever flew.

Beginnings

Early concepts

During World War II, some scientists at the Manhattan Project's Los Alamos Laboratory where the first atomic bombs were designed, including Stan Ulam, Frederick Reines and Frederic de Hoffmann, speculated about the development of nuclear powered rockets.[2] In 1946, Ulam and C. J. Everett wrote a paper in which they considered the using atomic bombs as a means of rocket propulsion. This would become the basis for Project Orion.[3] In December 1945, Theodore von Karman and Hsue-Shen Tsien wrote a report for the United States Army Air Forces. While they agreed that it was not yet practical, Tsien speculated that nuclear powered rockets might one day be powerful enough to launch satellites into orbit.[4]

In 1947, North American Aviation's Aerophysics Laboratory published a large paper surveying many of the problems involved in using nuclear reactors to power airplanes and rockets. The study was specifically aimed at a vehicle with a range of 16,000 kilometres (10,000 mi) and a payload of 3,600 kilograms (8,000 lb), and covered turbopumps, structure, tankage, aerodynamics and nuclear reactor design. They concluded that hydrogen was best as a propellant and that graphite would be the best reactor material, but they assumed an operating temperature of 5,700 °F (3,150 °C), a temperature beyond the capabilities of available materials. Their work used very conservative numbers and emerged with very unpromising results for the idea of nuclear-powered rockets.[4]

The public revelation of atomic energy at the end of the war generated a great deal of speculation, and in the United Kingdom, Val Cleaver, the chief engineer of the rocket division at De Havilland, and Leslie Shepard, a nuclear physicist at the University of Cambridge, independently considered the problem of nuclear rocket propulsion. They became collaborators, and in a series of papers published in the Journal of the British Interplanetary Society in 1948 and 1949, they outlined the design of a nuclear-powered rocket with a solid-core graphite heat exchanger. They reluctantly concluded that nuclear rockets were essential for deep space exploration, but not yet technically feasible.[5][6]

Bussard report

In 1953, Robert W. Bussard, a physicist working on the Nuclear Energy for the Propulsion of Aircraft (NEPA) project at the Oak Ridge National Laboratory wrote a detailed study. He had read Cleaver and Shepard's work,[7] that of the Tsien,[8] and a February 1952 report by engineers at Consolidated Vultee.[9] He used data and analyses from existing chemical rockets, along with specifications for existing components. His calculations were based on the state of the art for nuclear reactors.[10] Most importantly, the paper surveyed several ranges and payload sizes; Consolidated's pessimistic conclusions had partly been the result of considering only a narrow range of possibilities.[9]

The result, Nuclear Energy for Rocket Propulsion, stated that the use of nuclear propulsion in rockets is not limited by considerations of combustion energy and thus low molecular weight propellants such as pure hydrogen may be used. While a conventional engine could produce an exhaust velocity of 2,500 metres per second (8,300 ft/s), a hydrogen-fueled nuclear engine could attain an exhaust velocity of 6,900 metres per second (22,700 ft/s) under the same conditions. He proposed a graphite moderated reactor due to graphite's ability to withstand high temperatures and concluded that the fuel elements would have to be coated with protective cladding to withstand hydrogen propellants.[10]

Bussard's study had little impact at first, mainly because only 29 copies were printed, and it was classified as Restricted Data, and therefore could only be read by someone with the required security clearance.[11] In December 1953, it was published in Oak Ridge's Journal of Reactor Science and Technology. While still classified, this gave it a wider circulation.[7] Darol Froman, the Deputy Director of the Los Alamos Scientific Laboratory (LASL), and Herbert York, the director of the University of California Radiation Laboratory at Livermore, were interested, and established committees to investigate nuclear rocket propulsion. Froman brought Bussard out to Los Alamos to assist for one week per month.[12]

Official sanction

Bussard's study also attracted the attention of John von Neumann, and he formed an ad hoc committee on Nuclear Propulsion of Missiles. Mark Mills, the assistant director at Livermore was its chairman, and its other members were Norris Bradbury, Edward Teller, Herbert York, Abe Silverstein and Allan F. Donovan.[12] After hearing input on various designs, the Mills committee recommended that development proceed, with the aim of producing a nuclear upper stage for an intercontinental ballistic missile (ICBM). York created a new division at Livermore, and Bradbury created a new one called N Division at Los Alamos under the leadership of Raemer Schreiber, to pursue it.[13] In March 1956, the Armed Forces Special Weapons Project (AFSWP) recommended allocating $100 million to the nuclear rocket engine project over three years for the two laboratories to conduct feasibility studies and construct of test facilities.[14]

Eger V. Murphree and Herbert Loper at the Atomic Energy Commission (AEC) were more cautious. The Atlas missile program was proceeding well, and if successful would have sufficient range to hit targets in most of the Soviet Union. At the same time, nuclear warheads were becoming smaller, lighter and more powerful. The case for a new technology that promised heavier payloads over longer distances therefore seemed weak. However, the nuclear rocket had acquired a political patron in Senator Clinton P. Anderson from New Mexico (where LASL was located), the deputy chairman of the United States Congress Joint Committee on Atomic Energy (JCAE), who was close to von Neumann, Bradbury and Ulam. He managed to secure funding.[14]

All work on the nuclear rocket was consolidated at Los Alamos, where it was given the codename Project Rover; Livermore was assigned responsibility for development of the nuclear ramjet, which was codenamed Project Pluto.[15] Project Rover was directed by an active duty USAF officer seconded to the AEC, Lieutenant Colonel Harold R. Schmidt. He was answerable to another seconded USAF officer, Colonel Jack L. Armstrong, who was also in charge of Pluto and the Systems for Nuclear Auxiliary Power (SNAP) projects.[16]

Design concepts

Cutaway diagram of Kiwi rocket engine

In principle, the design of a nuclear thermal rocket engine is quite simple: a turbopump would force hydrogen through a nuclear reactor that would heat it to very high temperatures. Complicating factors were immediately apparent. The first was that a means had to be found of controlling reactor temperature and power output. The second was that a means had to be devised to hold the propellant. The only practical means of storing hydrogen was in liquid form, and this required a temperature below 20 K (−253.2 °C). The third was that the hydrogen would be heated to a temperature of around 2,500 K (2,230 °C), and materials would be required that could both withstand such temperatures and resist corrosion by hydrogen.[17]

Liquid hydrogen was theoretically the best possible propellant, but in the 1950s it was expensive, and available only in small quantities.[18] Before setting on hydrogen, LASL considered other propellants such as methane and ammonia. Ammonia was inexpensive, easy to obtain, liquid at 239 K (−34 °C), and easy to pump and handle. Tests conducted in 1955 to 1957 therefore used it. However, it is much heavier than hydrogen, which reduced the engine's impulse, and tests revealed that it was even more corrosive, and had undesirable neutronic properties.[19] For the fuel, they considered plutonium-239, uranium-235 and uranium-233. Plutonium was rejected because it tends to form compounds, and could not reach temperatures as high as those achievable by uranium. Uranium-233 was seriously considered, as it held the prospect of saving weight, but was not readily available.[20]

For structural materials in the reactor, the choice came down to graphite or metals.[20] Of the metals, tungsten emerged as the frontrunner. However, it was expensive and hard to fabricate. To get around its undesirable neutronic properties, it was proposed that tungsten-184, which does not absorb neutrons, be used.[21] Graphite actually gets stronger at temperatures up to 3,300 K (3,030 °C), and sublimes rather than melts at 3,900 K (3,630 °C).[22] To control the reactor, the core was surrounded by control drums coated with beryllium (a neutron moderator) on one side and boron (a neutron poison) on the other. The reactor's power output could be controlled by rotating the drums.[23]

LASL produced a series of design concepts, each with its own codename: Uncle Tom, Uncle Tung, Bloodhound and Shish.[24] By 1955, it had settled on a 1,500 MW design called Old Black Joe. In 1956, this became the basis of 2,700 MW design intended to be the upper stage of an ICBM. [20]

Test site

Arrangement of facilities at the Nuclear Rocket Development Station in Jackass Flats

Nuclear reactors for Project Rover were built at LASL Technical Area 18 (TA-18), also known as the Pajarito Site. Fuel and internal engine components were fabricated in the Sigma complex at Los Alamos.[25] Two reactors were built for each engine; one for "zero-power critical" experiments at Los Alamos and another used for full-power testing.[26] The reactors were tested at very low power before being shipped to Jackass Flats at the Nevada Test Site. Testing of fuel elements and other materials science was done by the LASL N Division at TA-46 using various ovens and later the Nuclear Furnace. Staff from the LASL Test (J) and Chemical Metallurgy Baker (CMB) divisions also participated in Project Rover.[25]

Work commenced on test facilities at Jackass Flats in mid-1957. All materials and supplies had to be brought in from Las Vegas. Test Cell A consisted of a farm of hydrogen gas bottles and a concrete wall 3 feet (0.91 m) thick to protect the electronic instrumentation from radiation from the reactor. The control room was located 2 miles (3.2 km) away. The plastic coating on the control cables was chewed by burrowing rodents and had to be replaced. The reactor was test fired with its exhaust plume in the air so that any radioactive fission products picked up from the core could be safely dispersed.[20]

The reactor maintenance and disassembly building (R-MAD) was in most respects a typical hot cell used by the nuclear industry, with thick concrete walls, lead glass viewing windows, and remote manipulation arms. It was exceptional only for its size: 250 feet (76 m) long, 140 feet (43 m) and 63 feet (19 m) high. This allowed the engine to be moved in and out on a railroad car.[20] The "Jackass and Western Railroad", as it was light-heartedly described, was said to be the world's shortest and slowest railroad.[27] There were two locomotives, the remotely controlled electric L-1, and the diesel/electric L-2, which was manually controlled, but had radiation shielding around the cab.[20]

Construction workers were housed in Mercury, Nevada. Later thirty trailers were brought to Jackass Flats to create a village named "Boyerville" after the supervisor, Keith Boyer. Work was completed in the fall of 1958.[20]

Transfer to NASA

President John F. Kennedy (right) visits the Nuclear Rocket Development Station. To the left of the president are Glenn Seaborg, Chairman of the U.S. Atomic Energy Commission; Senator Howard Cannon; Harold Finger, manager of the Space Nuclear Propulsion Office; and Alvin C. Graves, director of test activities at the Los Alamos Scientific Laboratory.

By 1957, the Atlas missile project was proceeding well, and with smaller and lighter warheads becoming available, the need for a nuclear upper stage had all but disappeared.[28][26] On 2 October 1957, the AEC proposed cutting Project Rover's budget, but its timing was off.[29] Two days later, the Soviet Union launched Sputnik 1, the first artificial satellite. This surprise space success fired fears and imaginations around the world. It demonstrated that the Soviet Union had the capability to deliver nuclear weapons over intercontinental distances, and contested cherished American notions of military, economic and technological superiority.[30] This precipitated the Sputnik crisis, and triggered the Space Race, an new area of competition in the Cold War.[31] President Dwight D. Eisenhower responded by creating the National Aeronautics and Space Administration (NASA).[32]

Donald A. Quarles, the Deputy Secretary of Defense, met with T. Keith Glennan, the new administrator of NASA, and Hugh Dryden, his deputy on 20 August 1958,[33] the day they after were sworn into office at the White House,[34] and Rover was the first item on the agenda. Quarles was eager to transfer Rover to NASA, as the project no longer had a military purpose.[16] Responsibility for the non-nuclear components of Project Rover was officially transferred from the United States Air Force (USAF) to NASA on 1 October 1958,[35] the day NASA officially became operational and assumed responsibility for the U.S. civilian space program.[36]

Project Rover became a joint NASA-AEC project.[35] Silverstein appointed Harold Finger to oversee the nuclear rocket development. On 29 August 1960, NASA created the Space Nuclear Propulsion Office (SNPO) to oversee the nuclear rocket project.[37] Finger was appointed as it manager, with Milton Klein from AEC as his deputy.[38] A formal "Agreement Between NASA and AEC on Management of Nuclear Rocket Engine Contracts" was signed by NASA Deputy Administrator Robert Seamans and AEC General Manager Alvin Luedecke on 1 February 1961. This was followed by an "Inter-Agency Agreement on the Program for the Development of Space Nuclear Rocket Propulsion (Project Rover)", which they signed on 28 July 1961.[39] SNPO also assumed responsibility for SNAP, with Armstrong becoming assistant to the drector of the Reactor Development Division at AEC, and Lieutenant Colonel G. M. Anderson, formerly the SNAP project officer in the disbanded Aircraft Nuclear Propulsion Office (ANPO), became chief of the SNAP Branch in the new division.[38] It soon became apparent that there were considerable cultural differences between NASA and AEC.[16]

On 25 May, President John F. Kennedy addressed a joint session of Congress. "First," he announced, "I believe 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." He then went on to say: "Secondly, an additional 23 million dollars, together with 7 million dollars already available, will accelerate development of the Rover nuclear rocket. This 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."[40]

Kiwi

The first phase of Project Rover, Kiwi was named after the New Zealand kiwi bird.[20] A kiwi cannot fly, and the Kiwi rocket engines were not intended to do so either. Their function was to verify the design and test the behavior of the materials used.[22] The Kiwi program developed of a series of non-flyable test nuclear engines, with the primary focus on improving the technology of hydrogen-cooled reactors. Between 1959 and 1964, a total of eight reactors were built and tested. Kiwi was considered to have served as proof of concept for nuclear rocket engines.[41]

Kiwi A

Raemer Schreiber with a Project Rover poster in 1959.

The first test of the Kiwi A, the first model of the Kiwi rocket engine, was conducted at Jackass Flats on 1 July 1959. Kiwi A had a cylindrical core 132.7 centimeters (52.2 in) high and 83.8 centimeters (33.0 in) in diameter. A central island containing heavy water that acted as a moderator to reduce the amount of uranium oxide required, and also as a coolant. The control rods were located inside the island, which was surrounded by 960 graphite fuel plates loaded with 4-micrometer (0.00016 in) uranium oxide fuel particles and a layer of 240 graphite plates.[42] The core was surrounded by 43.2 centimeters (17.0 in) of graphite wool moderator and encased in an aluminium shell. Gaseous hydrogen was used as a propellant, at a flow rate of 3.2 kilograms per second (7.1 lb/s). Intended to produced 100 MW, the engine ran at 70 MW for 5 minutes. Temperatures were much higher than expected, up to 2,900 K, due to cracking of the graphite plates, which was enough to cause some of the fuel to melt.[42]

A series of improvements were made for the next test on 8 July 1960 to create an engine known as Kiwi A Prime. The fuel elements were extruded into cylinders and coated with niobium carbide (NbC) to resist corrosion. Six were stacked end-to-end to and then placed in the seven holes in the graphite modules to create 137-centimeter (54 in) long fuel modules. This time the reactor attained 88 MW for 307 seconds, with an average core exit gas temperature of 2,178 K. The test was married by three core module failures, but the majority suffered little or no damage.[43]

The third and final test of the Kiwi A series was conducted on 19 October 1960. The Kiwi A3 engine used 68.58-centimeter (27.00 in) long cylindrical fuel elements in niobium carbide liners. The test plan called for the engine to be run at 50 MW (half power) for 106 seconds, and then at 92 MW for 250 seconds. The 50 MW power level was achieved with a propellant flow of 2.36 kilograms per second (312 lb/min), but exit gas temperature was 1,861 K, which was over 300 K higher than expected. After 159 seconds, the power was increased to 90 MW. To stabilize the exit gas temperature at 2,173 K, the fuel rate was increased to 3.81 kilograms per second (504 lb/min). It was later discovered that the neutronic power measuring system was incorrectly calibrated, and the engine was actually run at an average of 112.5 MW for 259 seconds, well above its design capacity. Despite this, the core suffered less damage than in the Kiwi A Prime test.[44] Finger called for bids from industry for the development of NASA's Nuclear Engine for Rocket Vehicle Application (NERVA) based upon the Kiwi engine.[45] Rover became part of NERVA; Rover dealt with the research into nuclear rocket reactor design, while NERVA involved the development and deployment of nuclear rocket engines, and the planning of space missions.[46]

Kiwi B

The Director of the Los Alamos National Laboratory, Norris Bradbury (left) in front of the Kiwi B4-A reactor

Kiwi A was considered a success as a proof of concept for nuclear rocket engines. It demonstrated that hydrogen could be heated in a nuclear reactor to the temperatures required for space propulsion, and that the reactor could be controlled.[47] LASL's original objective had been what it codenamed Condor (a large flying bird in contrast to the small flightless Kiwi): a 10,000 MW nuclear rocket engine capable of launching 11,000 kilograms (25,000 lb) onto a 480 kilometres (300 mi) orbit. However, in October 1958, NASA had studied putting a nuclear upper stage on a Titan I missile, and concluded that a 1,000 MW reactor stage could put 6,400 kilograms (14,000 lb) into orbit. This configuration was used in studies of Nova, and became the goal of Project Rover. LASL planned to conduct two tests with Kiwi B, an intermediate 1,000 MW design, in 1961 and 1962, followed by two tests of Kiwi C, a prototype engine, in 1963, and have a RIFT of a production engine in 1964.[23]

For Kiwi B, LASL made several design changes to get the required higher performance. The central core was eliminated, the number of coolant holes in each hexagonal fuel element was increased from four to seven, and the graphite reflector was replaced with a 20.3-centimetre (8.0 in) thick beryllium one.[44] Although beryllium was more expensive, more difficult to fabricate, and highly toxic, it was also much lighter, resulting in a saving of 2,500 pounds (1,100 kg). Due to the delay in getting Test Cell C ready, some features intended for Kiwi C were also incorporated in Kiwi B2. These included a nozzle cooled by liquid hydrogen instead of water, a new Rocketdyne turbopump, and a bootstrap start.[23]

The test of Kiwi B1A, the last test to use gaseous hydrogen instead of liquid, was initially scheduled for 7 November 1961, but on the morning of the test, a leaking valve resulted in a violent hydrogen explosion that blew out the walls of the shed and injured several workers. The reactor was undamaged, but there was extensive damage to the test car and the instrumentation, resulting in the test being postponed for a month. A second attempt on 6 December was aborted when it was discovered that many of the diagnostic thermocouples had been installed backwards. Finally, on 7 December, the test got under way. It was intended to run the engine at 270 MW for 300 seconds, but the test was scrammed after only 36 seconds at 225 MW because hydrogen fires started to appear. All the thermocouples performed correctly though, so a great deal of useful data was obtained. The average hydrogen mass flow during the full power portion of the experiment was 9.1 kilograms per second (1,200 lb/min).[48][49]

LASL next intended to test Kiwi B2, but structural flaws were found that required a redesign. Attention then switched to B4, a more radical design, but when it came to put the fuel clusters into the core, they were found to have too many neutrons, and it was feared that the reactor might unexpectedly start up. The problem was traced to absorption of water from the normally dry New Mexico air during storage. It was corrected by adding more neutron poison. After this, fuel elements were stored in an inert atmosphere. Starting with the Kiwi B4E test, the uranium carbide (UC
2) fuel elements were used, and given a protective coating. N Division then decided to test with the backup B1 engine, B1B, despite grave doubts about it based on the results of the B1A test, in order to obtain more data on the performance and behavior of liquid hydrogen.[50][51] On start up on 1 September 1962, the core shook but reached 880 MW. Flashes of light around the nozzle indicated that fuel pellets were being ejected; it was later determined that eleven had been. Rather than shut down, the testers rotated the drums to compensate, and were able to continue running at full power for a few minutes before a sensor blew and started a fire, and the engine was shut down. Most but not all of the test objectives were met. [51][52]

The next test of the series was of Kiwi B4A on 30 November 1962. A flame flash was observed when the reactor reached 120 MW. Power was increased to 210 MW, and held there for 37 seconds. Power was then increased to 450 MW, but flashes then became frequent, and the engine was shut down after 13 seconds. After the test it was discovered that 97 percent of the fuel elements were broken.[53] The difficulties of using liquid hydrogen were appreciated, and the cause of the vibration and failures was diagnosed as hydrogen leaking into the gap between the core and the pressure vessel.[54] Unlike a chemical engine that would likely have blown up after suffering damage, the engine remained stable and controllable throughout. The tests demonstrated that a nuclear rocket engine would be rugged and reliable in space.[51]

Kiwi A Prime is test fired

Kennedy visited Los Alamos on 7 December 1962 for a briefing on Project Rover.[55] It was the first time a president had visited a nuclear weapons laboratory. He brought with him a large entourage that included Lyndon Johnson, McGeorge Bundy, Jerome Wiesner, Harold Brown, Donald Hornig, Glenn Seaborg, Robert Seamans, Harold Finger and Clinton Anderson. The next day, the flew to Jackass Flats, making Kennedy the only president to ever visit a nuclear test site. Project Rover had received $187 million in 1962, and AEC and NASA were asking for another $360 million in 1963. Kennedy drew attention to his administration's budgetary difficulties, and his officials and advisors debated the future of Project Rover and the space program in general.[56]

Finger assembled a team of vibration specialists from other NASA centers, and along with staff from LASL, Aerojet and Westinghouse, conducted a series of "cold flow" reactor tests using fuel elements without fissionable material. Nitrogen, helium and hydrogen gas was pumped through the engine to induce vibrations. It was determined that they were caused by a dynamic flow instability in the clearance gaps between adjacent fuel elements. A series of minor design changes were made to address the vibration problem.[57][58] In the Kiwi B4D test on 13 May 1964, the reactor was automatically started and briefly run at full power (990 MW) with no vibration problems, although the test had to be terminated after 64 seconds when nozzle tubes ruptured and caused a hydrogen leak around the nozzle that started a fire. Cool down was performed with both hydrogen and 3,266 kilograms (7,200 lb) of nitrogen gas. On inspection after the test, no damaged fuel elements were found.[59]

The final test was the Kiwi B4E test on 28 August in which the reactor was operated for twelve minutes, eight of which were at full power (937 MW). This was the first test to use uranium carbide pellets instead of uranium oxide, with a 0.0508-millimetre (0.002 in) niobium carbide coating. These were found to oxidize on heating, causing a loss of carbon in the form of carbon monoxide gas. To minimize this, the particles were made larger (50 to 150 micrometres (0.0020 to 0.0059 in) in diameter), and given a protective coating of pyrolytic graphite . On 10 September, Kiwi B4E was restarted, and run at 882 MW for two and a half minutes, demonstrating the ability of a nuclear rocket engine to be shut down and restarted.[60][61]

In September 1964, tests were conducted with a Kiwi B4 engine and PARKA, a Kiwi reactor used for testing at Los Alamos. The two reactors were run 4.9 metres (16 ft), 2.7 metres (9 ft) and 1.8 metres (6 ft) apart, and measurements taken of reactivity. These tests showed that neutrons produced by one reactor did indeed cause fissions in another, but that the effect was negligible: 3, 12 and 24 cents respectively. The tests demonstrated that nuclear rocket engines can be clustered, just as chemical ones often are.[50][51][62][63]

Phoebus

Phoebus nuclear rocket engine on the Jackass & Western railroad

The next step in LASL's research program was to build a larger reactor. In 1960, it began planning a 4,000 MW reactor with an 890-millimetre (35 in) core as a successor to Kiwi. This ran into opposition from SNPO, which wanted a monster 20,000 MW reactor. LASL thought that the difficulties of building and testing such a large reactor were being taken too lightly. Just to build the 4,000 MW design required a new nozzle and improved turbopump from Rocketdyne. A prolonged bureaucratic conflict ensued.[64]

In March 1963, SNPO and the Marshall Space Flight Center (MSFC) commissioned Space Technology Laboratories (STL) to produce a report on what kind of nuclear rocket engine would be required for possible missions between 1975 and 1990. These missions included early manned planetary interplanetary round-trip expeditions (EMPIRE), planetary swingbys and flybys, and a lunar shuttle. The conclusion of this nine volume report, which was delivered in March 1965, and of a follow-up study, was that these missions could be carried out with a 4,100 MW engine with a specific impulse of 825 seconds (8.09 km/s). This was considerably smaller than had originally been thought necessary. From this emerged a specification for a 5,000 MW nuclear rocket engine, which became known as NERVA II.[65][66]

LASL and SNPO therefore moved to an agreement. LASL would build two versions of Phoebus: a small one (Phoebus I) with an 890-millimetre (35 in) core for testing advanced fuels, materials and concepts, and a larger 1,400-millimetre (55 in) one (Phoebus II) that would serve as a prototype for NERVA II. Both would be based on Kiwi. The focus was placed on achieving more power than was possible with Kiwi units and maintaining the maximum power for a longer duration. The work on Phoebus I was started in 1963, with a total of three engines being built, called 1A, 1B and 1C.[64]

Phoebus in the National Atomic Testing Museum in Las Vegas

Phoebus 1A on was tested on 25 June 1965, and run at full power (1,090 MW) for ten and a half minutes. Unfortunately, the intense radiation environment caused one of the capacitance gauges to produce erroneous readings. Even more unfortunately, when confronted by one gauge that said that the hydrogen propellant tank was nearly empty, and another that said that it was quarter full, the technicians in the control room chose to believe the faulty one. As a result, the propellant ran dry. Without liquid hydrogen to cool the 2,270 K (2,000 °C) engine quickly overheated and then exploded. About a fifth of the fuel was ejected; the rest was melted.[64] [67]

The test area was left for six weeks to give highly radioactive fission products time to decay. A grader with a rubber squeegee on its plow was used to pile up contaminated dirt so it could be scooped up. When this did not work, a 150 kW (200 hp) vacuum cleaner was used to pick up the dirt. Fragments on the test pad were initially collected by a robot, but this was too slow, and men in protective suits were used, picking up pieces with tongs and dropping then into paint cans surrounded by lead and mounted on small wheeled dollies. That took care of the main contamination; the rest was chipped, swept, scrubbed, washed or painted away. The whole decontamination effort took four hundred people two months to complete, and cost $50,000. The average dose of radiation received by the clean up workers was 0.66 rems (0.0066 Sv), while the maximum was 3 rems (0.030 Sv); LASL limited its employees to 5 rems (0.050 Sv) per annum.[64]

The next test was of Phoebus 1B. It was powered up on 10 February 1967, and run at 588 MW for two and a half minutes. To avoid a repeat of the mishap that had occurred to Phoebus 1A, a 30,280-litre (8,000 US gal), high pressure (5,171-kilopascal (750.0 psi) dewar was installed to provide an emergency liquid hydrogen supply in the event that there was a failure of the primary propellant supply system. A second test was conducted on 23 February 1967, when it was run for 46 minutes, of which 30 minutes were above 1,250 MW, and a maximum power of 1,450 MW and gas temperature of 2,444 K (2,171 °C) was achieved. The test was a success, but some corrosion was found.[68]

This was followed by a test of the larger Phoebus 2A. A preliminary low power (2,000 MW) run was conducted on 8 June 1968, then a full power run on 26 June. The engine was operated for 32 minutes, 12.5 minutes of which was above 4,000 MW, and a peak power of 4,082 MW was reached. At this point the chamber temperature was 2,256 K (1,983 °C), and total flow rate was 118.8 kilograms per second (15,710 lb/min). The maximum power level could not be reached because at this point the temperatures of the clamp band segments reached 417 K (144 °C), their red line limit. A third run was conducted on 18 July, reaching a power of 1,280 MW, a fourth later that day, with a power of around 3,500 MW.[69] A puzzling anomaly was that the reactivity was lower than expected. It was possible that the liquid hydrogen had overchilled the beryllium reflector, causing it to somehow lose some of its moderating properties. Alternatively, there are two spin isomers of hydrogen: parahydrogen is a neutron moderator but orthohydrogen is a poison, and perhaps the high neutron flux had changed some of the parahydrogen to orthohydrogen.[70]

Pewee

Pewee was the third phase of Project Rover. It was small, easy to test, and well-sized for uncrewed scientific interplanetary missions or small nuclear "tugs". Its main purpose was to test advanced fuel elements without the expense of a full-sized engine. Peewee took only nineteen months to develop from when SNPO authorized it in June 1967 to its first full-scale test in December 1968. Peewee had a 530-millimetre (21 in) core containing 36 kilograms (80 lb) 402 fuel elements and 132 support elements. Of the 402 fuel elements, 267 were fabricated by LASL, 124 by the Westinghouse Astronuclear Laboratory, and 11 at the AEC's Y-12 National Security Complex. Most were coated with niobium carbide (NbC) but some were coated with zirconium carbide (ZrC) instead; most also had a protective molybdenum coating. There were concerns that a reactor so small might not achieve criticality, so zirconium hydride (a good moderator) was added, and the thickness of the beryllium reflector was increased to 203 millimetres (8.0 in). There were nine control drums. The whole reactor, including the aluminium pressure vessel, weighed 2,570 kilograms (5,670 lb). [71][72][73]

Peewee 1 was started up three times: for check out on November 15, 1968, for a short duration test on November 21, and for a full power endurance test on December 4. The full power test had two hold in which the reactor was run at 503 MW (1.2 MW per fuel element). The average exit gas temperature was 2,550 K (2,280 °C), the highest ever recorded by Project Rover. The chamber temperature was 2,750 K (2,480 °C), another record. The test showed that the zircon carbide was more effective at preventing corrosion than niobium carbide. No particular effort had been made to maximize the specific impulse, that not being the reactor's purpose, but Peewee achieved a vacuum specific impulse of 901 seconds (8.84 km/s), well above the target for NERVA. So too was the average power density of 2,340 MW/m3; the peak density reached 5,200 MW/m3. This was 20% higher than Phoebus 2A, and the conclusion was that it might be possible to build a lighter yet more powerful engine still.[72][73]

LASL took a year to modify the Peewee design to solve the problem of overheating. In the fall of 1970, Peewee 2 was readied in Test Cell C for a series of tests. LASL planned to do twelve full-power runs at 2,427 K (2,154 °C), each lasting for ten minutes, with a cooldown to 540 K (267 °C) between each test. SNPO ordered LASL to return Peewee to E-MAD.[71] The problem was the National Environmental Policy Act (NEPA), which President Richard Nixon signed it into law on January 1, 1970.[74] SNPO believed that radioactive emissions were well within the guidelines, and would have no adverse environmental effects, but an environmental group claimed otherwise.[75] SNPO prepared a full environmental impact study for the upcoming Nuclear Furnace tests.[76] In the meantime, LASL planned a Peewee 3 test. This would be tested horizontally, with a scrubber to remove fission products from the exhaust plume. It also planned a Peewee 4 test to test fuels, and a Peewee 5 test to test afterburners. None of these tests were ever carried out.[71]

Nuclear Furnace

Two of the fuel forms tested by Project Rover: pyrolytic carbon-coated uranium carbide fuel particles dispersed in a graphite substrate, and "composite" which consisted of a uranium carbide-zirconium carbide dispersion in the graphite substrate.

The Nuclear Furnace was a test reactor only a tenth of the size of Peewee that was intended to provide an inexpensive means of conducting tests. Originally it was to be used at Los Alamos, but the cost of creating a suitable test site was greater than using Test Cell C. It had a tiny core 146 centimetres (57 in) long and 34 centimetres (13 in) in diameter that held 49 hexagonal fuel elements. Of these, 47 were uranium carbide-zirconium carbide "composite" fuel cells and two contained a seven-element cluster of single-hole pure uranium-zirconium carbide fuel cells. Neither type had previously been tested in a nuclear rocket propulsion reactor. In all, this was about 5 kg of highly enriched (93%) uranium-235. To achieve criticality with so little fuel, the beryllium reflector was over 360 millimetres (14 in) thick. Each fuel cell had its own cooling and moderating water jacket. Gaseous hydrogen was used instead of liquid to save money. A scrubber was developed.[71][73][77]

The objective of the Nuclear Furnace tests were to verify the design, and test the new composite fuels. Between June 29 and July 27, 1972, NF-1 was operated four times at full power (44 MW) and a fuel exit gas temperature of 2,444 K (2,171 °C) for a total of 108.8 minutes. The NF-1 was operated 121.1 minutes with a fuel exit gas temperature above 2,222 K (1,949 °C). It also achieved a near-record average power density 4,500 to 5,000 MW/m3 with temperatures up to 2,500 K (2,230 °C).[78] The scrubber worked well; although some krypton-85 leaked. The Environmental Protection Agency was able to detect minute amounts, but none outside the test range.[71] The tests indicated that composite fuel cells would be good for two to six hours operation at 2,500 to 2,800 K (2,230 to 2,530 °C), which the carbide fuels would give similar performance at 30,000 to 3,200 K (29,730 to 2,930 °C), assuming that problems with cracking could be overcome with improved design. For ten hours of operation, graphite-matrix would be limited to 2,200 to 2,300 K (1,930 to 2,030 °C), the composite could go up to 24,800 K (24,500 °C), and the pure carbide to 3,000 K (2,730 °C). Thus, the test program ended with three viable forms of fuel cell.[77]

Cancellation

Defending NERVA and Rover from critics required a series of bureaucratic and political battles as the rising cost of Vietnam War put pressure on budgets, but Anderson and Howard Cannon were up to the task. Congress defunded NERVA II in the 1967 budget, but President Johnson needed Anderson's support for his Medicare legislation, and on 7 February 1967 he provided the money for NERVA II from his own contingency fund.[79] Klein, who had succeeded Finger as head of the SNPO in 1967, faced two hours of questioning on NERVA II before the House Committee on Science and Astronautics, which cut the NASA budget. Defunding NERVA II saved $400 million, mainly in new facilities that would be required to test it. AEC and NASA acquiesced, because it had been demonstrated that NERVA I could perform the missions expected of NERVA II.[80]

Senator Clinton P. Anderson with a Kiwi rocket

NERVA had plenty of missions. NASA considered using Saturn V and NERVA on a "Grand Tour" of the Solar system. A rare alignment of the planets that occurs every 174 years occurred between 1976 and 1980, allowing a spacecraft to visit Jupiter, Saturn, Uranus and Neptune. With NERVA, that spacecraft could weigh up to 24,000 kilograms (52,000 lb). This was assuming NERVA had a specific impulse of only 825 seconds (8.09 km/s); 900 seconds (8.8 km/s) was more likely, and with that it could place a 77,000-kilogram (170,000 lb) space station the size of Skylab into orbit around the Moon. Repeat trips to the Moon could be made with NERVA powering a nuclear shuttle. There was also of course the mission to Mars, which Klein diplomatically avoided mentioning,[81] knowing that, even in the wake of the Apollo 11 Moon landing, the idea was unpopular with Congress and the general public.[82]

After Nixon replaced Johnson as president 1969, cost cutting became the order of the day. NASA program funding was reduced in the 1969 budget, shutting down the Saturn V production line,[83] but NERVA remained. Klein endorsed a plan whereby the Space Shuttle lifted a NERVA engine into orbit, then returned with the fuel and payload. This could be repeated, as the NERVA engine was restartable.[81][84] NERVA still had the steadfast support of Anderson, Cannon and Margaret Chase Smith in the Senate, although Anderson was ageing and tiring, and now delegated many of his duties to Cannon. NERVA received $88 million in fiscal year (FY) 1970 and $85 million in FY 1971, with funds coming jointly from NASA and the AEC.[85]

When Nixon tried to cancel NERVA in 1971, Anderson and Smith killed Nixon's pet project, the Boeing 2707 supersonic transport (SST), instead. It was a stunning defeat for the president.[86] In the budget for FY 1972, funding for the shuttle was cut, but NERVA survived.[87] Although its budget request was only $17.4 million, Congress allocated $69 million; Nixon only spent $29 million of it. (With the Congressional Budget and Impoundment Control Act of 1974, Congress would strip him of this ability.)[85]

In 1972, Congress again supported NERVA. A bi-partisan coalition headed by Smith and Cannon appropriated $100 million for it; a NERVA engine that would fit inside the shuttle's cargo bay was estimated to cost about $250 million over a decade. They added a stipulation that there would be no more reprogramming NERVA funds to pay for other NASA activities. The Nixon administration decided to cancel NERVA anyway. On 5 January 1973, NASA announced that NERVA (and therefore Rover) was terminated. Staff at LASL and SNPO were stunned; the project to build a small NERVA that could be carried on board the Space Shuttle had been proceeding well. Layoffs began immediately, and the SNPO was abolished in June.[88] After 17 years of research and development, Projects Rover and NERVA had spent about $1.4 billion, but no nuclear powered rocket never flown.[89]

Legacy

In 1983, the Strategic Defense Initiative ("Star Wars") identified missions that could benefit from rockets more powerful than chemical rockets, and some that could only be undertaken by more powerful rockets.[90] A nuclear propulsion project, SP-100, was created in February 1983 with the aim of developing a 100 KW nuclear rocket system. The concept incorporated a pebble-bed reactor, a concept developed by James R. Powell at the Brookhaven National Laboratory, which promised higher temperatures and improved performance over NERVA.[91] From 1987 to 1991 it was funded as a secret project codenamed Project Timber Wind.[92] The proposed rocket was later expanded into a larger design after the project was transferred to the Space Nuclear Thermal Propulsion (SNTP) program at the Air Force Phillips Laboratory in October 1991. NASA conducted studies as part of its Space Exploration Initiative (SEI) but felt that SNTP offered insufficient improvement over the nuclear rockets developed by Project Rover, and was not required by any SEI missions. The SNTP program was terminated in January 1994.[91] About $200 million was spent.[93]

An engine for interplanetary travel from Earth orbit to Mars orbit, and back, was studied in 2013 at the MSFC with a focus on nuclear thermal rocket engines.[94] Since they are at least twice as efficient as the most advanced chemical engines, they allow quicker transfer times and increased cargo capacity. The shorter flight duration, estimated at 3–4 months with nuclear engines,[95] compared to 8–9 months using chemical engines,[96] would reduce crew exposure to potentially harmful and difficult to shield cosmic rays.[97] Nuclear engines like the Pewee of Project Rover, were selected in the Mars Design Reference Architecture (DRA).[98] On 22 May 2019, Congress approved $125 million in funding for the development of nuclear rockets.[99][100]

Reactor test summary

Reactor Test date Starts Average
full power
(MW)		 Time at
full power
(s)		 Propellant
temperature
(chamber) (K)		 Propellant
temperature
(exit) (K)		 Chamber
pressure
(kPa)		 Flow rate
(kg/s)		 Vacuum
specific
impulse
(s)
Kiwi A July 1959 1 70 300 1778 3.2 724
Kiwi A Prime July 1960 1 88 307 2206 1125 3.0 807
Kiwi A3 October 1960 1 112.5 259 2172 1415 3.8 800
Kiwi B1A December 1961 1 225 36 1972 974 9.1 763
Kiwi B1B September 1962 1 880 2278 2413 34.5 820
Kiwi B4A November 1962 1 450 1556 1814 19.0 677
Kiwi B4D May 1964 1 915 64 2006 2378 3606 31.1 837
Kiwi B4E August 1964 2 937 480 1972 2356 3427 31.0 834
Phoebus 1A June 1965 1 1090 630 2278 2444 3772 31.4 849
Phoebus 1B February 1967 2 1290 1800 2094 2306 5075 38.1 825
Phoebus 2A June 1968 4 4082 744 2256 2283 3827 119.0 821
Peewee November 1968 3 503 2400 1803 2539 4344 18.8 865
NF-1 June 1972 5 44 6528 2444 1.7 849

Source: [1]

Notes

  1. ^ a b Finseth 1991, p. C-2.
  2. ^ Dewar 2007, p. 7.
  3. ^ Everett, C. J.; Ulam, S.M. (August 1955). "On a Method of Propulsion of Projectiles by Means of External Nuclear Explosions. Part I" (PDF). Los Alamos Scientific Laboratory. {{cite journal}}: Cite journal requires |journal= (help)
  4. ^ a b Dewar 2007, p. 8.
  5. ^ Dewar 2007, p. 4.
  6. ^ "Leslie Shepherd". Telegraph. 16 March 2012. Retrieved 6 July 2019.
  7. ^ a b Dewar 2007, pp. 10, 217.
  8. ^ Bussard 1953, p. 90.
  9. ^ a b Bussard 1953, p. 5.
  10. ^ a b Bussard 1953, pp. 1–2.
  11. ^ Bussard 1953, p. ii.
  12. ^ a b Dewar 2007, pp. 10–11.
  13. ^ Dewar 2007, pp. 11–13.
  14. ^ a b Dewar 2007, pp. 17–19.
  15. ^ Corliss & Schwenk 1971, pp. 13–14.
  16. ^ a b c Dewar 2007, pp. 29–30.
  17. ^ Spence 1968, pp. 953–954.
  18. ^ Dewar 2007, p. 45.
  19. ^ Dewar 2007, p. 221.
  20. ^ a b c d e f g h Dewar 2007, pp. 17–21.
  21. ^ Dewar 2007, pp. 171–174.
  22. ^ a b Corliss & Schwenk 1971, p. 14.
  23. ^ a b c Dewar 2007, p. 61.
  24. ^ Dewar 2007, pp. 21–22.
  25. ^ a b Sandoval 1997, pp. 6–7.
  26. ^ a b Fishbine et al. 2011, p. 20.
  27. ^ Corliss & Schwenk 1971, p. 41.
  28. ^ Corliss & Schwenk 1971, pp. 14–15.
  29. ^ Dewar 2007, p. 23.
  30. ^ Logsdon 1976, pp. 13–15.
  31. ^ Brooks, Grimwood & Swenson 1979, p. 1.
  32. ^ Swenson, Grimwood & Alexander 1966, pp. 101–106.
  33. ^ Rosholt 1969, p. 43.
  34. ^ Rosholt 1969, p. 41.
  35. ^ a b Rosholt 1969, p. 67.
  36. ^ Ertel & Morse 1969, p. 13.
  37. ^ Rosholt 1969, p. 124.
  38. ^ a b Engler 1987, p. 16.
  39. ^ Rosholt 1969, pp. 254–255.
  40. ^ "Excerpt from the 'Special Message to the Congress on Urgent National Needs'". NASA. 24 May 2004. Retrieved 10 July 2019.
  41. ^ Koenig 1986, p. 5.
  42. ^ a b Finseth 1991, pp. 12–14.
  43. ^ Finseth 1991, pp. 17–21.
  44. ^ a b Finseth 1991, pp. 21–24.
  45. ^ Heppenheimer 1999, p. 106.
  46. ^ Dewar 2007, p. 47.
  47. ^ Koenig 1986, pp. 7–8.
  48. ^ Finseth 1991, pp. 24–32.
  49. ^ Dewar 2007, p. 63.
  50. ^ a b Paxton 1978, p. 26.
  51. ^ a b c d Dewar 2007, p. 64.
  52. ^ Finseth 1991, pp. 32–40.
  53. ^ Finseth 1991, pp. 40–47.
  54. ^ Dewar 2007, p. 67.
  55. ^ "Los Alamos remembers visit by JFK". LA Monitor. 22 November 2013. Retrieved 15 July 2019.
  56. ^ Dewar 2007, pp. 66–67.
  57. ^ Finseth 1991, p. 47.
  58. ^ Dewar 2007, pp. 67–68.
  59. ^ Finseth 1991, pp. 47–51.
  60. ^ Koenig 1986, pp. 5, 9–10.
  61. ^ Finseth 1991, pp. 53–57.
  62. ^ Orndoff & Evans 1976, p. 1.
  63. ^ Finseth 1991, p. 59.
  64. ^ a b c d Dewar 2007, pp. 82–85.
  65. ^ Chovit, Plebuch & Kylstra 1965, pp. I-1, II-1, II-3.
  66. ^ Dewar 2007, p. 87.
  67. ^ Finseth 1991, pp. 63–67.
  68. ^ Finseth 1991, pp. 67–70.
  69. ^ Finseth 1991, pp. 72–78.
  70. ^ Dewar 2007, pp. 108–109.
  71. ^ a b c d e Dewar 2007, pp. 110–112.
  72. ^ a b Finseth 1991, pp. 78–83.
  73. ^ a b c Koenig 1986, pp. 11–12.
  74. ^ Council on Environmental Quality 2007, p. 2.
  75. ^ Dewar 2007, pp. 110–1121.
  76. ^ Newell & Hollingsworth 1971, pp. 1–6.
  77. ^ a b Finseth 1991, pp. 83–88.
  78. ^ Koenig 1986, pp. 15–16.
  79. ^ Dewar 2007, pp. 91–97.
  80. ^ Dewar 2007, pp. 99–101.
  81. ^ a b Dewar 2007, pp. 115–120.
  82. ^ Heppenheimer 1999, pp. 178–179.
  83. ^ Koenig 1986, p. 7.
  84. ^ Heppenheimer 1999, p. 139.
  85. ^ a b Heppenheimer 1999, pp. 423–424.
  86. ^ Dewar 2007, pp. 123–126.
  87. ^ Heppenheimer 1999, pp. 270–271.
  88. ^ Dewar 2007, p. 130.
  89. ^ Dewar 2007, p. 207.
  90. ^ Haslett 1995, p. 3-1.
  91. ^ a b Haslett 1995, pp. 1–1, 2-1–2-5.
  92. ^ Lieberman 1992, pp. 3–4.
  93. ^ Haslett 1995, p. 3-7.
  94. ^ Smith, Rick (10 January 2013). "NASA Researchers Studying Advanced Nuclear Rocket Technologies". space-travel.com. Retrieved 15 July 2019.
  95. ^ Fishbine et al. 2011, p. 17.
  96. ^ "How long would a trip to Mars take?". NASA. Retrieved 15 July 2019.
  97. ^ Burke et al. 2013, p. 2.
  98. ^ Borowski, McCurdy & Packard 2013, p. 1.
  99. ^ Cain, Fraser (1 July 2019). "Earth to Mars in 100 days: The Power of Nuclear Rockets". phys.org. Retrieved 10 July 2019.
  100. ^ Foust, Jeff (22 May 2019). "Momentum grows for nuclear thermal propulsion". SpaceNews. Retrieved 10 July 2019.

References

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