Project Rover
Project Rover was a United States project to develop a nuclear-thermal rocket that ran from 1955 to 1973 at the Los Alamos Scientific Laboratory. It began as a United States Air Force project to develop a nuclear-powered upper stage for an intercontinental ballistic missile. The project was transferred to NASA in 1958 after the Sputnik crisis triggered the Space Race. It was managed by the Space Nuclear Propulsion Office, a joint agency of the Atomic Energy Commission, and NASA. Project Rover became part of NASA's Nuclear Engine for Rocket Vehicle Application project and henceforth dealt with the research into nuclear rocket reactor design, while NERVA involved the overall development and deployment of nuclear rocket engines, and the planning for space missions.
Nuclear reactors for Project Rover were built at LASL Technical Area 18, also known as the Pajarito Canyon Site. They were tested there at very low power and then shipped to Area 25 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 a custom test reactor, the Nuclear Furnace. Project Rover resulted in the development of three reactor types: Kiwi, Phoebus, and Pewee. Kiwi and Phoebus were large reactors, while Pewee was much smaller, conforming to the smaller budget available after 1968.
The reactors were fueled by highly enriched uranium, 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 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 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, and his allies, Senators Howard Cannon from Nevada and Margaret Chase Smith from Maine. This enabled it to survive multiple cancellation attempts that became ever 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 canceled over their objection in January 1973, and none of the reactors ever flew.
Beginnings
Early concepts
During World War II, some scientists at the Manhattan Project's Los Alamos Laboratory, including Stan Ulam, Frederick Reines and Frederic de Hoffmann, speculated about the development of nuclear-powered rockets, and in 1947, Ulam and Cornelius Joseph "C. J." Everett wrote a paper in which they considered using atomic bombs as a means of rocket propulsion. This became the basis for Project Orion. 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.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 an aircraft with a range of and a payload of, 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 neutron moderator, but assumed an operating temperature of, which was beyond the capabilities of available materials. The conclusion was that nuclear-powered rockets were not yet practical.
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.
Bussard report
In 1953, Robert W. Bussard, a physicist working on the Nuclear Energy for the Propulsion of Aircraft project at the Oak Ridge National Laboratory, wrote a detailed study. He had read Cleaver and Shepard's work, that of Tsien, and a February 1952 report by engineers at Consolidated Vultee. 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 of nuclear reactors. 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.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, a hydrogen-fueled nuclear engine could attain an exhaust velocity of 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 require protective cladding to withstand corrosion by the hydrogen propellant.
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. 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. Darol Froman, the Deputy Director of the Los Alamos Scientific Laboratory, 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.
Approval
Robert 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 from LASL; Edward Teller and Herbert York from Livermore; Abe Silverstein, the associate director of the National Advisory Committee for Aeronautics Lewis Flight Propulsion Laboratory; and Allen F. Donovan from Ramo-Wooldridge.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. 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. In March 1956, the Armed Forces Special Weapons Project recommended allocating $100 million to the nuclear rocket engine project over three years for the two laboratories to conduct feasibility studies and construction of test facilities.
Eger V. Murphree and Herbert Loper at the Atomic Energy Commission 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 seemed weak. However, the nuclear rocket had acquired a powerful political patron in Senator Clinton P. Anderson from New Mexico, the deputy chairman of the United States Congress Joint Committee on Atomic Energy, who was close to von Neumann, Bradbury and Ulam. He managed to secure funding.
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. Project Rover was directed by an active duty USAF officer on secondment 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 projects.
Design concepts
In principle, the design of a nuclear thermal rocket engine is quite simple: a turbopump would force hydrogen through a nuclear reactor, where it would be heated by the reactor to very high temperatures and then exhausted through a rocket nozzle to produce thrust. 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 way to store hydrogen was in liquid form, and this required a temperature below. The third was that the hydrogen would be heated to a temperature of around, and materials would be required that could withstand such temperatures and resist corrosion by hydrogen.Liquid hydrogen was theoretically the best possible propellant, but in the early 1950s it was expensive, and available only in small quantities. In 1952, the AEC and the National Bureau of Standards had opened a plant near Boulder, Colorado, to produce liquid hydrogen for the thermonuclear weapons program. Before settling on liquid hydrogen, LASL considered other propellants such as methane and ammonia. Ammonia, used in the tests conducted from 1955 to 1957, was inexpensive, easy to obtain, liquid at, and easy to pump and handle. It was, however, much heavier than liquid hydrogen, reducing the engine's impulse; it was also found to be even more corrosive, and had undesirable neutronic properties.
For the fuel, they considered plutonium-239, uranium-235 and uranium-233. Plutonium was rejected because while it forms compounds easily, they could not reach temperatures as high as those of uranium. Uranium-233 was seriously considered, as compared to uranium-235 it is slightly lighter, has a higher number of neutrons per fission event, and a high probability of fission. It therefore held the prospect of saving some weight in fuel, but its radioactive properties make it more difficult to handle, and in any case it was not readily available. Highly enriched uranium was therefore chosen.
For structural materials in the reactor, the choice came down to graphite or metals. Of the metals, tungsten emerged as the frontrunner, but it was expensive, hard to fabricate, and had undesirable neutronic properties. To get around its neutronic properties, it was proposed to use tungsten-184, which does not absorb neutrons. Graphite was chosen as it is cheap, gets stronger at temperatures up to, and sublimes rather than melts at.
To control the reactor, the core was surrounded by control drums coated with graphite or beryllium on one side and boron on the other. The reactor's power output could be controlled by rotating the drums. To increase thrust, it is sufficient to increase the flow of propellant. Hydrogen, whether in pure form or in a compound like ammonia, is an efficient nuclear moderator, and increasing the flow also increases the rate of reactions in the core. This increased reaction rate offsets the cooling provided by the hydrogen. As the hydrogen heats up, it expands, so there is less in the core to remove heat, and the temperature will level off. These opposing effects stabilize the reactivity and a nuclear rocket engine is therefore naturally very stable, and the thrust is easily controlled by varying the hydrogen flow without changing the control drums.
LASL produced a series of design concepts, each with its own codename: Uncle Tom, Uncle Tung, Bloodhound and Shish. By 1955, it had settled on a 1,500 megawatt design called Old Black Joe. In 1956, this became the basis of a 2,700 MW design intended to be the upper stage of an ICBM.