NERVA
The Nuclear Engine for Rocket Vehicle Application was a nuclear thermal rocket engine development program that ran for roughly two decades. Its principal objective was to "establish a technology base for nuclear rocket engine systems to be utilized in the design and development of propulsion systems for space mission application". It was a joint effort of the Atomic Energy Commission and the National Aeronautics and Space Administration, and was managed by the Space Nuclear Propulsion Office until the program ended in January 1973. SNPO was led by NASA's Harold Finger and AEC's Milton Klein.
NERVA had its origins in Project Rover, an AEC research project at the Los Alamos Scientific Laboratory with the initial aim of providing a nuclear-powered upper stage for the United States Air Force intercontinental ballistic missiles. Nuclear thermal rocket engines promised to be more efficient than chemical ones. After the formation of NASA in 1958, Project Rover was continued as a civilian project and was reoriented to producing a nuclear powered upper stage for NASA's Saturn V Moon rocket. Reactors were tested at very low power before being shipped to Jackass Flats in the Nevada Test Site. While LASL concentrated on reactor development, NASA built and tested complete rocket engines.
The AEC, SNPO, and NASA considered NERVA a highly successful program in that it met or exceeded its program goals. It demonstrated that nuclear thermal rocket engines were a feasible and reliable tool for space exploration, and at the end of 1968 SNPO deemed that the latest NERVA engine, the XE, met the requirements for a human mission to Mars. The program had strong political support from Senators Clinton P. Anderson and Margaret Chase Smith but was cancelled by President Richard Nixon in 1973. Although NERVA engines were built and tested as much as possible with flight-certified components and the engine was deemed ready for integration into a spacecraft, they never flew in space.
Origins
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. In 1946, Ulam and C. J. Everett wrote a paper in which they considered the use of atomic bombs as a means of rocket propulsion. This would become the basis for Project Orion.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 Shepherd, 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 although nuclear thermal rockets were essential for deep space exploration, they were not yet technically feasible.
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 on "Nuclear Energy for Rocket Propulsion". He had read Cleaver and Shepard's work, that of the Chinese physicist Hsue-Shen Tsien, and a February 1952 report by engineers at Consolidated Vultee. Bussard's study had little impact at first 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. The paper was still classified, as was the journal, but 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 LASL to assist for one week per month.
Bussard's study also attracted the attention of John von Neumann, who formed an ad hoc committee for 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, a federal agency that conducted aeronautical research; and Allen F. Donovan from Ramo-Wooldridge, an aerospace corporation. After hearing input on several designs, the Mills committee recommended in March 1955 that development proceed, with the aim of producing a nuclear rocket upper stage for an intercontinental ballistic missile. York created a new division at Livermore, and Bradbury created a new one called N Division at LASL under the leadership of Raemer Schreiber, to pursue it. In March 1956, the Armed Forces Special Weapons Project, the agency responsible for the management of the national nuclear weapons stockpile, recommended allocating $100 million to the nuclear rocket engine project over three years for the two laboratories to conduct feasibility studies and the 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 therefore seemed weak. However, the nuclear rocket had acquired a political patron in Senator Clinton P. Anderson from New Mexico. The deputy chairman of the United States Congress Joint Committee on Atomic Energy, Anderson was close to von Neumann, Bradbury and Ulam. He managed to secure funding in January 1957.
All work on the nuclear rocket was consolidated at LASL, where it was given the codename Project Rover; Livermore was assigned responsibility for the development of the nuclear ramjet, which was codenamed Project Pluto. Project Rover was directed by an active duty United States Air Force 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 projects.
Project Rover
Underlying concepts
s create thrust by accelerating a working mass in a direction opposite to their desired trajectory. In conventional designs, this is accomplished by heating a fluid and allowing it to escape through a rocket nozzle. The energy needed to produce the heat is provided by a chemical reaction in the fuel, which may be mixed together as in the case of most solid fuel rockets, or separate tanks as in most liquid fuel rockets. Selecting the fuels to use is a complex task that has to consider the reaction energy, the mass of the fuel, the mass of the resulting working fluid, and other practical concerns like density and its ability to be easily pumped.Nuclear rocket engines use a nuclear reactor to provide the energy to heat the fuel instead of a chemical reaction. Because nuclear reactions are much more powerful than chemical ones, a large volume of chemicals can be replaced by a small reactor. As the heat source is independent of the working mass, the working fluid can be selected for maximum performance for a given task, not its underlying reaction energy. Due to its low molecular mass, hydrogen is normally used. This combination of features allows a nuclear engine to outperform a chemical one; they generally aim to have at least twice the specific impulse of a chemical engine.
Design concepts
In general form, a nuclear engine is similar to a liquid chemical engine. Both hold the working mass in a large tank and pump it to the reaction chamber using a turbopump. The difference is primarily in that the reaction chamber is generally larger, the size of the reactor. 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 temperatures below. The third was that the hydrogen would be heated to a temperature of around, and materials were required that could both withstand such temperatures and resist corrosion by hydrogen.For the fuel, plutonium-239, uranium-235 and uranium-233 were considered. Plutonium was rejected because it forms compounds easily and could not reach temperatures as high as those of uranium. Uranium-233 is slightly lighter than uranium-235, releases a higher number of neutrons per fission event on average, and has higher probability of fission, but its radioactive properties make it more difficult to handle, and it was not readily available. Uranium-235 was therefore chosen.
For structural materials in the reactor, the choice came down to graphite or metal. Of the metals, tungsten emerged as the front runner, but it was expensive, hard to fabricate, and had undesirable neutronic properties. To get around its neutronic properties, it was suggested tungsten-184, which does not absorb neutrons, should be used. On the other hand, graphite was cheap, actually gets stronger at temperatures up to, and sublimes rather than melts at. Graphite was therefore chosen.
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. Moreover, 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.
NERVA incorporated a radiation shield to protect personnel and external components from the intense neutron and photon radiation it emitted. An efficient lightweight shield material was developed by the Aerojet Nuclear Systems Company from a mixture of boron carbide, aluminum and titanium hydride, known as BATH after its components. Titanium hydride is an excellent neutron moderator and boron carbide an excellent neutron absorber. The three components were mixed in powdered form and a commercial extrusion machine was used to extrude them into the desired shape. BATH was found to be strong, with a tensile strength of up to, capable of withstanding high temperatures, and with superior radiation shielding properties.
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 MW 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.