SLAM Supersonic Low-Altitude Missile
Studies of the feasibility of using nuclear power for propulsion officially began in New York City in May 1946 but were moved to Oak Ridge, Tennessee, in September of that year to be at the source of nuclear technology. The NEPA (Nuclear Energy for Propulsion of Aircraft) Project made numerous studies of the direct air cycle in which air is heated by conduction as it passes through a nuclear reactor. The design of ceramic reactors led to the possibility of a nuclear ramjet with unlimited range. In November of 1955 the U.S. Office of Strategic Development asked the Atomic Energy Commission to determine the feasibility of this concept.
By October 1956 the world situation was such that the U.S. Air Force issued a System Requirement (SR #149) for a nuclear-powered winged missile. Further internal Air Force studies and reactor development at General Electric's Aircraft Nuclear Propulsion Project and later at the Lawrence Radiation Laboratory of the University of California indicated overall feasibility of the nuclear reactor. The Cold War situation at the time dictated the need for a strategic missile with positive deterrence or retaliation capability. Chance Vought also recognized the need and in 1957 formed a study group under Dr. Walt Hesse to do unfunded studies. These and studies at other aircraft companies resulted in the United States Air Force issuing Requests for Proposal which were sent out to the aircraft industry.
In August 1958, Chance Vought Aircraft, North American Aviation, and Convair were selected to conduct funded studies of a low-altitude nuclear-powered strategic missile for a mission no chemical-powered vehicle could perform.
In early 1961 another competition was held among the three aircraft companies for a contract to study and demonstrate the feasibility of the missile airframe and subsystems. This competition was won by Chance Vought Aircraft and a contract was awarded in April 1961, titled "Aerothermo-dynamics for Pluto". Pluto was the code name of the ceramic reactor development project then being done at Lawrence Radiation Laboratory.
Studies, design and tests of a nuclear- powered strategic missile weapon system were conducted at Chance Vought Aircraft during the period from early 1956 to mid-1964. During this period all the technical unknowns were evaluated and shown to be solvable. A conceptual design of a missile was completed and a test nuclear reactor for propulsion was operated at full power.
No airframe had been designed to operate in the environment of Mach 3 at sea level where skin temperatures reach 1,000 Fahrenheit and the sound pressure level is on the order of 162 db. Aerodynamics in this flight regime was little explored. Almost 1600 hours of wind tunnel testing in all the national laboratories resulted in a canard configuration design that could operate in the planned flight profile. The classical spike inlet was replaced with a scoop-type inlet invented in the program, which gave pitch/yaw performance over a wider range and a pressure recovery of 86% that was much higher than the initial program objective. An extensive materials investigative program resulted in the selection and fabrication of a section of fuselage using Rene 41 stainless steel with a skin thickness of 1/10 to ¼ inch. This was strength- tested in a furnace to simulate aerodynamic heating. Forward sections of the missile were to be gold plated to dissipate heat by radiation. A 1/3-scale model of the missile nose, inlet and duct was constructed and wind tunnel tested. A preliminary inboard design of the complete weapon system missile was made to show location of all equipment and hardware, including the hydrogen weapons. A detailed and final design would have been required.
To deliver multiple warheads with precision over long ranges required a dual guidance system. Inertial systems were available but were not capable of surviving in the harsh radiation environment. The impetus of the program resulted in the development of gas dynamic bearings for gyroscopes, and radiation-resistant, or "hardened" components which were evaluated in the Air Force NARF facility. These tests showed that inertial guidance systems could be made which would satisfy the mission requirements if midcourse and terminal corrections could be made. The Vought- funded studies associated with SLAM developed a precise system for such an application. This system was patented under the name of FINGERPRINT. The name was changed to TERCOM when the rights were assigned to the U.S.A.F. and is still known today by that name when used in the cruise missile. The system employs terrain contour information along the flight path stored in a digital matrix. A matrix of terrain elevations was concluded to be as distinctive as the human fingerprint. Elevations of all land areas of the earth were available from contour maps. Downward-looking radar on the missile then compares the real elevations with the stored data and the missile position is determined and corrections made to direct it toward the target.
Several TERCOM fixes could be made as SLAM proceeded to multiple targets. Extensive flight testing over all types of terrain, with and without snow cover, verified that accurate missile locations could be obtained. All the required hardware was verified in the NARF facility as being suitable for operation in a radiation environment.
The source of energy for SLAM propulsion was to be a nuclear fission reactor operating at a power level of 600 Megawatts. The reactor was not to have radiation shielding for the fission products of neutrons and gamma rays. This required careful selection of materials which could survive not only the high temperatures but also the high radiation levels. The study program investigated all missile subsystems. Some very sensitive ones required a feasible amount of local shielding. The result of the investigations led to the conclusion that missile subsystems were available or could be made available for the SLAM application. Flight testing of the missile was planned to be conducted over the northwest Pacific ocean with termination in deep ocean waters in the neighborhood where atmospheric testing of nuclear weapons had taken place.
The reactor development work for nuclear propulsion systems was started by the NEPA Project and specific development for nuclear ramjet application at the Aircraft Nuclear Propulsion Department of The General Electric Company. As the ramjet program gained in importance, it was moved to the Lawrence Radiation Laboratory (LRL) of the University of California in January 1957. In 1957, the Lawrence Radiation Laboratory (LRL) was requested by the U. S. Atomic Energy Commission to develop a nuclear reactor capable of propelling a ramjet vehicle. Attention was directed to a vehicle that would travel at supersonic speeds over long distances and at very low altitudes. Ambient air would enter at the intake end and pass through the reactor, where it would be heated to high temperatures and forcibly discharged rearward through a nozzle, imparting thrust to the vehicle. Enough thrust to transport large thermonuclear warheads was desired. To meet these ends, a program was established at LRL to design, develop, and test a series of high-power, high-temperature nuclear reactors. The first of these, Tory II-A, was successfully tested several times during 1961 at the Nevada Test Site (NTS). This reactor was designed with facility limitations in mind, but nevertheless provided pertinent information on fuel element integrity under realistic operating conditions. The engineering feasibility of the engine concept was thereby established, and invaluable design and operational experience was acquired. Not only were the desired operating conditions attained, but the reactor itself withstood extreme temperature and power levels for the desired time. The Tory II-C reactor consisted of a reflector core, structural components, and a reactor duct. The duct was a flanged circular cylinder 103 inches long and 57 inches in diameter, and contains the fueled core which was 51 inches long and 47-1/2 inches in diameter. Tory II-C, the second reactor of the Pluto program, was meant to be adequate as an engine for a ramjet vehicle. Problems such as control rod operation within the reactor environment, which were avoided in the Tory II-A design, are inherent in Tory II-C. The reactor was tested in the early months of 1964, before the Pluto program was terminated in July 1964. The tests were eminently successful.
The two reactor tests were conducted to verify feasibility. Tory II-A was a scaled-down test which was conducted in mid-1961 and operated at design conditions on October 5, 1961. Tory II-C was a full-scale reactor test for a period of 292 seconds which was the limit of the air supply from the storage facility. That facility stored 1.2 million pounds of air which had to be preheated to 943 degrees Fahrenheit and supplied at a pressure of 316 psi to simulate ramjet inlet diffuser conditions. Tests were conducted at Jackass Flats in the Nevada Test Station by Lawrence Radiation Laboratory. These tests demonstrated the feasibility of the nuclear power-plant for the SLAM weapon system.
LRL's working with Chance Vought for missile propulsion requirements resulted in the following nuclear reactor characteristics for the SLAM weapon system:
Diameter----------------------57.25 in. Fissionable Core-------------47.24 in. Length-------------------------64.24 in. Core Length------------------50.70 in. Critical Mass of Uranium--59.90 kg. Avg. Power Density---------10 MW/cubic foot Total Power-------------------600 MW Avg. Element Temperature- 2,330 deg. F
The fuel elements for the test reactors were made of the high-temperature ceramic beryllium oxide (BO). This was mixed with enriched uranium di-oxide (UO2) in a homogeneous mixture with a small amount of zirconium di-oxide (ZrO2) for stabilization. This mixture in a plastic mass was extruded by the Coors Porcelain Company under high pressure and then sintered to near theoretical minimum density. Each fuel element was a hollow hexagonal tube approximately 4 inches long, 0.3 inches across flats, and had an inside diameter of 0.227 inches. These were stacked end to end to provide the 50.7 inch length of heated air passage. There were 27,000 of these heated airflow channels and 465,000 individual fuel elements. The design with these small unattached pieces was such that the problems of thermal stress in ceramics was minimized.
The Tory II-C reactor was designed for flight capability at low-altitude, hot-day Mach-3 conditions for periods of 3 to 10 hours. It must sustain a thrust load of 280,000 pounds due to air flow, air pressure up to 342 psia, and lateral manuver loads as high as 4 g. This force picture appears in a high-temperature environment where naetal temperatures range to 2370° F and ceramic temperatures range to 265 0° F. All cooling is accomplished with ram air, the temperature of which is 1060°F. About 47.5% of the gross reactor volume consists of hollow, hexagonal, beryllium-oxide tubes. These comprise the homogeneously fueled moderator and most of the reflectors making up the reflected core.
All the major areas had been investigated by mid-1964 and the feasibility of nuclear flight was firmly established, laying a foundation for proceeding with a detailed design and flight test. But the world was beginning to change with the Cuban missile crisis in the past, the development of long- range ballistic missiles, and the advent of space exploration. The concept of releasing radioactive fission products in the atmosphere in any locale was being rejected as more was learned of the effects of their release.
The program was terminated in July 1964 by the Department of Defense and the State Department as "being too provocative". It was believed by many that if the U.S. deployed a missile of such awesome power against which there was no known defense, then the Soviets would be compelled to do so. At the end of the project, Chance Vought had 177 engineers and scientists involved in the program full time. It was called "a model technology program" by the Department of Defense. Much of the technology, especially that of the guidance system, TERCOM, is utilized in the cruise missile that is part of today's arsenal of U.S. weapons.
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