Project Pluto

During the 1950s, the United States Air Force (USAF) considered the use of nuclear powered aircraft and missiles. as part of its Aircraft Nuclear Propulsion project, which was coordinated by its Aircraft Nuclear Propulsion Office. Research into missiles was coordinated by its Missile Projects Branch.[1] The concept of using a nuclear reactor to provide a heat source for a ramjet was explored by Frank E. Rom and Eldon W. Sams at the National Advisory Committee for Aeronautics Lewis Research Center in 1954 and 1955.[2][3] The principle behind the nuclear ramjet was relatively simple: motion of the vehicle pushed air in through the front of the vehicle (the ram effect). If a nuclear reactor heated the air, the hot air expanded at high speed out through a nozzle at the back, providing thrust.[4] The concept appeared feasible, so in October 1956, the USAF issued a system requirement, SR 149, for the development of a winged supersonic missile.[1]

At the time, the United States Atomic Energy Commission (AEC) conducted studies of the use of a nuclear rocket as an upper stage of an intercontinental ballistic missile (ICBM) on behalf of the USAF. The AEC farmed this work out to its two rival atomic weapons laboratories: the Los Alamos Scientific Laboratory (LASL) and the Lawrence Radiation Laboratory, the predecessor of the Lawrence Livermore National Laboratory. By late 1956 improvements in nuclear weapon design had reduced the need for a nuclear upper stage, and the development effort was concentrated at LASL, where it became known as Project Rover.[5]

On 1 January 1957, the United States Air Force (USAF) and the United States Atomic Energy Commission (AEC) selected the Livermore Laboratory to study the design of a nuclear reactor to power ramjet engines. Keeping the theme of dog-related names, this research became known as Project Pluto.[4] It was directed by Theodore C. (Ted) Merkle, leader of the Livermore Laboratory's R-Division.[6]

Unlike commercial reactors, which are surrounded by concrete, the Pluto reactor had to be small and compact enough to fly, but durable enough to survive a 11,000-kilometre (7,000 mi) trip to a potential target. The nuclear engine could, in principle, operate for months, so a Pluto cruise missile could be left airborne for a prolonged time before being directed to carry out its attack.

The success of this project would depend upon a series of technological advances in metallurgy and materials science. Pneumatic motors necessary to control the reactor in flight had to operate while red-hot and in the presence of intense radiation. The need to maintain supersonic speed at low altitude and in all kinds of weather meant that the reactor, code-named "Tory", had to survive high temperatures and conditions that would melt the metals used in most jet and rocket engines. Ceramic fuel elements would have to be used; the contract to manufacture the 500,000 pencil-sized elements was given to the Coors Porcelain Company.[4]

Originally carried out at Livermore, California, the work was moved to new facilities constructed for $1.2 million on 21 square kilometres (8 sq mi) of Jackass Flats at the NTS, known as Site 401. The complex consisted of 10 kilometres (6 mi) of roads, critical-assembly building, control building, assembly and shop buildings, and utilities. Also required for the construction was 40 kilometres (25 mi) of oil well casing, which was necessary to store the approximately 450,000 kilograms (1,000,000 lb) of pressurized air used to simulate ramjet flight conditions for Pluto.[4]

Pluto SLAM (LASV) Test, 1963. The tests studied the aerodynamic characteristics of a Supersonic Low Altitude Missile (SLAM) configuration that was to be powered by the nuclear ramjet engines developed in Project Pluto

The proposed use for nuclear-powered ramjets would be to power a cruise missile, called SLAM, for Supersonic Low Altitude Missile. In order to reach ramjet speed, it would be launched from the ground by a cluster of conventional rocket boosters. Once it reached cruising altitude and was far away from populated areas, the nuclear reactor would be made critical. Since nuclear power gave it almost unlimited range, the missile could cruise in circles over the ocean until ordered "down to the deck" for its supersonic dash to targets in the Soviet Union. The SLAM, as proposed, would carry a payload of many nuclear weapons to be dropped on multiple targets, making the cruise missile into an unmanned bomber.

It was proposed that after delivering all its warheads, the missile could then spend weeks flying over populated areas at low altitudes, causing secondary damage from radiation. The output at exhaust of any value of M>1 with an unfiltered fuel source, as Pluto was designed, consisting of a reactor of any size sufficient to create the desired Mach number would create significant radioactive fallout along its trajectory regardless of exhaust nozzle configuration even though that configuration would affect dispersal patterns. However, it was believed the high speed of the missile would spread direct radiation from the reactor over a large territory, keeping ground exposure to low levels. Little to no fallout would be created as the reactor elements would have to be resistant to the airstream in order to function for any time. When the vehicle would eventually crash after exhausting its fuel or due to a mechanical failure, a local radiation hazard would be created by the reactor. Compared to the primary payload, the effect would not be significant.

The apparent discrepancy of information regarding collateral radioactive damage and the minimizing of its effects in online documents arises from the fact that most sources referencing the Pluto project use official government project information. This was released at a time when the project scientists themselves were still coming to a more accurate and consistent appreciation of radioactive fallout.

The Tory-IIA prototype

On 14 May 1961, the world's first nuclear ramjet engine, "Tory-IIA", mounted on a railroad car, roared to life for a few seconds. Three years later, "Tory-IIC" was run for five minutes at full power.

Despite these and other successful tests, Department of Defense, sponsor of the Pluto project, had second thoughts. The weapon was considered "too provocative",[7] and it was believed that it would compel the Soviets to construct a similar device.[8] At the same time Intercontinental ballistic missile technology had proven to be more easily developed than previously thought, reducing the need for such highly capable cruise missiles. The ICBM has several advantages over the SLAM. An ICBM required less ground support and maintenance, and could be launched in minutes instead of several hours, and so was less vulnerable to a nuclear first strike. An ICBM also traveled to its target faster and was less vulnerable to interception by Soviet air defenses. The main advantage of the SLAM was its ability to carry a larger payload (up to 16 warheads) but the value of this was diminished by improvements in nuclear weapon design, which made them smaller and lighter, and the subsequent development of multiple warhead capability in ICBMs.[9]

The other major problem with the SLAM concept was the environmental damage caused by radioactive emissions during flight, and the disposal of the reactor at the end of the mission. Although small compared to that produced by a thermonuclear explosion, it was a problem for testing. It was anticipated that numerous test flights would be required. The initial plan was to conduct them over a remote part of the North Pacific Ocean, ending with the test vehicle being dumped into the ocean in one of the areas where the US conducted atmospheric nuclear tests. However, by the early 1960s there was increasing awareness of the undesirable environmental impacts of radioactive contamination of the atmosphere and the ocean, and these were considered unacceptable wherever the tests were conducted. Coupled with the limitations of the concept, this led to the decision by the Department of Defense and the Department of State to terminate the project.[9] On 1 July 1964, seven years and six months after it was started, Project Pluto was canceled.[4]

• Dewar, James (2007). To The End of the Solar System: The Story of the Nuclear Rocket (2nd ed.). Burlington, Ontario: Apogee. ISBN 978-1-894959-68-1. OCLC 1061809723.
• Hacker, Barton C. (1995). "Whoever Heard of Nuclear Ramjets? Project Pluto, 1957—1964". Journal of the International Committee for the History of Technology. 1: 85–98. ISSN 1361-8113. JSTOR 23786203.
• Harkins, Hugh (2019). SLAM, Project Pluto and the Uninhabited Nuclear Powered Bomber. London: Centurion Publishing. ISBN 978-1-903630-50-1. OCLC 1286799595.
• Herken, Gregg (April–May 1990). "The Flying Crowbar". Air & Space Magazine. Vol. 5, no. 1. pp. 28–34, 54. Retrieved 5 April 2022.
• Merkle, T. C. (30 June 1959). The Nuclear Ramjet Propulsion System (Report). OSTI 4217328.
• United States Congress (1958). Hearings Before Subcommittees of the Joint Committee on Atomic Energy Congress of the United States Eighty-Fifth Congress Second Session On Outer Space Propulsion By Nuclear Energy January 22, 23, And February 6, 1958. Washington: U.S. Government Printing Office. Retrieved 6 April 2022.

Wikimedia Commons has media related to Project Pluto.

• Directory of U.S. Military Rockets and Missiles
• Vought SLAM pages
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