|Fig. 1: Schematic sketch of a shieldless nuclear ramjet missile, precursor to SLAM.  (Courtesy of NASA)|
Increasing concern about the human and economic cost of maintaining permanent land-based military forces in Asia led the Eisenhower administration (1953-1961) to abandon the "local defense" military doctrine (based on projecting power with ground-based forces) in favor of the "massive retaliation" paradigm, which relied on establishing the credible threat of a nuclear escalation in response to any hostile act from the U.S.S.R. or its allies. 
Central to this doctrine was the ability to deliver effective nuclear strikes in Soviet territory, both as a first strike and in a retaliatory manner. The U.S. Strategic Air Command (SAC) explored several offensive concepts, generally divided into two camps: "high altitude" attacks, carried out from above the stratosphere, and "low and fast" attacks where attackers would rely on speed and ground cover to escape detection. While the former paradigm eventually prevailed, with Inter-Continental Ballistic Missiles (ICBMs) providing the bulk of the U.S.'s first-strike and retaliation capability from the mid-Sixties to the present day, several fascinating concepts in the latter field were developed in the early years of the Cold War. The Supersonic Low Altitude Missile is one of the most fascinating products of this age: a Mach-3 nuclear-powered ramjet, it was designed to deliver up to 42 nuclear warheads deep into Soviet territory and simultaneously expose vast swaths of Soviet land to devastating sonic booms and highly radioactive exhaust products.
The concept was initially proposed in NACA reports in 1954 and 1955. [2,3] Fig. 1 shows a preliminary airframe design. Development of the reactor (codenamed Project Pluto) was entrusted to the Lawrence Radiation Laboratory, whereas airframes were developed by Ling-Temco-Vought and Convair.
|Fig. 2. Top: Diagram of a typical gas turbine jet engine. (Source: Wikimedia Commons). Bottom: Conceptual arrangement of a nuclear ramjet.  (Courtesy of the DOE)|
Conventional airbreathing turbojet engines consist of three main components: a compressor that increases the pressure and reduces the speed of the air entering the engine, a reaction chamber where the fluid's enthalpy is increased by adding heat (typically via chemical combustion) and a turbine that extracts part of the enthalpy from the fluid to power the compressor. The remaining enthalpy is converted to kinetic energy in a propulsive nozzle; by conservation of momentum, the acceleration imparted to the air results in a net thrust on the engine.
In ramjets, both the compressor and the turbine are removed: a static inlet with a special geometry is used to slow down the incoming airflow and increase its pressure, whereas the reaction chamber discharges directly in the propulsive nozzle. Ramjets require a high-speed impinging flow of air to operate: therefore, they cannot propel a vehicle from a standstill. On the other hand, they are especially efficient in supersonic flight, where shockwaves provide an effective way of reducing the speed of the impinging air and conventional compressors struggle to operate due to compressibility effects. 
In the SLAM concept (shown in Fig. 2), the combustion chamber was replaced by an open-core nuclear reactor: the airflow was allowed to traverse the core of the reactor, operating at 1650 K, and the resulting heated, radioactive fluid was then directed to a propulsive nozzle.
|Fig. 3: Core of the Tory II-C test reactor, a full-scale model of SLAM's proposed nuclear reactor.  (Courtesy of the DOE)|
Designing a nuclear reactor capable of supersonic flight presents a set of unique challenges: 
Strict mass and volume budgets. Both mass and volume are at a premium in any flying craft, even more so when high performance is desired.  This requires the design of a lightweight reactor with a small frontal cross-section.
Tremendous load from air drag. The airflow from the diffuser impinges directly on the head of the reactor, generating a pressure of 3.8 MPa at Mach 3, and crosses the entire reactor, subjecting the reactor to both compressive loads and shear loads due to internal ablation.
Very high thermal stresses. The efficiency of ramjet engines increases with temperature and in particular with the temperature difference between the core and the airflow: the temperature gradient induces extremely high mechanical loads on the reactor itself.
Chemical oxidation. Exposing the reactor core to a high-temperature oxygen-rich airflow significantly accelerates oxidation phenomena, which can compromise the mechanical properties of the reactor.
Very high lateral loads. The reactor is housed in a high-speed, maneuverable craft: it can be exposed to lateral accelerations of several g's during turns and in presence of turbulence.
Radiation leakage. In the SLAM ramjet concept, air passes directly through the reactor core and is then expelled through the nozzle. If the ramjet is to fly over allied territory, it is imperative to minimize radiation leakage (mainly from ablated reactor material) in the exhaust plume.
|Fig. 4: Ling-Temco-Vought proposal for the SLAM airframe. (Source: Wikimedia Commons).|
These constraints led to a unique design: a homogeneous reactor architecture, where the fuel and the moderator are mixed homogeneously, was selected because of its compactness. Beryllium oxide (BeO), a ceramic, was selected as the only suitable moderator able to withstand mechanical and thermal loads and oxidation. Furthermore, a complex reactor geometry with nearly 500,000 hexagonal fuel elements (shown in Fig. 3) was selected to ensure good structural and aerodynamic properties. We refer the interested reader to a technical report by the director of Project Pluto, Ted Merkle, for a fascinating and in-depth discussion of the constraints driving the design of the SLAM reactor. 
The twin issues of radiation containment and waste disposal, critical in civilian applications, were actually considered as part of the vehicle's offensive capability. The radioactive exhaust plume (together with the sonic boom) was meant as a way to inflict widespread damage to vast swaths of enemy land between nuclear bomb drops; conventional boosters would be used to propel the vehicle during its initial flight over allied territory.
Its mission exhausted, the SLAM would crash into the ground and its radioactive core would be destroyed: the resulting contamination would further contribute to the vehicle's destructive potential.
|Fig. 5: The Tory II-A test vehicle. (Source: Wikimedia Commons)|
|Fig. 6: Schematic of the Tory II-C test vehicle.  (Courtesy of the DOE)|
Designing a flying craft around an open-core reactor presents its own set of challenges: in particular, the vehicle's structure must withstand very high thermal loads due both to conduction, gamma and neutron heating (in addition to the external thermal loads due to aerodynamic effect) and navigation equipment must be radiation-hardened to survive the radiations emitted by the reactor. 
Airframe development was entrusted to two private corporations, Ling-Temco-Vought (LTV) and Corsair. However, no SLAM design was approved before the project's cancellation. As a result, relatively little information is available regarding the SLAM airframe concepts beyond general descriptions of the vehicle's capabilities. 
Fig. 4 shows LTV's proposed airframe for the SLAM missile. Three solid boosters (visible in the front section) were designed to propel the missile in the initial phase of flight over allied territory; the reactor would then be brought to criticality as the craft approached enemy territory.
The propulsion subsystem of SLAM was surprisingly mature at the time of the project's cancellation. [9-11] Both a subscale prototype (Tory II-A, shown in Fig. 5) and a full-scale prototype (Tory II-C, shown in Fig. 6) of the nuclear engine were designed and manufactured by the Lawrence Radiation Laboratory (now the Lawrence Livermore National Laboratory) and tested in an ad-hoc facility the Nevada desert (shown in Fig. 7). In order to simulate the airflow that the reactor would experience flying at Mach 3 at sea level, an air storage facility capable of storing 544,000 kg of air at a pressure of 25 MPa was manufactured; the air was heated to 810 K passing through a chamber containing 900,000 kg of heated steel ball bearings. A fully automated remote testing facility, including an unmanned railway to move the reactor from the assembly building to the test site, was built to ensure operator safety during reactor tests. Several tests were conducted, culminating in a full-power test on May 20, 1964 where the reactor achieved a steady-state power output of 461 MW and an internal fuel temperature of 1690 K for approximately three minutes. A fascinating report, declassified a decade after the cancellation of the project, gives a full account of the experimental activity conducted in the Nevada desert. 
|Fig. 7: Map of the Project Pluto test site in Jackass Flats, NV.  (Courtesy of the DOE)|
Project Pluto (and, as a result, SLAM) was canceled on July 1, 1964.  Advances in over-the-horizon radar technology made low-altitude cruise missiles vulnerable to anti-aerial defenses; at the same time, suborbital ICBMs reached sufficient maturity to present a credible threat to U.S.S.R. targets. Thus, the U.S. Air Force canceled virtually all programs developing "low and fast" weapons delivery systems, including SLAM.
While other canceled projects, e.g. the B-70 Valkyrie bomber, enjoyed a second life as civilian research programs, Project Pluto's legacy is virtually nonexistent. [8,12] The same reasons that made SLAM a formidable weapon strongly hindered adoption of its technology in peaceful applications.
Yet SLAM and in particular its nuclear engine, Project Pluto, remain a fascinating (if somewhat terrifying) testament to the ingenuity of nuclear scientists and military planners in the early years of the Cold War.
© Federico Rossi. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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