In the mid-1950s, nuclear ramjet powerplants for cruise missiles were studied, and in January 1957 the development of such a weapon system was officially initiated as Project Pluto. The reactor for the ramjet was developed at the Lawrence Radiation Laboratory (which eventually became the Lawrence Livermore National Laboratory, LLNL), while ramjet itself would be built by Marquardt.
The initial reactor prototype was called TORY-IIA and ran for the first time in May 1961. TORY-IIA was a proof-of-concept powerplant not intended for an actual flight-rated ramjet, and was followed by the larger and more powerful TORY-IIC. The latter was run-up on the ground to full power on 20 May 1964. The TORY-IIC consisted of 465000 tightly packed small fuel rods of hexagonal section, with about 27000 air-flow channels between them to heat the incoming high-pressure airflow. For the ground tests, the airflow was provided by a huge reservoir of compressed air, and TORY-IIC produced a thrust of about 170 kN (38000 lb) at a simulated airspeed of Mach 2.8. TORY-IIC was intended for use in the first flight tests, but operational missiles would probably have used a further improved design called TORY-III. The latter was however still in the design phase when the whole program was cancelled.
While reactor development was going on, the USAF had selected an airframe contractor for the actual cruise missile. The latter was known as SLAM (Supersonic Low-Altitude Missile), but the project name Pluto was sometimes also used when referring to the missile. In 1963, Ling-Temco-Vought (LTV) was awarded the SLAM development contract.
SLAM was a wingless design optimized for Mach 3+ flight at 300 m (1000 ft) altitude. It featured a ventral air intake for the ramjet, three fixed stabilizing fins at the rear, and three small all-moving control fins near the tip. SLAM was to be launched by multiple solid-fueled rocket boosters, which would propel the missile to ramjet ignition speed. Several basing options (including air-launch) were considered for SLAM but most likely it would have been launched from some sort of hardened shelters on the ground. Flying at Mach 3+ at very low level, the missile would have to withstand very severe aerodynamic and thermal stresses, and it was therefore designed with a very sturdy structure (yielding the nickname of “Flying Crowbar”).
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.
Because the SLAM reactor would operate at high radiation levels without shielding, finding suitable electronics that could operate even for the few hours lifetime required was a daunting task. Careful selection and substitution of insulation materials, potting compounds, and semiconductors in a full complement of missile electronics such as guidance and control, telemetry and instrumentation was made with industry assistance. The largest radiation effects test ever conducted took place in 1964 in the Air Force’s NARF facility at General Dynamics under SLAM Program sponsorship. It was demonstrated that suitable system electronics were or could be made available for the SLAM mission.
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.
After launch, SLAM would cruise at around Mach 4 at high altitude (10700 m (35000 ft)) to the general target area. Effective range at high altitude was so large (more than 100000 km), that the missile could actually loiter at a “fail safe” point for some time, before it was ordered either to abort the mission or continue to the target. Close to enemy air defenses, SLAM would descend to low level, and use its TERCOM (Terrain Contour Matching) guidance system to find its way to the targets. TERCOM uses pre-stored radar maps of the ground under the planned flight path, which are constantly compared by the missile’s guidance system to the actual radar images. SLAM was to be equipped with multiple (between 14 and 26) thermonuclear warheads which would be ejected one by one as the missile flew over its assigned targets. The warheads were to be ejected from hatches on the top of the missile to follow a lofted trajectory to the ground. This would give the low-flying SLAM a few seconds of time to escape the blast of its own bombs.
Apart from the thermonuclear warheads, SLAM itself was also a very formidable weapon. The sonic boom of a 25+ m long vehicle flying at Mach 3+ at 300 m altitude would cause severe destruction in non-hardened structures on the ground. Additionally, the nuclear ramjet continuously left a trail of highly radioactive dust, which would seriously contaminate the area below the missile. Finally, when the SLAM eventually crashed itself at the end of the mission, it would leave a wreckage of a very hot and radioactive (“dirty”) nuclear reactor.
The original development plans had called for a first nuclear powered flight test of SLAM in 1967. However, by 1964, serious questions about the viability of the SLAM concept had emerged. By that time, ICBMs had been fielded which had the necessary range to strike deep into the Soviet Union and were effectively unstoppable once launched. Compared to the ballistic missiles, SLAM was slow and vulnerable, and wouldn’t be in service before 1970. Another major problem was that of flight testing. As explained in the previous paragraph, it was impossible to test the SLAM over land, and about the only option was to fly it over U.S. possessions in the Pacific and then drop it into the ocean. However, the idea of dumping tons of highly radioactive junk into the biosphere wasn’t sounding good even in the 1960s, and the question what to do when a flight test missile ran out of control couldn’t be satisfactorily answered anyway. To top it off, the whole Pluto/SLAM program was rather expensive, and so it’s not surprising that the program was finally cancelled in July 1964.
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. As a result, the neutron flux was calculated to vary from 9 x 1017 N/CM2 in the aft section to 7 x 1014 N/CM2 in the nose. Gamma ray energy was expected to be 4 x 1011 MEV in the aft section and 1.2 x 108MEV in the electronics compartment.
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.