Directory of U.S. Military Rockets and Missiles
Appendix 4: Undesignated Vehicles
SLAM / Pluto
Copyright © 2003 Andreas Parsch

Vought SLAM (Pluto)

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").

Photo: Vought

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.


Note: Design of the SLAM had not been finalized at the time of cancellation. The data below represents one proposed configuration with a TORY-III reactor in the engine.

Length26.8 m (88 ft)
Diameter (payload section)1.5 m (58 in)
Weight27540 kg (60780 lb)
Speedat 300 m (1000 ft): Mach 3.5
at 9000 m (30000 ft): Mach 4.2
Ceiling10700 (35000 ft)
Rangeat 300 m (1000 ft): 21300 km (11500 nm)
at 9000 m (30000 ft): 182000 km (98300 nm)
PropulsionBooster: solid-fueled rockets
Sustainer: nuclear-powered ramjet
WarheadMultiple (up to 26) thermonuclear bombs

Main Sources

[1] Gregg Harken: "The Flying Crowbar", article in Air & Space Magazine, April/May 1990
[2] Scott Lowther: "Project Pluto", article in Aerospace Projects Review V2N1, Jan/Feb 2000
[3] Vought Heritage Website

Back to Directory of U.S. Military Rockets and Missiles, Appendix 4

Last Updated: 19 April 2003