M55 rocket

The M55 rocket was a chemical weapon developed by the United States in the 1950s. The United States Army produced both Sarin and VX unitary warheads for the M55.

In 1951 the US Army Chemical Corps and Ordnance Corps initiated a joint program to develop a 115mm chemical rocket. The US Army Ordnance Corps designed the 115mm T238 and launcher in 1957 to provide the army a means to attack large area targets with chemical agents. Artillery and mortars are for small area targets; and due to different spin stabilities, warheads intended for explosives are not ideal for chemical delivery. The 115mm rocket was subsequently accepted as the M55 rocket with M91 launcher. Produced from 1959–1965, the M55s were manufactured at Newport Army Ammunition Plant and tested at Aberdeen Proving Ground. The Army produced unitary warheads filled with Sarin (GB) and VX nerve agents for the M55.

During the 1960s the Army stored many M55s at Black Hills Army Depot. The M55 was also stored at the Rocky Mountain Arsenal and in Okinawa, Japan. The rockets in Japan were moved to Johnston Atoll during Operation Red Hat where they were destroyed during the 1990s.

Disposal operations for the M55 are made more difficult because of the rocket's design. The rocket propellant was a double base composition nitroglycerin (NG) and nitrocellulose (NC) propellant. Besides the NG and NC, M28 contains 2-nitrodiphenylamine (NDPA) as a stabilizer. The rocket propellant cannot be removed from the warhead without cutting open the rocket.

The propellant itself presents a hazard, because it becomes unstable as it ages. Specifically, the danger of autoignition is present as the stabilizer ages and becomes depleted. The U.S. National Research Council and other sources called the M55 the most dangerous weapon in the American chemical arsenal because of this and other hazards.

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