M55 (rocket)

An M55 rocket containing Sarin being destroyed at Johnston Atoll in 1990.

During the 1960s the Army stored many M55s at Black Hills Army Depot.[2] The M55 was also stored at the Rocky Mountain Arsenal and in Okinawa, Japan.[2] 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.[1] The rocket propellant was a double base composition nitroglycerin (NG) and nitrocellulose (NC) propellant.[2] Besides the NG and NC, M28 contains 2-nitrodiphenylamine (NDPA) as a stabilizer.[4] The rocket propellant cannot be removed from the warhead without cutting open the rocket.[5]

The propellant itself presents a hazard, because it becomes unstable as it ages.[6] Specifically, the danger of autoignition is present as the stabilizer ages and becomes depleted.[7] 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.[6][7]

Another danger is agent leakage. Army reports have indicated that nerve agent GB can corrode the metal casings of the munitions over time.[1] As Sarin decomposes it forms acids which can corrode the aluminum casings found around the agent in the M55.[6][8] M55 rockets containing GB have accounted for the majority of leaking American chemical weapons.[6] In mid-2002, over 4,000 munitions in the U.S. chemical stockpile were found to be leaking agent; of that number 2,102 were Sarin-containing M55s.[8]

The United States signed the Chemical Weapons Convention in 1993, which obligated it to destroy all chemical weapons by an extended deadline of April 2012. The U.S. missed the deadline, but completed the destruction of all declared chemical weapons on July 7, 2023 at Blue Grass Army Depot in Kentucky.[9][10][11] The final chemical weapon to be destroyed was a sarin nerve agent-filled M55 rocket.[12]