W78

W78 is a thermonuclear warhead deployed on the LGM-30G Minuteman III intercontinental ballistic missile (ICBM) as part of the United States strategic nuclear arsenal. Designed by the Lawrence Livermore National Laboratory, it entered the stockpile in 1979 to modernize the United States Air Force's land-based deterrent. The warhead is noted for its relatively high yield and its role in the Mk12A re-entry vehicle, forming a key component of America's nuclear triad during the late Cold War and beyond.

Design and development

The W78 was developed in the 1970s at the Lawrence Livermore National Laboratory under the auspices of the United States Department of Energy and its predecessor, the Energy Research and Development Administration. Its creation was part of a broader effort to enhance the accuracy, yield, and safety of the LGM-30G Minuteman III force, responding to advancements in Soviet Union missile defense and hardening. The design leveraged previous thermonuclear concepts, likely sharing some commonality with earlier warheads like the W62, but incorporated a new, more powerful secondary stage to achieve its specified yield. Development focused on improved reliability and integration with the advanced Mk12A re-entry vehicle, which offered better accuracy compared to the older Mk12 system used by the W62.

Specifications

The W78 is physically sized for the Mk12A re-entry vehicle, measuring approximately 68.9 inches in length and 21.3 inches in diameter, with an estimated weight between 700 and 800 pounds. Its explosive yield is reported in most declassified documents as 335 to 350 kilotons of TNT, significantly more powerful than the earlier 170-kiloton W62 it supplemented. The warhead uses an airburst detonation mode, optimized for maximum destructive effect against soft targets like industrial centers, airfields, and cities. It is a sealed-pit warhead, meaning its plutonium core is permanently encased, a design feature intended to enhance safety and security during handling and storage.

Deployment history

Production of the W78 occurred from 1979 through 1982, with approximately 1,080 units manufactured. Deployment began in 1979 with the United States Air Force's Strategic Air Command, initially equipping a portion of the LGM-30G Minuteman III missiles based at facilities such as Malmstrom Air Force Base, Minot Air Force Base, and F. E. Warren Air Force Base. The W78, housed in the Mk12A re-entry vehicle, was deployed alongside the lower-yield W62 in a mixed-force posture to complicate Soviet defense planning. Throughout the 1980s, it was a central element of the U.S. strategic posture during periods of heightened tension, such as the Soviet–Afghan War and the Strategic Defense Initiative era. Although some warheads were retired following the START I and New START treaties, the W78 remains in the active stockpile today, with ongoing programs like the W87-1 intended as its eventual replacement.

Variants

The W78 has no officially designated production variants, as it was produced to a single specification. However, it is part of a broader lineage of U.S. ICBM warheads. Its direct predecessor was the W62, and it shares a delivery system role with the later W87 warhead deployed on the LGM-118 Peacekeeper. There have been discussions and studies, such as those under the Stockpile Stewardship Program, concerning potential modifications or life-extension programs to maintain the weapon's reliability. The planned W87-1 warhead, intended for the new Ground Based Strategic Deterrent (GBSD) missile, is considered its functional successor, designed to offer similar or greater yield with enhanced safety features.

Category:Nuclear weapons of the United States Category:Nuclear warheads Category:Cold War nuclear weapons of the United States