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The W-78 Warhead

Intermediate Yield Strategic ICBM MIRV Warhead

Last updated 1 September 2001

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Three Mk-12A RVs mounted on bus, Minuteman III shroud at right in second picture. The W-78 warhead itself is not visible.

The W-78 is the warhead used with the Mk 12A reentry vehicle which arms a portion of the Minuteman III ICBM force. Designed for use on MIRV (multiple independently targeted reentry vehicle) bus upper stage. The W-78 replaced part of the W-62 to provide an increased hard target kill capability by increasing both accuracy and yield.

W-78/Mk-12A Warhead/Reentry Vehicle Package
Yield335-350 Kilotons
Weight700-800 lb
RV Length71.3 in (181 cm)
RV Base Diameter21.3 in (54.1 cm)
Warhead Length67.7 in (172 cm)
Warhead Diameter21.3 in (54.1 cm)
Number In Service920

Design Features

Two stage radiation implosion weapon.

RV CEP (circle error probability) is 720 feet

RV carries penetration aids (decoys etc.).


Contains plutonium as primary fissile material
Fissile core is beryllium reflected
Deuterium-tritium boosted
Probably contains lithium-6 (95% enrichment) deuteride fusion fuel

Delivery Method

Minuteman III ICBM. Each missile carries up to three W-78s, and is based in a hardened underground silo.

Safeguards and Arming Features


Fuzing and Delivery Mode



Designed and developed by Los Alamos National Laboratory (LANL)

The W-78 is based on earlier secondary stage designs, probably including the W-50 Pershing warhead. The primary of the W-78 has been test fired, but the warhead has never been successfully tested at full yield. The W-78 prototype was first fired shortly before the May 1976 Threshold Test Ban Treaty went into effect, possibly in Keelson (4 February 1976) at 200 Kt, which was apparently a disappointing yield. Redesign followed, and LANL challenged LASL's estimate of the design's yield capability. Following a special review by a panel headed by former LLNL director John Foster which supported LASL's claims, production engineering was resumed.
W-78 Development Schedule
July 1974 Development engineering begun at LANL
November 1975 Production engineering begun
February 1976 Production engineering suspended
March 1977 Production engineering resumed
August 1979 First production units completed
September 1979 Quantity production begins
October 1982/th> Quantity production ends


Initial manufacture August 1979
Initial deployment in December 1979, replacing W-62s already deployed on Minuteman IIIs. Deployment completed in February 1983.
1083 W-78 warheads have been manufactured.
Currently in service: 920 warheads
Some of these warheads will probably continue in service under START II, the reminder will be kept in the ready "hedge" stockpile.