W71

The W-71 nuclear warhead was a US thermonuclear warhead developed at Lawrence Livermore National Laboratory in California and deployed on the LIM-49A Spartan missile, a component of the Safeguard Program, an anti-ballistic missile (ABM) defense system briefly deployed by the US in the 1970s.

The W-71 warhead was designed to intercept incoming enemy warheads at long range, as far as 450 miles (720 km) from the launch point. The interception took place at such high altitudes, comparable to low earth orbit, where there is practically no air. At these altitudes, x-rays generated by the nuclear reactions can destroy incoming reentry vehicles at distances on the order of 10 miles (16 km), which made the problem of guiding the missile to the required accuracies much simpler than earlier designs that had lethal ranges of less than 1,000 feet (300 m).

The W-71 warhead had a yield of around 5 megatons of TNT (21 PJ). The warhead package was roughly a cylinder, 42 inches (1.1 m) in diameter and 101 inches (2.6 m) long. The complete warhead weighed around 2,850 pounds (1,290 kg). The W71 produced great amounts of x-rays, and needed to minimize fission output and debris to reduce the radar blackout effect that fission products and debris produce on anti-ballistic missile radar systems.

The W71 design emerged in the mid-1960s as the result of studies of earlier high-altitude nuclear tests carried out before the Partial Nuclear Test Ban Treaty of 1963. A number of tests, especially those of Operation Fishbowl in 1962, demonstrated a number of previously poorly understood or underestimated effects. Among these was the behaviour of x-rays created during the explosion. These tended to react with the atmosphere within a few tens of meters at low altitudes (see rope trick effect). At high altitudes, lacking an atmosphere to interact with, the mean free path of the x-rays could be on the order of tens of kilometers.

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