W71

At these altitudes, x-rays resulting from the nuclear explosion 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).

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.

[4] This presented a new method of attacking enemy nuclear reentry vehicles (RVs) while still at long range from their targets.

[4] Bell received a contract to begin conversion of the earlier LIM-49 Nike Zeus missile for the extended range role in March 1965.

Because of a perceived need to rapidly deploy the system, the team took a "do it once, do it right" approach in which the original test items were designed to be the production models.

[4] The warhead for Spartan was designed by Lawrence Livermore National Laboratory (LLNL), drawing on previous experience from Operation Plowshare.

Project Plowshares had previously explored the design of such clean bombs as part of an effort to use nuclear explosives for civilian uses where the production of long-lived radionuclides had to be minimized.

To maximize the production of x-rays, the W71 is reported to have used a gold tamper,[citation needed] rather than the usual depleted uranium or lead.

[7] In 2008, the United States Department of Energy declassified the fact that the radiation case of the W71 contained thorium metal.