The missile also had an optional nuclear warhead to improve the probability of a kill. The W-31 warhead had four variants offering 2, 10, 20 and 30 kiloton yields. The 20 kt version was used in the Hercules system. At sites in the United States the missile almost exclusively carried a nuclear warhead. Sites in foreign nations typically had a mix of high explosive and nuclear warheads. The fire control of the Nike system was also improved with the Hercules and included a surface-to-surface mode which was successfully tested in Alaska. The mode change was accomplished by changing a single plug on the warhead from the "Safe Plug" to "Surface to Air" or "Surface to Surface".