spectacularly when the second stage ignited on the pad and took off by itself. Range Safety blew up the errant rocket while the first stage sat on the pad and burned. The third and fourth tests (December 30 and January 9) had problems due to overheating in the boattail section. This necessitated adding extra shielding and insulation to wiring and other components. When the final AX flight was conducted a year after the program began, 17 Polaris missiles had been flown of which five met all of their test objectives. The first operational version, the Polaris A-1, had a range of and a single Mk 1 re-entry vehicle, carrying a single W-47-Y1 600 kt nuclear warhead, with an inertial guidance system which provided a circular error probable (CEP) of . The two-stage solid propellant missile had a length of , a body diameter of , and a launch weight of . was the first fleet ballistic missile submarine (SSBN in U.S. naval terminology) and she and all other Polaris submarines carried 16 missiles. Forty more SSBNs were launched in 1960 to 1966. Work on its W47 nuclear warhead began in 1957 at the facility that is now called the Lawrence Livermore National Laboratory by a team headed by John Foster and Harold Brown. The Navy accepted delivery of the first 16 warheads in July 1960. On May 6, 1962, a Polaris A-2 missile with a live W47 warhead was tested in the "Frigate Bird" test of Operation Dominic by in the central Pacific Ocean, the only American test of a live strategic nuclear missile. The two stages were both steered by thrust vectoring. Inertial navigation guided the missile to about a 900 m (3,000-foot) CEP, insufficient for use against hardened targets. They were mostly useful for attacking dispersed military surface targets (airfields or radar sites), clearing a pathway for heavy bombers, although in the general public perception Polaris was a strategic second-strike retaliatory weapon. After Polaris To meet the need for greater accuracy over the longer ranges the Lockheed designers included a reentry vehicle concept, improved guidance, fire control, and navigation systems to achieve their goals. To obtain the major gains in performance of the Polaris A3 in comparison to early models, there were many improvements, including propellants and material used in the construction of the burn chambers. The later versions (the A-2, A-3, and B-3) were larger, weighed more,