The B-52 first entered service in June 1955, now only 2 versions remain in service with its successor, ACC. A total of 193 J57 powered B-52G were built with integral wing tanks and short fin, and armed with four 0.50-in machine guns. In desert Storm, all missions were assigned to the G model, operating as free-fall bombers. The B-52G is well protected by numerous ECM systems, and 2 under nose 'blisters' house LLLTV and FLIR sensors - used with the terrain avoidance radar to provide low-level penetration capability. Some B-52 were used in many aircraft testing as motherships to carry them up to testing altitude, such as the X-15 program.d