In 1946 the Soviet Union developed a series of freefall bombs in four sizes 250 kg (550 lb), 500 kg (1,100 lb), 1,500 kg (3,300 lb), and 3,000 kg (6,600 lb) and sharing a single nose and a single tail fuze. The bomb could be dropped from up to 12,000 m (40,000 ft) and up to 1,000 km/h (625 mph). The original, 1946-series bombs had poor ballistic characteristics at supersonic speed, and their construction was fragile. As an interim measure, upgraded versions of the bombs were built with thicker walls and no nose fuze. The thick-walled version of the bombs were built until 1956.