RACER IV was a component of several of the first hydrogen bombs made by the United States during the 1950s. It was the first stage for three of the devices tested in four shots of the Castle series. They were shots Castle Bravo (Shrimp device), Romeo (Runt device), Union (Alarm Clock device) and Yankee (Runt II). The design was revised and RACER IV was the version used as the primary in the stockpiled Mark 14 and Mark 17 bombs.