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 Racer primary was developed in 1953 at Los Alamos for the first generation of US thermonuclear weapons, the Mark 14, Mark 16, and Mark 17 bombs. Racer was tested in the Upshot–Knothole series of tests with mock-up secondary stages as shots Nancy (Mark 14), Badger (Mark 16) and Simon (Mark 17). According to Chuck Hansen, during the Upshot-Knothole tests the Racer had proven to have inconsistent yield, varying from 23 kilotons in the Badger shot to 43 kilotons in the Simon shot.
The design was revised and RACER IV was the version used as the primary in the stockpiled Mark 14 and Mark 17 bombs.