RACER IV was a component of some of the first hydrogen bombs made by the United States during the 1950s. It was named after the snake of the same name as it was customary for Los Alamos to name primary devices after snakes and insects. RACER was the second fusion-boosted fission device type incorporating the principles tested first with the BOOSTER in the Greenhouse Item (the first was the modified MK-7 device tested in Snapper Dog in 1952). It was initially intended to be the first stage for all but two (the SHRIMP and ZOMBIE devices) of the devices tested in the Castle series. The shots intended to make use of it were Romeo (RUNT device), Union (ALARM CLOCK device), and Yankee (JUGHEAD, later RUNT II devices), which tested weapon systems aiming to ensure emergency capability, as well as