All of the compartments within the turrets were separated by flameproof bulkheads to prevent any flame or lethal gas from spreading throughout the turret. Each turret required a crew of 77–94 men to operate. The turrets were not actually attached to the ship, but sat on rollers, which meant that if the ship were to capsize the turrets would fall out. Each turret cost US$1.4 million, but this number did not include the cost of the guns themselves. The turrets were "three-gun," not "triple," because each barrel could be elevated and fired independently. The ship could fire any combination of its guns, including a broadside of all nine. The guns could be elevated from −5° to +45°, moving at up to 12° per second. The turrets could be rotated about 300° at a rate of about four degrees per second and can even be fired back beyond the beam, which is sometimes called "over the shoulder." The guns were never fired horizontally forward (in the 1980s refit, a satellite up-link antenna was mounted at the bow). To distinguish between the rounds fired from different battleships the "Iowa" class used dye bags which allowed artillery observers to determine which rounds had been fired by which ship. "Iowa", "New Jersey", "Missouri," and "Wisconsin" were assigned the colors orange, blue, red and green, respectively. Within each turret, a red stripe on the interior wall, inches from the railing, marked the boundary of the barrel's recoil, warning the crew to keep back. When brought into service during World War II the guns had a barrel life of roughly 290 rounds, limited in large part by the nitrated cellulose (NC) propellant. After World War II the Navy switched to smokeless powder diphenylamine (SPD), a cooler-burning propellant, which increased the barrel life from 290 to about 350 rounds. This was increased further by the introduction of a titanium dioxide and wax compound known as "Swedish Additive" on for her tour in Vietnam, and later used on all four "Iowa"s when they were reactivated in the 1980s. These measures were further augmented by the addition of polyurethane jackets, which were placed over the powder bags to reduce gaseous erosion during the firing of the guns. These measures greatly prolonged barrel life, and ultimately resulted in a shift from measuring barrel life in equivalent service rounds (ESR) to measuring barrel life in fatigue equivalent rounds (FER).