This is a Vernier engine, also designated LR101, for the Atlas intercontinental ballistic missile, the U.S.’s first operational ICBM. It entered the Air Force inventory in 1960. Two Vernier engines controlled attitude and yaw during the boost phase. Built by the Rocketdyne Division of North American Aviation, the Vernier produced 1000 pounds of thrust for 5.5 minutes and burned kerosene fuel drawn from the Atlas’s main propellant tanks. The first Atlas equipped with Vernier engines flew successfully in December 1957. Rocketdyne built this Vernier engine in November 1963.

The engine is named after the French mathematician Pierre Vernier (1580-1637), who devised a sliding scale used to mark small divisions on measuring instruments. Engineers and physicists adopted his techniques to make fine adjustments on larger, more complex devises, such as the screws on calipers. Vernier engines served the same general purpose on large launch vehicles.