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One of the signs accompanying the LR-105 engine. It reads

The Atlas Rocket Engine

The Atlas rocket was developed by the U.S. Air Force to be the nation's first intercontinental ballistic missile (ICBM) capable of boosting a nuclear warhead to any target on earth. The program began in the early 1950s with the first successful launch in December 1957. The liquid-fueled Atlas served as one of the primary ballistic missiles until it was phased out of strategic missile service in 1965. At that time, the missiles became available for use as boosters after refurbishment.

Built by General Dynamics (formerly Convair), the Atlas went on to become one of the nation's most important boosters for the Air Force, NASA, and the Department of Defense orbital payloads. It was first used as a non-strategic missile launcher in December 1958 for Project SCORE (Signal Communications Orbit Relay Equipment). The Atlas also became the launch vehicle for the Project Mercury piloted orbital flights, Project PRIME, the NAVSTAR Global Positioning System, Ranger, Mariner, Pioneer 10, Lunar Orbiter, the Surveyor spacecraft, and a variety of other military and civilian projects. More than 500 Atlas launches have taken place. To boost most of these payloads into orbit, upper stages were added to the Atlas to increase its lifting capacity. The Atlas used three main types of upper stages: the Agena, Centaur, and the Burner II.

The Atlas has a unique main engine arrangement often referred to as "stage-and-a-half." Upon launch, all three main engines -- two LR89 boosters and one LR105 sustainer -- ignite and lift the rocket to an altitude of approximately 35 miles (56 kilometers). At that point the two outer boosters shut down and are jettisoned along with the engine skirts. The central sustainer engine continues to run until the necessary speed is reached. With design improvements, the thrust of the three main engines (and the two small vernier or maneuvering rockets) was eventually increased from 389,000 pounds (1.7 million Newtons) to 439,000 pounds (1.9 million Newtons) at liftoff.

In its use as a space booster, the Atlas is used with the Centaur high-energy upper stage. With the upper stage, the Atlas can place 11,200 pounds (5,080 kilograms) into a 115-mile (185-kilometer)-high orbit, 4,100 pounds (1,860 kilograms) into a synchronous orbit, or send 1,300 pounds (590 kilograms) to a near planet.