The Atlas A was the first R&D configuration that ultimately led to the operational Atlas D, E, and F missiles. It consisted of minimum propellant, propulsion, and guidance systems. Its maximum range was only 600 nautical miles, and its maximum altitude was 57.5 nautical miles. A total of eight Atlas As were launched--all on the Atlantic Missile Range--during the period June 1957 to June 1958. The B series was the second Atlas developmental configuration. Its propulsion system was close to operational capability, and one series B missile traveled 5,500 nautical miles down the Atlantic Missile Range. Atlas 4-B, the second in the series B test flights, was launched successfully on 2 August 1958. The eighth missile in the series, Atlas 10-B, placed itself into orbit with the Project SCORE payload on 18 December 1958, becoming the world’s first communications satellite in the first successful use of the Atlas as a space launch vehicle.
The Convair Division of General Dynamics produced three different models of the Atlas ICBM destined for deployment with the Strategic Air Command. The first operational version of the Atlas, the "D" model, was a one and one-half stage, liquid-fueled, rocket-powered (360,000 pounds of thrust) ICBM equipped with radio-inertial guidance and a nuclear warhead. It was stored in a horizontal position on a "soft" above-ground launcher, unprotected from the effects of nuclear blast, and had an effective range, like all Atlas models, of approximately 6,500 nautical miles. The second Atlas ICBM configuration, the series E, possessed all-inertial guidance, improved engines (389,000 pounds of thrust), a larger warhead, and was stored in a horizontal position in a "semi-hard" coffin-type launcher. The series "F" missile was superior to its predecessors in several ways. Like the E model, the Atlas F was equipped with all-inertial guidance, but possessed improved engines (390,000 pounds of thrust) and a quicker reaction time due to its storable liquid fuel. The Atlas F missiles also were deployed in "hard" silo-lift launchers which stored the missiles vertically in underground, blast-protected silos and used elevators to raise the missiles to ground level for launch.
Meanwhile, considerable progress was made in developing second-generation ICBMs such as the Minuteman. Among the numerous advantages the newer missiles had over the Atlas was their ability to be launched from hardened and widely dispersed underground silos. Minuteman was also more economical to operate, more reliable, and because of its silo-launch capability, better able to survive a nuclear first strike than their first-generation counterparts.
Consequently, on 24 May 1963, General Curtis E. LeMay, Air Force Chief of Staff, approved the recommendations of the Air Force Ad Hoc Group for phaseout of Atlas D by the end of FY 1965 and the Atlas E's by the end of FY 1967. On 16 May 1964, Secretary of Defense Robert S. McNamara accelerated the phase-out of the Series E Atlas from the end of FY 1968 to the close of FY 1965. In addition, Secretary McNamara ordered the retirement of all Atlas F ICBMs by the end of FY 1968.
Project "Added Effort", the Air Force nickname for the programmed phaseout of all first-generation IC8Ms, began on 1 May 1964 when the first Atlas D's were taken off alert at the 576th Strategic Missile Squadron, Vandenberg AFB, California. Project Added Effort reached completion on 20 April 1965 when the last (first-generation) ICBM, an Atlas F. was shipped from the 551st Strategic Missile Squadron, Lincoln AFB, Nebraska, to Norton AFB, California, where it and other retired Atlas ICBMs were stored for future use as launch vehicles in research and development programs.