The SM-65F Atlas, or Atlas-F, was the final operational variant of the Atlas missile, only differing from the Atlas E in the launch facility and guidance package used. It first flew on 8 August 1961, and was deployed as an operational ICBM between 1961 and 1966. Following retirement as an ICBM, the Atlas-F, along with the Atlas-E, was refurbished for orbital launches as the Atlas E/F.
The Atlas E and F also differed in their launch facilities; Atlas E utilized the same coffin silos as Atlas D missiles, with the missile stored horizontally and raised upright for launch. Atlas F for comparison used a vertical silo with an elevator similar to the Titan I. The Atlas F was originally conceived when the Air Force decided that the coffin silos used for the Atlas D and E were too exposed and vulnerable to enemy attack.
Atlas E and F used the MA-3 propulsion system which had a separate gas generator for all three engines, unlike the Atlas D where one gas generator drove both booster turbopumps. In addition, they used pyrotechnic cartridges for rapid starting and the gas generators and pumps were mounted on each booster engine instead of being placed in the central thrust section. This setup was designed for ease of service and engine swapping in the field, while the Atlas D's simpler configuration was considered preferable for space launches. Total thrust of the MA-3 was .
Atlas E/F used the ARMA inertial guidance system rather than Atlas D's radio ground guidance; some IOC (Initial Operational Capability) Atlas D flights tested the ARMA system instead of the standard GE Mod II radio guidance. Aside from minor differences in the ARMA on the Atlas E and F, the major difference between the two consisted of differently-placed quick fill lines for the two silo systems used.
Most refurbished Atlas F space launches used solid-fueled upper stages, a notable exception being Missile 23F which launched Seasat, a NASA oceanography satellite, on June 27, 1978, the last Atlas-Agena vehicle flown. The final Atlas F launch took place on June 23, 1981 when Missile 87F successfully placed a NOAA weather satellite into orbit. Atlas E/F space launchers were given an extensive tear-down and rebuild by Convair and the ARMA system replaced with the GE Mod II guidance system.
1961-62
The first two Atlas F flights were Missile 2F on August 8, 1961 and Missile 4F on November 22, both from LC-11 at CCAS. Both tests were entirely successful, and on December 12, Missile 5F launched from LC-11. This test was a partial failure; an electrical short in the guidance system caused an early SECO command to be sent and the missile fell short of its planned range. On the 21st, Atlas 6F lifted off of LC-11 carrying a rhesus monkey named Scatback. This was the third launch of a primate on an Atlas in the past month - the first was Mercury-Atlas 5 which successfully lofted a chimpanzee on a two-orbit mission while the second, an Atlas E test, malfunctioned shortly after liftoff and had to be destroyed, claiming the life of the squirrel monkey it was carrying. Atlas 6F performed acceptably until after booster jettison when a hydraulic failure caused loss of sustainer thrust. The capsule separated and splashed down near Ascension Island in the South Atlantic, however the tracking beacon failed to operate and recovery crews could not locate it. They gave up the search and Scatback was officially lost at sea. The failure was traced to combustion of spent propellant following booster jettison which was damaging valves and/or ducting; it was solved by the installation of cutoff valves in the booster section.
So far, the Atlas F test program had gone well and nobody was prepared for the upcoming disaster on April 9, 1962 when Missile 11F exploded only one second after liftoff from LC-11. Examination of recovered missile parts found that some sort of explosion or detonation in the sustainer turbopump had occurred. The sustainer engine had shut down almost immediately at liftoff followed by loss of its telemetered engine parameter data. The booster engines operated satisfactorily until missile destruction at T+3 seconds.