Convair SM-65 E/F “Atlas” ICBM

As fully operational ICBMs, the Atlas E and F, which differed only in guidance systems, had upgraded engines and inertial control instead of the Atlas D's radio ground guidance. The ignition system was also different from the one used on the D-series, which used a "wet" start, meaning that the propellants were injected into the combustion chamber prior to ignition, and a hypergolic igniter on the fully developed version. The Atlas E/F for comparison used pyrotechnic cartridges and a dry start (ignition coming before propellant injection) for an extremely rapid ignition that required no hold-down time on the pad to prevent combustion instability. The booster engines had separate gas generators unlike the Atlas D which had one gas generator for both engines. The launcher system used for the E and F series was also different from the D series, eliminating the hold-down arms in favor of a mechanism that would immediately release the missile as thrust built up. Thus making the E/F a more effective missile.