The Atlas E/F, also designated SB-1A was an American expendable launch system and sounding rocket built using parts of decommissioned SM-65 Atlas missiles. It was a member of the Atlas family of rockets. The first stage was built using parts taken from decommissioned Atlas-E and Atlas-F missiles, with various solid propellant upper stages used, depending on the requirements of the payload. The Atlas E/F was also used without an upper stage for a series of re-entry vehicle tests. On a single launch, an RM-81 Agena liquid-propellant upper stage was used.