The Saturn V’s S-II stage was equipped with ullage rockets near the bottom and retrorockets neat the top.
After separation from the S-IC stage (aided by the S-IC retro rockets), four ullage rockets would fire to provide some slight forward acceleration for the S-II stage, allowing the liquid propellants to settle into the bottoms of their tanks. The ullage rockets were located on the interstage between the S-IC and the S-II, which remained temporarily attached to the S-II. When the J-2 engines fired, linear shaped charges would sever the connections between the interstage and the S-II. The combination of forward acceleration produced by the J-2’s and the rocket plumes impinging on the interstage assured separation.
The ullage motors each had 336 pounds of solid propellant and produced 22,700 pounds of thrust for 3.7 seconds.
At the forward end of the S-II stage was the conical interstage connecting to the S-IVb stage. After S-II burnout, the interstage would be severed at the base of the S-IVb, and would remain attached to the S-II. For retrorockets assured that the S-II would move backwards relative to the S-IVb. Each rocket had 268.2 pounds of solid propellant and produced 34,810 pounds of thrust for 1.52 seconds.
