A magazine ad from 1963 showing the S-IV stage and the X-20 Dyna Soar. The Dyna Soar is shown without its adapter section and Transtage, indicating that it is approaching re-entry (note that it is shown with the canopy heat shield still in place). The Saturn S-IV stage, used on a few Saturn I launches, was smaller than the S-IVB that was used on later Saturn Ib and Saturn V launches, and used six RL-10 rocket engines instead of the S-IVB’s single J-2. Also note the three prominent “ullage rockets” sticking out from the base of the stage. These were small solid rocket motors that would impart a slight forward acceleration to the stage prior to the ignition of the RL-10’s. The acceleration would be high enough and last long enough to settle the propellants into the rears of the tanks. Otherwise the liquid propellants would float around in microgravity and might very well not feed properly into the plumbing system; if a turbopump swallowed a large bubble of gas rather than liquid, it could be destroyed.
The Saturn I/S-IV never launched an actual Apollo CSM, but only boilerplate test articles. Interestingly, the BP-16 test article, launched May 25, 1965, stayed in orbit until July 8, 1989.
