The S-IVb stage of the Saturn Ib was equipped with three solid “ullage motors.” These relatively small solid rockets provided only a minimal acceleration to the vehicle; but that was enough. The purpose was not propulsive, per se, but just to give enough of an acceleration so that the liquid hydrogen and liquid oxygen propellants would settle into the aft portions of their tanks. If this was not done, it would be entirely possible that the turbopump for the J-2 main engine would swallow helium pressurant gas rather than propellant, causing, at the very least, the engine to fail to ignite and very likely severe structural damage.
The ullage motors were Thiokol TX-280-10’s developing an average thrust of 3,460 pounds for 3.9 seconds. A few seconds after burnout the ullage motors and their fairings were ejected to reduce weight.
