The S-IVB rocket stage played several important roles in NASA’s 1960s and 1970s manned space programs. The 58.4-foot-long, 21.7-foot-wide stage, which comprised a single restartable J-2 rocket engine, a forward liquid hydrogen tank, and an aft liquid oxygen tank, served as the second stage of the two-stage Apollo Saturn IB rocket and the third stage of the three-stage Apollo Saturn V.

The Saturn IB S-IVB’s J-2 engine would ignite at an altitude of about 42 miles and burn until it placed a roughly 23-ton payload into low-Earth orbit. After that, it would shut down and the spent stage would separate. The Saturn V S-IVB’s J-2, on the other hand, would ignite twice to accelerate the stage and its payload: once for 2.5 minutes at an altitude of about 109 miles and again for six minutes about two and a half hours later. The first burn would place the S-IVB and payload into a low parking orbit between 93 and 120 miles above the Earth; the second would place the S-IVB and payload onto a path that would intersect the moon, about 238,000 miles away, about three days after Earth launch. Departure for the moon was called Translunar Injection (TLI) ...

Following a safe landing and a period of surface exploration (less than one Earth day for the earliest Apollo landing missions), the LM ascent stage would lift off. About two hours later – again over the moon’s hidden hemisphere – the CSM would rendezvous and dock with the LM. The lunar landing crew would rejoin the CSM pilot, the astronauts would cast off the LM ascent stage, and preparations would begin to ignite the SPS to depart lunar orbit for Earth. The critical lunar-orbit departure maneuver, also carried out over Farside, was called Trans-Earth Injection (TEI).