The third stage: S-IVB – Extremes of temperature

The smallest stage of the Saturn V was the S-IVB (pronounced s-lour-b). and it was the earliest to fly. McDonnell Douglas had already been building them as the second stage of the Saturn IB rocket and little modification was needed to make it w ork with the rest of the Saturn V stack. It used the high-energy combination of liquid hydrogen and oxygen burning in a single restartable J-2 engine.

Liquid hydrogen is another cryogenic propellant, although in this case to become liquid it had to be brought down to minus 253’C. or 20 K; a mere 20 degrees above absolute zero. Rather than having two separate tanks that would require a heavy support structure between them, substantial mass was saved by fabricating one huge tank for both cryogenic propellants with an insulated bulkhead separating the lower, ellipsoidal compartment containing LOX from the liquid hydrogen in the larger upper section. As materials can have odd properties at these extremely low temperatures, insulation was painstakingly applied to the tank’s interior in the form of carefully machined polyurethane foam blocks to protect the adhesive and the tank’s aluminium skin. Including the conical interstage that joined it to the two low’er stages, the overall length of the stage was 18 metres with a 6.6-metre-diameter tank section.