The smallest stage of the Saturn V was the S-IVB (pronounced s-four-b), and 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 work 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, it had to be brought down to 20 K, only 20 degrees above absolute zero, or minus 253°C, to become liquid. Rather than having two separate tanks requiring a heavy support structure between them, mass was saved by fabricating one huge tank for both cryogenic propellants with an insulated bulkhead separating the LOX in the lower, ellipsoidal compartment from the liquid hydrogen in the 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 blocks to protect the tank's aluminium skin. Including the conical interstage that joined it to the rest of the vehicle, the overall length of the stage was 18 metres with a 6.6-metre-diameter tank section.
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