The second stage liquid hydrogen
booster of the Saturn V
launch vehicle used during the Apollo project
Somewhat the 'middle child' of the Saturn V, without the raw power of the S-IC or the glamour of the S-IVB that would propel humans out of Earth orbit, the S-II stage nonetheless became one of the make or break items for the U.S. during the space race.
The contract to develop the S-II was awarded to North American Aviation on 11 September, 1961. As North American was also developing the Apollo CSM spaceship, there were concerns in NASA early on that the S-II would not receive the management attention required. As problems surfaced, NASA felt that it's concerns were being realized and demanded (and got) substantive management and other personnel changes; as with the S-IC stage, the relationship between government contractee and private contractors was much closer than normal.
Any management issues aside, the S-II was faced with two severe problems. First, the performance required of the S-II dictated using liquid hydrogen as a fuel source, although the technology for using it was far from mature at the time. While the upper S-IVB stage also used the same propellants, it was a much smaller stage and so didn't face the S-II's problems of scale, and similarly, while the lower S-IC stage shared the problems of scale it used a conventional kerosene based fuel.
The propellants were not stored in two completely seperate tanks, as in the S-IC stage, but in tanks that were joined together, the propellants seperated by a common bulkhead.
Building this huge bulkhead - formed by two thin shells with a honeycomb material between them for strength - was a production nightmare. Thermal insulation between such large tanks was also an issue - at the temperature that hydrogen can be stored as a liquid, oxygen freezes into a solid. Ulitmately, North American's problems were solved only by the development of new techniques and construction apparatus specifically for the S-II.
Second, the S-II took much of the brunt of the Saturn V's weight control problem. The weight of the payload - the CSM and LM spacecraft - was steadily rising upward, which meant the weight of the launch vehicle had to come down if it was going to be able to take off at all. The most effective place to reduce weight would have been the S-IVB stage, but it was too far along for any substantive design changes. It wasn't practical to reduce much weight from the S-IC stage - a whopping 14 kilograms would have to be eliminated for every extra kilogram of payload. But in the S-II stage, only 5 kilograms had to be eliminated for every extra kilogram of payload.
The common bulkhead design reduced weight considerably. Additional weight was saved by building the tank walls - the frame of the stage - out of an alloy that got stronger as it got colder. By placing thermal insulation outside the tank, instead of inside as normal, the metal was chilled by the propellants and could be made about 30% thinner.
These and other solutions reduced the weight of the S-II's structure to the point that when fully fueled, the structure, plumbing, engines, etc, accounted for only 3% of the total weight of the stage.
The S-II used five J-2 engines (a single J-2 was used to power the S-IVB stage). The S-II's first flight was as part of an unmanned test of the Saturn V on November 9, 1967. Its last flight was as part of the modified Saturn V that orbited Skylab on May 14, 1973.
The S-II was 30.6 meters high (including the interstage adapters at either end) and 10 meters in diameter. It delivered 4.5 millions newtons of thrust for 270 seconds.