“The S-II stage was a nightmare the minute it was conceived, and it only got worse from there. During the course of its creation, it would grind up people and careers the way the transcontinental railway devoured laborers. Though the methods and materials used to build the S-II were reasonably well known, nobody had ever tried to apply them on such a titanic scale. Originally, it was to be somewhere around 8 stores tall with a diameter of 22 feet, but the width ballooned from there to 27 feet before the contract was even signed, then to 30, and finally to 33 feet. And all the while as the size of thing increased, NASA was trimming the allowable weight.” Harrison Storms of NAA.
The structural efficiency of the S-II stage, in terms of the weight and pressures taken by its extra-thin walls, was comparable only to the capacity of one of nature’s most refined examples of structural efficiency, the egg.
The Apollo contract specified a shirt-sleeve environment. For this reason, North American was told not to include in its design a hatch that opened by explosives, like Mercury’s. An accidentally blown hatch in space would cause an instant vacuum and certain death for an astronaunt not wearing his pressure suit.