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七十年代初,美国航宇局开始研制土星V火箭,用来发射重型有效载荷如轨道实验室、载人登月飞船等。土星V的第二级、S-Ⅱ是北美罗克威尔公司的空间分公司按照1961年10月与航宇局马歇尔宇宙飞行中心签订的合同研制的。第二级有五台J-2组合发动机,可携带267,700加仑的液氢,87,400加仑的液氧。发动机持续工作6分钟,能产生100万磅推力。第二级的质量系数约为0.914,它能把火箭从211,073英尺推到622,689英尺的高空,并使火箭的飞行速度从7,720英尺/秒提高到21,176英尺/秒。
In the early 1970s, NASA began to develop Saturn V rockets to launch heavy payloads such as orbital laboratories and manned spacecraft. The second level of Saturn V, S-II, is a space division of North American Rockwell Company developed under a contract with NASA Marshall Space Flight Center in October 1961. The second stage has five J-2 combined engines that carry 267,700 gallons of liquid hydrogen and 87,400 gallons of liquid oxygen. The engine lasts 6 minutes, producing 1 million pounds of thrust. The second-stage, with a mass factor of about 0.914, pushed the rockets from 211,073 feet to 622,689 feet and raised the rocket’s flying speed from 7,720 feet per second to 21,176 feet per second.