这是个it引导的强调句，However,（it was) not until the discovery of the reaction principle, which was the key to space travel and so represents one of the great milestones in the history of scientific thought, （that） rocket technology was able to develop.
是强调句句型。不是一般的系表结构。强调句型：“It is（was）＋被强调的部分（可以是主语。时间状语。地点状语。宾语。。等等。但不能是谓语动词）＋that（who）＋原句其它部分”来强调说话人的意愿： It was on Monday night that all this happened．所有这一切发生在周一晚上。 It's me who he blamed．他怪的是我。你问的这句话的主句是it wasn't until the discovery of the reaction principle that rocket technology was able to develop是 直到反应规则被发现，火箭科技才能发展（具体单词可能翻的不准，因为我不太了解航空术语。呵呵。不过主要是告诉你语法）。这里还有个句型，是NOT..UNTIL。。 含义是直到。。才。。which was the key to space travel and so represents one of the great milestones in the history of scientific thoug是定语从句，修饰名词词组the discovery of the reaction principle
