Hermann Oberth, the German father of rocketry, has a mixed legacy. On the one hand, he developed the V-2 rocket for Nazi Germany during World II. On the other hand, his ideas, rejected as implausible when he submitted them as part of his doctoral thesis, paved the way for the development of rocketry, and he later helped launch the United States into space.
Source:
https://www.space.com/20063-hermann-oberth.html