Helmut Gröttrup (1916–1981) was a German rocket engineer and assistant of Wernher von Braun in the V-2 rocket-project. Gröttrup was responsible for the guidance system.
After World War II, Gröttrup first lived in the western occupation zones. He refused to work for the Americans because he didn't want to be separated from his family. In that situation, an offer from the Soviets seemed to be right, who offered him not only a lot of money, but also the opportunity to continue his work in Germany and to stay with his family. He was the most important rocket specialist the Soviet Union could get and an important contributor to an early Soviet space program.
From September 9, 1945 through October 22, 1946, Gröttrup worked under the supervision of Sergey Korolyov in the Soviet Occupation Zone. Then, all scientists and engineers working for the Soviet Union were kidnapped and brought to the USSR by train along with their families.