Helmut Gröttrup (12 February 1916 – 5 July 1981) was a German electrical engineer. He was born in Cologne and died in Munich.
He was an assistant of Wernher von Braun in the V-2 rocket-project. Gröttrup was responsible for the radio guidance system. After the war, he worked in the Soviet rocketry program.
https://en.wikipedia.org/wiki/Helmut_Gr%C3%B6ttrup
Gröttrup provided to the Soviets with many details about geography of subcontractors involved into the A-4 program. He also contacted a number of qualified individuals outside of the Soviet zone, who could assist the Soviet rocket development effort. Apparently among people, who moved into the Soviet efforts to join A-4 program with the help of Gröttrup's references were aerodynamist Hans Zeise and the construction expert Anton Narr. Friz Fibach, the expert in launch operations, also crossed over into the Soviet zone from the British sector.
German legacy in the Soviet rocketry