During the 1930s Soviet rocket technology was comparable to Germany's, but Joseph Stalin's Great Purge severely damaged its progress. Many leading engineers were exiled, and Korolev and others were imprisoned in the Gulag. Although the Katyusha was very effective on the Eastern Front during World War II, the advanced state of the German rocket program amazed Soviet engineers who inspected its remains at Peenemünde and Mittelwerk after the end of the war in Europe. The Americans had secretly moved most leading German scientists and 100 V-2 rockets to the United States in Operation Paperclip, but the Soviet program greatly benefited from captured German manufacturing tools obtained from the V-2 production sites Mittelwerk in Eastern Germany. From July 1945, the Soviets recruited German scientists and workers for the "Institut Nordhausen" in Bleicherode to reestablish the lost design drawings and engineering data and to restore the manufacturing and assembly of V-2 components in Germany. This operation was set up by Dimitri Ustinov, Sergei Korolev, Valentin Glushko, and Boris Chertok. Helmut Gröttrup, a notable expert of control systems from Peenemünde, was appointed general director of Institut Nordhausen, also called "Zentralwerke," which grew to more than 5000 employees until October 1946.