Isayev and D D Sevruk at OKB-3 of NII-88 developed competing engine designs. In 1958, before development was completed, Sevruk's operation was closed down and his staff combined with Isayev's. Isayev considered his S5.1 design in a more advanced stage of development than Sevruk's S3.42A, so the Sevruk design was cancelled. The missile with the modified engine completed factory trials in 1958 and was accepted for PVO service in combination with the B-200M improved radar and other S-25 system elements. The new engine significantly increased the missile's performance, and this was the first version with hypersonic flight capability.