Delays in the UR-200 missile program prompted Chelomei to request R-7 rockets for prototype testing of the IS. Two such tests were carried out on 1 November 1963 and 12 April 1964. Later in the year Khrushchev cancelled the UR-200 in favour of the R-36, forcing the IS to switch to this launcher, whose space launcher version was developed as the Tsyklon-2. Delays in that program led to the introduction of a simpler version, the 2A, which launched its first IS test on 27 October 1967, and a second on 28 April 1968. Further tests carried out against a special target spacecraft, the DS-P1-M, which recorded hits by the IS warhead's shrapnel. A total of 23 launches have been identified as being part of the IS test series. The system was declared operational in February 1973.