February 1973, though some sources suggest that testing began in October 1972. The testing phase of the R-36M with various different types of warheads was finished in October 1975 and on 30 December 1975 deployment began (though some Western sources suggest that an initial operational capability was reached in early 1975). A total of 56 were deployed by 1977, though all were replaced by R-36MUTTKh missiles by 1984. These high-yield weapons were assessed in the West as possibly developed to attack American Minuteman ICBM launch control centers. R-36M (SS-18 Mod 2) Variant of the R-36M with a post-boost vehicle and up to eight reentry vehicles, each with a warhead yield estimated at between 0.5 and 1.5 Mt, with a range capability of about . The MIRVs were placed in pairs, and a post boost vehicle with a command structure and a propulsion system were contained in the nose cone of the R-36M. The flight tests of this variant began in September 1973 (though some Western sources suggest that the initial flight test occurred in August 1973), with IOC in 1975. Approximately 132 were deployed by 1978, but the post-boost vehicle design was seriously flawed, and all missiles were replaced by R-36MUTTKh by 1983. R-36MUTTKh (SS-18 Mod 3) Variant of the R-36MUTTKh with a single large reentry vehicle that was an improved version of the R-36M. On 16 August 1976, a few months after the R-36M entered service, the development of an improved modification of the R-36M (15A14) was approved. This missile subsequently received the designation R-36MUTTKh (15A18) and was developed by KB Yuzhnoye (OKB-586) through December 1976. The R-36MUTTKh was capable of carrying two different nose cones. On 29 November 1979, deployment of the improved R-36M with a single reentry vehicle carrying an 18–25 Mt warhead began. This variant is no longer in service. R-36MUTTKh (SS-18 Mod 4) Variant of the R-36MUTTKh with multiple warheads. It was probably designed to attack and destroy ICBMs and other hardened targets in the US. Its increasing accuracy made it possible to reduce the yield of the warheads and allowed an increase in the number of warheads from 8 to 10. According to some Western estimates, evidence suggested that it may be capable of carrying as many as 14 RVs (this may reflect observation of the deployment of countermeasures intended to overcome a ballistic missile defense, or to confuse American attack characterization systems). The