Missile R-27 Zyb (SS-N-6 Serb)

The R-27 (GRAU index 4K10, NATO reporting name SS-N-6 Serb) was a Soviet submarine-launched ballistic missile in service with the Soviet Navy from 1968 to 1988. Development produced a single-stage liquid-propellant missile sized specifically to fit within the launch tubes of the Project 667A submarine. Total production reached 1,800 missiles.

The airframe was an all-welded structure of hard-worked AMg6 aluminium-magnesium alloy panels, thinned by successive chemical etching and mechanical milling, and coated externally with a heat- and moisture-resistant asbestos-based layer. The missile carried no fins; rows of rubber shock absorbers protected the skin inside the launch tube. Oxidiser and fuel tanks shared a common bulkhead to maximise propellant volume. The guidance instrumentation occupied a hemispherical compartment formed by the upper oxidiser tank bulkhead, permitting removal for maintenance without a service hatch. Propulsion was a single fixed main thrust chamber operating on an oxidiser-rich staged combustion cycle, welded permanently inside the fuselage rather than mounted on an external thrust structure, plus two gimballed gas-generator chambers on the lower tank frustum. Both could be throttled. Propellants were unsymmetrical dimethylhydrazine and nitrogen tetroxide, hypergolic and ignited by squib. Missiles were fuelled at the factory and "ampulised" by welding the fill and drain valves shut, giving an initial service life of five years, later extended to fifteen.

The baseline R-27 carried a single detachable nuclear warhead of 1 Mt yield, separated from the missile body by a linear shaped charge. The R-27U (RSM-25) replaced it with three 200 kt warheads and was carried by Project 667AU boats.

Launch was performed submerged from 40 to 50 metres. The tube was pressurised, then flooded and equalised; the nozzles sat within a watertight air-filled adapter to limit water hammer at ignition. The two small chambers fired first, followed by the main chamber. Launch preparation took ten minutes, with an eight-second interval between missiles in a salvo. Between 1974 and 1990, 161 launches were conducted with a 93% success rate.

The R-27K (4K18, NATO SS-NX-13) was a two-stage anti-ship derivative, the second stage carrying the warhead, propulsion and terminal guidance. Cued by external targeting data, it delivered a manoeuvring re-entry vehicle with a warhead yield of 0.5 to 1 Mt. Trials were conducted from K-102, a Project 605 conversion of a Project 629 boat lengthened to accept four tubes, the Rekord-2 fire control system and the Kasatka B-605 target acquisition system. Testing ran from December 1972, with firings between September and December 1973, and the boat was accepted for service on 15 August 1975. The system never became operational because each R-27K tube counted against SALT strategic missile limits; Project 667A boats could accommodate the missile but lacked the targeting and fire control equipment.

The Soviet Navy was the sole operator. R-27 missiles were carried by Yankee I submarines, including K-219. The North Korean Hwasong-10 "Musudan" IRBM was derived from the R-27 under a 1992 contract, differing chiefly in a slightly lengthened fuselage. Surplus missiles were also converted into the Zyb sounding rocket, flown three times.