R-21 (missile)

Development of the R-15 and R-21 was initially assigned to Mikhail Yangel's Yuzhnoye Design Office (OKB-586) on 20 March 1958. The project was transferred to Viktor Makeyev's SKB-385 on 17 March 1959. It was intended for the Golf-class submarines (Project 629B); the complete missile system was called the D-4 weapon complex. The 4th Research Institute had been conducting trials of underwater missile launch systems since 1955 The R-21 was probably the basis for the design of the North Korean Hwasong-7 (No Dong) missile. ==Design==

Unlike Western designs, the R-21 used a cold launch solid rocket to eject the missile from the flooded launch tube before the main motor ignited. The propulsion system used an Inhibited red fuming nitric acid-amine propellants combination, AK-27I/TG-02. The AK-27I oxidizer was a mixture of 73% nitric acid, 27% nitrogen tetroxide, and an iodine inhibitory passivant. The fuel was originally used in the Wasserfall rocket under the name TONKA-250 and consisted of 50% triethylamine and 50% xylidine. This gave the R-21 a range of , double that of first-generation sub-launched missiles. The Naval Institute Guide suggests that the range was initially 1300 km, and extended to 1650 km later in the life of the missile. There was a single warhead of approximately 800 kilotons. ==Variants==

• R-21 (4K55) - original design • R-21A (4K55A) - subsequent modification but some variants were assigned the name 'Serb' typically used for the SS-N-6. Jane's uses 'Sark'. ==Operational history==

The missiles replaced first-generation R-11FM and R-13 missiles on some Golf (Project 629) and Hotel (Project 658) class SSBNs, with three missiles per submarine, Seven of the eight Hotel I (Project 658) subs were upgraded to Hotel II (Project 658M) standard, which were retired by 1991. ==Operators==

; • North Korean Navy ; • Soviet Navy == See also ==