While the R-11FM and R-13 provided the Soviet Union with successful submarine ballistic missiles, they also required a surface launch which made the submarine extremely vulnerable. Hence, almost simultaneously with development of those missiles, a development of an underwater launched version was begun. The development started in 1955 and the first stage of testing took place during 1956 and 1957. During this time four dummy missiles were successfully ejected from a specially designed submersible test stand. The Second stage of trials took place in 1958 when another 7 dummy missiles were successfully ejected, three from a test stand and four from submarine S-229 which was converted into a test sub by adding two launch tubes on the outside of the sub amidships.

The third stage of testing was carried out in 1959 an involved launching a live missile from underwater. This missile was essentially an R-11FM modified for underwater start. The first launch was unsuccessful and bizarre. While the submerged submarine made what appeared to its crew a successful launch, the missile never exited the water. The observation ship, packed with admirals and VIPs, pulled alongside the boat to find out what had happened. They found that the launch tube was opened but that the missile was still in the tube. Suddenly the rocket self-launched in full view of all of the dignitaries and VIPs. Fortunately, it fired at an angle away from the observation ship and there were no injuries. The second test in August 1959 also failed with the hull of missile being damaged during flooding of the tube. The third test succeeded and the first successful underwater launch of a Soviet submarine ballistic missile was made on 10 September 1960.

Design of an improved version, designated R-21, was started in 1959. This was completed by 1960 and trials were carried out until 1963. Out of 27 launches made, 24 were successful and Complex D-4 with the R-21 missile was accepted into service in May 1963. This missile could be launched from depths of 130 to 165 feet (40 ti 50 m) while submarine was moving up to 4 knots and in sea states as bad as 5. During active service between 1963 and 1982 a total of 228 launches were made of which 193 were successful. R-21 was a safe missile in service and there were no accidents with it during its service life.

Missile was single-stage, liquid fueled, with four chamber engine and a monobloc warhead.

Data from:

• "Ballisticheskie Rakety Otechestvennogo Flota" (Ballistic Missiles of the Fatherland Fleet) by Yu. L Korshunov and E.M. Kytovoy

Special help from Vladimir Yakubov