engine in its first stage and the RD-252 engine in its second stage. The weapon's third stage was related to the deorbiting process as well as warhead guidance and delivery; the Soviets referred to this system as OGCh. An aiming system inside the OGCh would check and rectify trajectory issues in relation to the desired target location through the use of various instruments (for instance, a radio altimeter used in conjunction with an inertial navigation system). The aiming system would make its checks directly after the missile came into orbit and immediately before third stage ignition. Deorbiting would be induced by the missile's retrorocket, the RD-854 engine, causing the warhead to take on a ballistic path towards its target. The warhead, retrorocket, and guidance system were each contained inside the OGCh module. A series of nozzles on the RD-854 enabled aerial maneuvering of the OGCh. Other nozzles facilitated the separation of the warhead from the rest of the OGCh, allowing it to fall alone on a ballistic path to the target. The R-36O's 8F021 warhead had an explosive yield between 5-20 megatons according to Soviet sources. Western intelligence suggests that the yield was smaller, being somewhere between 1-3.5 megatons. The missile was 32.60 meters long with a 3.00 meter diameter and a total launch mass of 180 tons. The R-36O used the same hypergolic propellants as the UR-200. Yangel design selection In 1965, Soviet military officials worked to select one of the three FOBS projects. The R-36O of Yangel's design bureau was chosen for further development over the others. The reasoning behind the Soviet FOBS design selection process remains somewhat unclear. A particularly confounding factor to consider is that none of the three proposed missiles had undergone a single test flight prior to the R-36O's selection. Even so, there are some explanations as to why Yangel's design was chosen in the end. These revolve around the occurrence of negative events in the competing GR-1 and UR-200A projects that effectively knocked them out of contention. One strike against Korolev's GR-1 was that it used a cryogenic propellant, thereby rendering the missile a poor candidate for operational storage in a missile silo as the Soviet military desired. Other negative factors include that the GR-1's 8D726 retrorocket was demonstrating a proclivity for failure in its initial tests and, as a cause for yet more worry, the troublesome R-9A, the subject of a different