There were competitors for the original military Global Rocket 2 (GR-2) requirement.

The OKB-1 of S P Korolev had begun design of the enormous N1 lunar rocket, and had already put the Soviet Union first in ballistic missiles and space through use of its R-7 ICBM. Korolev was working on the successor R-9 ICBM, and the NII variant of the N1(using the top two stages) could meet the GR-2 requirement.

M K Yangel’s KB Yuzhnoye proposed creation of two related launch vehicles to fulfil the military requirement - the R-46 heavy ICBM and the R-56 launch vehicle. These would cover the entire range of military requirements. Yangel’s OKB had already supplied the military with the great majority of its operational strategic rockets - the R-12 and R-14 IRBM’s and the R-16 ICBM.

OKB-52, under V N Chelomei, proposed to create a family of rockets - the medium ICBM UR-200, the heavy ICBM UR-500 (which would fulfil the heavy launch vehicle requirement) and the huge UR-700 for lunar requirements.

As payloads for the GR-2 Chelomei considered a broad spectrum of space craft, destined to solve defence, scientific investigation, and national economic tasks. These were to be called raketoplans - piloted spacecraft for solving military tasks in space. For example, orbital raketoplans were intended to fulfil intelligence, satellite inspection, and destruction tasks. For these purposes the raketoplan was to be equipped with an orbital manoeuvring engine, targeting systems, rendezvous systems, and space-to-space weapons. Later raketoplans would be used for scientific tasks, including flight to the moon and return to earth, and economic exploitation of near-earth space. Due to their high lift to drag ratios, raketoplans could, after completing their tasks in space, make a guided descent into the earth’s atmosphere with a landing on Soviet territory.

• 1961 August 15 - Chelomei begins UR-500 Proton design studies..  Launch Vehicle: Proton.

  At first the launch vehicle was simply to consist of 4 two-stage UR-200 rockets lashed together, the first and second stages working in parallel in clusters. A third stage would be modified from the UR-200 second stage. However study of this configuration, which included manufacturing of a dynamic test article, indicated that the payload capacity could not meet the military’s requirements.

• 1961 November 15 - Development of RD-253 engine begun..  Launch Vehicle: Proton.

  OKB-52 began to collaborate with V P Glushko’s OKB-456 in developing a high thrust storable propellant engine for the UR-500 Proton launch vehicle. Glushko had completed a storable liquid engine design of 150 tonnes for use in Korolev’s N1. However Korolev refused to accept this design, due to his categorical refusal to use toxic propellants in his rockets and his belief that such propellants could never deliver the required specific impulse. Korolev insisted on development of an oxygen-kerosene engine; Glushko categorically refused to do so. As a result, the two leading Soviet rocket designers irrevocably split. Korolev had to turn for development of his N1 engines to the aviation engine design OKB of N D Kuznetsov.

• 1962 January 15 - Proton configuration selected..  Launch Vehicle: Proton.

  The 'polyblock' design was chosen as most advantageous, following studies that indicated improved wind loads and bending moment characteristics compared to the conventional 'monoblock' design.

• 1962 May 15 - UR-500 advanced project published..  Launch Vehicle: Proton.

• 1963 June 15 - First tests of RD-253 engine cluster for Proton..  Launch Vehicle: Proton.

  Ground tests of the clustered engine assembly ran from June 1963 to January 1965.