Soviet nuclear thermal propulsion was initially pursued as an alternative to nuclear electric propulsion. The earliest work was a follow-on to OKB-1's late 1950's designs for missiles and launch vehicles using nuclear thermal engines with ammonia as propellant. This effort had been abandoned in 1960 due to the long development scale and the safety problems involved in testing nuclear systems in the first stage of rockets. Thereafter the engine bureaux of Bondaryuk (OKB-670) and Glushko (OKB-456) continued study of nuclear propulsion, but using liquid hydrogen for upper stage applications. Engines of 200 tonnes and 40 tonnes thrust with a specific impulse of 900 to 950 seconds were being considered. At the end of 1961 both bureaux completed their draft projects and it was decided to continue work on development of an engine in the 30 to 40 tonne thrust range. In the following year Korolev was asked to study application of such engines, followed by a demand in May 1963 from the Scientific-Technical Soviet for specific recommendations.
Korolev considered three variants based on the N1:
The study concluded that the two stage vehicle was the most promising. Compared to an equivalent vehicle using liquid oxygen/liquid hydrogen, mass in low earth orbit would be more than doubled. Optimal stage size was 700 to 800 tonnes for the Type A engines and 1,500 to 2,000 tonnes for the type V engines (this resulted in a extremely large number of nuclear engines by Western standards). Use of the nuclear stage would result in a single N1 launch being able to launch a round-trip lunar landing (mass landed on lunar surface over 24 tonnes with return of a 5 tonne capsule to earth).
For a Mars expedition, it was calculated that the AF engine would deliver 40% more payload than a chemical stage, and the V would deliver 50% more. But Korolev's study also effectively killed the program by noting that his favoured solution, a nuclear electric ion engine, would deliver 70% more payload than the Lox/LH2 stage.
Further investigation of nuclear thermal stages for the N1 does not seem to be pursued by OKB-1. Bondaryuk and Glushko turned to Chelomei and his MK-700 Mars spacecraft for future application of such stages. Glushko had designed his RD-410, 7 tonne thrust engine in the 1960's. But he also undertook an even more ambitious engine in the 1963 to 1970 period - the RD-600 gas core nuclear reactor. This exotic technology, also pursued in the United States, would have resulted in a 200 tonne thrust engine with a specific impulse of 2000 seconds. Although the draft project was completed and it was concluded the concept was entirely feasible, no funds for development were forthcoming.
It was not Glushko or Bondaryuk, but the Kosberg OKB that took concrete steps in the 1960's toward actually building nuclear thermal propulsion system hardware. In 1962 Kosberg, together with the Kurchatov Institute, Keldysh Scientific Centre, NPO Luch, and a half dozen other institutes, began experiments with nuclear thermal rockets for upper stage applications that used liquid hydrogen as the propellant.
However a 1972 review by a government expert commission of Chelomei's MK-700 Mars spacecraft design dealt a mortal blow to further rapid development of nuclear thermal propoulsion. Among other findings, the commission, noting the American abandonment of manned Mars plans and their NERVA engine, found no pressing need for an equivalent Soviet project. They noted that Russian nuclear thermal engines were only in the draft project stage and would take 15 to 20 years to reach technological maturity. It was felt that the radiation safety problem of nuclear thermal propulsion had only been solved theoretically. Negotiations with the United States would be required to achieve international permission for placing large nuclear reactors into orbit. The outcome of these negotiations was uncertain.
The state commission recommended that further work on manned Mars expeditions be deferred indefinitely. However Soviet development of nuclear-thermal propulsion was allowed to continue. Although Glushko abandoned, the technology, NPO Luch began tests of prototype Kosberg engines at a test stand 50 km Southwest of Semipalatinsk-21 in 1971. Tests continued there through 1978. Simultaneously a more elaborate facility was built 65 km south of Semipalatinsk-21 for comprehensive tests of the Baikal-1 prototype engine. Thirty simulated flights were conducted from 1970 to 1988 without failure. It was eventually proposed that two engines would be derived from this work: the RD-0410, a 'minimum' engine, of 3.5 tonnes thrust; and later the RD-0411, a 70 tonne thrust engine.
By 1989 work on both nuclear electric and nuclear thermal propulsion included bimodal use of the nuclear reactors to provide electrical power during dormant or ballistic cruise phases of flight. In the case of nuclear thermal engines this meant addition of a Brayton cycle turbine using xenon-helium coolant. A nuclear thermal Mars spacecraft proposed by the Kurchatov Institute in 1989 featured a new design powerplant of 20,000 kgf, a thermal power of 1200 MW, operating time of 5 hours, and a specific impulse of between 815 and 927 seconds. During cruise operations the turbine would provide 50-200 kW of electric power, requiring 600 square meters of radiators at the end of the spacecraft. Total mass of this combination power plant was estimated to be 50 to 70 tonnes.
The collapse of the Soviet Union ended further development work on nuclear thermal propulsion.