A version of the N1 with a nuclear upper stage was studied by Korolev in 1963. It was concluded that the optimum design would allow a single N1 to launch a direct manned lunar landing and return. However for manned Mars missions, a nuclear electric engine was found to be much more efficient. This essentially killed further consideration of thermal nuclear upper stages within the bureau.

Following abandonment of the nuclear-ammonia ICBM projects, the engine bureaus of Bondaryuk (OKB-670) and Glushko (OKB-456) continued study of nuclear propulsion, but using liquid hydrogen for upper stage applications. Engines of 200 metric tons and 40 metric tons thrust with a specific impulse of 900 to 950 seconds were being considered. At the end of 1961 both bureaus completed their draft projects and it was decided to continue work on development of an engine in the 30 to 40 metric ton thrust range. In the following year Korolev was asked to study application of such engines, followed by a specific demand in May 1963 from the Scientific-Technical Soviet for specific recommendations.

Korolev considered three variants based on the N1:

• A three stage vehicle using the N1 first and second stages and a nuclear third stage
• A three stage vehicle using the N1 first stage and nuclear second and third stages
• A two stage vehicle using the N1 first stage and a nuclear second stage

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 metric tons for the Type A engines and 1,500 to 2,000 metric tons for the type V engines (this resulted in a halaciously 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 metric tons with return of a 5 metric ton 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 favored 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. Bondaryuk and Glushko turned to Chelomei and his competing UR-700 rocket for future application of such stages.

Manufacturer: Korolev. LEO Payload: 270,000 kg (590,000 lb). to: 220 km Orbit. at: 51.60 degrees. Payload: 24,600 kg (54,200 lb). to a: lunar surface trajectory. Liftoff Thrust: 35,000.000 kN (7,868,000 lbf). Total Mass: 2,400,000 kg (5,200,000 lb). Core Diameter: 17.00 m (55.00 ft). Total Length: 180.00 m (590.00 ft).

• Stage Number: 1. 1 x Stage: N1 1962 - A. Gross Mass: 1,384,000 kg (3,051,000 lb). Empty Mass: 117,000 kg (257,000 lb). Thrust (vac): 39,420.000 kN (8,861,960 lbf). Isp: 331 sec. Burn time: 103 sec. Isp(sl): 296 sec. Diameter: 10.00 m (32.00 ft). Span: 17.00 m (55.00 ft). Length: 30.00 m (98.00 ft). Propellants: Lox/Kerosene. No Engines: 24. Engine: NK-15. Status: Study 1963. Includes 14,000 kg for Stage 1-2 interstage and payload fairing. Compared to total fuelled mass excludes 15,000 kg propellant expended in thrust build-up and boil-off prior to liftoff. Values as in draft project as defended on 2-16 July 1962.

• Stage Number: 2. 1 x Stage: N1 Nuclear A. Gross Mass: 700,000 kg (1,540,000 lb). Empty Mass: 250,000 kg (550,000 lb). Thrust (vac): 6,860.000 kN (1,542,180 lbf). Isp: 900 sec. Burn time: 570 sec. Diameter: 12.00 m (39.00 ft). Span: 12.00 m (39.00 ft). Length: 90.00 m (295.00 ft). Propellants: Nuclear/LH2. No Engines: 40. Engine: YaRD Type A. Status: Study 1963. N1 nuclear upper stage study, 1963. Figures calculated based on given total stage thrust, specific impulse, engine mass.

1963 May - Launch Vehicle: N1 Nuclear A, N1 Nuclear AF, N1 Nuclear V.

• Nuclear N1 designs Nation: USSR. At the end of 1961 the Glushko and Bondaryuk bureaux completed their draft projects on nuclear thermal engines for space vehicle upper stages. 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 specific demand in May 1963 from the Scientific-Technical Soviet for specific recommendations. 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. Bondaryuk and Glushko turned to Chelomei and his competing UR-700 rocket for future application of such stages.

• Vetrov, G S, S. P. Korolev i evo delo, Nauka, Moscow, 1998. ISBN: 5020036846. The collected papers of Soviet Chief Designer Korolev. A tremendous source of new information and insight on the Soviet space program. Russian language. More at amazon.com...