The serious development of nuclear electric propulsion began after issuance of the decree of 23 June 1960, as a result of which ten design bureaux and other organisations tackled technical questions related to its development. OKB-1 specialised in theoretical studies, experimental tests, materials technology, and equipment trials (including reactors). Korolev decided to collaborate with TsNII-58 (Chief Designer V G Grabin) for the reactor. This design bureau had designed the water moderated reactors which were already providing power in Tashkent, Riga, Kiev, Alma-Ata, Hungary, Rumania, DDR, Czechoslovakia, and Egypt. Grabin was at that time developing the first experimental fast neutron reactors using liquid metal cooling (SR-1 and SR-3) which were in operation in Obninsk at the Physics-Energy Institute (FEI).
In the beginning, development and test work was oriented towards providing power for an electric engine for manned interplanetary flight. The nuclear electric engines for the initial TMK-E Mars spacecraft design of 1960 used 7 MW of nuclear power. Later reactor research was expanded to cover application of nuclear power for scientific, economic, and military objectives in space.
OKB-1 and FEI studied various methods for transforming the reactor's thermal energy to electrical energy to power the engine (steam turbines, gas turbines, MHD, and direct thermo-electric conversion). This analysis indicated that direct thermo-electric conversion was clearly the best approach.
First stage testing of nuclear electric propulsion began in 1962 and the original draft project N1 of that year foresaw the use of this form of propulsion in multi-module orbital base stations and interplanetary spacecraft. In 1965 Section 12 (Manager I I Raikov) of OKB-1 completed work with FEI on a draft project for a nuclear electric propulsion engine YaERD-2200 for interplanetary crewed spacecraft. The YaERD-2200 consisted of two independent stages. Each had a nuclear reactor and an electric engine, with electrical output of each being 2,200 kW and total thrust 8.3 kgf.
The engine featured direct thermo-electric conversion using a fast neutron reactor; a coolant system using low activity isotope Lithium-7 in a single loop shared by both the reactor and engine; and an electro plasma engine with an efficiency of 55% and a specific impulse of 5500 sec
The reactor / engine design was upgraded to 5,000 kW total power in 1966-1970. The revised design could be used in single block (YaE-1 and YaE-1M) and multiple block (YaE-2 and YaE-3) applications. A single Block YaE-1 would have an electrical output of 2,500-3,200 kW with fuel for 4,000 to 8,000 hours of operation. Block YaE-1M would have an output of 5000 kW. Total thrust of the engine would be from 6.2 to 9.5 kgf with a specific impulse of from 5,000 to 8,000 sec. In three block applications, electric capacity would be 3 x 3,200 kW and 3 x 5,000 kW. The Aelita MEK design of 1969 used a total of 15,000 kW.
Development of nuclear electric propulsion continued throughout the 1970's. In accordance with the decrees of 8 June 1971 and 15 June 1976 this was now concentrated on development of the more modest nuclear electric rocket stage 11B97. This stage would have an electric capacity of 500-600 kW and would use specialised plasma-ion electric engines using standing plasma waves and anodes. In 1975 nuclear electric propulsion work was reorganised within NPO Energia into a special complex 7 (Manager M V Melnikov). In 1984 it was renamed section 7 (Manager P I Bistrov, and from 1993 Y A Bakanov). Through all these reorganisations functional test of the reactor and engine components continued. Concepts for the direct thermoelectric transformation of energy could not be realised without a new class of refractory and high temperature materials, new heat pipe concepts, and other new technology. To develop these technologies it was necessary to build new materials test facilities, high temperature tests stands, new experimental shops to develop methods to handle and work new refractory alloys (niobium, molybdenum, wolfram, vanadium) and insulative and magnetic materials.
From 1966 to 1982 many test stands were built to develop these materials and test components of the systems. The final result was the 11B97 engine, powered from a reactor with a 200 litre core containing 30 kg of uranium fuel. In 1978 this engine was studied for use as a reusable interorbital space tug for launch by Energia-Buran. In 1982, according to the decree of 5 February 1981, NPO Energia developed for the Ministry of Defence the interorbital tug Gerkules with 550 kW maximum output and continuous operation in the 50-150 kW range for 3 to 5 years. In 1986 an interorbital tug was studied to solve the specific application of transporting heavy satellites of 100 tonnes to geostationary orbit, launched by Energia.
In 1986 RKK Energia updated the 1969 MEK design for launch by the new Energia launch vehicle. The propulsion section was essentially the same except that for safety reasons two completely independent redundant reactor / engine assemblies were used in the place of the single unit of the MEK design.
Energia retained the electric engines of the 1969 MEK design but dropped the nuclear ractor for its 1989 Mars expedition design. This spacecraft used the same thruster arrays requiring the same power output (15 MW) as the 1986 nuclear design. But in this case two enormous panels, each 200 m x 200 m would generate a total of 15 MW of power at earth. The use of ultra-thin (less than 50 micrometer) / low mass (0.2 kg per square meter) photovoltaic cells with a high specific power value (up to 200 W per square meter) minimised the weight of these vast arrays. The total mass of the electric engines, structure, and solar panels was 40 tonnes. The power generated would be used primarily by two ion engine clusters mounted perpendicular to the living block. In high-power mode these would have a specific impulse of 3500 seconds. They would consume 165 tonnes of xenon propellant during the voyage (of 355 tonnes total spacecraft mass).
In the 1990's Energia studied use of nuclear electric propulsion for the scientific development project 'Mars - Nuclear electric propulsion Stage' under contract to the Russian Space Agency and the project 'Star - Soarer' under contract to the Ministry of Atomic Industry. These studies looked at designs for the 2005 period. At the beginning of the 1990's a new type of nuclear generator was studied, that would have a capacity of 150 kW in the transport role and provide 10-40 kW to power spacecraft systems while coasting. This was designated ERTA (Elecktro-Raketniy Transportniy Apparat). Technologies and concepts for this engine were studied by FEI and other organisations. A modular concept was adopted. In 1994 ERTA was studied for launch by Titan, Ariane 5, or Energia-M launch vehicles. The reactor weight was 7,500 kg and it could provide up to 10 years of electrical power traded off against 1.5 years of powered flight.
Aside from this work on the 150 kW design, there was also an examination at the same time of the use of nuclear electric propulsion for Mars expeditions. Single and multiple launch approaches were considered. For a single-launch complex of 150 tonnes a nuclear electric propulsion unit of 5 to 10 MW with enough fuel for 1.5 years would be required. For the multiple launch design, a power of 1 to 1.5 MW and fuel for three years would be required.
In 1994-95, RKK Energia, and NASA's Jet Propulsion Laboratory analysed the project 'Mars Together'. This studied the use of spacecraft using solar arrays or nuclear reactors of up to 30 to 40 kW for insertion into Martian orbit and operation of a side-looking radar to digitally map the surface. As a preliminary step a demonstration launch was proposed of a spacecraft with a mass of 120 to 150 kg, a solar panel area of 30 square meters and engines with a thrust of 3 kW. Objectives of the experiment would be understanding of the changing of the orbital altitude with continuous work of the ion engine for several hundred hours.