Gerkules Nuclear Tug Gerkules Nuclear Electric Interorbital Tug with 11B97 engine. Credit - © Mark Wade

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 (ElecktroRaketnovo Transportnovo 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 7500 tonnes and it could provide up to 10 years of electrical power traded off against 1.5 years of powered flight.