Design and Development of the MITEE-B Bi-Modal Nuclear Propulsion Engine

Abstract

Previous studies of compact, ultra‐lightweight high performance nuclear thermal propulsion engines have concentrated on systems that only deliver high thrust. However, many potential missions also require substantial amounts of electric power. Studies of a new, very compact and lightweight bi‐modal nuclear engine that provides both high propulsive thrust and high electric power for planetary science missions are described. The design is a modification of the MITEE nuclear thermal engine concept that provided only high propulsive thrust. In the new design, MITEE‐B, separate closed cooling circuits are incorporated into the reactor, which transfers useful amounts of thermal energy to a small power conversion system that generates continuous electric power over the full life of the mission, even when the engine is not delivering propulsive thrust. Two versions of the MITEE‐B design are described and analyzed. Version 1 generates 1 kW(e) of continuous power for control of the spacecraft, sensors, data transmission, etc. This power level eliminates the need for RTG’s on missions to the outer planets, and allowing considerably greater operational capability for the spacecraft. This, plus its high thrust and high specific impulse propulsive capabilities, makes MITEE‐B very attractive for such missions. In Version 2, of MITEE‐B, a total of 20 kW(e) is generated, enabling the use of electric propulsion. The combination of high open cycle propulsion thrust (20,000 Newtons) with a specific impulse of ∼1000 seconds for short impulse burns, and long term (months to years), electric propulsion greatly increases MITEE’s ΔV capability. Version 2 of MITEE‐B also enables the production and replenishment of H2 propellant using in‐situ resources, such as electrolysis of water from the ice sheet on Europa and other Jovian moons. This capability would greatly increase the ΔV available for certain planetary science missions. The modifications to the MITEE multiple pressure tube/fuel element assembly to achieve bi‐modal capability are modest. Small diameter coolant tubes are bonded to the surface of the MITEE cold frits that enclose the fuel elements. When the MITEE‐B is not operating with H2 propellant to generate high thrust, the reactor continues to operate at low thermal, which is transferred to the closed coolant circuit. Three electric power generations are examined for MITEE‐B: closed Brayton, Stirling, and a conventional steam cycle with a mini‐turbine. The Stirling and steam cycles have the lowest specific masses in kg/kW(e). Both appear practical for MITEE‐B.