A Nuclear Electric Propulsion Module for Outer Planets Exploration

Abstract

Nuclear electric propulsion offers unique advantages for the outer planets exploration. Electric propulsion enables to provide very high Delta V (10 to 50 km/s) while the onboard power is available for the rest of the mission. The system study showed that 80 to 100 kWe are needed for a BOM mission of 10 tons (ARIANE 5 ESCA capabilities). The proposed design implies a combination with a small cryogenic propulsion module both for interplanetary injection and for injection in safe orbit in case of failure. The mission profile enables to use existing electric thrusters designs, thus minimising development costs and risks. INTRODUCTION The outer planets exploration is presently depending on RTG for power supply, while chemical propulsion and gravity assist provide the required velocity increment (∆V). Most outer planet missions involved fly-bys or repeated fly-bys of their satellites. The possibility to orbit Europe or Titan will provide very important scientific results. In a longer term, Kuiper belt exploration will also provide invaluable data on early solar system composition. This type of new missions requires ∆V beyond the capability of chemical propulsion, so electric propulsion is mandatory. RTG are not available in Europe, therefore two options can be considered : an European RTG offering no possibility to perform electric propulsion, or a NEP stage offering major improvements for on board power and mission profile. The second option is presented in this work. The main optimisation parameters are determined in a first step. Then the different nuclear reactor and energy conversion options are described. Last, electric propulsion options are presented. The overall layout of a multi-missions bus is presented. The electrical power of the nuclear reactor is close to the one required for a manned base on Mars or Moon. Therefore this offers the possibility to reduce overall development cost. The NEP stage includes a cryogenic propulsion module : this combination enables to reduce the ∆V requirement when compared to an all electric mission profile. Anyway the chemical propulsion module is required to inject the stage in safe orbit in case of nuclear reactor or system failure. MAIN OPTIMISATION PARAMETERS In any electric propulsion system, the overall dry mass is a major design parameter. Since the mission involves a very high ∆V, the propellant mass is also a concern. However the selection of a high specific impulse system is not a panacea. At equal thrust, the power increases almost proportionally to the Isp, and the power source increases in the same proportion. As it is well known in electric propulsion, this results in an optimum specific impulse corresponding to a maximum payload (figure 1).