Recent advances in electric propulsion technologies such as magnetoplasma rockets gave a new momentum to the study of nuclear electric propulsion concepts for Mars missions. Some recent works have been focused on very short Earth-to-Mars transfers of about 40 days with high-power, variable speci¦c impulse propulsion systems [1]. While the interest of nuclear electric propulsion appears clearly with regard to the payload mass ratio (due to a high level of speci¦c impulse), its interest with regard to the transfer time is more complex to de¦ne, as it depends on many design parameters. In this paper, a general analysis of the capability of nuclear electric propulsion systems considering both criteria (the payload mass ratio and the transfer time) is performed, and the technological requirements for fast EarthMars transfers are studied. This analysis has been performed in two steps. First, complete trajectory optimizations have been performed by CNES-DCT in order to obtain the propulsion requirements of the mission for di¨erent technological hypotheses regarding the engine technology (speci¦c impulse levels and the throttling capability) and di¨erent mission requirements. The methodology used for designing fuel-optimal heliocentric trajectories, based on the Pontryagin£s Maximum Principle will be presented. Trajectories have been computed for various power levels combined with either variable or ¦xed I sp . The second step consisted in evaluating a simpler method that could easily link the main mission requirements , 2013 This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Progress in Propulsion Physics4 (2013) 313-336 DOI: 10.1051/eucass/201304313 © Owned by the authors, published by EDP SciencesArticle available at http://www.eucass-proceedings.eu or http://dx.doi.org/10.1051/eucass/201304313
PROGRESS IN PROPULSION PHYSICS(the transfer time and the payload fraction) to the main technological requirements (the speci¦c mass of the power generation system and the structure mass ratio of the whole vehicle, excluding the power generation system). Indeed, for power-limited systems, propulsion requirements can be characterized through the ¤trajectory characteristic¥ parameter, de¦ned as the integral over time of the squared thrust acceleration. Technological requirements for the vehicle can then be derived from the propulsion requirements using a simpli¦ed performance model designed by Onera [2]. This model yields the optimum vehicle design in terms of the payload mass ratio as well as the theoretical upper limit of the power source£s speci¦c mass as a function of the transfer time. Both studies show that the key to very fast EarthMars transfers (40 days, or less) is the reduction of the power source speci¦c mass below 1 kg/kW. On-going French studies [3] tend to show that speci¦c masses of nuclear reactors for exploration mission are expect...