Technological requirements of nuclear electric propulsion systems for fast Earth-Mars transfers

Abstract

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 specific 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 specific impulse), its interest with regard to the transfer time is more complex to define, 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 Earth-Mars 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 different technological hypotheses regarding the engine technology (specific impulse levels and the throttling capability) and different 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 fixed I sp . The second step consisted in evaluating a simpler method that could easily link the main mission requirements (the transfer time and the payload fraction) to the main technological requirements (the specific 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, defined 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 simplified 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 specific mass as a function of the transfer time. Both studies show that the key to very fast Earth-Mars transfers (40 days, or less) is the reduction of the power source specific mass below 1 kg/kW. On-going French studies [3] tend to show that specific masses of nuclear reactors for exploration mission are expected to be much higher, even at very high power levels, so this requirement is unlikely to be met at short or medium term. Finally, a synthesis of these results will outline the performance of a nuclear electric propulsion system for fast Earth-Mars transfer that could be achievable considering “reasonably optimistic” hypotheses for the specific mass of the power generator.