How Fast Can We Go To Mars Using High Power Electric Propulsion?

Abstract

Recent advances in electric propulsion technologies such as magnetoplasma rockets gave new momentum to the study of nuclear electric propulsion concepts for Mars missions.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 transfer time is more complex to define, as it depends on many design parameters. In this paper, we perform a general analysis of the mid-term capability of nuclear electric propulsion systems considering both payload mass ratio and transfer time. This study has been performed through a multidisciplinary analysis which combines general performance calculations for power-limited systems, an analysis of nuclear power-source that could be available in the future, and a series of mission analysis including trajectory optimization. The results obtained in this study emphasize and -most importantly -quantify the importance of the specific mass of the power and propulsion system with regards to the objective of a fast transfer. The objective of a very fast Earth-to-Mars transfer in less than six weeks appears unrealistic in a mid-term context, as it depends on a hypothetical breakthrough on the nuclear electric power source, with a specific mass typically lower than 1 kg/kW. The results obtained in this paper draw the contour of performance improvements that could be obtained in a mid-term horizon, using power generation technologies that are challenging but more commonly considered as reasonably optimistic, for example using a high temperature Brayton or Rankine conversion cycle. It appears that the shorter Earth-toMars transfer time that could be expected for missions with sufficient payload is about 120 days, compared to 180 days with chemical propulsion. Nomenclatureg 0 = Earth gravity acceleration at sea-level (9.80665 m/s 2 ) Isp = specific impulse k = tank structural mass ratio = M S / M P q = mass flow-rate (kg/s) M = mass (kg) M f = total final mass (kg) M i = total initial mass (kg) M P = propellant mass (kg) M PL = payload mass (kg) M W = power source mass (kg) M S = structure mass (kg) 2 Pe = input power delivered by the source (W) r = position vector t = time T = thrust U = thrust direction vector v = velocity vector α = specific mass of the power source and propulsion system (kg/kW) γ T = thrust acceleration (m/s 2 ) λ = trajectory characteristic parameter (m 2 /s 3 ) μ = Earth gravity constant (3.986.10 14 km 3 /s 2 ) η = global efficiency of the propulsion system