Feasibility Assessment of Rapid Earth–Mars Transfers Using High-Power Electric Propulsion Nicolas Bérend * ONERA–The French Aerospace Lab, F-91123 Palaiseau, France Elisa Cliquet Moreno † and Jean-Marc Ruault ‡ Centre National d’Études Spatiales, F-75612 Paris, France and Richard Epenoy § Centre National d’Études Spatiales, F-31401 Toulouse, France DOI: 10.2514/1.A32560 The aim of this paper is to investigate the feasibility of an Earth-to-Mars transfer with reduced transfer time using a high-power electric propulsion system. The study involved a multidisciplinary analysis combining general performance calculations for power-limited systems, an analysis of a nuclear power source that might be available in the future, and a series of mission analyses, including trajectory optimization. The study quantified the importance of the specific mass of the power and propulsion system with regard to the objective of a fast transfer. A very fast Earth- to-Mars transfer in less than six weeks appears to be unrealistic in the medium term because it depends on a hypothetical breakthrough with the nuclear electric power source, requiring a specific mass that would typically be less than 1 kg∕kW. The results obtained define the limits to improvements that could be obtained in the medium term, using power-generation technologies that, while challenging, are frequently considered as reasonably optimistic, for example using a high-temperature Brayton or Rankine conversion cycle. The shortest Earth-to-Mars transfer time that could be expected for missions with a large enough payload mass was found to be about 120 days, compared to 180 days with chemical propulsion. Nomenclature g 0 = 9.80665 m∕s 2 , terrestrial acceleration due to gravity at sea-level Isp = specific impulse k = M S ∕M P , tank structural mass ratio 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 S = structure mass, kg M W = power source mass, kg Pe = input power delivered by the source, W q = mass flow rate, kg∕s r = position vector, m T = thrust modulus, N t = time, s U = thrust direction vector v = velocity vector, m∕s α = specific mass of the power source and propulsion system, kg∕kW γ T = thrust acceleration, m∕s 2 η = overall efficiency of the propulsion system λ = trajectory characteristic parameter, m 2 ∕s 3 μ = gravitational parameter (1.327 · 10 20 × m 3 ∕s 2 for the Sun, and 3.986 · 10 14 m 3 ∕s 2 for the Earth) I. Introduction O NE of the challenges of human exploration of Mars lies in the long time required for interplanetary transfer, which is about 180 days for Earth-to-Mars using chemical or nuclear thermal propulsion. Indeed, classical manned Mars mission scenarios based on chemical or nuclear thermal propulsion typically allow for two types of mission profiles: either a conjunction class mission with a 500–600 day stay, requiring 180–220 days each way, or an opposition class mission with a 30–60 day stay, requiring 250–300 days in transit to Mars and 150– 300 days for the return leg [1–3]. Although not considered as strictly mandatory, any reduction in transfer time would be an improvement, as it would relieve many issues, both technical and psychological, for human Mars missions. High-power nuclear electric power systems, whose primary advantage is a high specific impulse, are considered as possible solutions to this problem [4,5] because they open the way to alternative, more flexible mission profiles. One example of fast Earth-to-Mars transfer was studied recently by the Ad Astra Rocket Company (AARC), which proposed a mission architecture for sending humans to Mars in 39 days using nuclear-generation for electrically powered variable-specific-impulse magnetoplasma rocket (VASIMR) engines [6]. The purpose of this paper is to analyze the medium-term capability of nuclear electric propulsion systems considering both payload mass ratio and transfer time with regard to Earth-to-Mars transfers of less than 6 months. In the first part of the paper (Sec. II), general mass target considerations for manned Mars missions are given, and the achievable specific mass of the nuclear power-generation system is discussed based not only on bibliographical sources but also on recent CNES–AREVA studies [7]. Based on this discussion, high- level assumptions are derived regarding the specific mass of the power source for credible medium-term and long-term mission Presented as Paper 2012-3889 at the 48th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, Atlanta, GA, 30 July–1 August 2012; received 5 December 2012; revision received 29 October 2013; accepted for publication 6 November 2013; published online 28 March 2014. Copyright © 2013 by ONERA and CNES. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 1533-6794/14 and $10.00 in correspondence with the CCC. *Senior Expert, System Design and Performance Calculation Department, hemin de la hunière et des joncherettes, BP 80100. Member AIAA. † Propulsion System Specialist, Directorate of Launchers, 52 Rue Jacques Hillairet. ‡ Project Manager, Directorate of Launchers, 52 Rue Jacques Hillairet. § Trajectory Optimization Expert, Toulouse Space Centre, 18 Avenue Edouard Belin. 946 JOURNAL OF SPACECRAFT AND ROCKETS Vol. 51, No. 3, May–June 2014 Downloaded by UNIV. DEGLI STUDI DI MILANO on June 10, 2014 | http://arc.aiaa.org | DOI: 10.2514/1.A32560