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I. Introduction The Mars Design Reference Architecture 5.0 (DRA-5) is a 2007 study guided by NASA’s Mars Architecture Steering Group1. It is the latest in a series of Reference Architectures and presents several options for key trades in the mission architecture. One of the most significant trades, and the one this paper concerns, is the in-space propulsion design. DRA-5 considers two options in depth: nuclear thermal propulsion and chemical propulsion using liquid hydrogen and liquid oxygen (LH2/LOX). This paper examines the chemical propulsion option and provide alternatives based on in-flight refueling of liquid water at key points during the mission. This paper begins with an overview of the Mars DRA-5 propulsion concept, followed by a summary of the particular challenges facing this architecture that the proposed approach addresses, then an overview of the electrolysis propulsion technology, which enables the use of liquid water as propellant. Section II presents the concepts of operation for several Mars transporation schemes using this technology with tanks of liquid water that are sent ahead of the crew vehicle to anticipate refueling. Section III compares the launch requirements for each. Section IV concludes and discusses future work. A. Overview of Propulsion in Mars Design Reference Architecture 5.0 The DRA-5 chemical propulsion design architecture is depicted in Figure 1. This design sends six crew to Mars for an approximately 1.5 year stay. The design uses three interplanetary vehicles per mission. Two are cargo vehicles sent ahead of the crew, with their cargo contained in an aeroshell for aerocapture at Mars. One includes a Mars surface habitat, while the other contains a descent/ascent vehicle (DAV). The third vehicle transports the crew and consists of a transit habitat and five propulsion modules: • Three for trans-Mars injection (TMI) o Two perform the first burn together and are then jettisoned; the third completes the maneuver. • One for Mars orbit injection (MOI) • One for trans-Earth injection (TEI). The additional modules exist because the crew vehicle does not use aerocapture to enter Mars orbit and, unlike the cargo, need to return to Earth. All three vehicles are assembled in orbit, over a period of 170 days for the cargo vehicles, and 120 days for the crew vehicle. The present study focuses on the crew vehicle because its five-module propulsion staging can be streamlined by the proposed refueling approach. As the next section makes clear, pre-positioning fuel to be collected after departure from Earth can eliminate several of these modules and dramatically reduces the size and mass of the crewed vehicle. Although the propellant being pre-positioned for refueling requires its own launch vehicles, the net result is a reduction in the total launch mass required and thus in the number of launches needed. Although the cargo vehicles have a longer combined assembly time than the crew vehicle, they each have only two propulsion modules, because they do not require MOI (aerocapture is used instead) or TEI (the cargo payloads do not return to Earth). Also, the mass of vehicle is lower, about 310 t, than the crewed vehicle at 486 t. There is correspondingly less propellant and propulsion dry mass as well. Therefore, the cargo vehicles do not benefit as much from refueling, and this study does not examine them further. Additionally, because the DRA-5 is based on a 2007 study, it uses the Ares launch vehicles from the since- cancelled Constellation program. Because “the gross [LEO] payload capability of the reference Ares V vehicle was assumed to be 131.4 t”12 for this study, and the Space Launch System (SLS) Block 2 has a LEO payload capability of 130 t, the two launch vehicles are comparable. Additionally, the Ares I was intended to have a 25 t LEO payload capability, which is comparable to the SpaceX Falcon 9 (F9) at nearly 23 t.14,15 Although the F9 is not human-rated at the time of this writing, SpaceX is pursuing this goal and intends to use F9 for its Commercial Crew Program launches.16 Therefore, this analysis hereafter substitutes SLS for Ares V, and F9 for Ares I. 2 American Institute of Aeronautics and Astronautics Figure 1: Baseline Mars transportation system architecture from the Mars DRA-5.1 Downloaded by NASA LANGLEY RESEARCH CENTRE on January 30, 2018 | http://arc.aiaa.org | DOI: 10.2514/6.2018-1537PDF Image | Water Electrolysis for Propulsion of a Crewed Mars
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