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The comparisons in Tables IV-1 through IV-4 show that the refueling architecture saves one or two SLS launches, depending on the performance of the electrolysis propulsion system that replaces the LH2/LOX stages in the baseline mission. Assembly times before departure are correspondingly reduced, up to 50%, because not only are there fewer launches in general, but at least one of them is a propellant delivery to Mars and not part of the assembly at Earth prior to departure. Whether performance is 450 s or 300 s, using FH for some assembly in Earth orbit or to deliver smaller amounts of propellant at different stages of the mission ultimately saves an additional SLS launch. The resulting difference in launch cost depends on the cost estimate chosen for SLS and FH. Because two FH launches replace the additional SLS launch in most cases, replacing an SLS launch with two FH launches is generally cost effective if, assuming our cost estimate for FH is accurate, each SLS costs more than $600M. There is the possibility of eliminating the SLS from the mission completely, and using only heavy lift launch vehicles such as the FH. The limiting factor here is the mass of the transit habitat, which is much greater than that which the FH can deliver to LL2 or HEO, coupled with the inefficiency of electrolysis propulsion for escaping from LEO. Thus, additional launch vehicles boost the transit habitat from LEO, where its mass can be delivered, to a higher orbit from which electrolysis propulsion can depart more quickly. The net result is that, while this may be the least expensive option in terms of launch costs, the assembly time is not reduced. V . Conclusion The Mars DRA-5 chemical propulsion reference is a multi-stage vehicle massing 486 t on departure from LEO. It takes five SLS launches and 120 days to assemble, requiring an additional LEO module to reboost its orbit during this period. There are also concerns about the storage lifetime of cryogenic hydrogen and oxygen as propellant, which have never before been used after a period of up to several years in space. This paper focuses on water electrolysis propulsion that enables strategic refueling from tanks pre-positioned ahead of the crewed vehicle by earlier launches. This propulsion technology enables alternative trajectories that were originally considered in the DRA-5, including HEO or LL2 departure to take advantage of the Deep Space Gateway as a staging area. The results show that at least one SLS launch can be eliminated at current electrolysis propulsion performance. Two SLS launches can be eliminated if performance improves or if two FH launches are substituted for one of the two SLS launches. Three SLS launches can be eliminated if performance improves and two FH launches are substituted for one of the three SLS launches. These alternatives reduce the assembly time of the mission and the total mass that must be lifted into orbit. The amount of propellant ever stored in cryogenic form, as well as the time it must be stored for, is dramatically reduced, because propellant can be stored in water form indefinitely until it is needed. This results in less boil-off, and a greater fraction of stored propellant mass usable for propulsion. These architectures can support an otherwise baseline Mars DRA-5 mission, take advantage of the Deep Space Gateway if it is available, and do so while significantly reducing launch costs due to fewer launch vehicles needed. For future work, we will investigate the possibility of in-situ resource utilization of water for the return trip from Mars. Both Mars and its moon Deimos are possible candidates for this, and each has their own advantages and disadvantages. Lifting water from the surface of Mars would require an additional ascent vehicle, and it is possible that simply delivering water massing the same as this ascent vehicle to Mars orbit would be a more efficient use of a launch vehicle. However, a visit to Deimos is not currently planned in the Mars DRA-5 and doing so would increase the scope of the mission, but this would also provide an opportunity to achieve additional scientific objectives. Additionally, the possibility of low-thrust trajectories using electrolysis propulsion should be examined further. While even the ideal specific impulse of 450 s is poor compared to that of other electric thrusters, electrolysis propulsion offers a much greater thrust per unit power, and also the flexibility to store electrolyzed propellant temporarily to perform impulsive maneuvers when needed. The relatively high thrust per unit power could permit continuous-thust trajectories that are otherwise impractical with current solar panel performance. If an entirely low- thrust trajectory is designed, and water for refueling is sourced elsewhere than from Earth, it could be possible to take advantage of water as an ISRU propellant without any cryogenic storage needed at all. Acknowledgments This work was conducted at Cornell University. We would like to acknowledge the contributions of Cornell undergraduate students who took the MAE 5160 Spacecraft Technology and Systems Architecture course in the spring 2017 semester. Their final project work for the course helped explore different configuration ideas for this paper. 23 American Institute of Aeronautics and Astronautics 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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