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D. Trajectory Orbits for assembly and staging, together with trajectories for departure to trans-Mars injection, constitute many options. This section considers three. First is the LEO assembly and departure baselined in the Mars DRA-5. Second is an Earth-Moon Lagrange point 2 (LL2) departure that requires less V from the crew vehicle, but more tightly constrains the available payload mass per launch vehicle, because all components must be delivered to LL2 for assembly. The same is true for the third trajectory, which departs from a highly elliptical Earth orbit (HEO). All trajectories are for the 2035 launch window, with the cargo vehicles, including any propellant depot sent to Mars, launching in the 2033 window. 1. DRA-5 Baseline (LEO Departure) In the DRA-5, the baseline crew trajectories are constrained to 180 days. The 2035 launch opportunity departs on 6/26/2035, and arrives at Mars on 12/23/2035.25 The necessary maneuvers are a trans-Mars injection from LEO in two parts, totaling 4 km/s V, Mars orbit injection with 1 km/s V, and a trans-Earth injection with 1.6 km/s V, for a total of 6.6 km/s. In Section III, the electrolysis propulsion architectures that depart from LEO use the same trajectory. When considering LEO departure for an electrolysis propulsion mission, the boiloff during electrolysis, versus inefficiency of low thrust spiraling orbits, becomes a consideration. With 4 km/s of V required for trans-Mars injection, a 58 t payload with a 5.1 t solar array require overs 134 days to electrolyze if the maneuver is to be performed all at once. But, a continuous thrusting transfer involves a spiral out of LEO, which is inefficient. A third possibility is to electrolyze continuously, but perform an apoapsis raising burn at each periapsis, to avoid storing cryogenic propellant for longer than a single orbit. While approach reduces boiloff compared to electrolyzing for 134 days before performing any maneuver, it still requires 134 days. An alternative trajectory avoids this problem entirely. The following subsections examine two possibilities with less V required from the electrolysis propulsion vehicle to reach trans-Mars injection. 2. Highly Elliptical Orbit Departure An alternative to assembling in, and departing from, LEO is to do so in a 10 day HEO orbit approximately to the lunar distance. In this case, for the 2035 launch opportunity, trans-Mars injection is reduced from 4 km/s to approximately 0.7 km/s.33 Combined with the same 1 km/s MOI and 1.6 km/s TEI as the previous subsection, the round trip V the crew vehicle must achieve under its own power is reduced to 3.6 km/s. This strategy requires slightly less V than the L2 departure described next, and has similar mass constraints for each launch vehicle. 3. Earth-Moon L2 Departure Another alternative is to assemble the crew vehicle in cislunar space, specifically, in a halo orbit using the Earth- Moon Lagrange point 2 (LL2). For departure, flybys of the Moon and Earth are used to escape the Earth-Moon system and reach trans-mars injection as shown in Figure 6. In this case, for the 2035 launch opportunity, trans-Mars injection is reduced from 4 km/s to approximately 1.1 km/s.33 Combined with the same 1 km/s MOI and 1.6 km/s TEI as the previous subsection, the round trip V the crew vehicle must achieve under its own power is reduced to 4 km/s. This strategy requires slightly more V than the HEO departure, but provides the opportunity to utilize the Deep Space Gateway (DSG), a planned cislunar space station, as a staging area if it is available. The DSG is intended to support buildup of deep space transports for Mars among other missions.36 Its presence to supervise stationkeeping can eliminate the need for independent RCS on propellant depots at LL2 while awaiting the crew, reducing their non- propellant mass and increasing the amount of propellant which can be delivered by each launch vehicle. 10 American Institute of Aeronautics and Astronautics Figure 5: 2035 trajectory from the Mars DRA-5.25 Figure 6: Departure from Lunar Lagrange point 2 from the Mars DRA-5.25 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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