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II. Mars Design Reference Architecture 5 Proposed Pre-positioning Propellant Architecture A. Refueling Depot Design Our concept pre-positions lightweight tanks of liquid water to strategic locations for the crew vehicle to refuel in between maneuvers. Propellant can be stored as water indefinitely if heated to prevent freezing, in contrast to cryogenic hydrogen and oxygen. An advanced cryogenic propellant storage system could keep boil-off as low as 0.1% per day, while present boil-off is on the order of 1%.32 In either case, if the propellant were stored in cryogenic form for a full synodic period in between launch windows, years would pass before it was used, and an unacceptably high fraction would be lost. Because water does not boil off in the same way, water depots can be delivered to Mars orbit in the same synod as the cargo vehicles in the DRA-5, over two years prior to crew arrival. Potential propellant destinations include several Earth orbits for integration prior to departure, the Earth-Moon Lagrange point 2 (LL2) to use the Deep Space Gateway as a staging area, and Mars orbit for refueling prior to trans- Earth injection for the return trip. There is also the option of delivering a complete propulsion module, including electrolyzers and thrusters, along with the tank. In this case, the delivered module is integrated into the crew vehicle stack before departure, instead of drained of propellant for a refueling operation. The propulsion module dry mass of the Mars DRA-5 trans-Mars injection propulsion modules is 15.1 t.24 This figure includes the dry mass of 5 engines massing 1 t each, as well as the truss to hold them and the dry mass of cryogenic tanks sized for 91.1 t of LH2/LOX. These features reduce the propulsion module dry mass to 6 t for the modules proposed here. The amount of launch mass dedicated to delivered propellant depends on where the propellant is being delivered and how long it remains there before being collected. The analysis uses the previously mentioned payload figures for SLS and FH to compute possible propellant mass delivery for each vehicle, shown in Tables II-1 and II-2, respectively. Tables II-3 and II-4 show the possible propellant mass delivery for each vehicle when a complete propulsion module is delivered instead of just a depot. Because of the density of water, the dimensions of the fairings do not constrain the amount of water propellant either vehicle can deliver even to LEO where the available mass is greatest. For example for the smallest, 5 m diameter fairing option for SLS, there are 250 m3 of mission volume available.30 Delivering the maximum possible propellant mass of 125 t to LEO fills only half of that volume, leaving plenty for the system dry mass. SLS mass delivery capabilities are published for LEO and to Mars orbit. Capability to GTO, HEO, and LL2 is estimated from the available charts of useful payload mass deliverable to an orbit with a given characteristic energy.30 FH delivery capabilities are published for LEO, GTO, and to Mars orbit.15 Here, the FH capability to LL2 and HEO is estimated to be 20 t. The greater payload capability of the SLS means that it can also deliver a greater mass fraction of propellant to each destination than the FH can. However, this choice may not be cost effective, and the payload capacity of the SLS is not always required for sufficient refueling for the return trip. Propellant deliveries to LEO take place around the same date as the crew vehicle is launched, so as to minimize drag-makeup. These propellant depots have the highest propellant mass fraction. Depots placed in GTO or HEO are insensitive to launch date relative to crew launch. Propellant deliveries to LL2 are launched ahead of the crew, to take advantage of a slow, low-V transfer trajectory that is unsuitable for a crew vehicle.33 The LL2 requires active stationkeeping, so propellant deliveries should not be made too far in advance if unsupported. If the Deep Space Gateway is available, as is assumed in Tables II-1 through II-4, depots or modules can be delivered to and then supervised by it instead, eliminating the need for them to have their own RCS and power systems. The Mars DRA-5 mission architecture includes cargo vehicles sent ahead of the crew at the Mars launch window preceding the crew mission. Any water propellant tanks delivered to Mars orbit are sent during the same launch window, so that they have confirmed successful arrival before the crew departs on the following launch window. This way, the crew will not arrive at Mars orbit only to find their return propellant unusable, leaving them stranded. This is in contrast to propellant tanks delivered to Earth orbit or the Earth-Moon system, where the crew can still abort the mission and return to Earth in the MPCV. 4 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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