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Figure 2: Electrolysis thruster design. Reproduced with permission from a course presentation.39 B. Thruster The use of water for chemical propulsion was described earlier in section I.C., βElectrolysis Propulsion Overview.β Liquid water is used to densely and inertly store hydrogen and oxygen for later separation into separate gases or a mixture of the two, which is then combusted to produce thrust. Theoretical performance is identical to that of LH2/LOX chemical propulsion, 450 s, while performance of 300 s has been reached in our work and others.19 Section I describes the advantages and disadvantages of this propulsion method relative to that of LH2/LOX; this section considers its use in the Mars DRA-5 mission specifically. Due to the large specific enthalpy of water (286 kJ/mol), substantial power is required to electrolyze enough of it to use for the propulsion of a large spacecraft such as the crewed Mars vehicle. With a specific impulse of 450 s, in order to maintain a steady thrust of 1 N, the spacecraft requires 3.6 kW of power for electrolysis, or 4.5 kW of power for an electrolysis propulsion system assuming 80% efficiency. We have measured about 90% efficiency in COTS electrolyzers; so, this figure is conservatively low37. The specific thrust is 222 mN/kW, a significantly greater thrust per unit power than other electric thrusters, such as high p erformance Hall effect thrusters achieving 68 mN/kW.38 Operation as a continuous thruster, or in short pulses using electrolyzed gases, completely eliminates the need for cryogenic propellant storage. However, with a specific impulse of only 450 s, the οV required for many low-thrust trajectories is prohibitive, even if the propellant for the return trip is pre-deployed. Storing the propellant temporarily in cryogenic form after electrolysis addresses this issue by allowing impulsive burns. The concept of operations is shown in Figure 2. A lightweight tank stores water at low pressure. When the thruster is in use, water continuously passes from the storage tank to a separate electrolysis chamber. The plumbing from the tank to the chamber serves as a heat sink for the nozzle of the thruster, similar to other regeneratively cooled nozzles. Electrolysis produces hydrogen and oxygen gas, which are separated into temporary storage tanks. In this way, usable propellant can be stored in the short term prior to maneuvers. C. Electrolysis Power Tradeoff An important consideration is the tradeoff between the mass of solar panels required for rapid electrolysis, versus the loss of propellant due to boil-off during longer storage periods. The specific mass of solar panels at the 100 kW level is between 30 and 40 kg/kW.29 The Mars DRA-5 assumes 30 kg/kW is available, and that assumption is in force here.25 With a specific enthalpy of 286 kJ/mol, 184 kW electrolyzes 1 t of water per day with 100% efficiency, or 5.52 t of solar panel mass. An approximate propellant mass lost to boil-off during electrolysis is a function of boiloff rate (at least 0.1% per day32) and electrolysis rate. D days of power are required to electrolyze at least N t of propellant at a rate R determined by the electrolysis power P (or solar panel mass S), and efficiency Ξ·: π·=π,π = ππ = ππ (1) π (184 kW) 5.52 However, some of the propellant is lost during electrolysis. So, the end result is MPDF Image | Water Electrolysis for Propulsion of a Crewed Mars
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