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hydrogepnumpd,rawinagfewmilliwatatssures thatno hydrogebnuildsup in thewaterfeed chamber. The reliabilityof the watervaporfeed electrolysisystemhas beendemonstrated previousilny anaccelerattedstsimulatin1g0 yearsworthof propellanptroductiofonr North South station keeping (NSSK) on a geosynchronousastellit_e°. Utilizing the electrochem"icpaulmpinga"ctionof theSPE electrolyzgear,seouhsydrogeandoxygeunpto pressureosf 2.72MPa(20 MPahasbeen demonstratwede)reproducewd,ithsubsequent burnsconsuminpgropellandtsownto 0.7MPa tankpressurSeP.E-basfeudelcellshaveflown onsevenGeminmiissionl_s,butSPE-baseddevelopment in valve and regulator vaporfeedelectrolyzehrsavenotbeenflight qualifiedyet.Sizingof the electrolyzeforr selectemdissiondsependosnthesystemdsesign approachE.ithertheelectrolyzeisr scaled accordintogtheavailabpleowearndthemission is accomplishewdith the givenpropellant generatiorante,or the electrolyzeisr scaled accordintgo themissionrequiremenwthsich dictatetherequirepdropellangteneratiorante andtherefopreowerIn.thiscasea,dditionsaollar collectotrosdrivetheelectrolyzaerreaddedO.n highdelta-Vmissionsth,ehigherIspof the hydrogen/oxygperonpellanctsompensaftoers theadditionamlassofcomponen(tes.g., electrolyzersg,as tanks,additionalsolar collectors)that state-of-the-acrthemical propulsiosnystemdsonotrequire. Gas Dryers Both the hydrogen and the oxygen leaving the electrolysis unit contain small quantities of water vapor. If not removed, this water vapor could condense inside the tanks and propellant lines. Furthermore, the presence of water vapor inside the propellants will reduce thruster performance. The installation of propellant dryers based on desiccant bed is a simple solution. This would be a highly reliable passive component. For small spacecraft applications, the amount of water vapor will be low, so this component will be small with relatively low weight. The amount of water vapor depends on gas pressure. A conservative estimate is that for a 7.0 MPa system, approximately 2% need to be added to the propellant mass in order to account for the desiccant mass. The amount of water absorbed in the desiccant under these conditions is approximately 0.25 % of the total water wet mass. technologies. of leak rates valves and respectively), 9 Watts), and minimizing mass (10-100 gms). 12 In order to satisfy even near term developments electromechanical systems to further reduce the mass and achievable flow rates. The biggest obstacle with MEMS, however, is the leak rate, which has been greater than for conventionally manufactured valves, and the need to filter even the smallest particles. Water and Propellant Tankage Because the vapor feed electrolyzer pressurizes the propellant, the water supply can be stored at ambient pressures in thin-walled, light weight tanks. The storage of gaseous reactants, especially hydrogen, however, has always been a problem for on orbit applications. For missions in which a velocity change must be accomplished in a single, large AV burn, the required tank mass to contain the required gaseous hydrogen is high. If multiple bums are possible to accomplish the mission, filling and draining gas storage pressure vessels multiple times can effectively reduce the mass penalty of gaseous hydrogen storage. The propellant tanks are now sized to accommodate only the largest bum of the mission, the required mass is effectively "amortized" over the number of times that the tank gets refilled during the mission. The figure of merit for lightweight pressure tanks is the performance factor, which is the burst pressure multiplied by the internal volume and divided by the tank weight (Pb.V/W). Recent NASA TM-113157 4 a Propellant Feed System The propellant feed designed for maximum of the propellants is electrolyzer. Direct electrolyzer to the tanks supply propellants. For highly controllable impulse bits and maximum combustion efficiency, regulators are needed between the tanks and thruster to control the propellant mass flow rates. For less restrictive needs, a blowdown system could be used to simplify the operation and reduce system weight resulting in some performance reduction. This has (internal resulted leakage in the reduction <10 -6 scc/h He for system described here is simplicity. accomplished Pressurization through the feed lines from the Over the last several years, strict micro- propulsion requirements have driven the < 1 minimizing power requirements (< scc/h for regulators, are focused on micro- (MEMS) technology stricter requirements,PDF Image | Electrolysis Spacecraft Propulsion Applications
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