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consisteodfthestoragtaenkp,ropellalninteand headspaceinsidethewatertowerof the electrolysuisnit.Thevolumeoccupiebdy the oxygenconsisteodf thestoragteankandthe propellalnintesD.uringtheinitialtankfill,from highaltitudaembienutpto 1 MPat,hepressure insidethehydrogetannkincreasemdoreslowly thaninsidethe oxygentanks dueto the additionhaeladspacTe.hepressuirnebothtanks remainesdteadayfterclosingthesupplyvalves. Thiscausetdhemixturreatioof thefirsthot-fire inatesstequentcoebeoxygernich(O/F-9.2). Becausheighpressurheydrogewnastrapped insidetheelectrolystoiswert,hehydrogetannk experiencaendincreasinepressuereachtimethe supplyvalveopenedA.sa resultt,hepressure insidethehydrogetannkswashighetrhaninthe oxygetnanksduringsubsequtensttsc,ausinag slightlyhydrogerinchmixtureratio(O/F-7.6). Abou8t testsequencweesrerequiretdoreach marginaelquilibriumconditionbs,ecaustehe spaciensidethewatetrowecrhangeadsaresult of waterconsumptioEnv.enthoughconditions changedslightlyduringcontinuingtesting, chambperessurOe,/Fratiosa,ndcharacteristic velocitiedsidnotchangneoticeabTlyh.roughout thefullcoursoeftestingth,ethrusteprerformed well. sccm, respectively, which correspond to 2, 5, and 10 A cell current. These cell currents translate to approximately 3, 8 and 16 W available power, typical for small spacecraft. The horizontal axis displays the pressure in the oxygen storage volume. The range displayed is from 0.6 to 1.0 MPa, approximately the pressure range when cycling between electrolysis charge and hot-fire discharge. The vertical axis shows the power required. Fig. 6 shows that the required power increases, as expected, with increasing storage pressure, and that this increase is larger for higher showed efficiency pressure, energy internal design system pressure in order stepwise increase in Fig. 6a is due to the characteristics of the data acquisition equipment. Increasing the input power leads to an increase in propellant generation rate. Fig. 7a shows the average power required as a function of oxygen generation rate. The vertical axis of this figure is taken as the average power required between 0.6 and 1.0 MPa oxygen storage pressure. The figure shows an approximately linear relationship between input power and generation rate, with a value of 0.46 W/(sccm oxygen). As was shown in Fig. 6, a difference exists between power required at 0.6 MPa, and 1.0 MPa. This is displayed in Fig. 7b, which shows that the absolute difference between power levels required at 0.6 MPa and 1.0 MPa increases with increasing average minimum electrolysis divided by the actual power used. The remaining power is rejected as heat. Typical efficiency values for electrolysis are between 85 and 90%. Rocket Testing As noted previously, initial rocket test sequences were executed with a copper heat-sink chamber. Temperature, pressure, and propellant mixture ratio data were obtained to verify that test conditions remained within their expected ranges. Typical copper combustion chamber pressures are shown in Fig. 8 as a function of time. At the initiation of testing, the thruster valves opened, the spark igniter was turned on, and the chamber pressure increased as the result Electrolysis System Performance Key parameters during an electrolysis tank fill were the supplied voltage, the current through the cells, the pressure build-up inside the oxygen and hydrogen storage tanks, the electrolysis unit temperature, and the rate of propellant generation (measured in total fill time to an oxygen tank pressure of 1 MPa). The electrolysis voltage provides a measure of cell conversion efficiency This current slightly ranged from 1.47 V at 1 kPa and IA to 1.81 V at 1 MPa and 10 A. Electrolysis tests were performed at a variety of different cell currents, from 2 to 10 A. The current was kept at a constant value during each test. The increasing efficiency decreases with increasing cell and electrolysis pressure, and decreases propellant generation rates. electrolyzer efficiency is defined The as the with cell temperature. Cell voltage power theoretically required for water pressure inside the electrolysis test caused the cell voltage to gradually increase, requiring a slightly higher power for conversion than at lower operating pressures. The constant current assured a constant propellant generation rate. Fig. 6a, b, and c show the electrolysis power required to maintain constant propellant generation rate with increasing pressure, for oxygen generation rates of 7.5, 18.7, and 37.5 NASA TM-113157 8 unit during each generation that the rates. These electrolysis experiments conversion increasing tradeoffs toward of propulsion electrolysis with gas compression decreased as expected by theory, required hardware for due and to the to the gradually configuration. an electrolysis lower maximum This pushes to maximize efficiency. ThePDF Image | Electrolysis Spacecraft Propulsion Applications
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