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have been used extensively for orbit insertion of products from ambient pressure up to pressures of at least 20 MPa. The absence of a pressurization system simplifies the propellant feed significantly and eliminates components that must have long-term compatibility with propellants. For deep space missions, water is significantly easier to contain than the hypergolic Earth storables, offering stability over a relatively wide temperature range. A final advantage of the water rocket is its dual mode potential. For relatively high thrust applications, the system can be used as a bipropellant engine. For low thrust levels and/or small impulse bit requirements, cold gas oxygen can be used alone. The potential of the water electrolysis rocket as a medium to propulsion in in turn are monopropellant large satellites and planetary spacecraft. more costly and systems. is toward the for These complex primary systems than of electric thruster systems For example, arcjets are already used for North- South station High power developed planetary propulsion thrusters are poised to be flight tested for A recent trend use precision on-orbit Water electrolysis functions on smaller satellites. propulsion can provide higher high performance recognized for water electrolysis systems (RCS) in 1965. demonstrated that 500,000 N-s of total impulse could be obtained with a water electrolysis satellite propulsion system during laboratory tests with 20 N and 0.5 N engines. Such a propulsion system, however, was never accepted for a flight program. This was partly due to the decision that the improved performance was not sufficient to mitigate the perceived increase in complexity. Other disadvantages included: the large tankage needed for gaseous storage, the increased weight due to the need to pressure feed the electrolyzer, the limited power available for propellant generation, the propellant utilization penalty of gas dryers, and the ignition requirement. Recent advances in propellant storage technology, 6 water vapor feed electrolysis, 7's and solar array performance, along with a flurry of research in GH2/GO2 ignition (e.g. the LEAP program and SSTO, 9 among others) have made the use of electrolysis propulsion more attractive from a mass standpoint. In addition, there now exists an innovative new system which improves the performance of small spacecraft called the Unitized Regenerative Fuel Cell (URFC), an integrated electrolyzer and fuel cell in a single reversible unit. 7 This system offers the potential for dual use (power and propulsion) and a for orbit transfer and missions. 2 Pulsed primary plasma for satellite on-orbit functions. keeping ion and Hall thrusters are being of geostationary satellites. performance propulsion requirements (-0.17 N/kW) are greatly below those of electric propulsion devices (-0.08 N/kW for 2.2 kW arcjets, and 0.03 N/kW for 2.6 kW ion thrusters). These advantages become more pronounced at lower power levels, where efficiencies of electric propulsion devices are significantly reduced. In a water electrolysis propulsion system, water stored in a lightweight, low pressure tank is fed to an electrolyzer. The electrolyzer consumes electrical energy to decompose the water into pressurized hydrogen and oxygen. If solar energy is available, these devices can also serve as a load leveling function, storing the energy as hydrogen and oxygen gases. The propellant is propulsion device has been some time. Newman 4 discussed than the options. At equal thrust of water electrolysis propulsion propulsion for reaction control Stechman et al. 5 clean and inexpensive, reducing with propellant acquisition, maintenance, and launch. Water compact, lightweight tanks at relatively high density (1.0 g/cc). Storage requirements for propulsion are set by one or more high impulse '"ourns", where the hydrogen and oxygen are stored in separate tanks, to be mixed and ignited inside the combustion chamber of a conventional rocket engine. The gaseous hydrogen/gaseous oxygen (GH2/GO2) propellants have performance measured at an Isp of over 350 s (at thrust levels of 0.5 to 15 N), 3 which is superior to earth storable chemical alternatives. The products of combustion are clean and free of carbon, sparing from water vapor condensation are mission dependent and need to be investigated. Neither mechanical pumps nor pressurant gas are required to feed a water electrolysis rocket system, because electrolyzers are now able to electrochemically "pump" water decomposition weight savings propulsion and over power established, systems in optics and other sensitive instruments degradation. Contamination issues with NASA TM-113157 2 established chemical levels, power costs ground can associated handling, be stored in substantial separate, certain study 8 showed that for low-earth-orbit satellites, the specific energy (energy per weight of storage unit) of a water fuel cell was better than state-of-the an NiCad batteries and approximately equal to that of NiH batteries, about 15 W-hr/kg. This study did not include the mission scenarios. A Hamilton Standard (LEO) capacityPDF Image | Electrolysis Spacecraft Propulsion Applications
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