lecture10b - MECH 7221 MECH 6251/498D Liquid Rocket Engine...

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MECH 7221 Liquid Rocket Engine Systems RL-10 rocket engine (Pratt & Whitney) • Overview of basic component in liquid rocket engine systems • Liquid propellant choices Reading: Sutton and Biblarz Chapter 6, 7, 8.1-8.2 Objective
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Bipropellant Systems Definition: Liquid oxidizer and liquid fuel feed into a thrust chamber where they mix and react chemically; the combustion gases then accelerate and are exhausted through a converging-diverging nozzle.
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Bipropellant Systems Benefits • Relatively high achievable performance for chemical propulsion Drawback • high cost and complexity (“twice” as many components as a monopropellant system, many more variables to be controlled/analyzed) • Bipropellants are often dangerous from a system safety/handling standpoint • Reaction temperatures are often high enough that combustion temperatures are too high to be accommodated by the thruster materials • Sometimes need extremely tight mixture ratio control • Nasty exhaust products (contamination of spacecraft’ external surface or the surface of an ephemeral body)
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Typically chosen as the propulsion system when: • high thrust levels, total impulse, and/or Δ Vs are required to offset dry mass • Operational lifetime of the system is long • The spacecraft temperature is tightly maintained. Bipropellant Systems • Basic biprop propulsion system (with pressure fed system types)
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Unless thruster inlet conditions are carefully controlled, thrusters can burn through or otherwise fail: • not a problem with monopropellant thruster • many variables affect thruster inlet conditions - magnitude and location of pressure drops - relative pressure drops in oxidizer and fuel feed systems - Propellant and propellant tank temperature and pressure - propellant vapor pressure and state of pressurant saturation. Bipropellant Systems
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Due to typically large propellant masses and need to carefully control inlet conditions, most biprop systems include a pressurization system • Usually can’t afford the mass or volume penalty, or large excursion in inlet pressure associated with blow-down operation for the entire mission. Oxidizer is not compatible with most elastomers, so all-metal propellant management devices are typical in oxidizer tanks. Separate propellants need to be kept apart both in liquid and vapor forms. Bipropellant Systems
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• Bipropellant systems will always have some hold-up/residual remaining in the propellant tanks at the end of the mission that must be accounted for (more so than monopropellant systems due to mixture ratio uncertainties) • Chugging (feed system frequency interaction with engines) • Vapor migration in the feed system • Center of gravity shift during operation (must number, locate, and size tanks to account for this) Bipropellant Systems
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• A repeatable minimum impulse bit is difficult to achieve due to mixture ratio and combustion uncertainties • The design of the propellant management devices (PMDs) in the tanks are
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lecture10b - MECH 7221 MECH 6251/498D Liquid Rocket Engine...

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