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Lesson 11: Hybrid Propulsion System Basics

Lesson 11: Hybrid Propulsion System Basics. Dr. Andrew Ketsdever. Hybrid Propulsion Systems. A hybrid propulsion system is one in which one propellant is stored in liquid (or gaseous) state while the other is stored in solid phase. Solid Propellant / Liquid (or gas) Oxidizer Most Common

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Lesson 11: Hybrid Propulsion System Basics

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  1. Lesson 11: Hybrid Propulsion System Basics Dr. Andrew Ketsdever

  2. Hybrid Propulsion Systems • A hybrid propulsion system is one in which one propellant is stored in liquid (or gaseous) state while the other is stored in solid phase. • Solid Propellant / Liquid (or gas) Oxidizer • Most Common • Solid Oxidizer / Liquid Propellant • Less Common

  3. Advantages • SAFETY: Literally no possibility of explosion • Controllable • Throttle • Stop / Re-start • Safe exhaust products • Higher Isp than solids • Higher density Isp than liquids • Lower complexity than liquids • Lower inert mass fraction than liquids

  4. Disadvantages • More complex than solids • Lower Isp than liquids • Lower density Isp than solids • Lower combustion efficiency than either liquids or solids • O/F variability • Poor propellant utilization • Higher inert mass fraction than solids

  5. Hybrid Schematic

  6. Fuels • Hydroxy Terminated Poly-Butadiene (HTPB) • Polyethylene • Polymethyl Methacrylate (Plexiglass, PMMA) • Paraffin • Metallic additives (Aluminum)

  7. Oxidizers • LOx (liquid oxygen) • O2 (gas) • H2O2 (hydrogen peroxide) • N2O (nitrous oxide) • N2O4 (nitrogen tetroxide)

  8. Hybrid Facts • Some hypergolic fuel/oxidizer combinations have been studied • Most hybrid systems require an igniter to initiate combustion • Fuel regression rates are typically 1/3 less than solid propellants • For high thrust, multi-port configurations are needed • Surface area driven mass flow rates

  9. Solid Rocket Motor • Oxidizer and Fuel ingredients are mixed in the grain • Combustion occurs as a result of heterogeneous chemical reactions near the propellant surface • Propellant burn rate controlled by combustion chamber pressure (St. Robert’s Law) • Throttling or extinguishment is difficult since fuel and oxidizer can not be separated

  10. Hybrid Rocket Motor • Fuel grain contains no oxidizer • Solid fuel must first vaporize before combustion can occur • Port fluid dynamics • Port heat transfer mechanisms • Drivers for fuel regression

  11. Hybrid Ballistics • Primary combustion region over the fuel grain is limited to a very narrow flame zone • Within boundary layer formed from gaseous oxidizer flow over solid fuel surface • Boundary layer and thus regression rate is influenced by • Local turbulence • Port pressure • Port temperature • Oxidizer mass flow rate • Fuel grain composition

  12. Hybrid Ballistics • Performance depends critically on: • Flow mixing degree in the combustion chamber • Residence time of the combustion gases • Fuel grain regression rate is largely a function of the energy required to convert the fuel from solid to vapor phase • Material dependent

  13. Hybrid Ballistics

  14. Hybrid Ballistics • Fuel is vaporized as a result of heat transferred from the flame zone to the fuel grain • Convection • Radiation • Vaporized fuel and oxidizer mix in the port • The flame is established at a location within the boundary layer determined by stoichiometric conditions

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