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Hybrid Propulsion System Basics Dr. Andrew Ketsdever.

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1 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) O 2 (gas) H 2 O 2 (hydrogen peroxide) N 2 O (nitrous oxide) N 2 O 4 (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 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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