Lesson 11 Hybrid Propulsion System Basics Dr Andrew

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 gas) Oxidizer • Most Common – Solid Oxidizer / Liquid Propellant • Less Common (or

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

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

Hybrid Schematic

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

Oxidizers • • • LOx (liquid oxygen) O 2 (gas) H 2 O 2 (hydrogen peroxide) N 2 O (nitrous oxide) N 2 O 4 (nitrogen tetroxide)

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

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

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

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

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

Hybrid Ballistics

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