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 – Solid Oxidizer / Liquid Propellant • Less Common

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