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) 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