Solid-fuel Rocket - Grain Geometry

Grain Geometry

Solid rocket fuel deflagrates from the surface of exposed propellant in the combustion chamber. In this fashion, the geometry of the propellant inside the rocket motor plays an important role in the overall motor performance. As the surface of the propellant burns, the shape evolves (a subject of study in internal ballistics), most often changing the propellant surface area exposed to the combustion gases. The mass flux (kg/sec) of combustion gases generated is a function of the instantaneous surface area, (m2), and linear burn rate (m/sec):

Several geometric configurations are often used depending on the application and desired thrust curve:

• Circular Bore Simulation

• C-Slot Simulation

• Moon Burner Simulation

• 5-point Finocyl Simulation

• Circular Bore: if in BATES configuration, produces progressive-regressive thrust curve.
• End Burner: propellant burns from one axial end to other producing steady long burn, though has thermal difficulties, CG shift.
• C-Slot: propellant with large wedge cut out of side (along axial direction), producing fairly long regressive thrust, though has thermal difficulties and asymmetric CG characteristics.
• Moon Burner: off-center circular bore produces progressive-regressive long burn, though has slight asymmetric CG characteristics
• Finocyl: usually a 5- or 6-legged star-like shape that can produce very level thrust, with a bit quicker burn than circular bore due to increased surface area.