Hybrid Rocket Motor: Propellant Selection and Fuel Grain Design

Abstract:
Hybrid rocket motor propellant offers a number of advantages over the traditional liquid and solid propellants, particularly in the areas of safety, cost and environmental effects. The combustion mechanism and internal ballistics of hybrid rocket motor (HRM) differs markedly from those in liquid engine and solid motor. Prediction of internal ballistic parameters of HRM is greatly affected by the instantaneous port geometry of the solid fuel grain during firing and types of oxidizer and fuel.
This paper discusses the design and selection criteria of HRM propellant (solid fuel, gas or liquid oxidizer and energetic additives), fuel grain geometry and operating parameters. The three basic elementary geometries namely, tube, pie-shaped, and triangular ports are used to describe more complicated hybrid grain configurations. Consequently burning perimeter, effective port area, fuel grain cross section area, filling coefficient (greater than 50%) and sliver ratio (about 6%) can be calculated.
Comprehensive static tests for small scale hybrid rocket motor have been carried out for various solid fuel materials (PMMA, PE, PE+AL, paraffin, bees wax) with gas oxygen as oxidizer. Finally, the designer selects a propellant type that is likely to give the required performance, considering cost, mechanical properties as well as the necessary storage stability and the best safety properties (non-toxic and smokeless).