M.Sc Thesis


M.Sc StudentOr Livne
SubjectCombustion Model of a Solid Propellant Augmented by a
Liquid Oxidizer
DepartmentDepartment of Aerospace Engineering
Supervisor Professor Emeritus Gany Alon
Full Thesis textFull thesis text - English Version


Abstract

The simplicity of solid propelled motors makes them widely used for various missions. However, due to the poor energetic performance of the available solid oxidizers, solid propellants exhibit poor energetic performance compared to liquid propellants. Over the past 20-30 years significant efforts have been made in order to improve the energetic performance of solid propellants, with no practical impact on the world of rocketry.

Recently, Pelosi and Gany introduced a new concept of solid propellants improved by liquid encapsulated oxidizer, showing higher energetic potential than regular solid propellants (up to 20% theoretically). Their work included a 1-D time averaged model for the burning of liquid oxidizer droplets within a solid fuel matrix.

The aim of the present work is to create a new 2-D model for the burning of droplets of various liquid oxidizers, within a solid fuel matrix. This model also takes into consideration the changes with time during the burning process.

The model considers a unit cell within the propellant consisting of an oxidizer (encapsulated) droplet surrounded by a stoichiometric amount of a solid fuel binder. The combustion proceeds via three stages. In the first stage, the liquid oxidizer absorbs heat until it reaches its boiling temperature. In the second stage, the liquid oxidizer and the solid fuel gasify simultaneously due to heat absorption from the flame. After all of the oxidizer has evaporated, during the third stage, the fuel residues gasify and react with the oxidizing environment that surrounds the propellant.

Three known liquid oxidizers were modeled - HNO3, H2O2 and N2O4 - within an HTPB matrix. The burning properties of the different combinations were studied for under critical pressures considering various droplet diameters, compositions, and initial temperatures.

As expected for solid propellants, the burn rate was found to be faster for smaller capsule sizes or higher oxidizer concentration. Regarding Vieille's Burn Rate Law, the exponential parameter (n) was found to be small (~0.3) for big capsules (200-500 mm) and close to unity for small capsules (d?20 mm), apparently due to rapid gaseous ingredients mixing for the small size capsules. Burn rate sensitivity for initial propellant temperature showed a milder effect than common solid propellants (about 0.1% vs. 0.3% per degree, respectively). The main reason for this behavior is attributed to the relatively short oxidizer heat up stage.

Finally, the resulting burning properties are close to those of regular solid propellants, and consistent with Pelosi and Gany’s research.