Presented in this thesis is the development of a new method to achieve plateau burning profiles in composite solid propellants. Propellants were formulated using ammonium perchlorate (AP) as the oxidizer and hydroxyl terminated polybutadiene (HTPB) for the binder and fuel. Unlike the linear relationship of a typical propellant, plateauing propellants will have a period of flat burning or pressure insensitivity, on a log-log plot. This phenomenon has been extensively studied to determine the cause of the plateau effect; the known methods limit the formulations to a few well developed batches. There are three main mechanisms responsible for the plateauing behavior, in particular, the binder melt layer, oxidizer combustion as a monopropellant, and catalyst effects. This thesis examines previous methods to formulate plateau propellants, and then introduces new methods to achieve a plateau propellant.
Propellants were formulated using various AP sizes and size distributions along with a non-catalytic additive. The baseline propellant consisted of both 80% monomodal and 85% bimodal AP and 20% or 15% binder composed of isophorone diisocyanate (IPDI)-cured HTPB with Dioctyl Adipate. A trimodal mixture was introduced with oxidizer mass loading at 85% and 15% binder concentration. Propellant burning rates were tested using a strand bomb pressurized at varying conditions between 500 and 3000 psi. Catalytic additives were also introduced to the propellant to improve the burning rate, and in this study titania was chosen for its uniformity and reliability. First, baselines propellants with wide AP distributions were made to check the plateau effect with AP. Then, a non-catalytic additive was added to promote the plateau effect. Once the plateau additive was proven to be effective, titania was introduced to the propellant formulation to observe how the nano-additive affects the plateau behavior.
Analysis of various propellant formulations determined that plateau is most easily achieved through a wide size distribution of AP, but can also be obtained with a narrower AP distribution using an additive. When titania is added to the formulation, the plateau effects could be reduced or eliminated. The mechanism that best describes the ability for a propellant to plateau is the melt layer theory. This theory describes how the melt layer causes the AP to retreat in the melt layer and confines the oxidizer crystals preventing the proper mixing of combustion gases in the three-flame structure. The additive is believed to lower the viscosity, allowing more oxidizer to recess into the binder. Plateau is also achieved with the addition of burning rate modifier titania, but in small concentrations.
