The Relationship Between Flame Structure and Burning Rate for Ammonium Perchlorate Composite Propellants

Abstract

The burning rate of a propellant is one of the most desired pieces of information for rocket motor design. Propellant burning rate is known to be linked to the microscale flame structures located just above the propellant surface. Flame structure and burning rate for an ammonium perchlorate composite propellant depend in large part on three factors: ammonium perchlorate particle size, propellant formulation, and pressure. Propellant burning rates are in general higher with decreasing AP particle size and increasing pressure. When the microscale flame structures sit higher, on average, above the propellant surface, the propellant will have slower burning rates due (in part) to decreased heat feedback to the propellant surface. The addition of burning rate modifiers to the propellant will also change the flame structure, and therefore the burning rate. Currently, propellants are developed using iterations of mixing and testing to obtain burning rates and physical parameters for computer models. However, this method is not optimal due to the large amount of time and cost involved with this highly empirical approach. Ideally, modelers would be able to make a priori predictions of formulation burning rates, but we are far from that currently. Modelers do desire to create high-fidelity computer models to simulate burning rocket propellants, and much progress has been made in recent years; however, relatively little is known about the actual flame structure in composite propellants which has had limited advances. Knowledge of the variation of flame structure with pressure and propellant formulation will not only assist in the validation of these high-fidelity computer models but will also provide insight to propellant formulators as they seek to use alternate ingredients and methods. This chapter seeks to describe the current data we have on the flame structures in ammonium perchlorate composite propellants and how microscale flame structure affects global burning rate. We review the status of current modeling, diagnostics that have been applied, simplified configurations that have been considered, and recent in situ measurements that are now available for at least the final diffusion flame. Although much remains to be done, significant advancement has been made to reach the ultimate goal of truly predictive propellant simulation and design.