Acoustic pressure oscillation effects on mean burning rate of solid propellants

Abstract

Solid propellants are widely used in aerospace applications such as launch vehicle main stages, booster, and missiles etc. This is mainly due to higher thrust generation in its early stages. The performance of solid rocket design depends on its internal ballistic characteristics such as combustion characteristics of propellants, mean burning rates, grain geometry and shape. One of the most important ballistic properties used in the design and development of solid rocket motor is mean burning rate of the propellant composition. The main factor affecting the mean burning rate of the propellants are propellant composition, initial surface temperature, chamber pressure, metal powder, additives and combustion instability. The process of combustion occurs in thin region of reaction zone adjacent to the propellant burning surfaces. The instabilities are excited and self-sustained due to the unsteady energy release from the reaction zone of the propellants. This unsteady heat release causes the combustion instability . There are several forms of combustion instabilities associated with solid rocket motor which includes non-acoustic instabilities, acoustic instabilities, vortex shedding instabilities, L* instabilities etc. One of the most frequently occurring instability is the low frequency acoustic instability lying between 5 Hz and 400 Hz, in which pressure oscillation occurs along the longitudinal axis of the motor, usually called as longitudinal instability. This longitudinal instability occurs in all types of solid rocket motors, large solid propellant boosters, solid propellant based ramjet engine etc. The present work is focussed on low frequency combustion instability and characteristics in long rocket motors and booster configurations. Experiments has been carried out to investigate the effects of acoustic pressure oscillations on mean burning rate of solid propellant combustion under quasi and unsteady conditions at elevated mean chamber pressures conditions of typical rocket operations. Experiments are conducted in a window bomb test facility to measure mean burning rate for the mean chamber pressures ranging from 1 MPa to 7 MPa. Five different propellant types are considered here, out of which four are in-house propellants and one novel propellant. The in-house propellants mainly consist of ammonium perchlorate / hydroxyl terminated poly butadiene (AP/HTPB) based composite propellants, two propellants with non - plateau and other two with plateau burning behaviours. Each case of plateau and non- plateau propellant contains one non - aluminized and aluminized propellant as well. Window bomb facility is coupled with a rotating valve, which is designed to operate at maximum pressure of 11 MPa. The rotary valve is used to generate periodic acoustic pressure oscillations and required frequencies inside the window bomb at selected mean chamber pressures conditions. Three different frequencies are considered in the study which includes 140 Hz, 180 Hz and 240 Hz with different acoustic pressure amplitudes imposed on the propellant burning surfaces. The imposed pressure amplitude oscillations range from 0.1 % to 1.5 % of mean chamber pressure. newline