High pressure pulsed motor firing combustion instability investigations

For the last several years the Naval Air Warfare Center has participated in an extensive effort to understand nonlinear pulsed instability in tactical sized solid rocket motors. The purpose of this work is to broaden the knowledge of design factors that influence non-linear axial mode combustion instability. This effort concentrated on reduced smoke propellant systems at pressures around 1000 psi. A new effort has been undertaken to examine pulsed instability in reduced smoke systems operating at higher pressure. This paper is a progress report on results of this multi-year effort. In this paper, combustion instability data will be presented on three tactical size motor firings at 500,1500 and 2000 psi. Each motor was pulsed three times during burn. The motors were instrumented with two high frequency piezoelectric quartz pressure transducers. The acoustic data will be presented and compared with linear stability predictions made by the Standard Performance Prediction/Standard Stability Program (SPP/SSP) computer code. Included in the paper will be laboratory response testing results and future plans. Results indicate a significant adverse effect on stability as pressure is increased. Knowledge produced by this program will make it easier for solid rocket motor designers to avoid instability problems, particularly at higher operating pressures, and to solve such problems when they do occur. BACKGROUND AND INTRODUCTION The Naval Air Warfare Center (NAWCWPNS) at China Lake has participated in a program to develop an improved understanding of linear and non-linear combustion instability in solid propellant rocket motors. One primary goal of this program was to develop a systematic data base of motor and stability data for future analysis. Earlier papers have reported on previous NAWCWPNS work on this program." The motors fired in the past program were 5 inches in diameter and 67 inches in length. The majority were loaded with an 88% solids reduced smoke AP/HTPB propellant with a nominal burning rate of 0.240 in/sec at 1,000 psi. In addition, motors were fired which contained 1 percent 8 micron aluminum oxide, 90 micron aluminum oxide, and 3 micron zirconium carbide as stability additives in place of 1 percent ammonium perchlorate. Motor pressures ranged from 500 to 1500 psi. Pressure coupled combustion response measurements were made at the nominal motor operating pressures for all propellants. Several motor configurations and propellant variations were included in the program. A total of 23 motors were fired and each motor was typically pulsed three times during burn. The baseline grain geometry was a six-point star in the aft two-thirds of the motor and a cylindrical section in the forward end. Most of these motors were fired using the baseline reduced smoke composite propellant and three were fired with propellants containing stability additives. Three motors with star-forward grains, one motor with a full star grains, two motors with cylindrical cross sections, and four half length higher frequency motors were also fired. The pulsing produced 10 unstable pulses (pulses that grew to a limiting oscillatory amplitude) and 32 stable pulses (pulses that decayed). A complete description of the motors can be found in references 8, 9,11 and 12. This effort was sponsored by the Air Launched Weaponry Technology Block Program Office under the authority of Tom Loftus and Fred Zarlingo, Code 474T60D, Naval Air Warfare Center, China Lake, CA 93555-6100. * Research Scientist, Research and Technology Division, Senior Member AIAA Approved for Public Release; Distribution is Unlimited.