Pulsed motor firings - Pulse amplitude, formulation, and enhanced instrumentation

Over the last ten years the Naval Air Warfare Center has participated in an extensive effort to understand non-linear pulsed instability in tactical sized solid rocket motors. The purpose of this work was 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 over the last several years 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 five tactical size motor firings, two at 1000 and three at 1500 psi. Each motor was pulsed two times during bum. The motors were instrumented with high frequency piezoelectric quartz pressure transducers. In addition to pressure, pulse amplitude, propellant and geometry were varied. The motors were instrumented with two or three high frequency Kistler type piezoelectric pressure transducers. The motors with three transducers had them mounted at the forward, middle and aft end of the motor. Several significant achievements were made during this years program. First, the stability boundary was determined by bracketing the pulse amplitude required to trigger a motor into high levels of combustion instability. Second, placement of three transducers mounted along the length of the motor presents hard to obtain wave-form and phase data of a motor undergoing combustion instability. Third, results showed that it is possible to pulse a motor into nonlinear limiting amplitude oscillations whose propellant contains a stability additive. 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 hi 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 hi 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 This effort was sponsored by the Air Launched Weaponry Technology Block Program Office under the authority of Tom Loftus and Scott Fuller, Naval Air Warfare Center, China Lake, CA 93555-6100. * Research Scientist, Research and Technology Division, Senior Member A1AA Approved for Public Release. Distribution is Unlimited. 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. In addition to the work done at China Lake, non-linear stability analysis was performed by Phillips Laboratory and motor firings and analysis were performed by Canada and the United Kingdom. Australia also participated by performing propellant response testing." Some of the data generated and analysis performed during the course of this program indicated a possible increase in instability tendencies at high motor operating pressures. In addition, there have been concerted efforts to develop tactical solid rocket systems which have greatly reduced plume signatures and use light weight composite motor cases. The pressure. These motors were also pulsed. The results of this study, besides providing needed motor combustion instability data, showed clearly that increasing a motor's operating pressure increases a motors susceptibility to pulsed instability. During the course of this years work five motors were fired and pulsed. The paragraphs below will discuss the motor hardware, pulsing methods, test matrix, propellants and, most importantly, the acoustic analysis of the motor firings data. MOTOR FIRINGS DETAILS Propellant: Table 1 shows the two different formulations used during these motor firings. Unfortunately, these propellants differ in more that one way, making conclusions about formulation effects difficult to make. There is one important difference, however, that conclusions can be drawn from. Propellant A contains one percent of the Table 1. Baseline Propellant Properties Ingredient AP RDX HTPB Carbon Black ZrC Burning Rate @ 1000 psi Exponent @ 1000 psi Propellant Density Flame Temperature @ 1000 psi Speed of Sound (a), 1000 osi Motors 1-4 Approximate % 82.0 4.0 12.5 0.5 1.0 0.267 in/sec 0.360 0.0648 lbs/in 4915 °F 3554 ft/sec Motor 5 Approximate % 86.5