Peter J. Ferrara

Evaluation of Nozzle Erosion Characteristics Utilizing a Rocket Motor Simulator

This research addresses scientific understanding and methods for mitigation of rocket nozzle erosion by solid-propellant combustion products in order to substantially increase the operating pressures of future missiles. Several processes can affect the nozzle erosion rate at high-pressure and high-temperature conditions. To characterize the nozzle erosion processes at both traditional operating pressures and at substantially increased pressure levels, two separate test facilities have been planned for operation at chamber pressures up to 8000 psi (55.2 MPa). The focus of this paper is the design and development of one of these test rigs, the rocket motor simulator (RMS). The RMS is a gaseous reactant combustor used to simulate propellant product species generated from a selected non-metallized Propellant S. Utilizing a bi-directional counter-flow vortex combustion chamber, the gaseous reactants are mixed and combusted producing a gaseous mixture with temperature and concentrations of selected oxidizing species similar to those of Propellant S. The ability to control the product species concentrations through gaseous reactant flow rates allows the evaluation of the effect individual chemical species have on the nozzle erosion process. As a means of mitigating erosion of the nozzle throat, a nozzle boundary-layer control system is also adopted in the nozzle assembly design to evaluate the methodology of boundary-layer cooling as a means of controlling erosion rates. Instantaneous erosion rates will be measured using a real-time Xray radiography system in combination with the X-ray translucent nozzle assembly.

Development of a Correlation Between Internal Flow Field and Heat-Flux Measurements in a Simulated Fin-Slot Rocket Motor

Fin-slot propellant grains have been used in a variety of solid rocket propulsion systems. Advantages of fin-slot propellant grains include: a constant total burning surface area and thrust level, large burning surface area, large free volume, and greater reliability for ignition. It is known that the fin-slot region has confined space and a complex geometry and the influence of the igniter jet has a profound effect on the flow-field re-circulating patterns, due to its impingement angle, degree of under-expansion, and strength of the induced vortex. In order to accurately predict the overall ignition transient for the reusable solid rocket motors (RSRM) of the Space Shuttle booster with head-end fin slots, it is necessary to have the knowledge of the energy transfer rates in the fin-slot region. An approximate 1:10 th scale pie-shaped fin-slot motor was designed to simulate the first segment of the fin-slot RSRM and to perform diagnostic measurements for studying the flow and heat transfer behavior on the exposed propellant surface. The simulation motor consisted of a single, inert triangular fin section mounted in a horizontal, 2-D axisymmetric stainless steel chamber with an observation window. Opposite to this flow-visualization window was an array of 36 flushmounted heat-flux gauges installed on a diagnostic panel in a perpendicular orientation to detect the local temperature rise rates at representative regions of the fin-slot propellant surface. Clean air was compressed in a storage tank and allowed to pass through a heated blow-down wind tunnel for supplying hot airflow through the igniter section and into the fin-slot region at multiple temperature levels, simulating the hot gas products from the igniter. Data from the direct discharge of a live igniter onto an inert fin-slot propellant sample were also collected for comparison with the hot-air heat transfer experiments. Results were used to develop a correlation between the internal flow-field and heat-transfer within the fin-slot region. The heat-transfer rates evaluated from this correlation matched the measured data trend within the experimental error.

Flame-Spreading Behavior in a Fin-Slot Solid Propellant Rocket Motor Grain (Part II)

To accurately predict the overall ignition transient for the reusable solid rocket motors of the space shuttle booster with head-end fin slots, it is necessary to have the knowledge of the flame-spreading rates in the fin-slot region. This paper is the second of a two part study and deals with the development of a flame-spreading correlation in the fin-slot region. A subscale (1:10) pie-shaped fin-slot motor was designed to perform diagnostic measurements for studying the flame-spreading behavior on the exposed propellant surface. Dynamic similarity was considered in the igniter design so the impinging jet had a similar exit angle onto the propellant surface in the fin-slot section. Flame-spreading measurements were gathered using a high-speed digital camera and nonintrusive optical measurement methods through an array of 36 near-infrared fast-response photodetectors installed perpendicular to representative regions of the propellant surface. Results showed that the flame-spreading phenomena was highly nonuniform, starting in the downstream portion of the fin-slot region before traveling back toward the igniter. A correlation was developed for the dimensionless flame-spreading time interval showing that it was inversely proportional to the pressurization rate to a power of 0.62, which depends strongly upon the flow parameters of the igniter induced flow and local propellant grain geometry.

Flowfield Structure in a Fin-Slot Solid Rocket Motor (Part I)

To accurately predict the overall ignition transient for the reusable solid rocket motor of the space shuttle booster with head-end fin slots, it is necessary to acquire detailed flowfield structure and energy transfer rates on the exposed inert fin-slot propellant surfaces. This paper is the first of a two-part study and deals with the internal flowfield structure and heat-transfer characteristics in the fin-slot region. A subscale (1:10) pie-shaped fin-slot motor was designed to perform diagnostic measurements. An array of 36 flush-mounted heat-flux gauges was installed to detect the local temperature-rise rates at representative regions perpendicular to the propellant surface. Flowfield visualizations were conducted by applying either a chalk-powder/kerosene mixture or many small threads taped to various locations on the inner surface of the sacrificial window of the fin-slot region for high-speed video camera recording. Computational fluid dynamics simulations were performed for modeling the internal flowfield of the test rig. Results were used to develop a heat-transfer correlation governed by the internal flowfield structure within the fin-slot region. The theoretically calculated and experimentally observed internal flowfield patterns were similar in nature. The heat-transfer rates determined from the developed correlation matched the measured data trend within the experimental error. The flowfield structure and heat-transfer rate distribution are mainly governed by the major recirculating flow induced by the igniter jet.

Comprehensive Three Dimensional Mortar Interior Ballistics Model for 120mm Mortar System with Experimental Validation

A three-dimensional mortar interior ballistic (3D-MIB) model and code have been developed and stage-wise validated with multiple sets of experimental data in close collaboration between The Pennsylvania State Univ. (PSU) and Army Research and Development Engineering Center. This newly developed MIB model and numerical code realistically simulates the combustion and pressurization processes in various components of the 120mm mortar system. Due to the complexity of the overall interior ballistic processes in the mortar propulsion system, the overall problem has been solved in a modular fashion, i.e., simulating each component of the mortar propulsion system separately. The physical processes in the mortar system are two-phase and were simulated by considering both phases as an interpenetrating continuum. Mass and energy fluxes from the flash tube into the granular bed of M1020 ignition cartridge were determined from a semi-empirical technique. For the tail-boom section, a transient one-dimensional two-phase numerical code based on method of characteristics (MOC) was developed and validated by experimental test results. The mortar tube combustion processes were modeled and solved by using a twophase Roe-Pike method with van Leer flux limiter, a fourth-order Runge-Kutta scheme, and an adaptive mesh generator to account for the projectile motion. For each component, the predicted pressure-time traces showed significant pressure wave phenomena, which closely simulated the measured pressure-time traces. The experimental data for the flash tube and ignition cartridge were obtained at PSU whereas the pressure-time traces at the breech-end of the mortar tube were obtained from the tests conducted at Yuma Proving Ground (YPG) and by using an instrumented mortar simulator at Aberdeen Test Center (ATC). The 3DMIB code was also used to simulate the effect of flash tube vent-hole pattern on the pressurewave phenomenon in the ignition cartridge. A comparison of the pressure difference between primer-end and projectile-end locations of the original and modified ignition cartridges with each other showed that the early-phase pressure-wave phenomenon can be significantly reduced with the modified pattern on the flash tube. The flow property distributions predicted by the 3D-MIB for a zero charge increment case are explained in details in this work.