Carmine Carmicino

Role of Injection in Hybrid Rockets Regression Rate Behavior

A series of firing tests was carried out to investigate the influence of the oxidizer injection on the solid fuel regression rate behavior in a hybrid rocket engine. For this purpose, a conical subsonic nozzle as the injector of the gaseous oxidizer was selected to generate nonuniform conditions at the entrance of the fuel port. Gaseous oxygen and polyethylene fuel cylindrical grains were used. When the oxygen was fed by this kind of injector, the fuel regression in the region of the oxygen impingement on the grain’s surface was increased several times, which led to irregular fuel consumption with the characteristic afterburn port shape typical of solid fuel ramjets having a rearward-facing step at the air inlet. The local instantaneous regression rates, measured by means of the ultrasound pulse-echo technique, showed regression rate-time profiles strongly dependent on the geometric configuration and helped to explain the complex regression phenomenon deriving from the impingement zone displacement during the time. Empirical correlations for the prediction of the average regression rate were developed taking into account the influence of mass flux, pressure, and port diameter. Finally, a nondimensional semi-empirical correlation involving the effect of the blowing number yielded improved accuracy.

Acoustics, Vortex Shedding, and Low-Frequency Dynamics Interaction in an Unstable Hybrid Rocket

This paper deals with an experimental investigation into the stability behavior of a hybrid rocket where gaseous oxygen is fed with either an axial conical subsonic nozzle or a radial injector. The influence of the oxidizer-injection configurations on the motor stability is thoroughly examined. These distinct oxidizer-injection techniques allowed unveiling key and so far unreported features of the hybrid rocket combustion stability, especially emphasizing the role of vortex shedding which occurs in both the pre- and postcombustion chamber. Axial and radial injectors caused completely stable and unstable combustor operations, respectively, and this fact has been attributed to the fluid dynamics and unsteady heat release at the entrance of the fuel grain port. In particular, the unstable combustion in the radial-flow injector motor was dominated by low-frequency pressure oscillations, around 10-20 Hz. These low-frequency pressure oscillations were always accompanied by longitudinal acoustic modes. In some cases, the pressure oscillations abruptly increased, reaching peak-to-peak amplitude close to 70% of the mean chamber pressure, which is somewhat unusual for hybrid engines. Vortex shedding in the aft-mixing chamber is considered as the main driving mechanism of this latter behavior.

Influence of a Conical Axial Injector on Hybrid Rocket Performance

This paper was aimed at analyzing the static engine firings results obtained by means of a hybrid rocket where gaseous oxygen was supplied into axial-symmetric polyethylene cylindrical grains through two different injector configurations: an axial conical subsonic nozzle and a radial injector. The axial injector is considered interesting because of its easy design and the remarkable feature that it produces no significant pressure oscillations for the stabilizing effect due to the hot gas recirculation zone established within the combustion port. To take advantage of its qualities, the assessment of the regression rate variations under the flow field generated by this configuration is required. For the investigated set of operating conditions, the instantaneous regression rates exhibit a time-and-space dependence caused by the impinging jet zone dynamics, while the average regression rates are higher and less mass flux dependent than those achieved with the radial injection motor and expected from the turbulent flow through pipes. A comparison to the data from the radial injector was further drawn in terms of combustion efficiency, fuel consumption profiles, and combustion stability. The radial injector, at the same mass flux and pressure, produces lower regression rates, high pressure oscillations and worse combustion efficiency at the same L*, but more uniform fuel consumption.

Simulation of Gaseous Oxygen/Hydroxyl-Terminated Polybutadiene Hybrid Rocket Flowfields and Comparison with Experiments

Numerical simulations of the flowfield in a gaseous oxygen/hydroxyl-terminated polybutadiene hybrid rocket engine are carried out with a Reynolds-averaged Navier–Stokes solver including detailed gas/surface interaction modeling based on surface mass and energy balances. Fuel pyrolysis is modeled via finite-rate Arrhenius kinetics. A simplified two-step global reaction mechanism is considered for the gas-phase chemistry to model the combustion of 1,3-butadiene in oxygen. Results are compared with the firing test data obtained from a laboratory-scale hybrid rocket in which gaseous oxygen is fed into axisymmetric hydroxyl-terminated polybutadiene cylindrical grains through an axial conical subsonic nozzle. With the oxidizer fed by this injector, which generates nonuniform conditions at the entrance of the fuel port, the fuel regression rate is shown to increase several times with respect to the case of homogeneous injection of the oxidizer through all the grain port area, in agreement with the experimental f...

Experimental Investigation into the Effect of Solid-Fuel Additives on Hybrid Rocket Performance

An experimental performance evaluation, conducted with static firings of a laboratory-scaled hybrid rocket burning gaseous oxygen and several fuel combinations, aimed at raising the fuel regression rate is addressed in this paper. Pure hydroxyl-terminated polybutadiene has been tested as a baseline fuel and compared to hydroxyl-terminated polybutadiene loaded with nano- or micrometer-sized aluminum, nanosized Viton®-coated aluminum, magnesium hydride, magnesium, nanosized iron, and iron with magnesium. After a first exploratory test series in which both radial- and axial-injection arrangements have been analyzed, an axial nozzle has been selected to feed oxygen into single-port fuel grains; in that configuration, the port diameter effect upon the regression rate has been further investigated, revealing faster regression with larger port diameters at a given mass flux. The outcomes in terms of regression rate, combustion efficiency, and motor stability are examined. A thorough comparison with the available...

Acoustic Analysis of Hybrid Rocket Combustion Chambers

A method based on passive linear stability measurement has been developed to investigate combustion dynamics in hybrid rocket motors. A one-dimensional model is used to describe the acoustic motions inside the rocket chamber as a function of the motor axial coordinate. The equations of mass, momentum, and energy are solved for the average flow. A linearized analysis, which takes combustion response functions into account, provides pressure and velocity acoustic oscillations. When the boundary conditions at the nozzle inlet are enforced and the response functions are known, the acoustic modes and growth constant can be derived, whereas the coupled responses can be determined from the experimental measurement of the mode frequencies. If noise sources in the control volume are accounted for, then it is theoretically possible to determine the response functions at all frequencies simply by measuring the pressure fluctuations at the head end of the motor. Experimental data are analyzed and contrasted to the proposed one-dimensional model, showing a good agreement.

Transient Computational Thermofluid-Dynamic Simulation of Hybrid Rocket Internal Ballistics

A computational thermofluid-dynamic model of hybrid rocket internal ballistics is developed. Numerical simulations of the flowfield in a laboratory small-scale hybrid rocket motor, operated with gaseous oxygen and high-density polyethylene propellants, are carried out with the aim of predicting the solid fuel regression rate experimentally achieved with two different oxidizer injectors. The fuel regression rate is the main parameter for the hybrid rocket design. Here, it is calculated with a detailed gas/surface interface characterization based on local mass and energy balances. The combustion of oxygen and gaseous ethylene injected from the fuel wall is modeled by means of the probability-density-function approach coupled to chemical equilibrium. Two oxidizer-injection configurations, which generate either a two-dimensional axially symmetric or three-dimensional flowfield, are analyzed. The local regression rate is evaluated along both the fuel grain axis and inner circumference in the three-dimensional ...

DESIGN OF A LAB -SCAL E COOLED TWO -DIMENSIONAL PLUG NOZZLE FOR EXPERIMEN TAL TESTS

Present space transportation systems though, on the one hand, are able to solve many economical and managerial problems, on the other have given rise to some new ones of purely propulsive nature. In fact, the engines of these vehicles have to perform high thrusts and have to operate at very different altitudes (0 400 km ), therefore their nozzles have to generate high expansion ratios and, as a consequence, have to work for long time in over -expansion. Furthermore, the use of highly energetic pr opellants (H 2-O2), but easily dissociable (H 2), leads to heavy losses for frozen effect. Unconventional nozzles seem to meet very well these requirements, ensuring small losses in terms of specific impulse in overexpansion conditions and the almost total and immediate recombination of dissociated species. However one of the most serious problems of unconventional nozzles is the higher thermal stress on the wall, particularly in the throat region whose dimensions are very small. Then a cooling system is cert ainly required to ensure a long operation time. Actual numerical models concerning hot flow in advanced nozzles have to be improved and they need experimental tests that can show the feasibility of the realization of this kind of configuration, even from a technological point of view. For this reason a two -dimensional plug nozzle has been designed and constructed so that the thermo -fluid dynamic behavior of truncated plug nozzles can be tested in order to try of reducing the related technological troubles. The nozzle shape has been chosen following some basic criteria for making easier the plug cooling. In this paper some numerical results of the flow -field in over -expanded condition with turbulent reacting flow and the heat flux distribution along the plug have been reported. The nozzle will be tested on the hybrid rocket set up in the Propulsion Laboratory of DISIS, that allows to perform tests safely and cheaply at different running pressures up to 30 atm and different mass flow rates up to 0.3 kg/s . The acq uisition system employed allows to measure the motor thrust and to calculate the specific

Analysis of Acoustics and Vortex Shedding Interactions in Hybrid Rocket Motors

Hybrid rocket engines usually have an aft-mixing chamber to improve combustion efficiency. The presence of a sudden expansion at the exit of the fuel port determines the formation of vortices, whose vigorous burning may drive acoustic waves in the chamber. The shedding of vortices itself is then affected by the flow fluctuations, producing a well-known feedback loop. A reduced-order model is used here to analyze this phenomenon. It is assumed that vortex burning is localized in space and time, and a kicked oscillator model is used. A one-dimensional model proposed by the authors is used to determine the values of the eigenacoustic modes and corresponding damping coefficients. Numerical results are compared with experimental data.

Novel Comprehensive Technique for Hybrid Rocket Experimental Ballistic Data Reconstruction

A novel internal ballistics technique for determining the time-resolved spatially averaged regression rate of the solid-fuel grain in a hybrid rocket engine is presented. This technique makes use of the measurements of the oxidizer mass flow rate and the pressure levels in both the motor prechamber and postchamber, and it allows estimation of the combustion efficiency evolution over the burning time as well. A one-dimensional steady-state model, which takes into account the variation of the gas mixture thermodynamic properties along the fuel grain port, is adopted to predict thermofluid-dynamic parameters across the combustion chamber as a function of the regression rate and pressure. Two experimental test cases, both realized on a laboratory-scale rocket (one of which shows the typical features of the axial-injection motor burning oxygen with pure fuel, and the other the different behavior ensuing from a radial injection and fuel loaded with aluminum particles), are successfully analyzed, revealing the c...