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.

Role of Injection in Hybrid Rockets Regression Rate Behavior

Ultra-High-Temperature Ceramic (UHTC) materials, because of their high temperature resistance, are suitable as thermal protection systems for re-entry vehicles or components for space propulsion. Massive UHTC materials are characterized by poor thermal shock resistance, which may be overcome using C or SiC fibers in a UHTC matrix (UHTCMC). The University of Naples “Federico II” has a proven experience in the field of material characterization in high-enthalpy environments. A hypersonic arc-jet facility allows performing tests in simulated atmospheric re-entry conditions. The Aerospace Propulsion Laboratory is employed for testing rocket components in a representative combustion environment. Ad-hoc computational models are developed to characterize the flow field in both facilities and perform thermal analysis of solid samples. Current research programs are related to a new-class of UHTCMC materials, for rocket nozzles and thermal protection systems. The activities include design of the prototypes for the test campaign, numerical simulations and materials characterizations.

Aero-thermo-chemical characterization of ultra-high-temperature ceramics for aerospace applications

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.

Influence of a Conical Axial Injector on Hybrid Rocket Performance

Ablative materials are commonly used to protect the nozzle metallic housing and to provide the internal contour to expand the exhaust gases in both solid and hybrid rockets. Because of interaction with hot gas, these materials are chemically eroded during rocket firing, with a resulting nominal performance reduction. The objective of the present work is to study the erosion behavior of graphite nozzles in hybrid engines at different operating conditions and compare results with those obtained for solid motors. A main distinctive feature of hybrid engine operating conditions is, in fact, a greater concentration of oxygen-containing combustion products than solid motors. The adopted approach relies on a validated full Navier–Stokes flow solver coupled with a thermochemical ablation model that takes into account heterogeneous chemical reactions at the nozzle surface, rate of diffusion of the species through the boundary layer, ablation species injection in the boundary layer, heat conduction inside the nozzl...

Numerical Analysis of Nozzle Material Thermochemical Erosion in Hybrid Rocket Engines

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.

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

Trade-off between paraffin-based and aluminium-loaded HTPB fuels to improve performance of hybrid rocket fed with N2O

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...

Carmine Carmicino

Papers

Role of Injection in Hybrid Rockets Regression Rate Behavior

Influence of a Conical Axial Injector on Hybrid Rocket Performance

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

Trade-off between paraffin-based and aluminium-loaded HTPB fuels to improve performance of hybrid rocket fed with N2O

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