M. Arif Karabeyoglu

Evaluation of the Homologous Series of Normal Alkanes as Hybrid Rocket Fuels

The liquid layer hybrid combustion theory which was developed to predict the regression rate behavior of hybrid rocket fuels burning by forming a liquid layer on their surfaces has been improved. In the enhanced version of the theory, the regression rate equations are cast in a non-dimensional format, normalized by the classical regression rate, and a universal law for the non-dimensional regression rate has been derived. Comprehensive prediction methods for the surface temperature have been developed for the subcritical and the supercritical operating conditions for the molten fuel. Note that each regime is quite different in terms of the underlying surface phenomenon. In the subcritical operation, the temperature is dictated by the physical phase transformation process whereas in the supercritical case, pyrolysis chemistry governs the surface phenomenon. The enhanced theory has been applied to the homologous series of normal alkanes (C2H2n+2) which are fully saturated hydrocarbons with varying numbers of carbon atoms. It has been demonstrated that the regression rate predicted by the theory matches the data obtained from the motor tests with reasonable accuracy. The motor test data used in the comparison was based on a wide range of practical fuel systems composed of n-alkane molecules including liquid pentane (n=5), paraffin waxes (n=28-32), PE waxes (n=60-80) and, finally, the high density polyethylene polymer (n > 100,000).

Development of Scalable Space-Time Averaged Regression Rate Expressions for Hybrid Rockets

DOI: 10.2514/1.19226 The fuel regression rate expressions reported in the hybrid literature often depend explicitly on the physical dimensions of the system such as the fuel port diameter. Typically, when these dimensional formulas are applied to systems with significantly different scales, they produce grossly inaccurate results. This paper addresses the development of scalable space–time averaged regression rate formulas for hybrid rockets. The derivation process hingesonthe assumption thatthe local instantaneous regression rateisa function of the local mass fluxandthe axial port distance in the power law format as predicted by the classical theory developed by Marxman. In this study, we have developed physics-based nondimensional formulas for the space–time averaged regression rate and used these expressions to develop a scalable regression rate law for a selected propellant combination, paraffin-based SP-1a/ GOX,fromlimitedmotordata.Initially,spaceandtimeaveragingaretreatedseparately,whichwerelatercombined to develop a technique that allows for the coupling between the spatial and time variations to predict the port diameter and mass flow rate profiles as functions of time. Finally, a comprehensive technique to estimate the systematic and random errors on the regression rate and mass flux data is also outlined.

Modeling of Hybrid Rocket Low Frequency Instabilities

A comprehensive dynamic model of a hybrid rocket has been developed to understand and predict the transient behavior including instabilities. A linearized version of the transient model predicted the low-frequency chamber pressure oscillations that are commonly observed in hybrids. The source of the instabilities is based on a complex coupling of thermal transients in the solid fuel, the wall heat transfer blocking due to fuel regression rate, and the transients in the boundary layer that forms on the fuel surface. The oscillation frequencies predicted by the linearized theory are in very good agreement with 43 motor test results obtained from the hybrid propulsion literature. The motor test data used in the comparison cover a very wide spectrum of parameters including 1) four separate research and development programs; 2) three different oxidizers (liquid oxygen, gaseous oxygen, and nitrous oxide); 3) a wide range of motor dimensions, that is, from 5 in. (12.7 cm) diameter to 72 in. (182.9 cm) diameter, and operating conditions; and 4) several fuel formulations. A simple universal scaling formula for the frequency of the primary oscillation mode is also suggested.

Peregrine Hybrid Rocket Motor Ground Test Results

To further develop and demonstrate the applicability of liquefying-fuel hybrid rocket technology to low-cost launch applications, a small team of engineers is developing a medium-scale liquefying-fuel hybrid sounding rocket using storable propellants (paraffin wax and N2O) that will carry a 5 kg payload to the edge of space. This rocket, known as Peregrine, is being developed by engineers from NASA Ames, Stanford University, Space Propulsion Group Inc. (SPG, Sunnyvale, CA) and NASA Wallops, with a launch from Wallops anticipated at some point in the future. This paper focuses on the propulsion ground test results obtained to date.

Transient Modeling of Hybrid Rocket Low Frequency Instabilities

A comprehensive dynamic model of a hybrid rocket has been developed in order to understand and predict the transient behavior including instabilities. A linearized version of the transient model predicted the low-frequency chamber pressure oscillations that are commonly observed in hybrids. The source of the instabilities is based on a complex coupling of thermal transients in the solid fuel, wall heat transfer blocking due to fuel regression rate and the transients in the boundary layer that forms on the fuel surface. The oscillation frequencies predicted by the linearized theory are in very good agreement with 43 motor test results obtained from the hybrid propulsion literature. The motor test results used in the comparison cover a very wide spectrum of parameters including: 1) four separate research and development programs, 2) three different oxidizers (LOX, GOX, N2O), 3) a wide range of motor dimensions (i.e. from 5 inch diameter to 72 inch diameter) and operating conditions and 4) several fuel formulations. A simple universal scaling formula for the frequency of the primary oscillation mode is suggested.