Robert A. Frederick

Direct ultrasonic measurement of solid propellant ballistics

This article illustrates the application of an ultrasonic pulse–echo technique to determine the burning rate of a composite solid propellant as a function of pressure. An evaluation of the measurement uncertainty of the method is also presented. Unlike the more traditional strand burner techniques, where dozens of constant pressure tests are necessary, the ultrasonic technique measures the burn surface position thousands of times per second as the pressure varies. This reduces the number of tests necessary to determine the ballistic characteristics of the propellant by an order of magnitude. This work presents new methods to characterize the changing speed of sound in the propellant and quantitative estimates of the measurement uncertainty in the burning rate measurement. The results of the uncertainty analysis showed that the measurement is accurate to around 4%. The propellant samples were tested in a closed-combustion vessel, under pressurization rates of up to 15.8 MPa/s. The data obtained with the cl...

Mechanistic Assessment of Swirl Coaxial Injection by Quantitative X-Ray Radiography

Detailed x-ray radiographic experiments were conducted to evaluate the time-averaged spray characteristics of a liquid rocket swirl injector. Sprays issued from a single liquid-centered swirl coaxial element—with and without coannular gas flow—were exposed to focused, monochromatic x rays produced by a synchrotron light source. Two-dimensional attenuation data are presented at various axial stations and reveal both projected mass distribution of liquid in the injector near field and mass-weighted axial velocity integrated over time. Measurements describing spray morphology are also inferred from the x-ray data and compared with those measured through objective image processing of visible light imagery. The gas flow is observed to reduce spray cone angle up to 50% and increase liquid film thickness up to 20% in the near field. Measurements compare well with those extracted from imagery of the pressure-swirl spray; however, the x-ray technique is more robust in resolving liquid film thickness for the optica...

Pulse-echo measurements of unsteady propellant deflagration

This article discusses a laboratory test method for the measurement of nonsteady deflagration rates and combustion-stability properties of solid propellants. The method combines ultrasonic pulse-echo measurement, fluidic pressure modulation, and digital signal processing techniques to compute the real and imaginary components of pressure-coupled response functions of solid propellants. A description of the apparatus identifies the major components and their functions. A section on the experimental procedure illustrates the steps necessary to conduct a test, and a section on the data reduction technique describes the sequence of steps employed to compute the nonsteady burn rate and the pressure-coupled response. A detailed implementation of the technique as it applies to two representative tests follows, in which graphs and tables illustrate the intermediate data reduction steps. Details of the evaluation of the uncertainties associated with the technique and the data reduction algorithms are also presented.

Jet Breakup Length to Impingement Distance Ratio for Like Doublet Injectors

Cold-flow experiments using water at atmospheric pressure were conducted to systematically determine the effect of the jet breakup length to impingement distance ratio for like-doublet injectors. The ratio had a large effect on the breakup characteristics of the spray. For ratios greater than one, a flat sheet was formed that subsequently disintegrated into waves of ligaments and droplets. The breakup length of the sheet increased with the Weber number up to a transition point where it began to decrease with further increases of the Weber number. For ratios equal to one, an unsteady flat sheet was formed due to the jets intermittently disintegrating before reaching the impingement point. New empirical sheet breakup length and transition correlations were developed for these configurations. For ratios less than one, no sheet was formed. In addition, measurements of the ligament wavelengths and centerline droplet diameters were conducted for jet breakup length to impingement distance ratios greater than or ...

An Analytical Investigation Characterizing the Application of Single Frequency Acoustic Modulation for High Frequency Combustion Instability Suppression

This research investigation presents a one dimensional acoustically consistent linear modal analysis based analytical method to analyze the stability of a fundamental longitudinal combustion instability mode when acoustic modulation is introduced. This analytical method was developed to qualitatively describe high frequency instability suppression trends seen in experiments in which strategic acoustic modulation was applied. This experimental liquid rocket engine combustor uses a speaker located in the oxidizer post, upstream of an injector to acoustically modulate the incoming propellant. This study provides a thorough derivation of an analytical method that was utilized to study the acoustic stability of the instability mode as single frequency acoustic modulation from the speaker was applied. Two ducts with different diameters were used to approximate the experimental rocket engine combustor geometry, with acoustically consistent matching conditions applied at the interface of the ducts. The boundary condition of the exit of the chamber was assumed to be acoustically closed, while the inlet boundary condition was derived in order to simulate a speaker introducing single frequency acoustic modulations into the combustor. Five case studies considering a fundamental longitudinal mode from f = 2, 300 − 2, 500Hz were investigated in order to determine the stability trend of the instability as acoustic forcing is applied. All cases consistently predicted a naturally unstable mode in the absence of acoustic forcing from the speaker, which is consistent with the spontaneously excited combustion instability observed experimentally without modulation from the speaker. It was found that the model was able to successfully predict trends in the stability of the mode observed in the experimental tests, which provides insight into the causes of suppression phenomena in the spontaneous longitudinal instability.

MEASURING THE REGRESSION OF A BURNING SOLID PROPELLANT

A new instrument is presented that measures the regression of a burning solid propellant surface. The system extracted the diffuse reflection of a laser beam from the burning surface using synchronous detection. The incoming beam had a 4000 Hz angular modulation that served as the basis of a synchronous detection scheme. A galvanometer kept the beam spot centered under a sensor as the burn surface regressed. Theoretical analysis of the system using the component response properties resulted in the design of a controller for optimal bandwidth and stability. Laboratory evaluation on diffuse surfaces revealed a system bandwidth of 200 Hz, a range of 50 mm, and a resolution of 0.03 mm. Experiments conducted on burning propellants showed that the instrument successfully tracked the position of the surface at combustion pressures up to 17 atm. Under these conditions, the combustion environment reduced the beam transmission by up to 24 dB. This lowered the system bandwidth to 20 Hz and the resolution to 0.08 mm....