Hybrid rocket engines are promising propulsion systems which present appealing features such as safety, low cost, and environmental friendliness. On the other hand, certain issues hamper the development hoped for. The present paper discusses approaches addressing improvements to one of the most important among these issues: low fuel regression rate. To highlight the consequence of such an issue and to better understand the concepts proposed, fundamentals are summarized. Two approaches are presented (multiport grain and high mixture ratio) which aim at reducing negative effects without enhancing regression rate. Furthermore, fuel material changes and nonconventional geometries of grain and/or injector are presented as methods to increase fuel regression rate. Although most of these approaches are still at the laboratory or concept scale, many of them are promising.

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Approaches to Low Fuel Regression Rate in Hybrid Rocket Engines

Much Research on composite solid propellants has been performed over the past few decades and much progress has been made, yet many of the fundamental processes are still unknown, and the development of new propellants remains highly empirical. Ways to enhance the performance of solid propellants for rocket and other applications continue to be explored experimentally, including the effects of various additives and the impact of fuel and oxidizer particle sizes on burning behavior. One established method to measure the burn rate of composite propellant mixtures in a controlled laboratory setting is to use a constant-volume bomb, or strand burner. To provide high-pressure burn rate data at pressures up to 360 atm, the authors have installed and characterized a new strand burner facility at the University of Central Florida. Details on the new facility and the measurement procedures are summarized. Repeatability between different batches of the same mixture has been demonstrated to be very good, and two common HTPB/Ammonium Perchlorate (AP) propellant mixtures containing 7/3 and 5/5 bimodal AP distributions were tested in the burner. The resulting burn rates are compared to data from the literature with good agreement in burn rate exponent and the impact of changing from a 7/3 to a 5/5 AP split.

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High Pressure Testing Of Composite Solid Rocket Propellant Mixtures: Burner Facility Characterization

The development of an ultrasound technique for precisely measuring the instantaneous regression rate of a solid-rocket propellant under transient conditions is reviewed. The technique is used to measure the burning-rate response of several solid propellants to an oscillatory chamber pressure with a frequency of up to 300 Hz. This measurement is known as the propellant's pressure-coupled response function and is used as an input into rocket stability prediction models. The ultrasound waveforms are analyzed using cross-correlation and other digital signal processing techniques to determine burning rate. Digital methods are less prone to bias and offer greater flexibility than other techniques previously used. The resulting data are corrected for compression effects. The effects of a changing thermal profile on the measurement are discussed. Other phenomena that may corrupt the measurement, such as surface roughness, are also covered. Results of the experiments are compared to data from two other measurement techniques: a T-burner and a magnetic flowmeter.

Evaluation of ultrasound technique for solid-propellant burning-rate response measurements

Review of ultrasonic technique for steady state burning rate measurements

The measurement of solid propellant burning rates using ultrasound requires the simultaneous acquisition and analysis of ultrasonic signals and pressure data simultaneously in a wide range of pressure values during the process of propellant burning. Recently, this method has been proposed as an effective approach based on an analysis of full waveforms of ultrasonic signals together with a laboratory prototype system in which the proposed approach has been implemented. However, this prototype system had limitations in terms of data processing speed and signal processing procedures. To overcome such limitations, in the present study, we develop a dedicated, high speed system that can acquire ultrasonic full waveforms and pressure data up to 2,000 times per second. Our system can also estimate the burning rate as a function of pressure using a special software based on ultrasonic full waveform analysis. This paper describes the approach adopted in this high speed system, along with the burning rate measurement results obtained from three propellants with different burning characteristics.

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Measurement of solid propellant burning rates by analysis of ultrasonic full waveforms

Determination of the ultrasonic burning rate technique resolution

Ultrasonic, pulse-echo transducers send acoustic waves through a solid propellant to determine the web thickness and the burning rate. The acoustic speed in the propellant increases with increasing pressure and must be quantified to make accurate burning-rate calculations. This experimental work determined the speed of sound as a function of pressure for cured and uncured binder materials hydroxyl-terminated polybutadiene (HTPB) and polytetrahydrofuran (PTHF) and epoxy. A nitrogen system pressurized the 3-cm-diam, 2-47-mm-thick, cylindrical samples to pressures of 15 MPa at pressurization rates of 2 or 14 MPa/s. The results show that the acoustic velocity of cured HTPB, PTHF, and epoxy are 1510, 1650, and 2600 m/s, respectively, at 6.9 MPa. Velocities for uncured HTPB and PTHF range from 1560 to 1610 m/s. The uncertainty is estimated at 1% for a 15-mm sample thickness. The pressure sensitivity is linear above 3 MPa at 1.4-4.3 m/s . MPa. A study of cured and uncured HTPB samples having lengths from 2 to 47 mm revealed a 20% variation in the sound speed at sample lengths below 10 mm. This effect could be the result of a measurement bias for the cured samples and surface tension for the uncured samples.

Ultrasonic Properties of Propellant Ingredients

The pressure coupled response function was measured for a composite non-metallized propellant in a vented bomb. The technique described is unique in the manner in which the pressure oscillations are generated. An inert gas is injected into the combustion chamber to generate the necessary pressure oscillations. The burning rate was measured using the ultrasonic technique. Testing was done at various frequencies from lo75 Hz and pressures from 600-1200 psig. The pressure coupled response function was evaluated for each test together with the random error. An analytic model of the conditions within the combustion chamber has also been developed to aid in determining the settings needed to obtain the desired conditions in the test. These results seem to be reasonable. The data at high frequency approachs the signal to noise limit.

Direct Ultrasonic Measurement of Solid Propellant Combustion Transients

The pulse-echo ultrasonic technique has been successfully applied to solid propellant for burning rate measurement. However, no one has ever performed an uncertainty analysis of this technique. This study presents a detailed uncertainty assessment of the technique and shows the results for a set of three selected propellants. Direct Monte-Carlo simulations were performed to determine the uncertainty in the burning rate. The total uncertainty was shown to range between 0.01 in/set and 0.035 in/set and the relative uncertainty from 3.5 to 5%. In general, when propellant burns faster, the total uncertainty is larger and the relative uncertainty is smaller. The total uncertainty in the burning rate tends to follow the same variation with pressure as the burning rate and the relative uncertainty stays pretty much constant during an entire test. A sensitivity analysis shows that the parameters which have the most influence on the mcertainty in the burning rate are the propagation time of the ultrasonic pulses through the propellant and the initial thickness of the propellant. The thicker the initial length of the propellant sample is, the lower the uncertainty will be.

Marlow Moser

Papers

Determination of the ultrasonic burning rate technique resolution

Ultrasonic Properties of Propellant Ingredients

Direct Ultrasonic Measurement of Solid Propellant Combustion Transients

Uncertainty assessment of the pulse-echo ultrasonic burning rate measurement technique

Mixed Hybrid Propellants