Matthew Stephens

High-Pressure Testing of Composite Solid Propellant Mixtures: Burner Facility Characterization

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.

Comparison of Hand and Mechanically Mixed AP/HTPB Solid Composite Propellants

The Rocket Propulsion Laboratory (RPL) at the University of Central Florida has recently produced a lab-scale propellant mixing unit. This new method of mechanically mixing the propellant is proposed as a more realistic, safe, and efficient method of propellant production than the previous method of hand mixing. To test for verification of the mechanically produced propellant and as a verification of data repeatability, results from both methods were compared. A baseline HTPB/AP, non-metallized propellant was used. The burning rate curves from two mixtures of each mixing method were collected and compared. Collected data showed that both methods produced extremely similar burning rate curves and that the mechanical mixing method had a slightly higher correlation coefficient than hand mixing.

Nano Additives and Plateau Burning Rates of Ammonium-Perchlorate-Based Composite Solid Propellants

Plateau propellants exhibit burning rate curves that do not follow the typical linear relationship between burning rateandpressurewhenplottedonalog–logscale,andbecauseofthisdeviation,theirburningbehaviorisclassifiedas anomalous burning. This paper contains a literature review chronicling the last half-century of research to better understand the mechanisms that govern anomalous burning and to shed light on current research into plateau and related propellants. In addition to the review, a series of experiments investigating the use of nanoscale TiO2-based additivesinammonium-perchloratehydroxyl-terminatedpolybutadienecompositepropellantswasperformed.The baseline propellant consisted of either 70 or 80% monomodal ammonium perchlorate (223 � m) and 30 or 20% binder composed of isophorone-diisocyanate-cured hydroxyl-terminated polybutadiene with Tepanol. Propellants were tested using a strand bomb between 3.45 and 17.24 MPa (34.0–170.1 atm). Analysis of the burning rate data shows that the crystal phase and synthesis method of the TiO2 additive are influential to plateau tailoring and to the apparent effectiveness of the additive in altering the burning rate of the composite propellant. Some of the discrepancy in the literature regarding the effectiveness of TiO2 as a tailoring additive may be due to differences in how the additive was produced. Doping the TiO2 with small amounts of metallic elements (Al, Fe, or Gd) showed additional effects on the burning rate that depend on the doping material and the amount of the dopant. This work providesthe firstpublishedpropellantmixturesandburningrateresultsforcompositepropellantsemployingmetaldoped nanoparticle additives.

Performance of Ammonium-Perchlorate-Based Composite Propellant Containing Nanoscale Aluminum

Several composite propellant mixtures of hydroxyl-terminated polybutadiene, ammonium perchlorate, and aluminum were prepared with and without the addition of small percentages of nanoscale aluminum and tested in a strandburneratpressuresupto34.5MPa.Theeffectofmonomodalversusbimodalammoniumperchlorateparticle size, coarse aluminum particle size, nano aluminum particle size, and coarse-to-fine ratios on burning rate and manufacturability were explored. A significant conclusion of the present study is that the addition of nanoscale aluminum does not always ensure an increase in the propellant’s burning rate when produced using conventional methods. It was observed that over the range of mixtures and pressures explored, a bimodal oxidizer is required for the nanoscale aluminum to affect the burning rate, and that a monomodal oxidizer tended to nullify any influence of thenanoscalealuminum.Insomecases,theadditionofnanosizedaluminumdecreasedtheburningrate.Thelevelof burning-rateincreaseordecreasedependedonthebimodalormonomodalammoniumperchlorateparticlesizes,the coarse aluminum particle size, and the pressure range.

Performance of AP-Based Composite Propellant Containing Nanoscale Aluminum

Several HTPB/AP/Al-based composite propellant mixtures were prepared with and without the addition of small percentages of nanoscale aluminum and burned in a strand burner at pressure up to 5000 psi. The effect of monomodal versus bimodal AP particle size, course aluminum particle size, nano aluminum particle size, and course-to-fine ratios on burn rate and manufacturability were explored. A significant conclusion of the present study is that the addition of nano-sized aluminum does not necessarily increase a propellants burn rate when prepared using conventional methods. It was observed that, over the range of mixtures and pressures explored, a bimodal oxidizer is required for the nanoscale Al to affect the burning rate, and that a monomodal oxidizer tended to nullify any influence. In some cases, the addition of nano-sized aluminum actually decreased the burn rate. The level of burn rate increase or decrease depended on the bimodal or monomodal AP particle sizes, the coarse Al particle size, and the pressure range.

Assessing the Mixedness of Composite Solid Rocket Propellants Using Fluorescent Particles

A new diagnostic technique was developed for assessing the effectiveness of mixing techniques of solid composite propellants using nanoparticle additives. The diagnostic uses nanosized quantum dots in suspension or micron-sized powders that are mixed into the propellant in place of the additives. Upon exposure to an ultraviolet light source, the particles fluoresce, hence serving as tracers to assess the uniformity of the mixture and therefore the effectiveness of the mixing procedure. Collection of the image using a digital camera provides data on intensity variations in the fluorescent signal, allowing for quantitative assessment of uniformity and mixedness. Various mixtures involving hydroxyl-terminated polybutadiene binder and ammonium perchlorate oxidizer were manufactured at various levels of mixing to test the diagnostic. In addition to confirming the uniformity of the nanosized particles using the target mixing procedure, variations in mixing quality and comparisons between mechanically and hand-mixed propellants showed distinct differences correlating to the mixedness of each propellant that was supported with data from burning rate studies. The present diagnostic can therefore also be used to assess the mixedness of propellants that do not contain nanoparticle additives. Other potential applications include curing agent dispersion assessment and linking homogeneity to mixedness and mechanical properties.

New Additives for Modifying the Burn Rate of Composite Solid Propellants

The ability of nano-particle additives to tailor the burning rate of composite solid rocket propellants is being explored in the authors’ laboratories. In the present study, two different nano-particles and two separate styles of mixing in the additive were investigated. These variations were tested on a typical HTPB/AP composite rocket propellant with varying characteristics. After a baseline study was completed without additives, a Taguchi L8 matrix was designed to fully test the effect of six different mixture variables and the two different additives on the burning rate. The propellants were tested in a strand bomb at pressures ranging from approximately 34 to 136 atm, and some representative data are provided. The results thus far indicate that small levels of nano-particle additives (∼ 1 wt %) can have significant effects on the magnitude and pressure dependence of the composite propellant.

NANO ADDITIVES AND PLATEAU BURNING RATES IN COMPOSITE SOLID PROPELLANTS

Plateau propellants exhibit burning rate curves that do not follow the typical linear relationship between burning rate and pressure when plotted on a log-log scale, and because of this deviation their burning behavior is classified as anomalous burning. This paper contains a literature review chronicling the last half-century of research to better understand the mechanisms that govern anomalous burning and to shed light on current research into plateau and related propellants. In addition to the review, a series of experiments investigating the use of nanoscale TiO2-based additives in AP-HTPB composite propellants was performed. The baseline propellant consisted of either 70% or 80% monomodal AP (223 μm) and 30% or 20% binder composed of IPDI-cured HTPB with Tepanol. Propellants were tested using a strand bomb between 500 and 2500 psi (34.0-170.1 atm). Analysis of the burning rate data shows that the crystal phase and synthesis method of the TiO2 additive are influential to plateau tailoring and to the apparent effectiveness of the additive in altering the burning rate of the composite propellant. Some of the discrepancy in the literature regarding the effectiveness of TiO2 as a tailoring additive may be due to differences in how the additive was produced. Doping the TiO2 with small amounts of metallic elements (Al, Fe, or Gd) showed additional effects on the burning rate that depend on the doping material and the amount of the dopant. This work provides the first published propellant mixtures and burning rate results for composite propellants employing metaldoped nanoparticle additives.