Developments in processing and ballistics of dicyclopentadiene-based composite solid propellants

Patrick T O'Neil, Purdue University

Abstract

Over the past several years, dicyclopentadiene has been researched for use as a potential binder in composite solid propellant formulations. The binder of a composite solid propellant is very important because it plays a significant role in defining the mechanical, combustion, and ballistic characteristics of a propellant. Dicyclopentadiene, a hydrocarbon heterodimer that is capable of forming a highly-cross-linked polymer network through ring opening metathesis polymerization, has been investigated as a potential binder due to it's low viscosity, ability to withstand high solids loading, and favorable mechanical properties. While the continued work with dicyclopentadiene-based composite solid propellants has shown marginal improvements in propellant quality, the purpose of this work was to investigate methods that would produce substantial gains in propellant quality. A major short coming of dicyclopentadiene is the limited time permitted for mixing and casting of composite solid propellents due to the expediency of the polymerization reaction. In an effort to better understand, and predict, the amount of time available for mixing and casting operations, dicyclopentadiene gumstock polymerization tests were conducted to evaluate the influence of mixture temperature and inhibitor concentration on polymerization rate. Experiments have shown that an Arrhenius and power-law equation adequately model the influence of mixture temperature and inhibitor concentration, respectively, on dicyclopentadiene's polymerization rate. Paradoxically, recent research has shown that dicyclopentadiene's low viscosity acts more as a hinderance than an advantage. This is because the low viscosity allows particle setting to occur, loss of dicyclopentadiene into mold tooling due to capillary action, and evaporation of liquid dicyclopentadiene during the propellant's cure cycle, all of which contributes to reduce propellant quality. To remedy these issues, gelling agents were investigated as a means to modify the rheological characteristics of dicyclopentadiene. Experimental research showed that, due to dicyclopentadiene's non-polar nature, only non- or weakly-polar gelling agents were capable of forming a solution with dicyclopentadiene. All gelling agents that were capable of forming a solution also had the desired effect of modifying dicyclopentadiene's rheological characteristics. Furthermore, these gelling agents had the positive effect of lowering dicyclopentadiene's freezing point but, unfortunately, significantly extend the time required for dicyclopentadiene to fully polymerize. A limited-scope aging study was conducted involving dicyclopentadiene-based gumstocks and propellants. Uniaxial tensile tests and surface hardness measurements were used to monitor changes in polymerized dicyclopentadiene's mechanical properties over the course of weeks to years. Results from these tests indicate that polymerization is still underway, thus, the current cure cycle is insufficient. Another aspect of this work was to evaluate how propellant formulation variables affect ballistic and mechanical properties of dicyclopentadiene-based propellants. The addition of 0.25 mass% nano aluminum was found to greatly increases the burning rate of dicyclopentadiene-based propellants but, had a negative impact on propellant quality, attributed to the high surface area of the nano-sized particles. As a result of this line of research, a strong correlation between propellant quality and the specific surface area of solids added to dicyclopentadiene was discovered. In general, high-quality polymerized-dicyclopentadiene-based propellants are only achievable when the specific surface area of all solids remains under 4.82E-02 m2 g−1, regardless of total solids loading. Iron oxide is also capable of providing increases in burning rate but, appears to make the propellant more sensitive to impact stimuli and reduces the stress-strain capabilities of dicyclopentadiene-based propellants. Additionally, test conducted revealed that mechanical properties and impact stimuli have a parabolic relationship with coarse-to-fine ammonium perchlorate ratio; maximum values were observed to occur at a ratio of 13-to-7.

Degree

M.S.M.E.

Advisors

Heister, Purdue University.

Subject Area

Aerospace engineering|Mechanical engineering

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