Development and Characterization of High Performance Ammonia Borane Based Rocket Propellants

Michael Baier, Purdue University

Abstract

Historically, hypergolic propellants have utilized fuels based on hydrazine and its derivatives due to their good performance and short ignition delays with the commonly used hypergolic oxidizers. However, these fuels are highly toxic and require special handling precautions for their use. In recent years, amine-boranes have begun receiving attention as potential alternatives to these more conventional fuels. The simplest of these materials, ammonia borane (AB, NH3BH3) has been shown to be highly hypergolic with white fuming nitric acid (WFNA), with ignition delays as short as 0.6 milliseconds being observed under certain conditions. Additionally, thermochemical equilibrium calculations predict net gains in specific impulse when AB based fuels are used in place of the more conventional hydrazine-based fuels. As such, AB may serve as a relatively less hazardous alternative to the more standard hypergolic fuels. Presented in this work are the results of five major research efforts that were undertaken with the objective of developing high performance fuels based on ammonia borane as well as characterizing their combustion behavior. The first of these efforts was intended to better characterize the ignition delay of ammonia borane with WFNA as well as investigate various fuel binders for use with ammonia borane. Through these efforts, it was determined that Sylgard-184 silicone elastomer produced properly curing fuel samples. Additionally, a particle size dependency was observed for the neat material, with the finer particles resulting in ignition delays as short as 0.6 milliseconds, some of the shortest ever reported for a hypergolic solid fuel with WFNA. The objective of the second area of research was intended to adapt and demonstrate a temperature measurement technique known as phosphor thermography for use with burning solid propellants. Using this technique, the surface temperature of burning nitrocellulose (a homogeneous solid propellant) was successfully measured through a propellant flame. During the steady burning period, average surface temperatures of 534 K were measured across the propellant surface. These measured values were in good agreement with surface temperature measurements obtained elsewhere with embedded thermocouples (T = 523 K). While not strictly related to ammonia borane, this work demonstrated the applicability of this technique for use in studying energetic materials, setting the groundwork for future efforts to adapt this technique further to studying the hypergolic ignition of ammonia borane. The third research area undertaken was to develop a novel high-speed multi-spectral imaging diagnostic for use in studying the ignition dynamics and flame structure of ammonia borane. Using this technique, the spectral emissions from BO, BO2, HBO2, and the B-H stretch mode of ammonia borane (and its decomposition products) were selectively imaged and new insights offered into the combustion behavior and hypergolic ignition dynamics of ammonia borane. After the fuel and oxidizer came into contact, a gas evolution stage was observed to precede ignition. During this gas evolution stage, emissions from HBO2 were observed, suggesting that the formation of HBO2at the AB-nitric acid interface may help drive the initial reactant decomposition and thermal runaway that eventually results in ignition.

Degree

Ph.D.

Advisors

Son, Purdue University.

Subject Area

Chemical engineering

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