DEVELOPMENT AND CHARACTERIZATION OF HIGH PERFORMANCE AMMONIA BORANE BASED ROCKET PROPELLANTS
Creators: name: Baier, Michael J
Publisher: name: Purdue University Graduate School
Year: 2021
Subjects: Aerospace engineering not elsewhere classified

description: Historically, hypergolic propellants have utilized fuels based on hydrazine and its<br>derivatives due to their good performance and short ignition delays with the commonly used<br>hypergolic oxidizers. However, these fuels are highly toxic and require special handling<br>precautions for their use.<br>In recent years, amine-boranes have begun receiving attention as potential alternatives to<br>these more conventional fuels. The simplest of these materials, ammonia borane (AB, NH3BH3)<br>has been shown to be highly hypergolic with white fuming nitric acid (WFNA), with ignition<br>delays as short as 0.6 milliseconds being observed under certain conditions. Additionally,<br>thermochemical equilibrium calculations predict net gains in specific impulse when AB based<br>fuels are used in place of the more conventional hydrazine-based fuels. As such, AB may serve as<br>a relatively less hazardous alternative to the more standard hypergolic fuels.<br>Presented in this work are the results of five major research efforts that were undertaken<br>with the objective of developing high performance fuels based on ammonia borane as well as<br>characterizing their combustion behavior. The first of these efforts was intended to better<br>characterize the ignition delay of ammonia borane with WFNA as well as investigate various fuel<br>binders for use with ammonia borane. Through these efforts, it was determined that Sylgard-184<br>silicone elastomer produced properly curing fuel samples. Additionally, a particle size dependency<br>was observed for the neat material, with the finer particles resulting in ignition delays as short as<br>0.6 milliseconds, some of the shortest ever reported for a hypergolic solid fuel with WFNA.<br>The objective of the second area of research was intended to adapt and demonstrate a<br>temperature measurement technique known as phosphor thermography for use with burning solid<br>propellants. Using this technique, the surface temperature of burning nitrocellulose (a homogeneous solid propellant) was successfully measured through a propellant flame. During the<br>steady burning period, average surface temperatures of 534 K were measured across the propellant<br>surface. These measured values were in good agreement with surface temperature measurements<br>obtained elsewhere with embedded thermocouples (T = 523 K). While not strictly related to<br>ammonia borane, this work demonstrated the applicability of this technique for use in studying<br>energetic materials, setting the groundwork for future efforts to adapt this technique further to<br>studying the hypergolic ignition of ammonia borane.<br>The third research area undertaken was to develop a novel high-speed multi-spectral<br>imaging diagnostic for use in studying the ignition dynamics and flame structure of ammonia<br>borane. Using this technique, the spectral emissions from BO, BO2, HBO2, and the B-H stretch<br>mode of ammonia borane (and its decomposition products) were selectively imaged and new<br>insights offered into the combustion behavior and hypergolic ignition dynamics of ammonia<br>borane. After the fuel and oxidizer came into contact, a gas evolution stage was observed to<br>precede ignition. During this gas evolution stage, emissions from HBO2 were observed, suggesting<br>that the formation of HBO2 at the AB-nitric acid interface may help drive the initial reactant<br>decomposition and thermal runaway that eventually results in ignition. After the nitric acid was<br>consumed/dispersed, the AB samples began burning with the ambient air, forming a quasi-steady<br>state diffusion controlled flame. Emission intensity profiles measured as a function of height above<br>the pellet revealed the BO/BO2-based emissions to be strongest in the flame zone (corresponding<br>to the highest gas temperatures). Within the inner fuel-rich region of the flame, the HBO2 emission<br>intensity peaked closer to the fuel surface after which it unexpectedly began to decrease across the<br>flame zone. This is seemingly in contradiction to the current understanding that HBO2 is a stable product species and may suggest that for this system it is consumed to form BO2 and other boron oxides.<br>The fourth area of research undertaken during this broader research effort investigated the<br>use of ammonia borane and other amine borane additives on the ignition delay and predicted<br>performance of novel hypergolic fuels based on tetramethylethylenediamine (TMEDA). Despite<br>these materials being in some cases only sparingly soluble in TMEDA, solutions of ammonia<br>borane, ethylenediamine bisborane, or tetramethylethylenediamine bisborane in TMEDA resulted<br>in reductions of the mean ignition delays of 43-51%. These ignition delay reductions coupled with<br>the significantly reduced toxicity of these fuels compared to the conventional hydrazine-based<br>hypergolic fuels make them promising, safer alternatives to the more standard hypergolic fuels.<br>Attempts were made to improve these ignition delays further by gelling the TMEDA, allowing for<br>amine borane loadings beyond their respective solubility limits. Moving to these higher loadings<br>had mixed results however, with the ignition delays of the AB/EDBB-based fuels increasing<br>significantly with higher AB/EDBB loadings. The ignition delays of the TMEDABB-based fuels<br>on the other hand decreased with increasing TMEDABB loadings, though the shortest were still<br>comparable to those found with the saturated fuel solutions.<br>The final research area that was undertaken was focused on scaling up and developing fuel<br>formulations based on ammonia borane for use in a small-scale hypergolic hybrid rocket motor.<br>Characterization of the regression rate behavior of these fuels under motor conditions suggested<br>the fuel mass flow rate was driven primarily by the thermal decomposition of the ammonia borane.<br>This mechanism is fundamentally different from that which governs the regression rate of most<br>conventional solid fuels used in hybrid rockets as well as that of ethylenediamine bisborane, a<br>similar material in the amine borane family of fuels. Understanding this governing mechanism further may allow for its exploitation to enable high, nearly constant fuel mass flow rates<br>independent of oxidizer mass fluxes. If successful, this would enable further optimization of the<br>design for rocket systems utilizing these fuels, resulting in levels of performance that rival that of<br>the more conventional hydrazine-based fuels.<br>