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 HBO2 at the AB-nitric acid interface may help drive the initial reactant decomposition and thermal runaway that eventually results in ignition. After the nitric acid was consumed/dispersed, the AB samples began burning with the ambient air, forming a quasi-steady state diffusion controlled flame. Emission intensity profiles measured as a function of height above the pellet revealed the BO/BO2-based emissions to be strongest in the flame zone (corresponding to the highest gas temperatures). Within the inner fuel-rich region of the flame, the HBO2 emission intensity peaked closer to the fuel surface after which it unexpectedly began to decrease across the 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.
The fourth area of research undertaken during this broader research effort investigated the use of ammonia borane and other amine borane additives on the ignition delay and predicted performance of novel hypergolic fuels based on tetramethylethylenediamine (TMEDA). Despite these materials being in some cases only sparingly soluble in TMEDA, solutions of ammonia borane, ethylenediamine bisborane, or tetramethylethylenediamine bisborane in TMEDA resulted in reductions of the mean ignition delays of 43-51%. These ignition delay reductions coupled with the significantly reduced toxicity of these fuels compared to the conventional hydrazine-based hypergolic fuels make them promising, safer alternatives to the more standard hypergolic fuels. Attempts were made to improve these ignition delays further by gelling the TMEDA, allowing for amine borane loadings beyond their respective solubility limits. Moving to these higher loadings had mixed results however, with the ignition delays of the AB/EDBB-based fuels increasing significantly with higher AB/EDBB loadings. The ignition delays of the TMEDABB-based fuels on the other hand decreased with increasing TMEDABB loadings, though the shortest were still comparable to those found with the saturated fuel solutions.
The final research area that was undertaken was focused on scaling up and developing fuel formulations based on ammonia borane for use in a small-scale hypergolic hybrid rocket motor. Characterization of the regression rate behavior of these fuels under motor conditions suggested the fuel mass flow rate was driven primarily by the thermal decomposition of the ammonia borane. This mechanism is fundamentally different from that which governs the regression rate of most conventional solid fuels used in hybrid rockets as well as that of ethylenediamine bisborane, a 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 independent of oxidizer mass fluxes. If successful, this would enable further optimization of the design for rocket systems utilizing these fuels, resulting in levels of performance that rival that of the more conventional hydrazine-based fuels.