DEVELOPMENT AND CHARACTERIZATION OF HIGH PERFORMANCE AMMONIA BORANE BASED ROCKET PROPELLANTS

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.