Jacob D. Dennis

Amine–Boranes: Green Hypergolic Fuels with Consistently Low Ignition Delays

Complexation of amines with borane converts them to hypergols or decreases their ignition delays (IDs) multifold (with white fuming nitric acid as the oxidant). With consistently low IDs, amine-boranes represent a class of compounds that can be promising alternatives to toxic hydrazine and its derivatives as propellants. A structure-hypergolicity relationship study reveals the necessary features for the low ID.

Characterization of Ethylenediamine Bisborane as a Hypergolic Hybrid Rocket Fuel Additive

A boron-based catalytic and energetic fuel additive is explored as a mechanism to enhance performance of hybrid motors using nitric-acid-based oxidizers. Ethylenediamine bisborane is highly hypergolic with nitric acid, thereby eliminating the need for a separate ignition system and providing the potential for restart. Ignition delays of 2.9±0.3 ms for ethylenediamine bisborane powder and 31.7±19.6  ms for ethylenediamine bisborane/hydroxyl-terminated polybutadiene combinations have been measured using white fuming nitric acid as the oxidizer. Theoretical specific impulse values higher than any other nitric-acid-based hypergolic hybrid combination tested are obtained; in addition, ethylenediamine bisborane is air stable and not toxic. Also presented is a method of manufacturing large quantities of high-purity ethylenediamine bisborane.

Rheological Characterization of Monomethylhydrazine Gels

In this paper, viscometry, yield stress, and small shear strain oscillation experiments on monomethylhydrazine gels are presented. The goal is to define a gel formulation with a measurable yield stress under low-shear conditions and shear-thinning behavior at shear rates representative of typical rocket injectors. Hydroxypropylcellulose, fumed silica, and a hybrid of the two materials are investigated as gelling agents for monomethylhydrazine. The two gel formulations that included hydroxypropylcellulose exhibited similar shear-thinning behavior with power-law flow indices of ∼0.1 to 0.2. The pure fumed silica gel samples exhibited highly shear-thinning behavior with negative power-law flow indices indicative of thixotropic behavior. Tests performed after a preshear cycle showed shear-thinning power-law flow indices of ∼0.3. Unlike the pure hydroxypropylcellulose and hybrid gels, the fumed silica gel exhibited a well-defined, measureable yield stress. Small-strain oscillatory experiments showed that hydro...

Ignition of Gelled Monomethylhydrazine and Red Fuming Nitric Acid in an Impinging Jet Apparatus

Ignition characteristics of gelled hypergolic propellants have been investigated through the use of an impinging jet apparatus. Monomethylhydrazine, gelled with both hydroxypropylcellulose and fumed silica, was tested with red fuming nitric acid, gelled with two versions of fumed silica. Rheological measurements were made prior to injection. Inspection of high speed video results showed that the type of gelling agent used has a major impact on the ignition and combustion of gelled hypergolic propellants. Hydroxypropylcellulose fuel gels impinged onto fumed silica oxidizer gels did not result in ignition due to poor mixing between the propellant jets. Fumed silica gels with mismatched rheological properties also resulted in long ignition delays. Fumed silica gels with proper rheological properties resulted in repeatable ignition delays similar to those observed with neat propellants.

Combustion Characterization of Amine Borane Hypergolic Propellants

The efforts described in this paper focus on the investigation of new green propellants based on parameters important to rocket applications including ignition delay, characteristic velocity, and specific impulse. Such nontoxic hypergolic propellants are necessary for the development of more effective, affordable, and sustainable space exploration capabilities. Candidate propellants are identified using high speed video through drop tests of oxidizer falling into a small pool of fuel. Triethylamine borane (TEB), solid ethylenediamine bisborane (EDBB), and diisopropylethylamine borane (DPEB) have been found to react vigorously and ignite with adequately small delays, well below 10 ms, with white-fuming nitric acid (WFNA). TEB was selected to be further evaluated in a small combustor at pressures up to 200 psia. Two-plane optical access of an impinging spray sheet provides qualitative assessment of the atomization and ignition. Precision measurements of pressure, nozzle throat diameter, and injector piston size and velocity allow determination of characteristic velocity. These findings are compared against the industry standard of monomethylhydrazine (MMH) and red-fuming nitric acid (RFNA) measured in the same test article for relative performance and against predictions from a NASA combustion code as a measure of combustion efficiency. Across all propellants, the peak efficiency was found to depend greatly on injection conditions. Arrays of orifice tips and nozzle throat sizes grant the variability to test the different propellant combinations across a range of mass flow rates and pressures with similar injection parameters. Rapid iteration on many propellants within this new portfolio can be performed to reduce, and ideally eliminate, toxicity and the performance gap relative to MMH/RFNA.

Critical Ignition Criteria for Monomethylhydrazine and Red Fuming Nitric Acid in an Impinging Jet Apparatus

The ignition and combustion of monomethylhydrazine (MMH) and red fuming nitric acid (RFNA) has been investigated in an impinging jet apparatus. Variation of the jet contact area and total propellant mass flow rate at a constant oxidizer to fuel and jet momentum ratio allowed for investigation of critical ignition thresholds. Ignition probability was determined for each condition and separated into three categories: good ignition, a transition region and failed ignition. Two conditions resulted in reactive stream separation. These two cases are analyzed in comparison to reported cases of reactive stream separation with hydrazine/NTO in literature. Two critical MMH/RFNA ignition thresholds were identified: a residence time of approximately 0.3 ms and a non-dimensional sheet growth rate of 2-4 s -1 . The interaction between these two terms was also investigated and a power law correlation was identified. This correlation allows for prediction of a MMH/RFNA sheet growth rate given a residence time and jet diameter. Nomenclature

Performance of neat and gelled monomethylhydrazine and red fuming nitric acid in an unlike-doublet combustor

ABSTRACT Ignition and combustion characteristics of neat and gelled monomethylhydrazine (MMH) and red fuming nitric acid (RFNA) were investigated through the use of an unlike doublet injector. MMH gelled with either hydroxypropylcellulose or fumed silica was tested with RFNA gelled with fumed silica. High-speed video results showed that the type of gelling agent has a major impact on the ignition and combustion of the hypergolic propellants. Mismatched rheological properties hinder rapid and repeatable ignition events. Gels with matched rheological properties produce repeatable ignition delays similar to those observed with neat propellants. An elevated pressure combustion chamber was developed to allow direct comparison of combustion efficiency between neat and gelled propellants. Experimental combustion efficiencies were 83% for neat propellants compared to 68% for fumed silica gelled propellants. The results indicate that gelled propellants can operate with acceptable ignition delays but suffer from reduced combustion efficiency and stability when compared to neat propellants.

Design and Validation of a Bomb Reactor for Liquid Hypergolic Propellants

Inspired by early research in hypergolic propellants, a bomb reactor was developed to investigate their combustion under conditions of rapid mixing. The goal of this effort is to provide insight into ignition mechanisms for hypergolic propellants and allow for comparison between potential propellant combinations. In this work we present the first step in this effort focused on measuring the liquid phase reaction rates of monomethylhydrazine and red fuming nitric acid. To accomplish this goal, various concentrations of monomethylhydrazine, diluted with deionized water, were reacted with pure red fuming nitric acid. Chamber pressure and temperature were measured during experiments and the resulting pressurization rates were analyzed to provide insights into the reaction rates. In addition, since the mixing time is a controlling variable in the ignition process of hypergolic propellants, several experiments were performed with simulants to characterize the system operation prior to reactive testing. Experiments with concentrations of monomethylhydrazine greater than 80% appeared to have pressurization rates dominated by vigorous gas phase reactions while 60% showed a repeatable induction time followed by a rapid pressure rise. A lack of rapid pressure rise was observed for concentrations of 20% and 40% but the pressurization rate did increase with concentration. Future work is discussed to improve understanding of the liquid phase reaction rates investigated in this work.