The objective of the WP-2400 Environmentally Sustainable Liquid Gas Generator Formulations Program is to investigate ionic liquids (IL) for use as hydrazine replacements while retaining the desirable features of the hydrazine system (low flame temperature, catalyst bed ignition, commercial materials). Ionic liquid monopropellants achieve technical performance through increased density and improved packing. In addition, low volatility aids in operator handling and minimum exposure.

AF-M315E, US Air Force blend of HydroxylAmmonium Nitrate (HAN), HydroxyEthyl Hydrazinium Nitrate (HEHN), Ammonium Nitrate, NH₄NO₃ (AN), and H₂O, is a recently-developed liquid monopropellant which is demonstrating some success at replacing hydrazine. It is much less toxic, non-flammable and has higher output as a rocket propellant. However, it may not represent the optimal blend of performance and safety properties. AF-M315E achieves its high output at the expense of high combustion temperature and slow ignition response. In addition, since it is classified as an explosive material, there are restrictions on its storage and handling. The program seeks to retain some of the performance and handling improvements demonstrated by AF-M315E while recouping some of the operability and versatility associated with hydrazine. The program is striving to reduce the combustion temperature and ignition response characteristics of the non-toxic monopropellants so that it can approach the expectations established from hydrazine-based systems. 

During the course of this four-year effort, material candidates based on ionic liquids were evaluated as possible hydrazine replacements. The program evaluated cations from the triazole, tetrazole, pyrimidium, hydrazide and imidazole family, and the perchlorate, dinitramides and nitrate anions. Initial evaluations examined the performance, sensitivity, catalytic response, thermal stability and toxicity of each candidate. Overall the triazoles and hydrazides exhibited acceptable properties. Both candidate families exhibited good ignition, heat of formulation, and reactivity. The imidizoles were the first to be eliminated from further evaluations due to poor performance while the tetrazoles were eliminated due to thermal stability concerns and the pyrimidiums due to poor catalytic ignition. Nitrates were determined as the only acceptable cation. Perchlorates were eliminated due to environmental concerns while dinitramides exhibited unacceptable low exotherm temperatures.

The program approach starts with modeling efforts (thermodynamic performance, Environmental Safety and Occupational Health (ESOH)/toxicological) on individual components and compositions; candidates that pass this preliminary screening are only then admitted to the laboratory, in small amounts, for sensitivity and stability screening. Acceptable stability and sensitivity/handling characteristics allow quantities to scale up to levels suitable for small scale mix studies and performance assessments. Successful mixes and scale-up efforts on materials and compositions exhibiting suitable performance in crude laboratory testing would then be subjected to larger scale sensitivity testing, required to transport materials from their point of origin to the site of testing, in a heavyweight “workhorse” thruster. Those scaled-up materials are also subjected to more rigorous in vitro and in vivo ESOH testing.

Overall the program was able to select less toxic candidate formulations as hydrazine replacements for Hot Fire testing. All candidates exhibited acceptable sensitivity (i.e., electrostatic discharge (ESD), impact, thermal stability) and were successfully scaled-up for Hot Fire testing. With the appropriately heated catalyst bed, the hot fire testing showed rapid ignition and stable combustion. Each candidate selected for testing had unique characteristics: Both HAN oxidized and non-HAN oxidized formulations were evaluated with a variety of cations including HEHN, Hydroxy-Ethyl-Amino-Triazolium Nitrate (HEATN) and carbohydrazinium nitrate (CDN). The cation MATN was too difficult to purify and the impurities represented a toxicity challenge. The WP-2400 effort brought together unique organizations and technical expertise to tackle the complex technical challenge of providing a monopropellant formulation with the attributes of hydrazine while eliminating its characteristic toxicity. This successful effort provides the foundation for the elimination of hydrazine as a monopropellant fuel.

Candidate formulations were identified that met program requirements for performance, flame temperature, and showed significantly improved toxicity over hydrazine. During the course of the WP-2400 effort, the partnership between US Air Force Research Laboratory (AFRL), US Army Missile R+D and Engineering Command (AMRDEC), University of Delaware, Army Public Health Center (APHC) and Aerojet Rocketdyne (AR) has allowed for the evaluation of over 70 Energetic Ionic Liquids (EIL) and the synthesis of over 20 candidates. This successful effort was only possible by the collaboration combining propulsion, predictive modeling and toxicity testing. The program assessed triazoles, tetrazoles, pyrimidiums, dinitramides, and imidazoles cations and nitrate anions as possible less toxic hydrazine replacements. The formulations identified were not fully optimized; performance measurements in flight weight hardware need to be made to provide accurate performance assessment. Since the candidate formulations possess low flame temperature they should be compatible in existing hardware and therefore can be readily assessed. This technology will require heated catalyst beds; tuning of the catalyst bed technology will further improve design performance.