Ammonium perchlorate (AP) is the primary oxidizer in most class 1.3 Department of Defense (DoD) solid rocket propellants. An estimated 24 million pounds of AP are manufactured per year. The ammonium (NH4+) and sodium (Na+) salts of perchlorate are extremely soluble and can easily diffuse into both groundwater and surface water. Recently, perchlorates have been detected in the groundwater at numerous hazardous waste sites in North America that are now designated Superfund sites. This contamination is associated with past rocket motor manufacturing and testing. The perchlorate anion is stable in the aqueous environment, and migrates easily into drinking water, so health care officials are regulating and monitoring this compound as an environmental hazard. The issue of perchlorate contamination is a topic of widespread interest due to potential health risks.
The objective of this project was to develop a series of environmentally benign solid rocket propellant formulations that incorporated fewer toxic ingredients and zero AP, as well as reducing hazardous waste in manufacture. This series of propellants would perform comparably to existing propellants, or, alternately, be candidates for programs for the development of propellants having advanced performance. Specific propellants were chosen within the first year of the project depending on opportunity at the time. Ultimately, a high-performance propellant, designed to replace both conventional reduced-smoke and minimum-smoke propellants of lesser performance, was selected for development.
A technology to replace AP with ingredients such as ammonium nitrate (AN), ammonium dinitramide (ADN), and more advanced organic oxidizers, was the focus of this research. ADN at the time was the most promising in terms of energy content, clean, agile manufacture and environmental innocuousness, so the emphasis was placed on this compound as the most likely AP replacement candidate. Development of alternate triazole cure chemistry for the propellant binder, replacing the conventional urethane cure, was undertaken to allow utilization of the greater chemical reactivity which the more energetic new oxidizers might display when incorporated into propellant formulations. Other ingredients, which would have the best potential in terms of high performance and low environmental burden, were tested for effect in these triazole-cured binder formulations as they emerged from the laboratories. At the same time, methods of prilling ADN were developed to allow easier processing of this energetic oxidizer in high-solids-loaded formulations. Processing aids, bonding agents, ballistic catalysts, and other formulation enhancements also came under development. Such materials had already been developed for AP, enhancing its efficacy as a propellant oxidizer, but they did not work on the new oxidizers. Analogous materials that had the same beneficial effects on the new oxidizers thus needed to be tested in the new formulations having the potential to replace AP formulations currently in use. Development of the necessary polymer derivatives and curatives for the binders was done by a University heterocyclic chemistry group and two commercial synthesis facilities, as well as work done in-house. Working closely with these synthesis laboratories allowed the rapid screening of many materials on the small scale, the determination of fundamental necessities for success, and ultimately the scaling up of the successful combinations to obtain sufficient quantities of the best performing formulations to be tested. Rocket motors made with these scaled-up mixtures were tested at an Air Force laboratory as well as in-house. A pilot-scale prill plant, which quenched small droplets of molten ADN into spheres for the best processibility, was demonstrated. Coatings and bonding agent candidates for the ADN and catalysts to modify its ballistic properties were tested, in order to increase the utility of this material to the level that AP currently enjoys. In addition to these characteristics, insensitivity to unwanted initiation, safety in handling, aging, environmental issues associated with manufacture and use of the replacement candidates, and their likely cost and availability had to be addressed.
Although this project was not successful in its goal of developing a viable AP-free propellant formulation of interest to another program for transition, the concepts that were explored here have generated interest among sponsors and are, predominantly, continuing under other programs. This SERDP effort was successful in the sense that it helped to move the opinion of the propulsion community to the idea that no miraculous ingredients exist that embody all the virtues of the chemicals that they are to replace, while simultaneously possessing all the desired new properties. The concept of “enablers”—i.e., coating and bonding agents, alternative binders and curatives, and particle sizing methods that will allow the use of currently available AP-replacement candidates—was, except for particle sizing and some aspects of the alternative binder cure, largely neglected and unexplored previous to this effort. Such developments will constitute the required technology base for the truly futuristic and advanced properties required of advanced, “green” propellants. This technology base will need to be developed from the beginning by such teams as the ones that supported this effort.
The triazole binder and the coating and bonding agent work is continuing in other programs under other funding, as are efforts to mitigate the sensitivities of the energetic materials that are available for use so that their performance and environmental advantages can be realized in fielded munitions. Efforts to interest the community in a prill plant for ADN will also continue.
This project began as an effort to develop an environmentally benign rocket propellant, having comparable performance properties the current conventional propellants, but with reduction of the contamination associated with current propellants in manufacture and use. However, in order to get the necessary leverage from other Programs, the replacement formulation had to be modified for a maximum performance replacement for the conventional propellants. The best oxidizer candidate to replace AP, in terms of performance, cost-effectiveness, and environmental burden, is ADN. ADN is a high-energy oxidizer that is easily made from low-cost ingredients which, when exposed to sunlight and water, is fairly quickly decomposed into AN. A triazole cyclization to cure the propellant binder is less affected by such reactive formulation ingredients than the conventional urethane cure and provides an environmentally friendly alternative to the toxic hazards of the polyisocyanates used in this cure. In addition, the triazole cure showed the promise of allowing uncured material to be saved from one day's production, and if necessary, recycled into that of the following day without degradation of quality of the cured material. With the new binders and curatives, and the processing and developing of enabling technology for the use of ADN, this oxidizer could be used as a replacement for AP and higher energy as well as lower environmental burdens would result. Given that the goals are significantly challenging, much basic understanding must yet be gained before such propellant performance goals can be met.