Perchlorate is found in groundwater and drinking water throughout the United States. This contamination is largely attributed to the use of ammonium perchlorate (AP) in the solid fuel for rockets and missiles. The primary health concern is that perchlorate competes with iodine for uptake into the thyroid gland, which can result in an iodine deficiency. The defense industry must meet the standards adopted by the U.S. Environmental Protection Agency for perchlorate in groundwater. The EPA has issued an Interim Drinking Water Health Advisory for perchlorate of 15 µg/L. The recommended Reference Dose (RfD) is 0.7 µg/kg/day.
The objective of this project was to develop environmentally benign solid rocket propellant formulations that do not use AP as an oxidizer.
Many programs and patents have explored the feasibility of replacing AP with non-halogenated oxidizers, including hydrazinium nitroformate (HNF), ammonium nitrate (AN), ammonium dinitramide (ADN), and hydroxyl ammonium nitrate (HAN). Each of these compounds present significant problems in performance, sensitivity, toxicity, or stability. Using combinations of organic and inorganic fuels and oxidizers, AP-free propellants can be developed with the same performance characteristics as AP-based formulations. The environmental fate and transport and toxicity of all candidates were evaluated prior to formulation work. Propellants using a chemically cross-linked rubber binder with ball powder [60/40 nitrocellulose (NC)/nitroglycerine (NG)] as filler are an effective medium in which to exploit new oxidizers. Making the rubber binder with plasticizers that do not swell the NC enables high performance and good mechanical properties. ATK Thiokol has made propellants with 50% ball powder and 10% tetramethylene tetranitramine (HMX) in a plasticized binder that have go/no-go values in the 70 plus or minus 10 cards range in the Naval Ordnance Laboratory (NOL) Large Scale Gap Test (LSGT). The NOL LSGT at 70 cards is a critical safety threshold for new formulations that hope to replace AP-based versions. The ball powder system is low cost and sensitive to bismuth compounds for ballistic modification. The selected new organic and inorganic oxidizers were thought to have sufficient oxygen balances, heats of formation, and density to help replace AP when used as supplemental oxidizers in ball powder propellants.
This project investigated the synthesis of three new oxidizers to use as supplements to ball powder propellants: diammonium di(nitramido) dinitoethylene (ADNDNE), 1,3,3,5,7,7-hexanitro-1,5-diazacyclooctane (HCO), and cyclic nitramine/gem-dinitro compounds such as 1,3,5,5-tetranitrohexahydropyrimidine (DNNC). All attempts to synthesize ADNDNE were unsuccessful. A small amount of HCO was made, but the intermediates for HCO were too hazardous and new synthetic methods would be needed to make scale up feasible. DNNC was produced at the 25 gram scale, but synthesis at larger scales was not pursued. The technologies assessed in this project to replace AP result in propellant formulations that have the desired Class 1.3 performance, but with higher Class 1.1 hazard properties. Since Class 1.3 is a requirement, it does not appear practical to use this technology to replace AP.
It has been estimated that more than 24 million pounds of AP are produced each year. Recent concerns about AP emissions have resulted in the elimination of most release sources. However, when AP is used as an oxidizer in rocket propellants, contamination of groundwater at very low levels cannot be ruled out. This project was designed to develop propellants that have a modifiable burn rate, meet or exceed current performance specifications, and are environmentally benign.