A TREATISE ON NANOALUMINUM FOR SOLID ROCKET PROPULSION

The first solid propellant samples loaded with nanosized aluminum (nAl) were burned at the Semenov Institute of Chemical Physics (Russia), in 1970. Remarkable increases in burning rate and decreases in condensed combustion products size prompted excitation in solid rocket practitioners. Intense research and development programs started worldwide. Unfortunately, after about 50 years, pristine nanosized energetic materials ingredients still are not used in propulsive devices at industrial levels. Technical issues prevent extensive applications. To overcome these difficulties, several approaches were tested. But it was not enough to fulfill promises. What is the reality today? After recalling the unique applications for steady and unsteady burning made possible by nAl, the basic reasons hindering its large-scale use and possible fixes, when available, will be discussed. Use of advanced manufacture techniques and refined binder systems, with a suitable plasticizer or double curative, allows processing and manufacturing of metallized AP/HTPB grains up to 18% nAl mass fraction. But hazards remain ubiquitous: nAl particles are overly susceptible to ignition by electrostatic charges or friction, even if partially passivated. Likewise, nAl presents explosivity and flammability risks significantly higher than standard micron-sized aluminum (mu Al). Specific impulse efficiency and ecological risks have still to be assessed in a convincing way. DoD Hazard Division classification of 1.3 is imperative for propellants intended for space exploration missions and this currently precludes the use of nAl. Overall, while nAl-based formulations satisfactory for industrial use can be prepared, work is still needed for nAl scalability and commercialization in high-performance propulsive systems.