Prospects of Aluminum Modifications as Energetic Fuels in Chemical Rocket Propulsion

The use of metals as high-energy fuels has been for long time a common approach to increase performance of chemical rocket propulsion in general. This effort was initially triggered by theoretical thermochemical considerations, but under real operating conditions, a series of collateral and unforeseen effects occurred, with both positive and negative consequences. After six decades, the use of micron-sized Al is the most common practice at industrial level for solid rocket propulsion in particular. Yet attempts are under way to mitigate some of the most deleterious effects: notably, the two-phase flow losses and slag accumulation taking place in gasdynamic nozzles. In this paper, a range of modified Al powders is discussed, going from nano-sized uncoated to coated Al particles and from chemically to mechanically activated micron-sized Al. These variants are duly characterized and comparatively tested under laboratory burning conditions. Due to page limitations, mainly the class of aluminized composite propellants (ammonium perchlorate/inert binder) and operating conditions often used in space applications are investigated. The reader is cautioned to avoid making generalizations to other formulations and conditions based on this limited dataset. Each of the tested Al variants has its own properties, and implementation in full-scale propulsive systems needs to be carefully evaluated for an overall assessment. The recommended strategy for best results is a dual mode Al mixture, synergistically exploiting each component. Other metal fuels, especially hydrides and boron compounds, are examined as well. New trends, capable of drastically changing the current situation but still in their infancy as of this writing, are briefly discussed at the end of the paper.