Hybrid rockets have distinct advantages over their pure solid or liquid propellant counterparts, and their performance can be improved by inclusion of metal additives. Several metallic additives (micro-Al, micro-Ti, micro-Mg, micro-Zr, nano-Al, nano-B, and Mg-coated nano-B) were selected as potential candidates for hybrid rocket applications and characterized by applicable microscopy techniques. The regression rates and combustion efficiencies of plain HTPB and HTPB loaded with each additive at various concentrations (10, 20, and 30 by mass) burning in GOX were evaluated at moderate oxidizer mass fluxes (10-150 kg/m~2-s) and pressures (≤0.86 MPa, 125 psia). In general, the inclusion of any of the metallic additives led to a reduction in the regression rate and did not significantly change the combustion efficiency. The only exceptions were fuel formulations containing micro-Zr, which yielded a moderate (10-20) increase in the regression rate at a concentration of 10. The observed trends were more prevalent at higher oxidizer mass fluxes and higher additive loadings. The reductions in regression rate were attributed to heat transfer blocking effects derived from accumulation of additive particles on the fuel surface layer. These phenomena were especially prevalent in highly loaded fuel formulations containing the nano-additives that exhibited unstable combustion and periodic surface-layer shedding. Zirconium appears to be the best metallic additive available since it can yield the highest theoretical density-specific impulse under the lowest O/F operation ratio without resulting in decrements to overall performance. Notably, combustion efficiency data for all fuel formulations were well correlated to the combustion residence time, and high combustion efficiencies (＞95) were achievable when a satisfactory residence time (～75 ms) was realized.
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