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Low regression rates limit application of hybrid rocket engines (HREs). In this paper, combustion of HTPB-and solid para©n wax-based fuels is investigated in a lab-scale burner by a time-resolved optical technique. The e¨ects of pressure are explored over the range 7 to 16 bar. Nanosized (ALEX TM , 100 nm, uncoated) and micronsized (MgB composite, 5 µm) metal additives are tested. For all runs, the instantaneous regression rate is not constant but higher at the beginning of tests. Overall, MgB performance and relatively ease of manufacturing enable to consider this additive as an interesting novel candidate to enhance hybrid rocket engines performance. NOMENCLATUREA space-averaged value (of parameter A) A time-averaged value (of parameter A) 2D two-dimensional a multiplicative factor [Eqs. (1) and (5)