In order to clarify flame behavior in the combustion chamber of swirling-oxidizer-flow-type hybrid rocket engines, a small hybrid rocket engine with a large quartz glass window was fabricated to observe the inside of the combustion chamber from the front directly. PP and PMMA fuel grains were burned under the swirling and non-swirling oxygen flow conditions. For both PP and PMMA combustion, the disturbed swirling flames were found to develop closer to the grain surface than those without swirl, resulting in an increase of the fuel regression rates. The swirling flames consisted of not a single large flame but an annular aggregate of small striated flames on the grain surface. Emission spectra of counter-diffusion flames over the PP and PMMA rods showed that the strong continuum due to hot carbon particles in the flames, and the weak C 2 and CH emissions were also visible. The filtered photographs which took the continuum and C 2 emissions in the hybrid rocket combustion chamber suggested that the gas-phase reactions of PMMA with oxygen substantially occurred not in the center region of the combustion chamber, but near the PMMA gain surface. For PP combustion, the gas-phase reactions with oxygen may also occur in some degree near the central region.

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