Coreless Combustor Operation in 10-mm and 25-mm Rotating Detonation Rocket Engines

Rotating detonation rocket engines (RDRE) for space applications that generate less than 200 N of thrust generally have small combustor diameters with even smaller inner cores when operating with an annular combustor. The heat transfer to the inner core during high mass flux operation is a significant challenge for missions requiring long operational lifetimes and multiple restarts. Coreless combustors have been shown to operate with spinning detonation waves and comparable chamber pressures to those of annular RDREs operating under the same mass flux and equivalence ratio conditions. The elimination of inner core burnout risk with small-scale coreless RDREs, while maintaining the efficiency and performance advantages of rotating detonative combustion, is a strong motive for in-depth investigations of this configuration. Toward this end, experimental studies of coreless RDREs having 25 mm and 10 mm cylindrical combustors have been carried out with methane-oxygen and hydrogen –oxygen propellant. The 25-mm-combustor was operable from very fuel-lean to very fuel-rich conditions with CH4-O2 propellant whereas spinning detonation waves in the 10-mm-combustor could not be stabilized with the same propellant when the equivalence ratio was less than 1.15. It was shown that slightly leaner propellant compositions would sustain RDRE operation when the CH4 was pre-heated prior to injection. With H2-O2 propellant, however, RDRE operation in the 10-mm-combustor was maintained over a wide range of equivalence ratios with wave numbers of one, two, or three being established as mass flux was increased. The limits of spinning wave operation in 10-mm coreless combustors were found to require the combustor diameter-to-detonation cell width ratio be greater than 10.