Abstract: A rotating detonation engine includes an annulus having a first wall, a second wall, and a volume having a detonation region in which a mixture of an oxidizer and a fuel detonate in a rotating fashion to create a pressure wave and detonation exhaust, the volume defining a downstream outlet through which detonation exhaust flows. The engine further includes an oxidizer outlet to output oxidizer and a fuel outlet to output fuel into the volume. The engine further includes an obstacle positioned in the volume and extending for an obstacle distance between the first wall and the second wall that is at least twenty five percent of an annulus distance from the first wall to the second wall, the obstacle designed to reflect the pressure wave such that a reflection of the pressure wave travels downstream and reduces an amount of the detonation exhaust that travels upstream.