A pressure gain combustor comprises a detonation chamber, a pre-combustion chamber, an oxidant swirl generator, an expansion-deflection (E-D) nozzle, and an ignition source. The detonation chamber has an upstream intake end and a downstream discharge end, and is configured to allow a supersonic combustion event to propagate therethrough. The pre-combustion chamber has a downstream end in fluid communication with the detonation chamber intake end, an upstream end in communication with a fuel delivery pathway, and a circumferential perimeter between the upstream and downstream ends with an annular opening in communication with an annular oxidant delivery pathway. The oxidant swirl generator is located in the oxidant delivery pathway and comprises vanes configured to cause oxidant flowing past the vanes to flow tangentially into the pre-combustion chamber thereby creating a high swirl velocity zone around the annular opening and a low swirl velocity zone in a central portion of the pre-combustion chamber. The E-D nozzle is positioned in between the pre-combustion chamber and detonation chamber and provides a diffusive fluid pathway therebetween. The ignition source is in communication with the low swirl velocity zone of the pre-combustion chamber. This configuration is expected to provide a combustor with a relatively low total run-up DDT distance and time, thereby enabling high operating frequencies and corresponding high combustor performance.