In an effort to increase thrust per weight ratio and decrease pollutant emissions of aero-turbine jet engines, a circumferentially burning Ultra Compact Combustor (UCC) with a Cavity-in-a-Cavity design has been developed. A numerical analysis of this design has been conducted and compared with experimental results. The CFD model has been validated through a wide range of conditions and four alternative physical configurations of the UCC have been modeled. Emissions, combustor efficiencies, temperature and velocity profiles, and pressure drop values were used as comparison parameters. Numerical results indicate that increasing the outflow area will increase the pressure drop over the combustor and decrease the combustor efficiency. A significant decrease (250%) in the cavity circumferential velocity effectively decreased the fuel-air mixing in the cavity resulting in decreased combustion efficiencies. A decreased cavity length reduced combustor pressure drop significantly with only minimal increases in pollutant emissions. The addition of a curved vane to the decreased cavity length configuration further decreased the pressure drop.